1.
Iapetus (moon)
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Iapetus is best known for its dramatic two-tone coloration. Discoveries by the Cassini mission in 2007 revealed several other unusual features, Iapetus was discovered by Giovanni Domenico Cassini, an Italian astronomer, in October 1671. He had discovered it on the side of Saturn and tried viewing it on the eastern side some months later. This was also the case the year, when he was again able to observe it on the western side. Cassini finally observed Iapetus on the side in 1705 with the help of an improved telescope. Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and this means that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and that the dark hemisphere is visible when Iapetus is on the eastern side. The dark hemisphere was later named Cassini Regio in his honour, Iapetus is named after the Titan Iapetus from Greek mythology. When first discovered, Iapetus was among four Saturnian moons labelled the Sidera Lodoicea by their discoverer Giovanni Cassini after King Louis XIV, however, astronomers fell into the habit of referring to them using Roman numerals, with Iapetus being Saturn V. Once Mimas and Enceladus were discovered in 1789, the scheme was extended. And with the discovery of Hyperion in 1848, Iapetus became Saturn VIII, geological features on Iapetus are named after characters and places from the French epic poem The Song of Roland. Examples of names used include the craters Charlemagne and Baligant, the one exception is Cassini Regio, the dark region of Iapetus, named after the regions discoverer, Giovanni Cassini. The orbit of Iapetus is somewhat unusual, although it is Saturns third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan. It has also the most inclined orbital plane of the regular satellites, the cause of this is unknown. From Iapetus, Saturn would appear to be 1°56 in diameter, the low density of Iapetus indicates that it is mostly composed of ice, with only a small amount of rocky materials. These features often lead it to be characterized as walnut-shaped, Iapetus is heavily cratered, and Cassini images have revealed large impact basins, at least five of which are over 350 km wide. The largest, Turgis, has a diameter of 580 km, its rim is extremely steep, Iapetus is known to support long-runout landslides or sturzstroms, possibly supported by ice sliding. In the 17th century, Giovanni Cassini observed that he could see Iapetus only on the west side of Saturn and he correctly deduced that Iapetus is locked in synchronous rotation about Saturn and that one side of Iapetus is darker than the other, conclusions later confirmed by larger telescopes. The difference in colouring between the two Iapetian hemispheres is striking, the leading hemisphere and sides are dark with a slight reddish-brown coloring, while most of the trailing hemisphere and poles are bright
2.
Impact crater on Mars
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This is a list of craters on Mars. Impact craters on Mars larger than 1 km exist by the hundreds of thousands, names are assigned by the International Astronomical Union after petitioning by relevant scientists, and in general, only craters that have a significant research interest are given names. Martian craters are named after scientists and science fiction authors. Craters cannot be named for living people, and names for small craters are rarely intended to commemorate a specific town, latitude and longitude are given as planetographic coordinates with west longitude
3.
Ray system
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A ray system comprises radial streaks of fine ejecta thrown out during the formation of an impact crater, looking somewhat like many thin spokes coming from the hub of a wheel. The rays may extend for lengths up to several times the diameter of their originating crater, Ray systems have been identified on the Moon, Earth, Mercury, and some moons of the outer planets. Rays appear at visible, and in some infrared wavelengths. Typically, visible rays have a higher albedo than the surrounding surface, more rarely an impact will excavate low albedo material, for example basaltic-lava deposits on the lunar maria. Thermal rays, as seen on Mars, are apparent at night when slopes. The layering of rays across other surface features can be useful as an indicator of the age of the impact crater. Micrometeorites in particular produce a glassy melt in the regolith that lowers the albedo, Rays can also become covered by lava flows, or by other impact craters or ejecta. The physical nature of lunar rays has historically been a subject of speculation, early hypotheses suggested that they were deposits of salt from evaporated water. Later they were thought to be deposits of ash or streaks of dust. After the impact origin of craters became accepted, Eugene Shoemaker suggested during the 1960s that the rays were the result of fragmented ejecta material, recent studies suggest that the relative brightness of a lunar ray system is not always a reliable indicator of the age of a ray system. Instead the albedo also depends on the portion of iron oxide, low portions of FeO result in brighter materials, so such a ray system can retain its lighter appearance for longer periods. Thus the material needs to be factored into the albedo analysis to determine age. Among the lunar craters on the side with pronounced ray systems are Aristarchus, Copernicus, Kepler, Proclus, Dionysius, Censorinus, Glushko. Similar ray systems also occur on the far side of the Moon, such as the rays radiating from the craters Giordano Bruno, Necho, Ohm, and the small but prominent Pierazzo. North Ray and South Ray craters, each with a ray system, were observed from the ground by the astronauts of Apollo 16 in 1972. List of craters with ray systems Reiner Gamma Martel, L. M. V, Lunar Crater Rays Point to a New Lunar Time Scale, Planetary Science Research Discoveries. Chipping pieces off Mars, Martian craters with rays may be the sources for the Mars meteorites found on Earth
4.
Meteor Crater
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Meteor Crater is a meteorite impact crater approximately 37 miles east of Flagstaff and 18 miles west of Winslow in the northern Arizona desert of the United States. The site was known as the Canyon Diablo Crater and fragments of the meteorite are officially called the Canyon Diablo Meteorite. Scientists refer to the crater as Barringer Crater in honor of Daniel Barringer, the crater is privately owned by the Barringer family through their Barringer Crater Company, which proclaims it to be the best preserved meteorite crater on Earth. Despite its importance as a site, the crater is not protected as a national monument. It was designated a National Natural Landmark in November 1967, Meteor Crater lies at an elevation of about 1,740 m above sea level. It is about 1,200 m in diameter, some 170 m deep, the center of the crater is filled with 210–240 m of rubble lying above crater bedrock. One of the features of the crater is its squared-off outline. The crater was created about 50,000 years ago during the Pleistocene epoch, the area was an open grassland dotted with woodlands inhabited by woolly mammoths and giant ground sloths. Since the craters formation, the rim is thought to have lost 15–20 m of height at the rim crest due to natural erosion, similarly, the basin of the crater is thought to have approximately 30 m of additional post-impact sedimentation from lake sediments and of alluvium. These erosion processes are the reason that very few remaining craters are visible on Earth, the relatively young age of Meteor Crater, paired with the Arizona climate, have allowed this crater to remain almost unchanged since its formation. The lack of erosion that preserved the craters shape helped lead to this crater being the first crater recognized as an impact crater from a natural celestial body. The object that excavated the crater was a meteorite about 50 meters across. The speed of the impact has been a subject of some debate, modeling initially suggested that the meteorite struck at up to 20 kilometers per second but more recent research suggests the impact was substantially slower, at 12.8 kilometers per second. It is believed that half of the impactors bulk was vaporized during its descent. Impact energy has been estimated at about 10 megatons, the meteorite was mostly vaporized upon impact, leaving little remains in the crater. The crater came to the attention of scientists following its discovery by American settlers in the 19th century and this was not an unreasonable assumption, as the San Francisco volcanic field lies only about 40 miles to the west. In 1891, the mineralogist Albert E. Foote presented the first scientific paper about the meteorites of Northern Arizona, several years earlier, Foote had received an iron rock for analysis from a railroad executive. Foote immediately recognized the rock as a meteorite and led an expedition to search, the team collected samples ranging from small fragments to over 600 pounds
5.
Moon
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The Moon is an astronomical body that orbits planet Earth, being Earths only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, following Jupiters satellite Io, the Moon is second-densest satellite among those whose densities are known. The average distance of the Moon from the Earth is 384,400 km, the Moon is thought to have formed about 4.51 billion years ago, not long after Earth. It is the second-brightest regularly visible celestial object in Earths sky, after the Sun and its surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its cycle of phases have made the Moon an important cultural influence since ancient times on language, calendars, art. The Moons gravitational influence produces the ocean tides, body tides, and this matching of apparent visual size will not continue in the far future. The Moons linear distance from Earth is currently increasing at a rate of 3.82 ±0.07 centimetres per year, since the Apollo 17 mission in 1972, the Moon has been visited only by uncrewed spacecraft. The usual English proper name for Earths natural satellite is the Moon, the noun moon is derived from moone, which developed from mone, which is derived from Old English mōna, which ultimately stems from Proto-Germanic *mǣnōn, like all Germanic language cognates. Occasionally, the name Luna is used, in literature, especially science fiction, Luna is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of our moon. The principal modern English adjective pertaining to the Moon is lunar, a less common adjective is selenic, derived from the Ancient Greek Selene, from which is derived the prefix seleno-. Both the Greek Selene and the Roman goddess Diana were alternatively called Cynthia, the names Luna, Cynthia, and Selene are reflected in terminology for lunar orbits in words such as apolune, pericynthion, and selenocentric. The name Diana is connected to dies meaning day, several mechanisms have been proposed for the Moons formation 4.51 billion years ago, and some 60 million years after the origin of the Solar System. These hypotheses also cannot account for the angular momentum of the Earth–Moon system. This hypothesis, although not perfect, perhaps best explains the evidence, eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists, You have eighteen months. Go back to your Apollo data, go back to computer, do whatever you have to. Dont come to our conference unless you have something to say about the Moons birth, at the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most popular. Afterward there were only two groups, the giant impact camp and the agnostics. Giant impacts are thought to have been common in the early Solar System, computer simulations of a giant impact have produced results that are consistent with the mass of the lunar core and the present angular momentum of the Earth–Moon system
6.
Depression (geology)
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A depression in geology is a landform sunken or depressed below the surrounding area. Erosion-related, Blowout, a depression created by wind erosion typically in either a partially vegetated sand dune ecosystem or dry soils, glacial valley, a depression carved by erosion by a glacier. River valley, a depression carved by erosion by a river. Area of subsidence caused by the collapse of a structure such as sinkholes in karst terrain. Sink, an endorheic depression generally containing a persistent or intermittent lake, collapse-related, Sinkhole, a depression formed as a result of the collapse of rocks lying above a hollow. This is common in karst regions, kettle, a shallow, sediment-filled body of water formed by melting glacial remnants in terminal moraines. Thermokarst hollow, caused by loss of the ground as the result of permafrost thawing. Impact-related, Impact crater, a depression created by an impact such as a meteorite crater, sedimentary-related, Sedimentary basin, in sedimentology, an area thickly filled with sediment in which the weight of the sediment further depresses the floor of the basin. Structural or tectonic-related, Structural basin, a depression, a region of tectonic downwarping as a result of isostasy or subduction. Graben or rift valley, fallen and typically linear depressions or basins created by rifting in a region under tensional tectonic forces, pull-apart basin caused by offset in a strike slip or transform fault. Oceanic trench, a linear depression on the ocean floor. Oceanic trenches are caused by subduction of oceanic crust beneath either oceanic crust or continental crust, pit crater, a volcanic depression smaller than a caldera formed by a sinking, or caving in, of the ground surface lying over a void. Maar, a resulting from phreatomagmatic eruption or diatreme explosion. Cryptodepression List of places on land with elevations below sea level
7.
Planet
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The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Several planets in the Solar System can be seen with the naked eye and these were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, in 2006, the International Astronomical Union officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of mass based on where or what they orbit. The planets were thought by Ptolemy to orbit Earth in deferent, at about the same time, by careful analysis of pre-telescopic observation data collected by Tycho Brahe, Johannes Kepler found the planets orbits were not circular but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the planets share characteristics such as volcanism, hurricanes, tectonics. Planets are generally divided into two types, large low-density giant planets, and smaller rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System, in order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites, several thousands of planets around other stars have been discovered in the Milky Way. e. in the habitable zone. On December 20,2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way, around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone. The idea of planets has evolved over its history, from the lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include not only in the Solar System. The ambiguities inherent in defining planets have led to much scientific controversy, the five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the fixed stars, ancient Greeks called these lights πλάνητες ἀστέρες or simply πλανῆται, from which todays word planet was derived. In ancient Greece, China, Babylon, and indeed all pre-modern civilizations, it was almost universally believed that Earth was the center of the Universe and that all the planets circled Earth. The first civilization known to have a theory of the planets were the Babylonians
8.
Natural satellite
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A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet. In the Solar System there are six planetary satellite systems containing 178 known natural satellites, four IAU-listed dwarf planets are also known to have natural satellites, Pluto, Haumea, Makemake, and Eris. As of January 2012, over 200 minor-planet moons have been discovered, the Earth–Moon system is unique in that the ratio of the mass of the Moon to the mass of Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System. At 3,474 km across, Earths Moon is 0.27 times the diameter of Earth, the first known natural satellite was the Moon, but it was considered a planet until Copernicus introduction of heliocentrism in 1543. Until the discovery of the Galilean satellites in 1610, however, galileo chose to refer to his discoveries as Planetæ, but later discoverers chose other terms to distinguish them from the objects they orbited. The first to use of the satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus in 1610. He derived the term from the Latin word satelles, meaning guard, attendant, or companion, the term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of moon. In 1957, however, the launching of the artificial object Sputnik created a need for new terminology, to further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earths natural satellite, but not when referring to other natural satellites. A few recent authors define moon as a satellite of a planet or minor planet, there is no established lower limit on what is considered a moon. Small asteroid moons, such as Dactyl, have also been called moonlets, the upper limit is also vague. Two orbiting bodies are described as a double body rather than primary. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon, some authors consider the Pluto–Charon system to be a double planet. In contrast, irregular satellites are thought to be captured asteroids possibly further fragmented by collisions, most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits. The Moon and possibly Charon are exceptions among large bodies in that they are thought to have originated by the collision of two large proto-planetary objects. The material that would have placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies, asteroid moons are thought to form by this process. Triton is another exception, although large and in a close, circular orbit, its motion is retrograde, most regular moons in the Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. The only known exception is Saturns natural satellite Hyperion, which rotates chaotically because of the influence of Titan
9.
Solar System
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The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun directly, the largest eight are the planets, with the remainder being significantly smaller objects, such as dwarf planets, of the objects that orbit the Sun indirectly, the moons, two are larger than the smallest planet, Mercury. The Solar System formed 4.6 billion years ago from the collapse of a giant interstellar molecular cloud. The vast majority of the mass is in the Sun. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being composed of rock. The four outer planets are giant planets, being more massive than the terrestrials. All planets have almost circular orbits that lie within a flat disc called the ecliptic. The Solar System also contains smaller objects, the asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptunes orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets, identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds. Six of the planets, at least four of the dwarf planets, each of the outer planets is encircled by planetary rings of dust and other small objects. The solar wind, a stream of charged particles flowing outwards from the Sun, the heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium, it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, the Solar System is located in the Orion Arm,26,000 light-years from the center of the Milky Way. For most of history, humanity did not recognize or understand the concept of the Solar System, the invention of the telescope led to the discovery of further planets and moons. The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99. 86% of the known mass. The Suns four largest orbiting bodies, the giant planets, account for 99% of the mass, with Jupiter. The remaining objects of the Solar System together comprise less than 0. 002% of the Solar Systems total mass, most large objects in orbit around the Sun lie near the plane of Earths orbit, known as the ecliptic
10.
Hypervelocity
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Hypervelocity is very high velocity, approximately over 3,000 meters per second. In particular, hypervelocity is velocity so high that the strength of materials upon impact is small compared to inertial stresses. Thus, even metals behave like fluids under hypervelocity impact, extreme hypervelocity results in vaporization of the impactor and target. For structural metals, hypervelocity is generally considered to be over 2,500 m/s, meteorite craters are also examples of hypervelocity impacts. Hypervelocity refers to velocities in the range from a few kilometers per second to tens of kilometers per second. The impactor, as well as the surface it hits, can undergo temporary liquefaction, the impact process can generate plasma discharges, which can interfere with spacecraft electronics. Hypervelocity usually occurs during meteor showers and deep space reentries, as carried out during the Zond, Apollo, given the intrinsic unpredictability of the timing and trajectories of meteors, space capsules are prime data gathering opportunities for the study of thermal protection materials at hypervelocity. Hypervelocity collisions can be studied by examining the results of naturally occurring collisions, currently the primary tool for laboratory experiments is a light-gas gun, but some experiments have used linear motors to accelerate projectiles to hypervelocity. The properties of metals under hypervelocity have been integrated with weapons, the vaporization upon impact and liquefaction of surfaces allow metal projectiles formed under hypervelocity forces to penetrate vehicle armor better than conventional bullets. NASA studies the effects of simulated orbital debris at the White Sands Test Facility Remote Hypervelocity Test Laboratory, objects smaller than a softball cannot be detected on radar. This has prompted spacecraft designers to develop shields to protect spacecraft from unavoidable collisions, at RHTL, four two-stage light-gas guns propel 0.05 mm to 22.2 mm diameter projectiles to velocities as fast as 8.5 km/s. According to the United States Army, hypervelocity can also refer to the velocity of a weapon system. Hypervelocity star Kinetic energy penetrator Terminal velocity Hypersonic Impact depth
11.
Collision
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A collision or crash is an event in which two or more bodies exert forces on each other for a relatively short time. Some examples of interactions that scientists would consider collisions, An insect touches its antenna to the leaf of a plant. The antenna is said to collide with leaf, a cat walks delicately through the grass. Each contact that its paws make with the ground is a collision, each brush of its fur against a blade of grass is a collision. When a boxer throws a punch, his fist is said to collide with the opponents face, the magnitude of the velocity difference just before impact is called the closing speed. What distinguishes different types of collisions is whether they also conserve kinetic energy, the Line of impact is the line which is collinear to the common normal of the surfaces that are closest or in contact during impact. This is the line along which internal force of collision acts during impact, specifically, collisions can either be elastic, meaning they conserve both momentum and kinetic energy, or inelastic, meaning they conserve momentum but not kinetic energy. An inelastic collision is sometimes called a plastic collision. A “perfectly inelastic” collision is a case of inelastic collision in which the two bodies stick together after impact. The degree to which a collision is elastic or inelastic is quantified by the coefficient of restitution, a perfectly elastic collision has a coefficient of restitution of one, a perfectly inelastic collision has a coefficient of restitution of zero. In reality, any macroscopic collision between objects will convert some kinetic energy to internal energy and other forms of energy, so no large-scale impacts are perfectly elastic, however, some problems are sufficiently close to perfectly elastic that they can be approximated as such. In this case, the coefficient of restitution equals one, an inelastic collision is one in which part of the kinetic energy is changed to some other form of energy in the collision. Momentum is conserved in collisions, but one cannot track the kinetic energy through the collision since some of it is converted to other forms of energy. In this case, coefficient of restitution does not equal one, collisions in ideal gases approach perfectly elastic collisions, as do scattering interactions of sub-atomic particles which are deflected by the electromagnetic force. Some large-scale interactions like the slingshot type gravitational interactions between satellites and planets are perfectly elastic, collisions between hard spheres may be nearly elastic, so it is useful to calculate the limiting case of an elastic collision. The assumption of conservation of momentum as well as the conservation of kinetic energy makes possible the calculation of the velocities in two-body collisions. In maritime law, it is desirable to distinguish between the situation of a vessel striking a moving object, and that of it striking a stationary object. The word allision is then used to mean the striking of a stationary object, relatively few problems involving collisions can be solved analytically, the remainder require numerical methods
12.
Volcanic crater
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A volcanic crater is a roughly circular depression in the ground caused by volcanic activity. It is typically a bowl-shaped feature within which occurs a vent or vents, a volcanic crater can be of large dimensions, and sometimes of great depth. During certain types of eruptions, a volcanos magma chamber may empty enough for an area above it to subside. In most volcanoes, the crater is situated at the top of a formed from the erupted volcanic deposits such as lava flows. Volcanoes that terminate in such a summit crater are usually of a conical form, other volcanic craters may be found on the flanks of volcanoes, and these are commonly referred to as flank craters. Some volcanic craters may fill either fully or partially with rain and/or melted snow, a crater may be breached during an eruption, either by explosions or by lava, or through later erosion. Breached craters have a lower rim on one side. Some volcanoes, such as maars, consist of a crater alone and these volcanic explosion craters are formed when magma rises through water-saturated rocks and causes a phreatic eruption. Volcanic craters from phreatic eruptions often occur on plains away from other obvious volcanoes
13.
Mercury (planet)
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Mercury is the smallest and innermost planet in the Solar System. Its orbital period around the Sun of 88 days is the shortest of all the planets in the Solar System and it is named after the Roman deity Mercury, the messenger to the gods. Like Venus, Mercury orbits the Sun within Earths orbit as a planet, so it can only be seen visually in the morning or the evening sky. Also, like Venus and the Moon, the displays the complete range of phases as it moves around its orbit relative to Earth. Seen from Earth, this cycle of phases reoccurs approximately every 116 days, although Mercury can appear as a bright star-like object when viewed from Earth, its proximity to the Sun often makes it more difficult to see than Venus. Mercury is tidally or gravitationally locked with the Sun in a 3,2 resonance, as seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. As seen from the Sun, in a frame of reference that rotates with the orbital motion, an observer on Mercury would therefore see only one day every two years. Mercurys axis has the smallest tilt of any of the Solar Systems planets, at aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercurys surface appears heavily cratered and is similar in appearance to the Moons, the polar regions are constantly below 180 K. The planet has no natural satellites. Mercury is one of four planets in the Solar System. It is the smallest planet in the Solar System, with a radius of 2,439.7 kilometres. Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede, Mercury consists of approximately 70% metallic and 30% silicate material. Mercurys density is the second highest in the Solar System at 5.427 g/cm3, Mercurys density can be used to infer details of its inner structure. Although Earths high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller, therefore, for it to have such a high density, its core must be large and rich in iron. Geologists estimate that Mercurys core occupies about 55% of its volume, Research published in 2007 suggests that Mercury has a molten core. Surrounding the core is a 500–700 km mantle consisting of silicates, based on data from the Mariner 10 mission and Earth-based observation, Mercurys crust is estimated to be 35 km thick. One distinctive feature of Mercurys surface is the presence of narrow ridges
14.
Callisto (moon)
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Callisto /kəˈlɪstoʊ/ is the second-largest moon of Jupiter, after Ganymede. It is the third-largest moon in the Solar System and the largest object in the Solar System not to be properly differentiated, Callisto was discovered in 1610 by Galileo Galilei. At 4821 km in diameter, Callisto has about 99% the diameter of the planet Mercury and it is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about 1883000 km. It is not in an orbital resonance like the three other Galilean satellites—Io, Europa, and Ganymede—and is thus not appreciably tidally heated. Callistos rotation is locked to its orbit around Jupiter, so that the same hemisphere always faces inward. It is less affected by Jupiters magnetosphere than the inner satellites because of its more remote orbit. Callisto is composed of equal amounts of rock and ices, with a density of about 1.83 g/cm3. Compounds detected spectroscopically on the surface water ice, carbon dioxide, silicates. Investigation by the Galileo spacecraft revealed that Callisto may have a small silicate core, the surface of Callisto is the oldest and most heavily cratered in the Solar System. Its surface is covered with impact craters. Prominent surface features include multi-ring structures, variously shaped impact craters, at a small scale, the surface is varied and made up of small, sparkly frost deposits at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material. The absolute ages of the landforms are not known, Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere. Callisto is thought to have formed by accretion from the disk of the gas. Callistos gradual accretion and the lack of tidal heating meant that not enough heat was available for rapid differentiation, the likely presence of an ocean within Callisto leaves open the possibility that it could harbor life. However, conditions are thought to be less favorable than on nearby Europa, various space probes from Pioneers 10 and 11 to Galileo and Cassini have studied Callisto. Because of its low levels, Callisto has long been considered the most suitable place for a human base for future exploration of the Jovian system. Callisto was discovered by Galileo in January 1610, along three other large Jovian moons—Ganymede, Io, and Europa. Callisto is named one of Zeuss many lovers in Greek mythology
15.
Asteroid
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Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids and these terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet. As minor planets in the outer Solar System were discovered and found to have volatile-based surfaces that resemble those of comets, in this article, the term asteroid refers to the minor planets of the inner Solar System including those co-orbital with Jupiter. There are millions of asteroids, many thought to be the remnants of planetesimals. The large majority of known asteroids orbit in the belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter. However, other orbital families exist with significant populations, including the near-Earth objects, individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups, C-type, M-type, and S-type. These were named after and are identified with carbon-rich, metallic. The size of asteroids varies greatly, some reaching as much as 1000 km across, asteroids are differentiated from comets and meteoroids. In the case of comets, the difference is one of composition, while asteroids are composed of mineral and rock, comets are composed of dust. In addition, asteroids formed closer to the sun, preventing the development of the aforementioned cometary ice, the difference between asteroids and meteoroids is mainly one of size, meteoroids have a diameter of less than one meter, whereas asteroids have a diameter of greater than one meter. Finally, meteoroids can be composed of either cometary or asteroidal materials, only one asteroid,4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the eye for a short time. As of March 2016, the Minor Planet Center had data on more than 1.3 million objects in the inner and outer Solar System, the United Nations declared June 30 as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, the first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet. In the early half of the nineteenth century, the terms asteroid. Asteroid discovery methods have improved over the past two centuries. This task required that hand-drawn sky charts be prepared for all stars in the band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, the expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers
16.
Earth
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Earth, otherwise known as the World, or the Globe, is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets, according to radiometric dating and other sources of evidence, Earth formed about 4.54 billion years ago. Earths gravity interacts with objects in space, especially the Sun. During one orbit around the Sun, Earth rotates about its axis over 365 times, thus, Earths axis of rotation is tilted, producing seasonal variations on the planets surface. The gravitational interaction between the Earth and Moon causes ocean tides, stabilizes the Earths orientation on its axis, Earths lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earths surface is covered with water, mostly by its oceans, the remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earths polar regions are covered in ice, including the Antarctic ice sheet, Earths interior remains active with a solid iron inner core, a liquid outer core that generates the Earths magnetic field, and a convecting mantle that drives plate tectonics. Within the first billion years of Earths history, life appeared in the oceans and began to affect the Earths atmosphere and surface, some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earths distance from the Sun, physical properties, in the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely, over 7.4 billion humans live on Earth and depend on its biosphere and minerals for their survival. Humans have developed diverse societies and cultures, politically, the world has about 200 sovereign states, the modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most often spelled eorðe. It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō, originally, earth was written in lowercase, and from early Middle English, its definite sense as the globe was expressed as the earth. By early Modern English, many nouns were capitalized, and the became the Earth. More recently, the name is simply given as Earth. House styles now vary, Oxford spelling recognizes the lowercase form as the most common, another convention capitalizes Earth when appearing as a name but writes it in lowercase when preceded by the. It almost always appears in lowercase in colloquial expressions such as what on earth are you doing, the oldest material found in the Solar System is dated to 4. 5672±0.0006 billion years ago. By 4. 54±0.04 Gya the primordial Earth had formed, the formation and evolution of Solar System bodies occurred along with the Sun
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Europa (moon)
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Europa /jʊˈroʊpə/, is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet. It is also the sixth-largest moon in the Solar System, Europa was discovered in 1610 by Galileo Galilei and was named after Europa, the legendary mother of King Minos of Crete and lover of Zeus. Slightly smaller than Earths Moon, Europa is primarily made of rock and has a water-ice crust. It has an atmosphere composed primarily of oxygen. Its surface is striated by cracks and streaks, whereas craters are relatively rare, in addition to Earth-bound telescope observations, Europa has been examined by a succession of space probe flybys, the first occurring in the early 1970s. Europa has the smoothest surface of any solid object in the Solar System. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, sea salt from a subsurface ocean may be coating some geological features on Europa, suggesting that the ocean is interacting with the seafloor. This may be important in determining if Europa could be habitable, in addition, the Hubble Space Telescope detected water vapor plumes similar to those observed on Saturns moon Enceladus, which are thought to be caused by erupting cryogeysers. The Galileo mission, launched in 1989, provides the bulk of current data on Europa, no spacecraft has yet landed on Europa, although there have been several proposed exploration missions. The European Space Agencys Jupiter Icy Moon Explorer is a mission to Ganymede that is due to launch in 2022, NASAs planned Europa Multiple-Flyby Mission will be launched in the mid-2020s. Europa, along with Jupiters three other large moons, Io, Ganymede, and Callisto, was discovered by Galileo Galilei on 8 January 1610, and possibly independently by Simon Marius. The first reported observation of Io and Europa was made by Galileo Galilei on 7 January 1610 using a 20×-magnification refracting telescope at the University of Padua. However, in observation, Galileo could not separate Io and Europa due to the low magnification of his telescope. The following day,8 January 1610, Io and Europa were seen for the first time as separate bodies during Galileos observations of the Jupiter system, Europa is named after Europa, daughter of the king of Tyre, a Phoenician noblewoman in Greek mythology. Like all the Galilean satellites, Europa is named after a lover of Zeus, Europa was courted by Zeus and became the queen of Crete. The naming scheme was suggested by Simon Marius, who discovered the four satellites independently, Marius attributed the proposal to Johannes Kepler. The names fell out of favor for a time and were not revived in general use until the mid-20th century. In much of the astronomical literature, Europa is simply referred to by its Roman numeral designation as Jupiter II or as the second satellite of Jupiter
18.
Io (moon)
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Io /ˈaɪ. oʊ/ is the innermost of the four Galilean moons of the planet Jupiter. It is the fourth-largest moon, has the highest density of all the moons and it was discovered in 1610 and was named after the mythological character Io, a priestess of Hera who became one of Zeuss lovers. With over 400 active volcanoes, Io is the most geologically active object in the Solar System, several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km above the surface. Ios surface is dotted with more than 100 mountains that have been uplifted by extensive compression at the base of Ios silicate crust. Some of these peaks are taller than Mount Everest, unlike most satellites in the outer Solar System, which are mostly composed of water ice, Io is primarily composed of silicate rock surrounding a molten iron or iron-sulfide core. Most of Ios surface is composed of extensive plains coated with sulfur, Ios volcanism is responsible for many of its unique features. Numerous extensive lava flows, several more than 500 km in length, the materials produced by this volcanism make up Ios thin, patchy atmosphere and Jupiters extensive magnetosphere. Ios volcanic ejecta also produce a large torus around Jupiter. Io played a significant role in the development of astronomy in the 17th and 18th centuries and it was discovered in January 1610 by Galileo Galilei, along with the other Galilean satellites. This discovery furthered the adoption of the Copernican model of the Solar System, the development of Keplers laws of motion, and the first measurement of the speed of light. In 1979, the two Voyager spacecraft revealed Io to be a geologically active world, with numerous volcanic features, large mountains, the Galileo spacecraft performed several close flybys in the 1990s and early 2000s, obtaining data about Ios interior structure and surface composition. These spacecraft also revealed the relationship between Io and Jupiters magnetosphere and the existence of a belt of high-energy radiation centered on Ios orbit, Io receives about 3,600 rem of ionizing radiation per day. Further observations have made by Cassini–Huygens in 2000 and New Horizons in 2007, as well as from Earth-based telescopes. From the surface of Io, Jupiter would subtend an arc of 19. 5°, although Simon Marius is not credited with the sole discovery of the Galilean satellites, his names for the moons were adopted. Based on a suggestion from Johannes Kepler in October 1613, he devised a naming scheme whereby each moon was named for a lover of the Greek mythological Zeus or his Roman equivalent. He named the innermost large moon of Jupiter after the Greek mythological figure Io, since the surface was first seen up close by Voyager 1, the International Astronomical Union has approved 225 names for Ios volcanoes, mountains, plateaus, and large albedo features. The approved feature categories used for Io for different types of volcanic features include patera, fluctus, vallis, named mountains, plateaus, layered terrain, and shield volcanoes include the terms mons, mensa, planum, and tholus, respectively. Named, bright albedo regions use the term regio, examples of named features are Prometheus, Pan Mensa, Tvashtar Paterae, and Tsũi Goab Fluctus
19.
Titan (moon)
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Titan is the largest moon of Saturn. It is the only known to have a dense atmosphere. Titan is the sixth ellipsoidal moon from Saturn, frequently described as a planet-like moon, Titan is 50% larger than Earths Moon, and it is 80% more massive. It is the second-largest moon in the Solar System, after Jupiters moon Ganymede, and is larger than the smallest planet, Mercury, discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn, and the sixth known planetary satellite. Titan orbits Saturn at 20 Saturn radii, from Titans surface, Saturn subtends an arc of 5.09 degrees and would appear 11.4 times larger in the sky than the Moon from Earth. Titan is primarily composed of ice and rocky material. The geologically young surface is smooth, with few impact craters, although mountains. The atmosphere of Titan is largely nitrogen, minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas, and deltas, Huygens was inspired by Galileos discovery of Jupiters four largest moons in 1610 and his improvements in telescope technology. Christiaan, with the help of his brother Constantijn Huygens, Jr. began building telescopes around 1650 and it was the sixth moon to be discovered. He named it Saturni Luna, publishing in the 1655 tract De Saturni Luna Observatio Nova, after Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686, astronomers fell into the habit of referring to these and Titan as Saturn I through V. Other early epithets for Titan include Saturns ordinary satellite, Titan is officially numbered Saturn VI because after the 1789 discoveries the numbering scheme was frozen to avoid causing any more confusion. Numerous small moons have been discovered closer to Saturn since then and he suggested the names of the mythological Titans, brothers and sisters of Cronus, the Greek Saturn. In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, Titan orbits Saturn once every 15 days and 22 hours. Because of this, there is a point on its surface. Longitudes on Titan are measured westward, starting from the passing through this point. Its orbital eccentricity is 0.0288, and the plane is inclined 0.348 degrees relative to the Saturnian equator. Viewed from Earth, Titan reaches a distance of about 20 Saturn radii from Saturn
20.
Erosion
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In earth science, erosion is the action of surface processes that remove soil, rock, or dissolved material from one location on the Earths crust, then transport it away to another location. Eroded sediment or solutes may be transported just a few millimetres, the rates at which such processes act control how fast a surface is eroded. Feedbacks are also possible between rates of erosion and the amount of eroded material that is carried by, for example. Processes of erosion that produce sediment or solutes from a place contrast with those of deposition, while erosion is a natural process, human activities have increased by 10-40 times the rate at which erosion is occurring globally. At well-known agriculture sites such as the Appalachian Mountains, intensive farming practices have caused erosion up to 100x the speed of the rate of erosion in the region. Excessive erosion causes both on-site and off-site problems, on-site impacts include decreases in agricultural productivity and ecological collapse, both because of loss of the nutrient-rich upper soil layers. In some cases, the end result is desertification. Off-site effects include sedimentation of waterways and eutrophication of bodies, as well as sediment-related damage to roads. Intensive agriculture, deforestation, roads, anthropogenic climate change and urban sprawl are amongst the most significant human activities in regard to their effect on stimulating erosion, however, there are many prevention and remediation practices that can curtail or limit erosion of vulnerable soils. Rainfall, and the surface runoff which may result from rainfall, produces four types of soil erosion, splash erosion, sheet erosion, rill erosion. Splash erosion is generally seen as the first and least severe stage in the erosion process. In splash erosion, the impact of a falling raindrop creates a crater in the soil. The distance these soil particles travel can be as much as 0.6 m vertically and 1.5 m horizontally on level ground. If the soil is saturated, or if the rate is greater than the rate at which water can infiltrate into the soil. If the runoff has sufficient flow energy, it will transport loosened soil particles down the slope, sheet erosion is the transport of loosened soil particles by overland flow. Rill erosion refers to the development of small, ephemeral concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hillslopes, generally, where water erosion rates on disturbed upland areas are greatest, rills are active. Flow depths in rills are typically of the order of a few centimetres or less and this means that rills exhibit hydraulic physics very different from water flowing through the deeper, wider channels of streams and rivers. Gully erosion occurs when water accumulates and rapidly flows in narrow channels during or immediately after heavy rains or melting snow
21.
Impact structure
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Impact structure is synonymous with the less commonly used term astrobleme meaning star wound. In an impact structure, the visible and topographic expressions of an impact crater are no longer obvious. Any meteorite fragments that may once have been present would be long since eroded away, possible impact structures may be initially recognized by their anomalous geological character or geophysical expression. These may still be confirmed as impact structures by the presence of shocked minerals, shatter cones, geochemical evidence of extraterrestrial material or other methods
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Cryptoexplosion
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The term cryptoexplosion structure means an explosion of unknown cause. The term is now largely obsolete, the assumption was that some unusual form of volcanism, or a gas explosion originating within the crust, was the cause. The use of the term went away with the rise of the science of impact crater recognition in the late 20th century, most structures described as cryptoexplosions turned out to be eroded impact craters, caused by the impact of meteorites. Today geologists discount former cryptoexplosion theories, Impact crater Impact structure Astrobleme Impact event Traces of Catastrophe
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Late Heavy Bombardment
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The Late Heavy Bombardment is an event thought to have occurred approximately 4.1 to 3.8 billion years ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. The LHB happened after the Earth and other rocky planets had formed and accreted most of their mass, Evidence for the LHB derives from lunar samples brought back by the Apollo astronauts. Isotopic dating of Moon rocks implies that most impact melts occurred in a narrow interval of time. Several hypotheses attempt to explain the apparent spike in the flux of impactors in the inner Solar System and they also note that the rate of impact cratering could differ significantly between the outer and inner zones of the Solar System. The main piece of evidence for a lunar cataclysm comes from the ages of impact melt rocks that were collected during the Apollo missions. The majority of impact melts are believed to have formed during the collision of asteroids or comets tens of kilometers across. The Apollo 15,16, and 17 landing sites were chosen as a result of their proximity to the Imbrium, Nectaris, under study on Earth, the ages of impact melts collected at these sites clustered between about 3.8 and 4.1 Ga. The apparent clustering of ages of these led to postulation that the record an intense bombardment of the Moon. They called it the lunar cataclysm and proposed that it represented an increase in the rate of bombardment of the Moon around 3.9 Ga. At the time, the conclusion was considered controversial, as more data has become available, particularly from lunar meteorites, this theory, while still controversial, has gained in popularity. The lunar meteorites are believed to randomly sample the lunar surface, many of the feldspathic lunar meteorites probably originated from the lunar far side, and impact melts within these have recently been dated. Consistent with the hypothesis, none of their ages was found to be older than about 3.9 Ga. Nevertheless, the ages do not cluster at this date. Dating of howardite, eucrite and diogenite meteorites and H chondrite meteorites originating from the asteroid belt reveal numerous ages from 3. 4–4.1 Ga, impact velocities above 10 km/s require very high inclinations or the large eccentricities of asteroids on planet crossing orbits. Such objects are rare in the current asteroid belt but the population would be increased by the sweeping of resonances due to giant planet migration. Studies of the highland crater size distributions suggest that the family of projectiles struck Mercury. The first criticism concerns the origin of the impact melt rocks that were sampled at the Apollo landing sites. While these impact melts have been attributed to having been derived from the closest basin. According to this hypothesis, the cluster of impact melt ages near 3.9 Ga simply reflects material being collected from a single impact event, Imbrium
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Chicxulub impact
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The Chicxulub crater is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named and it was formed by a large asteroid or comet about 10 kilometres in diameter, the Chicxulub impactor, striking the Earth. The crater is more than 180 kilometers in diameter and 20 km in depth and it makes the feature the third of the largest confirmed impact structures on Earth. The crater was discovered by Antonio Camargo and Glen Penfield, geophysicists who had been looking for petroleum in the Yucatán during the late 1970s, Penfield was initially unable to obtain evidence that the geological feature was a crater and gave up his search. Later, through contact with Alan Hildebrand in 1990, Penfield obtained samples that suggested it was an impact feature, evidence for the impact origin of the crater includes shocked quartz, a gravity anomaly, and tektites in surrounding areas. Penfields job was to use data to scout possible locations for oil drilling. In the data, Penfield found a huge underwater arc with extraordinary symmetry in a ring 70 km across and he then obtained a gravity map of the Yucatán made in the 1960s. A decade earlier, the same map suggested a feature to contractor Robert Baltosser. Penfield found another arc on the peninsula itself, the ends of which pointed northward, Pemex disallowed release of specific data but let Penfield and company official Antonio Camargo present their results at the 1981 Society of Exploration Geophysicists conference. That years conference was underattended and their report attracted scant attention, coincidentally, many experts in impact craters and the K–Pg boundary were attending a separate conference on Earth impacts. Although Penfield had plenty of data sets, he had no rock cores or other physical evidence of an impact. He knew Pemex had drilled exploratory wells in the region, in 1951, one bored into what was described as a thick layer of andesite about 1.3 kilometres down. This layer could have resulted from the heat and pressure of an Earth impact. Penfield tried to secure site samples, but was told such samples had been lost or destroyed, when attempts at returning to the drill sites and looking for rocks proved fruitless, Penfield abandoned his search, published his findings and returned to his Pemex work. At the same time, in 1980, geologist Walter Alvarez and his father, Nobel Prize-winning scientist Luis Walter Alvarez and their evidence included greenish-brown clay with surplus iridium containing shocked quartz grains and small weathered glass beads that looked to be tektites. Thick, jumbled deposits of rock fragments were also present, thought to have been scoured from one place. Such deposits occur in locations but seem concentrated in the Caribbean basin at the K–Pg boundary. So when Haitian professor Florentine Morás discovered what he thought to be evidence of an ancient volcano on Haiti, tests on samples retrieved from the K–Pg boundary revealed more tektite glass, formed only in the heat of asteroid impacts and high-yield nuclear detonations
25.
Sikhote-Alin meteorite
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An iron meteorite fell on the Sikhote-Alin Mountains, in southeastern Russia, in 1947. Though large iron meteorite falls had been witnessed previously and fragments recovered, an estimated 70 tonnes of material survived the fiery passage through the atmosphere and reached the Earth. The bright flash and the sound of the fall were observed for 300 kilometres around the point of impact not far from Luchegorsk. A smoke trail, estimated at 32 km long, remained in the sky for several hours, as the meteor, traveling at a speed of about 14 km/s, entered the atmosphere, it began to break apart, and the fragments fell together. At an altitude of about 5.6 km, the largest mass apparently broke up in a violent explosion called an air burst, on November 20,1957 the Soviet Union issued a stamp for the 10th anniversary of the Sikhote-Alin meteorite shower. It reproduces a painting by P. I, medvedev, a Soviet artist who witnessed the fall, he was sitting in his window starting a sketch when the fireball appeared, so he immediately began drawing what he saw. Because the meteor fell during daytime, it was observed by many eyewitnesses, evaluation of this observational data allowed V. G. Fesenkov, then chairman of the meteorite committee of the USSR Academy of Science, to estimate the meteoroids orbit before it encountered the Earth. This orbit was ellipse-shaped, with its point of greatest distance from the sun situated within the asteroid belt, such an orbit was probably created by collisions within the asteroid belt. Sikhote-Alin is a fall with the overall size of the meteoroid estimated at approximately 90,000 kg. A more recent estimate by Tsvetkov puts the mass at around 100,000 kg, krinov had estimated the post-atmospheric mass of the meteoroid at some 23,000 kg. The strewn field for this meteorite covered an area of about 1.3 km2. Some of the fragments made impact craters, the largest of which was about 26 m across and 6 m deep, fragments of the meteorite were also driven into the surrounding trees. The Sikhote-Alin meteorite is classified as an iron meteorite belonging to the chemical group IIAB and with a coarse octahedrite structure. It is composed of approximately 93% iron,5. 9% nickel,0. 42% cobalt,0. 46% phosphorus, minerals present include taenite, plessite, troilite, chromite, kamacite, and schreibersite. These pieces are characterized by regmaglypts in the surface of each specimen, the second type are fragments which were either torn apart during the atmospheric explosions or blasted apart upon impact on the frozen ground. Most were probably the result of the explosion at 5.6 km altitude, a large specimen is on display in Moscow. Many other specimens are held by Russian Academy of Science and a number of smaller specimens have made their way into the collectors market. Glossary of meteoritics Meteorite Meteorite fall 2013 Russian meteor event List of meteor air bursts Exposition Meteorites, Moscow, Russia, Russian Academy of Sciences
26.
Craton
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A craton is an old and stable part of the continental lithosphere, where the lithosphere consists of the Earths two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and rifting of continents, cratons are found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by sedimentary rock. They have a thick crust and deep roots that extend as much as several hundred kilometres into the Earths mantle. The term craton is used to distinguish the stable portion of the continental crust from regions that are geologically active. Cratons can be described as shields, in which the basement rock crops out at the surface, the word craton was first proposed by the Austrian geologist Leopold Kober in 1921 as Kratogen, referring to stable continental platforms, and orogen as a term for mountain or orogenic belts. Later authors shortened the term to kraton and then to craton. Cratons are subdivided geographically into geologic provinces, a geologic province is a spatial entity with common geologic attributes. A province may include a single dominant structural element such as a basin or a fold belt. Adjoining provinces may appear similar in structure but be considered due to differing histories. At that depth, craton roots extend into the asthenosphere, craton lithosphere is distinctly different from oceanic lithosphere because cratons have a neutral or positive buoyancy, and a low intrinsic isopycnic density. This low density offsets density increases due to contraction and prevents the craton from sinking into the deep mantle. Cratonic lithosphere is much older than oceanic lithosphere—up to 4 billion years versus 180 million years, rock fragments carried up from the mantle by magmas containing peridotite have been delivered to the surface as inclusions in subvolcanic pipes called kimberlites. These inclusions have densities consistent with composition and are composed of mantle material residual from high degrees of partial melt. Peridotite is strongly influenced by the inclusion of moisture, craton peridotite moisture content is unusually low, which leads to much greater strength. It also contains high percentages of low-weight magnesium instead of higher-weight calcium, peridotites are important for understanding the deep composition and origin of cratons because peridotite nodules are pieces of mantle rock modified by partial melting. Harzburgite peridotites represent the crystalline residues after extraction of melts of compositions like basalt, an associated class of inclusions called eclogites, consists of rocks corresponding compositionally to oceanic crust that has metamorphosed under deep mantle conditions. Isotopic studies reveal that many eclogite inclusions are samples of ancient oceanic crust subducted billions of years ago to depths exceeding 150 km into the deep kimberlite diamond areas and they remained fixed there within the drifting tectonic plates until carried to the surface by deep-rooted magmatic eruptions
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Subduction
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Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced or sinks due to gravity into the mantle. Regions where this occurs are known as subduction zones. Rates of subduction are typically in centimeters per year, with the rate of convergence being approximately two to eight centimeters per year along most plate boundaries. Plates include both oceanic crust and continental crust, stable subduction zones involve the oceanic lithosphere of one plate sliding beneath the continental or oceanic lithosphere of another plate due to the higher density of the oceanic lithosphere. That is, the lithosphere is always oceanic while the overriding lithosphere may or may not be oceanic. Subduction zones are sites that have a rate of volcanism, earthquakes. Subduction zones are sites of convective downwelling of Earths lithosphere, subduction zones exist at convergent plate boundaries where one plate of oceanic lithosphere converges with another plate. The descending slab, the plate, is over-ridden by the leading edge of the other plate. The slab sinks at an angle of approximately twenty-five to forty-five degrees to Earths surface and this sinking is driven by the temperature difference between the subducting oceanic lithosphere and the surrounding mantle asthenosphere, as the colder oceanic lithosphere is, on average, denser. At a depth of approximately 80–120 kilometers, the basalt of the oceanic crust is converted to a rock called eclogite. At that point, the density of the oceanic crust increases and provides additional negative buoyancy and it is at subduction zones that Earths lithosphere, oceanic crust, sedimentary layers and some trapped water are recycled into the deep mantle. Earth is so far the only planet where subduction is known to occur, subduction is the driving force behind plate tectonics, and without it, plate tectonics could not occur. Subduction zones dive down into the mantle beneath 55,000 kilometers of convergent plate margins, subduction zones burrow deeply but are imperfectly camouflaged, and geophysics and geochemistry can be used to study them. Not surprisingly, the shallowest portions of subduction zones are known best, subduction zones are strongly asymmetric for the first several hundred kilometers of their descent. They start to go down at oceanic trenches and their descents are marked by inclined zones of earthquakes that dip away from the trench beneath the volcanoes and extend down to the 660-kilometer discontinuity. Subduction zones are defined by the array of earthquakes known as the Wadati–Benioff zone after the two scientists who first identified this distinctive aspect. Subduction zone earthquakes occur at greater depths than elsewhere on Earth, such deep earthquakes may be driven by deep phase transformations, thermal runaway, the subducting basalt and sediment are normally rich in hydrous minerals and clays. Additionally, large quantities of water are introduced into cracks and fractures created as the slab bends downward
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Plate tectonics
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The theoretical model builds on the concept of continental drift developed during the first few decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s, the lithosphere, which is the rigid outermost shell of a planet, is broken up into tectonic plates. The Earths lithosphere is composed of seven or eight major plates, where the plates meet, their relative motion determines the type of boundary, convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The relative movement of the plates typically ranges from zero to 100 mm annually, tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction carries plates into the mantle, the material lost is balanced by the formation of new crust along divergent margins by seafloor spreading. In this way, the surface of the lithosphere remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle, earlier theories, since disproven, proposed gradual shrinking or gradual expansion of the globe. Tectonic plates are able to move because the Earths lithosphere has greater strength than the underlying asthenosphere. Lateral density variations in the result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from the ridge and drag, with downward suction. Another explanation lies in the different forces generated by forces of the Sun. The relative importance of each of these factors and their relationship to other is unclear. The outer layers of the Earth are divided into the lithosphere and asthenosphere and this is based on differences in mechanical properties and in the method for the transfer of heat. Mechanically, the lithosphere is cooler and more rigid, while the asthenosphere is hotter, in terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, Plate motions range up to a typical 10–40 mm/year, to about 160 mm/year. The driving mechanism behind this movement is described below, tectonic lithosphere plates consist of lithospheric mantle overlain by either or both of two types of crustal material, oceanic crust and continental crust. Average oceanic lithosphere is typically 100 km thick, its thickness is a function of its age, as passes, it conductively cools
29.
Calderas
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A caldera is a large cauldron-like depression that forms following the evacuation of a magma chamber/reservoir. When large volumes of magma are erupted over a time period. The ground surface then collapses downward into the emptied magma chamber. Although sometimes described as a crater, the feature is actually a type of sinkhole, as it is formed through subsidence, only seven known caldera-forming collapses have occurred since the start of the 20th century, most recently at Bárðarbunga volcano in Iceland. The word comes from Spanish caldera, and this from Latin caldaria, in some texts the English term cauldron is also used. If enough magma is ejected, the chamber is unable to support the weight of the volcanic edifice above it. A roughly circular fracture, the fault, develops around the edge of the chamber. Ring fractures serve as feeders for fault intrusions which are known as ring dykes. Secondary volcanic vents may form above the ring fracture, as the magma chamber empties, the center of the volcano within the ring fracture begins to collapse. The collapse may occur as the result of a cataclysmic eruption. The total area that collapses may be hundreds or thousands of square kilometers, some calderas are known to host rich ore deposits. One of the worlds best-preserved mineralized calderas is the Sturgeon Lake Caldera in northwestern Ontario, Canada, if the magma is rich in silica, the caldera is often filled in with ignimbrite, tuff, rhyolite, and other igneous rocks. Silica-rich magma has a high viscosity, and therefore does not flow easily like basalt, as a result, gases tend to become trapped at high pressure within the magma. Further lava flows may be erupted, if volcanic activity continues, the center of the caldera may be uplifted in the form of a resurgent dome such as is seen at Cerro Galán, Lake Toba, Yellowstone, etc. by subsequent intrusion of magma. A silicic or rhyolitic caldera may erupt hundreds or even thousands of kilometers of material in a single event. Even small caldera-forming eruptions, such as Krakatoa in 1883 or Mount Pinatubo in 1991, may result in significant local destruction, large calderas may have even greater effects. When Yellowstone Caldera last erupted some 650,000 years ago, it released about 1,000 km3 of material, by comparison, when Mount St. Helens erupted in 1980, it released ~1.2 km3 of ejecta. The ecological effects of the eruption of a large caldera can be seen in the record of the Lake Toba eruption in Indonesia, more recently several geneticists, including Lynn Jorde and Henry Harpending have proposed that the human species was reduced to approximately five to ten thousand people
30.
Sinkholes
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A sinkhole, also known as a cenote, sink, sink-hole, shakehole, swallet, swallow hole, or doline, is a depression or hole in the ground caused by some form of collapse of the surface layer. Most are caused by karst processes—for example, the dissolution of carbonate rocks or suffosion processes. Sinkholes vary in size from 1 to 600 m both in diameter and depth, and vary in form from soil-lined bowls to bedrock-edged chasms, sinkholes may form gradually or suddenly, and are found worldwide. Sinkholes may capture surface drainage from running or standing water, but may form in high. Sinkholes that capture drainage can hold it in limestone caves. These caves may drain into tributaries of larger rivers, the formation of sinkholes involves natural processes of erosion or gradual removal of slightly soluble bedrock by percolating water, the collapse of a cave roof, or a lowering of the water table. Sinkholes often form through the process of suffosion, for example, groundwater may dissolve the carbonate cement holding the sandstone particles together and then carry away the lax particles, gradually forming a void. Occasionally a sinkhole may exhibit a visible opening into a cave below, sinkholes also occur in sandstone and quartzite terrains. As the rock dissolves, spaces and caverns develop underground and these sinkholes can be dramatic, because the surface land usually stays intact until there is not enough support. Then, a collapse of the land surface can occur. Sinkholes also form from human activity, such as the collapse of abandoned mines and salt cavern storage in salt domes in places like Louisiana, Mississippi, more commonly, sinkholes occur in urban areas due to water main breaks or sewer collapses when old pipes give way. They can also occur from the overpumping and extraction of groundwater, sinkholes can also form when natural water-drainage patterns are changed and new water-diversion systems are developed. Sinkholes tend to occur in karst landscapes, karst landscapes can have up to thousands of sinkholes within a small area, giving the landscape a pock-marked appearance. These sinkholes drain all the water, so there are only subterranean rivers in these areas, examples of karst landscapes with a plethora of massive sinkholes include Khammouan Mountains and Mamo Plateau. The largest known sinkholes formed in sandstone are Sima Humboldt and Sima Martel in Venezuela, some sinkholes form in thick layers of homogenous limestone. On the contact of limestone and insoluble rock below it, powerful underground rivers may form, in such conditions, the largest known sinkholes of the world have formed, like the 662-metre deep Xiaozhai Tiankeng, giant sótanos in Querétaro and San Luis Potosí states in Mexico and others. The state of Florida in the United States is known for having frequent sinkhole collapses, the Murge area in southern Italy also has numerous sinkholes. Sinkholes can be formed in retention ponds from large amounts of rain, an analysis of a case of sinkhole formation under a retention pond due to a large amount of rain can be seen in a sinkhole collapse study
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Glacial cirque
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A cirque is an amphitheatre-like valley formed by glacial erosion. Alternative names for this landform are corrie and cwm, a cirque may also be a similarly shaped landform arising from fluvial erosion. The concave shape of a glacial cirque is open on the downhill side, cliff-like slopes down which ice and glaciated debris combine and converge from the three or more higher sides. The fluvial cirque or makhtesh, found in karst landscapes, is formed by intermittent river flow cutting through layers of limestone, a common feature for all fluvial-erosion cirques is a terrain which includes erosion resistant upper structures overlying materials which are more easily eroded. Glacial cirques are found amongst mountain ranges throughout the world, classic cirques are typically about one kilometer long, situated high on a mountainside near the firn line, they are typically partially surrounded on three sides by steep cliffs. The highest cliff often is called a headwall, the fourth side forms the lip, threshold or sill, the side at which the glacier flowed away from the cirque. Many glacial cirques contain tarns dammed by either till or a bedrock threshold, when enough snow accumulates it can flow out the opening of the bowl and form valley glaciers which may be several kilometers long. These areas are sheltered from heat, encouraging the accumulation of snow, if the accumulation of snow increases, the process of nivation follows, whereby a hollow in a slope may be enlarged by ice segregation weathering and glacial erosion. Ice segregation erodes the rock vertical rock face and causes it to disintegrate, eventually, this hollow may become large enough that glacial erosion intensifies. The enlarging of this open ended concavity creates a larger leeward deposition zone, debris in the ice also may abrade the bed surface, should ice move down a slope it would have a ‘sandpaper effect’ on the bedrock beneath, on which it scrapes. Eventually, the hollow may become a bowl shape in the side of the mountain, with the headwall being weathered by ice segregation. The basin will become deeper as it continues to be eroded by ice segregation and abrasion, should ice segregation, plucking and abrasion continue, the dimensions of the cirque will increase, but the proportion of the landform would remain roughly the same. A bergschrund forms when the movement of the glacier separates the moving ice from the ice forming a crevasse. The method of erosion of the headwall lying between the surface of the glacier and the floor has been attributed to freeze-thaw mechanisms. The temperature within the bergschrund changes very little, however, studies have shown that ice segregation may happen with only changes in temperature. Water that flows into the bergschrund can be cooled to freezing temperatures by the ice allowing freeze-thaw mechanisms to occur. If two adjacent cirques erode toward one another, an arête, or steep sided ridge, forms, when three or more cirques erode toward one another, a pyramidal peak is created. In some cases, this peak will be accessible by one or more arêtes
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Ring dike
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A ring dike or ring dyke is an intrusive igneous body that is circular, oval or arcuate in plan and has steep contacts. While the widths of ring dikes differ, they can be up to several thousand meters, the most commonly accepted method of ring dike formation is directly related to collapse calderas. Collapse calderas form due to the emptying of a magma chamber, effusive eruptions that take place on the flanks of the associated volcano and a fissure system that direct magma away from the chamber are both mechanisms that can empty a magma chamber. As pressure in the magma chamber changes, an increase in tensile stresses create tension fractures at the surface of the volcano, the geometry of the top of the magma chamber dictate the location and magnitude of the tension fractures. In addition, it was found that the higher the radius to depth ratio of the magma chamber, once a tension threshold is approached, the roof of the magma chamber collapses in on itself, and is known as calderon subsistence. Tension fractures extend deeper in the profile and shear fractures or dip-slip faults form in a pattern around the caldera and are known as ring faults. Ring faults can be vertical or steeply dipping faults. When they are inward dipping, they are known as normal faults, ring faults then allow magma to rise through the fractures, forming a ring dike. These dikes can form as a result of collapse caldera formation. The magma of a dike is typically composed of acidic or intermediate composition due to the less dense melt that exists at the top of the magma chamber. It has been hypothesized that ring dikes may form when inclined sheets are captured within a fault system. The deflection of the sheets may be caused by the difference in properties between and within the fault zone. Whether or not a ring fault dips inward or outward from the center of subsidence is a highly contentious issue. Ring faults near the surface are subject to erosion and mass wasting, changing the morphology of the caldera walls, about 36 ring dikes have been found in the Ossipee Mountains in New Hampshire. Caldera subsistence appears to be the leading to the formation of some of the ring dikes. The composition ranges from monzonite to quartz syenite, the Loch Bà ring dike, found on the Isle of Mull in Scotland, serves as a classic example of a well formed ring dike. This intrusive body forms an oval and its diameter can be measured at roughly 5.8 km by 8.5 km, the width of the dike varies throughout the profile, with a maximum width of approximately 300 meters. The composition varies from rhyolite to felsite, with phenocrysts of alkali feldspar, batholith Cone sheet Fissure vent Laccolith Billings, Marland P
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Salt domes
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A salt dome is a type of structural dome formed when a thick bed of evaporite minerals found at depth intrudes vertically into surrounding rock strata, forming a diapir. It is important in geology because salt structures are impermeable. The formation of a salt dome begins with the deposition of salt in a marine basin. This indicates that a period of episodic flooding and evaporation of the basin must occur. Over time, the layer of salt is covered with deposited sediment, the overlying sediment will undergo compaction, causing an increase in density and therefore a decrease in buoyancy. Unlike clastics, pressure has a smaller effect on the density of salt due to its crystal structure. The ductility of salt initially allows it to deform and flow laterally. The vertical growth of these salt pillows creates pressure on the surface, causing extension. Possible forces that drive the flow of salt are differential loading on the source layer, forces that resist this flow are the mass of the roof block and the blocks inherent resistance to faulting, aka strength. If the diapir is narrow its height must be greater, eventually, over millions of years, the salt will pierce and break through the overlying sediment, first as a dome-shaped and then a mushroom-shaped - fully formed salt diapir. If the rising salt diapir breaches the surface, it can become a salt glacier. In cross section, these large domes may be anywhere from 1 to 10 kilometres across, typical structures of active diapirism are a central crestal graben flanked by flaps that rotate upward and outward. Reverse faults can separate the flaps from the overburden, normal faults create the crestal graben and propagate downward. New faults form farther outward as the dome arch becomes more intense and these structures occur beneath the surface and are not necessarily associated with the dome at the surface. Emergence of the dome will not occur unless the dome is very wide or tall relative to the overburdens thickness, if a salt dome has not pierced the surface they can found located beneath the surface in various ways. The unique surficial structures can be observed as indicating the salt dome beneath the surface, Salt domes can also be interpreted from seismic reflection where the stark density contrast between the salt and surrounding sediments outlines the salt structures. Salt domes can also be associated with springs and natural gas vents. The first salt dome was discovered in 1890 when an oil well was drilled into Spindletop Hill near Beaumont TX
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Daniel Barringer (geologist)
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For other people named Daniel Barringer, see Daniel Barringer. Daniel Moreau Barringer was a geologist best known as the first person to prove the existence of a crater on the Earth. The site, owned privately by Barringer and now by his family, has renamed the Barringer Crater in his honor. He later studied geology and mineralogy at Harvard University and at the University of Virginia, in 1892, Barringer, along with his friend Richard A. F. Penrose, Jr. and others, purchased a gold and silver mine near Cochise, Arizona. Later, Barringer also discovered the Commonwealth Silver Mine in Pearce and these mining ventures made him a wealthy man. In 1902 Barringer learned of the existence of a crater, located 35 miles east of Flagstaff. The crater, known as Coon Mountain, had previously studied by the geologist Grove Karl Gilbert in 1891. Gilbert had hypothesized that the crater must have been the result of either a gas explosion or a meteorite. After performing experiments in the crater, however, Gilberts conclusion was that the crater could not be the result of an impact and he concluded this despite the clear presence of thousands of small meteoritic particles in the vicinity of the crater. Upon hearing of the existence of the crater and the meteoritic iron, with both scientific and monetary aims in mind, Barringer created the Standard Iron Company in order to mine the crater for the iron that he assumed must be buried below its surface. The Standard Iron Company conducted drilling operations in and around the crater between 1903 and 1905, and concluded that the crater had indeed caused by a violent impact. It was unable to find the meteorite, however, in 1906, Barringer and his partner, the mathematician and physicist Benjamin C. Tilghman, presented their first papers to the U. S. Geological Survey outlining the evidence in support of the impact theory, the papers were published in the Proceedings of the Academy of Natural Sciences in Philadelphia. The mining of the crater continued until 1929 without ever finding the ten-million ton meteorite that Barringer assumed must be hidden. At this time the astronomer Forest Ray Moulton performed calculations on the energy expended by the meteorite on impact, by this point Barringer had spent over $600,000 in mining the crater, nearly bankrupting him, with no iron profits to show for it. Barringer died of an attack on November 30,1929. He was survived by his wife, Margaret Bennett, and eight children, who, with their descendants, formed the Barringer Crater Company, by the time of his death, Barringer had convinced most of the scientific community that his impact theory was correct. The theory has been confirmed with new evidence since then
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Arizona
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Arizona is a state in the southwestern region of the United States. It is also part of the Western United States and the Mountain West states and it is the sixth largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix, Arizona is one of the Four Corners states. It has borders with New Mexico, Utah, Nevada, California, and Mexico, Arizonas border with Mexico is 389 miles long, on the northern border of the Mexican states of Sonora and Baja California. Arizona is the 48th state and last of the states to be admitted to the Union. Historically part of the territory of Alta California in New Spain, after being defeated in the Mexican–American War, Mexico ceded much of this territory to the United States in 1848. The southernmost portion of the state was acquired in 1853 through the Gadsden Purchase, Southern Arizona is known for its desert climate, with very hot summers and mild winters. There are ski resorts in the areas of Flagstaff, Alpine, in addition to the Grand Canyon National Park, there are several national forests, national parks, and national monuments. To the European settlers, their pronunciation sounded like Arissona, the area is still known as alĭ ṣonak in the Oodham language. Another possible origin is the Basque phrase haritz ona, as there were numerous Basque sheepherders in the area, There is a misconception that the states name originated from the Spanish term Árida Zona. See also lists of counties, islands, rivers, lakes, state parks, national parks, Arizona is in the Southwestern United States as one of the Four Corners states. Arizona is the sixth largest state by area, ranked after New Mexico, of the states 113,998 square miles, approximately 15% is privately owned. The remaining area is public forest and park land, state trust land, Arizona is well known for its desert Basin and Range region in the states southern portions, which is rich in a landscape of xerophyte plants such as the cactus. This regions topography was shaped by volcanism, followed by the cooling-off. Its climate has hot summers and mild winters. The state is well known for its pine-covered north-central portion of the high country of the Colorado Plateau. Like other states of the Southwest United States, Arizona has an abundance of mountains, despite the states aridity, 27% of Arizona is forest, a percentage comparable to modern-day France or Germany. The worlds largest stand of pine trees is in Arizona
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Barringer Crater
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Meteor Crater is a meteorite impact crater approximately 37 miles east of Flagstaff and 18 miles west of Winslow in the northern Arizona desert of the United States. The site was known as the Canyon Diablo Crater and fragments of the meteorite are officially called the Canyon Diablo Meteorite. Scientists refer to the crater as Barringer Crater in honor of Daniel Barringer, the crater is privately owned by the Barringer family through their Barringer Crater Company, which proclaims it to be the best preserved meteorite crater on Earth. Despite its importance as a site, the crater is not protected as a national monument. It was designated a National Natural Landmark in November 1967, Meteor Crater lies at an elevation of about 1,740 m above sea level. It is about 1,200 m in diameter, some 170 m deep, the center of the crater is filled with 210–240 m of rubble lying above crater bedrock. One of the features of the crater is its squared-off outline. The crater was created about 50,000 years ago during the Pleistocene epoch, the area was an open grassland dotted with woodlands inhabited by woolly mammoths and giant ground sloths. Since the craters formation, the rim is thought to have lost 15–20 m of height at the rim crest due to natural erosion, similarly, the basin of the crater is thought to have approximately 30 m of additional post-impact sedimentation from lake sediments and of alluvium. These erosion processes are the reason that very few remaining craters are visible on Earth, the relatively young age of Meteor Crater, paired with the Arizona climate, have allowed this crater to remain almost unchanged since its formation. The lack of erosion that preserved the craters shape helped lead to this crater being the first crater recognized as an impact crater from a natural celestial body. The object that excavated the crater was a meteorite about 50 meters across. The speed of the impact has been a subject of some debate, modeling initially suggested that the meteorite struck at up to 20 kilometers per second but more recent research suggests the impact was substantially slower, at 12.8 kilometers per second. It is believed that half of the impactors bulk was vaporized during its descent. Impact energy has been estimated at about 10 megatons, the meteorite was mostly vaporized upon impact, leaving little remains in the crater. The crater came to the attention of scientists following its discovery by American settlers in the 19th century and this was not an unreasonable assumption, as the San Francisco volcanic field lies only about 40 miles to the west. In 1891, the mineralogist Albert E. Foote presented the first scientific paper about the meteorites of Northern Arizona, several years earlier, Foote had received an iron rock for analysis from a railroad executive. Foote immediately recognized the rock as a meteorite and led an expedition to search, the team collected samples ranging from small fragments to over 600 pounds
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Eugene Shoemaker
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Eugene Merle Shoemaker, also known as Gene Shoemaker, was an American geologist and one of the founders of the field of planetary science. He is best known for co-discovering the Comet Shoemaker–Levy 9 with his wife Carolyn S. Shoemaker and David H. Levy. Shoemaker was born in Los Angeles, California, the son of Muriel May, a teacher, and George Estel Shoemaker, who worked in farming, business, teaching, and motion pictures. For his Ph. D. degree at Princeton, Shoemaker studied the dynamics of Barringer Meteor Crater, located near Winslow. To understand the dynamics, Shoemaker inspected craters that remained after underground atomic bomb tests at the Nevada Test Site at Yucca Flat and he found a ring of ejected material that included shocked quartz, a form of quartz that has a microscopically unique structure caused by intense pressure. Shoemaker helped pioneer the field of astrogeology by founding the Astrogeology Research Program of the United States Geological Survey in 1961 at Flagstaff, Arizona and he was its first director. He was prominently involved in the Lunar Ranger missions to the Moon, Shoemaker was also involved in the training of the American astronauts. Shoemaker would train astronauts during field trips to Meteor Crater and Sunset Crater near Flagstaff and he was a CBS News television commentator on the early Apollo missions, especially the Apollo 8 and Apollo 11 missions, appearing with Walter Cronkite during live coverage of those flights. He was awarded the John Price Wetherill Medal from the Franklin Institute in 1965, coming to Caltech in 1969, he started a systematic search for Earth orbit-crossing asteroids, which resulted in the discovery of several families of such asteroids, including the Apollo asteroids. Shoemaker advanced the idea that sudden geologic changes can arise from asteroid strikes, previously, astroblemes were thought to be remnants of extinct volcanoes — even on the Moon. Shoemaker received the Barringer Medal in 1984 and a National Medal of Science in 1992, in 1993, he co-discovered Comet Shoemaker–Levy 9 using the 18 Schmidt camera at Palomar Observatory. This comet was unique in that it provided the first opportunity for scientists to observe the impact of a comet. Shoemaker–Levy 9 collided with Jupiter in 1994, the resulting impact caused a massive scar on the face of Jupiter. Most scientists at the time were dubious of whether there would even be any evident markings on the planet, Shoemaker spent much of his later years searching for and finding several previously unnoticed or undiscovered impact craters around the world. Shoemaker died on July 18,1997 during one such expedition in a head on car accident while on the Tanami Road northwest of Alice Springs and his vehicle and another were thought to be using the center, relatively smooth part of a heavily rutted, unimproved road. On seeing Shoemaker approaching, the driver of the vehicle pulled hard to his left, and had Shoemaker done the same. But Shoemaker, as an American accustomed to driving on the side of the road, reflexively pulled hard to his right. A head-on collision in which Shoemakers vehicle was traveling at 80 km/h, the occupants of the other vehicle were rendered unconscious, but not seriously injured
38.
Volcano
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A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. Earths volcanoes occur because its crust is broken into 17 major, therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging. This type of volcanism falls under the umbrella of plate hypothesis volcanism, Volcanism away from plate boundaries has also been explained as mantle plumes. These so-called hotspots, for example Hawaii, are postulated to arise from upwelling diapirs with magma from the boundary,3,000 km deep in the Earth. Volcanoes are usually not created where two plates slide past one another. Erupting volcanoes can pose hazards, not only in the immediate vicinity of the eruption. Historically, so-called volcanic winters have caused catastrophic famines, the word volcano is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn comes from Vulcan, the god of fire in Roman mythology. The study of volcanoes is called volcanology, sometimes spelled vulcanology, at the mid-oceanic ridges, two tectonic plates diverge from one another as new oceanic crust is formed by the cooling and solidifying of hot molten rock. Most divergent plate boundaries are at the bottom of the oceans, therefore, most volcanic activity is submarine, black smokers are evidence of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. In a process called flux melting, water released from the subducting plate lowers the temperature of the overlying mantle wedge. This magma tends to be very viscous due to its high content, so it often does not reach the surface. When it does reach the surface, a volcano is formed, typical examples of this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire. Because tectonic plates move across them, each volcano becomes dormant and is eventually re-formed as the plate advances over the postulated plume and this theory is currently under criticism, however. The most common perception of a volcano is of a mountain, spewing lava and poisonous gases from a crater at its summit, however. The features of volcanoes are more complicated and their structure. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have features such as massive plateaus
39.
Shoemaker-Levy 9
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This generated a large amount of coverage in the popular media, and the comet was closely observed by astronomers worldwide. The collision provided new information about Jupiter and highlighted its role in reducing space debris in the inner Solar System, the comet was discovered by astronomers Carolyn and Eugene M. Shoemaker and David Levy. Shoemaker–Levy 9 had been captured by Jupiter and was orbiting the planet at the time and it was located on the night of March 24,1993 in a photograph taken with the 40 cm Schmidt telescope at the Palomar Observatory in California. It was the first comet observed to be orbiting a planet, calculations showed that its unusual fragmented form was due to a previous closer approach to Jupiter in July 1992. At that time, the orbit of Shoemaker–Levy 9 passed within Jupiters Roche limit, the comet was later observed as a series of fragments ranging up to 2 km in diameter. These fragments collided with Jupiters southern hemisphere between July 16 and July 22,1994 at a speed of approximately 60 km/s or 216,000 km/h. The prominent scars from the impacts were more visible than the Great Red Spot. The comet was thus a serendipitous discovery, but one that quickly overshadowed the results from their main observing program, comet Shoemaker–Levy 9 was the ninth periodic comet discovered by the Shoemakers and Levy, hence its name. It was their eleventh comet discovery overall including their discovery of two comets, which use a different nomenclature. The discovery was announced in IAU Circular 5725 on March 27,1993, because of this, he suggested that the Shoemakers and David Levy had discovered the fragments of a comet that had been disrupted by Jupiters gravity. Orbital studies of the new comet soon revealed that it was orbiting Jupiter rather than the Sun and its orbit around Jupiter was very loosely bound, with a period of about 2 years and an apoapsis of 0.33 astronomical units. Its orbit around the planet was highly eccentric, tracing back the comets orbital motion revealed that it had been orbiting Jupiter for some time. It is likely that it was captured from an orbit in the early 1970s. No precovery images dating back to earlier than March 1993 have been found, before the comet was captured by Jupiter, it was probably a short-period comet with an aphelion just inside Jupiters orbit, and a perihelion interior to the asteroid belt. The volume of space within which an object can be said to orbit Jupiter is defined by Jupiters Hill sphere, when the comet passed Jupiter in the late 1960s or early 1970s, it happened to be near its aphelion, and found itself slightly within Jupiters Hill sphere. Jupiters gravity nudged the comet towards it, although the comet had approached Jupiter closely before, the July 7 encounter seemed to be by far the closest, and the fragmentation of the comet is thought to have occurred at this time. Each fragment of the comet was denoted by a letter of the alphabet, from fragment A through to fragment W, studies suggested that the train of nuclei would plow into Jupiters atmosphere over a period of about five days. Intense studies of the comet were undertaken, and as its orbit became more accurately established, the most optimistic prediction was that large, asymmetric ballistic fireballs would rise above the limb of Jupiter and into sunlight to be visible from Earth
40.
Shock metamorphism
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Shock metamorphism or impact metamorphism describes the effects of shock-wave related deformation and heating during impact events. PDFs are only produced by extreme shock compressions on the scale of meteor impacts and they are not found in volcanic environments. This form of twinning in quartz is common but the occurrence of close-spaced Brazil twins parallel to the basal plane, has only been reported from impact structures. The very high pressures associated with impacts can lead to the formation of high-pressure polymorphs of various minerals, quartz may occur as either of its two high-pressure forms, coesite and stishovite. Coesite occasionally occurs associated with eclogites formed during very high pressure regional metamorphism but was first discovered in a crater in 1960. Stishovite, however, is known from impact structures. Two of the polymorphs of titanium dioxide, one with a baddeleyite-like form. Diamond, the high-pressure allotrope of carbon, has been associated with many impact structures. Shatter cones have a conical shape that radiates from the top of the cones repeating cone-on-cone. They are only known to form in rocks beneath meteorite impact craters or underground nuclear explosions and they are evidence that the rock has been subjected to a shock with pressures in the range of 2-30 GPa. The hypothetical wavefronts are composed of many microcavities and their origin is seen in a physical phenomenon of ultrasonic cavitation, which is well known from the technical practice. The effects described above have been found singly, or more often in combination, the search for such effects therefore forms the basis for identifying possible candidate impact structures, particularly to distinguish them from volcanic features. Impactite Meteorite shock stage Suevite Chapter 4, Shock-Metamorphic Effects in Rocks and Minerals of the book, French. Traces of Catastrophe, A handbook of shock-metamorphic effects in terrestrial meteorite impact structures, Lunar and Planetary Institute 120pp
41.
Shocked quartz
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Shocked quartz is a form of quartz that has a microscopic structure that is different from normal quartz. Under intense pressure, the structure of quartz is deformed along planes inside the crystal. These planes, which show up as lines under a microscope, are called planar deformation features, Shocked quartz was discovered after underground nuclear bomb testing, which caused the intense pressures required to form it. Eugene Shoemaker showed that shocked quartz is found inside craters created by meteor impact, such as the Barringer Crater. The presence of shocked quartz proves that these craters were formed by an impact, Shocked quartz is found worldwide, in the thin Cretaceous–Paleogene boundary layer, which occurs at the contact between Cretaceous and Paleogene rocks. This is further evidence that the transition between the two periods was caused by a large impact. Though shocked quartz is only recognized, Eugene Shoemaker discovered it in building stones in the Bavarian town of Nördlingen. Shocked quartz is associated with two high-pressure polymorphs of silicon dioxide, coesite and stishovite and these polymorphs have a crystal structure different from standard quartz. This structure can be formed only by pressure, but at moderate temperatures. High temperatures would anneal the quartz back to its standard form, Coesite and stishovite are also indicative of impact events. Breccia Lechatelierite Shatter cone Shock metamorphism Shocked quartz page Coesite page Stishovite page
