Enceladus is the sixth-largest moon of Saturn. It is about 500 kilometers about a tenth of that of Saturn's largest moon, Titan. Enceladus is covered by fresh, clean ice, making it one of the most reflective bodies of the Solar System, its surface temperature at noon only reaches −198 °C, far colder than a light-absorbing body would be. Despite its small size, Enceladus has a wide range of surface features, ranging from old cratered regions to young, tectonically deformed terrains. Enceladus was discovered on August 28, 1789, by William Herschel, but little was known about it until the two Voyager spacecraft, Voyager 1 and Voyager 2, passed nearby in the early 1980s. In 2005, the Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered water-rich plumes venting from the south polar region. Cryovolcanoes near the south pole shoot geyser-like jets of water vapor, molecular hydrogen, other volatiles, solid material, including sodium chloride crystals and ice particles, into space, totaling about 200 kg per second.
Over 100 geysers have been identified. Some of the water vapor falls back as "snow". According to NASA scientists, the plumes are similar in composition to comets. In 2014, NASA reported that Cassini found evidence for a large south polar subsurface ocean of liquid water with a thickness of around 10 km; these geyser observations, along with the finding of escaping internal heat and few impact craters in the south polar region, show that Enceladus is geologically active. Like many other satellites in the extensive systems of the giant planets, Enceladus is trapped in an orbital resonance, its resonance with Dione excites its orbital eccentricity, damped by tidal forces, tidally heating its interior and driving the geological activity. On 27 June 2018, scientists reported the detection of complex macromolecular organics on Enceladus' jet plumes, as sampled by the Cassini orbiter. Enceladus was discovered by William Herschel on August 28, 1789, during the first use of his new 1.2 m 40-foot telescope the largest in the world, at Observatory House in Slough, England.
Its faint apparent magnitude and its proximity to the much brighter Saturn and Saturn's rings make Enceladus difficult to observe from Earth with smaller telescopes. Like many satellites of Saturn discovered prior to the Space Age, Enceladus was first observed during a Saturnian equinox, when Earth is within the ring plane. At such times, the reduction in glare from the rings makes the moons easier to observe. Prior to the Voyager missions the view of Enceladus improved little from the dot first observed by Herschel. Only its orbital characteristics were known, with estimations of its mass and albedo. Enceladus is named after the giant Enceladus of Greek mythology; the name, like the names of each of the first seven satellites of Saturn to be discovered, was suggested by William Herschel's son John Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope. He chose these names because Saturn, known in Greek mythology as Cronus, was the leader of the Titans.
Features on Enceladus are named by the International Astronomical Union after characters and places from Burton's translation of The Book of One Thousand and One Nights. Impact craters are named after characters, whereas other feature types, such as fossae, planitiae and rupes are named after places; the IAU has named 85 features on Enceladus, most Samaria Rupes called Samaria Fossa. Enceladus is one of the major inner satellites of Saturn along with Dione and Mimas, it orbits at 238,000 km from Saturn's center and 180,000 km from its cloud tops, between the orbits of Mimas and Tethys. It orbits Saturn every 32.9 hours, fast enough for its motion to be observed over a single night of observation. Enceladus is in a 2:1 mean-motion orbital resonance with Dione, completing two orbits around Saturn for every one orbit completed by Dione; this resonance maintains Enceladus's orbital eccentricity, known as a forced eccentricity. This non-zero eccentricity results in tidal deformation of Enceladus; the dissipated heat resulting from this deformation is the main heating source for Enceladus's geologic activity.
Enceladus orbits within the densest part of Saturn's E ring, the outermost of its major rings, is the main source of the ring's material composition. Like most of Saturn's larger satellites, Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. Unlike Earth's Moon, Enceladus does not appear to librate more than 1.5° about its spin axis. However, analysis of the shape of Enceladus suggests that at some point it was in a 1:4 forced secondary spin–orbit libration; this libration could have provided Enceladus with an additional heat source. Plumes from Enceladus, which are similar in composition to comets, have been shown to be the source of the material in Saturn's E ring; the E ring is the widest and outermost ring of Saturn. It is an wide but diffuse disk of microscopic icy or dusty material distributed between the orbits of Mimas and Titan. Mathematical models show that the E ring is unstable, with a lifespan between 10,000 and 1,000,000 years. Enceladus is orbiting at its narrowest but highest density point.
In the 1980s some suspected that Enceladus is the
Schiaparelli (Martian crater)
Schiaparelli is an impact crater on Mars, located near the planet's equator at latitude 3° south and longitude 344° in the Sinus Sabaeus quadrangle. It measures 459 kilometers in diameter and was named after Italian astronomer Giovanni Schiaparelli, known for his observations of the Red Planet and his mistranslated term "canali"; the name was adopted by IAU's Working Group for Planetary System Nomenclature in 1973. A crater within Schiaparelli shows many layers that may have formed by the wind, volcanoes, or deposition under water. Layers can be a few meters tens of meters thick. Recent research on these layers suggests that ancient climate change on Mars, caused by regular variation in the planet's tilt, may have caused the patterns in layers. On Earth, similar changes of climate results in ice-age cycles; the regular appearance of rock layers suggests that regular changes in climate may be the root cause. Regular changes in climate may be due to variations of a planet's tilt; the tilt of the Earth's axis changes by only a little more than 2 degrees since our moon is large.
In contrast Mars's tilt varies by tens of degrees. When the tilt is low, the poles are the coldest places on the planet, while the equator is the warmest; this could cause gases in the atmosphere, like water and carbon dioxide, to migrate poleward, where they would freeze. When the obliquity is higher, the poles receive more sunlight, causing those materials to migrate away; when carbon dioxide moves from the Martian poles, the atmospheric pressure increases causing a difference in the ability of winds to transport and deposit sand. With more water in the atmosphere sand grains may stick and cement together to form layers. In the 2011 novel The Martian by Andy Weir, the 2015 feature film adapted from it, Schiaparelli is the landing site for Ares 4, the fourth manned mission to Mars; the protagonist, Mark Watney, an astronaut from Ares 3, stranded on Mars, must travel from Acidalia Planitia to Schiaparelli, a journey of 3,200 kilometres. 4062 Schiaparelli, asteroid Climate of Mars Geology of Mars HiRISE HiWish program Impact event List of craters on Mars Ore resources on Mars Planetary nomenclature Layers, Bedrock Ridges, Dark Sand in Schiaparelli Crater, LPL HiRISE, includes large color photos
Tikhonravov is a large, eroded crater in the Arabia quadrangle of Mars. It is 344 kilometres in diameter and was named after Mikhail Tikhonravov, a Russian rocket scientist. Tikhonravov is believed to have once held a giant lake that drained into the 4500 km long Naktong-Scamander-Mamers lake-chain system. An inflow and outflow channel has been identified. Many craters once contained lakes; some craters in Tikhonravov are classified as pedestal craters. A pedestal crater is a crater with its ejecta sitting above the surrounding terrain, they form when an impact crater ejects material which forms an erosion resistant layer, thus causing the immediate area to erode more than the rest of the region. The result is that both its ejecta blanket stand above the surroundings. List of craters on Mars
E. M. Antoniadi
Eugène Michel Antoniadi was a Greek astronomer. Antoniadi was born in Istanbul but spent most of his adult life in France, after being invited there by Camille Flammarion, he became a Fellow of the Royal Astronomical Society on 10 February 1899, in 1890 he became one of the founding members of the British Astronomical Association. In 1892, he joined the BAA’s Mars Section and became that section's Director in 1896, he became a member of the Société astronomique de France in 1891. Flammarion hired Antoniadi to work as an assistant astronomer in his private observatory in Juvisy-sur-Orge in 1893. Antoniadi worked there for nine years. In 1902, he resigned from both the Juvisy observatory and from SAF. Antoniadi rejoined SAF in 1909; that same year, Henri Deslandres, Director of the Meudon Observeratory, provided him with access to the Grande Lunette He became a reputed observer of Mars, at first supported the notion of Martian canals, but after using the 83 centimeter telescope at Meudon Observatory during the 1909 opposition of Mars, he came to the conclusion that canals were an optical illusion.
He observed Venus and Mercury. He made the first map of Mercury, but his maps were flawed by his incorrect assumption that Mercury had synchronous rotation with the Sun; the first standard nomenclature for Martian albedo features was introduced by the International Astronomical Union when they adopted 128 names from the 1929 map of Antoniadi named La Planète Mars. He is famed for creating the Antoniadi scale of seeing, used by amateur astronomers, he was a strong chess player. His best result was equal first with Frank Marshall in a tournament in Paris in 1907, a point ahead of Savielly Tartakower, he died in Paris, aged 73. His full name was Eugène Michel Antoniadi, however he was known as Eugenios Antoniadis, his name is sometimes given as Eugène Michael Antoniadi or as Eugène Marie Antoniadi. 1925 - Prix Jules Janssen from the Société astronomique de France. 1926 – Prix Guzman of 2,500 Francs from the Académie des Sciences. 1932 - Prix La Caille from the Académie des Sciences. 1970 - Antoniadi impact crater on the Moon named in his honor by the International Astronomical Union.
1973 - Antoniadi crater on Mars named in his honor by the International Astronomical Union. 1976 - Antoniadi Dorsum wrinkle ridge on Mercury named in his honor by the International Astronomical Union. Antoniadi was a prolific writer of books; the subjects included astronomy and architecture. He wrote articles for L'Astronomie of the Société astronomique de France, Astronomische Nachrichten, the Monthly Notices of the Royal Astronomical Society, among others. Notable works include: Sur une Anomalie. La planète Mars, 1659-1929. La Planète Mercure et la rotation des satellites. Etude basée sur les résultats obtenus avec la grande lunette de l'observatoire de Meudon. Abetti, Giorgio. "Antoniadi, Eugène M.". Dictionary of Scientific Biography. 1. New York: Charles Scribner's Sons. P. 172. ISBN 978-0-684-10114-9. McKim, Richard J.. "The Life and Times of E. M. Antoniadi, 1870-1944. Part I: An Astronomer in the Making". Journal of the British Astronomical Association. 103: 164–170. Bibcode:1993JBAA..103..164M. McKim, Richard J..
"The Life and Times of E. M. Antoniadi, 1870-1944. Part II: The Meudon Years". Journal of the British Astronomical Association. 103: 219–227. Bibcode:1993JBAA..103..219M. Edward Winter, A Chessplaying Astronomer
Tethys is a mid-sized moon of Saturn about 1,060 km across. It is named after the titan Tethys of Greek mythology. Tethys has a low density of 0.98 g/cm3, the lowest of all the major moons in the Solar System, indicating that it is made of water ice with just a small fraction of rock. This is confirmed by the spectroscopy of its surface, which identified water ice as the dominant surface material. A small amount of an unidentified dark material is present as well; the surface of Tethys is bright, being the second-brightest of the moons of Saturn after Enceladus, neutral in color. Tethys is cratered and cut by a number of large faults/graben; the largest impact crater, Odysseus, is about 400 km in diameter, whereas the largest graben, Ithaca Chasma, is about 100 km wide and more than 2000 km long. These two largest surface features may be related. A small part of the surface is covered by smooth plains. Like all other regular moons of Saturn, Tethys formed from the Saturnian sub-nebula—a disk of gas and dust that surrounded Saturn soon after its formation.
Tethys has been approached by several space probes including Pioneer 11, Voyager 1, Voyager 2, multiple times by Cassini since 2004. Tethys was discovered by Giovanni Domenico Cassini in 1684 together with Dione, another moon of Saturn, he had discovered two moons and Iapetus earlier, in 1671–72. Cassini observed all of these moons using a large aerial telescope he set up on the grounds of the Paris Observatory. Cassini named the four new moons as Sidera Lodoicea to honour king Louis XIV of France. By the end of the seventeenth century, astronomers fell into the habit of referring to them and Titan as Saturn I through Saturn V. Once Mimas and Enceladus were discovered in 1789, the numbering scheme was extended to Saturn VII by bumping the older five moons up two slots; the discovery of Hyperion in 1848 changed the numbers one last time, bumping Iapetus up to Saturn VIII. Henceforth, the numbering scheme would remain fixed; the modern names of all seven satellites of Saturn come from John Herschel.
In his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope, he suggested the names of the Titans and brothers of Kronos, be used. Tethys is named after the titaness Tethys of Greek mythology, it is designated Saturn III or S III Tethys. The correct adjectival form of the moon's name is Tethyan, although other forms are used. Tethys orbits Saturn at a distance of about 295,000 km from the center of the planet, its orbital eccentricity is negligible, its orbital inclination is about 1°. Tethys is locked in an inclination resonance with Mimas, however due to the low gravity of the respective bodies this interaction does not cause any noticeable orbital eccentricity or tidal heating; the Tethyan orbit lies deep inside the magnetosphere of Saturn, so the plasma co-rotating with the planet strikes the trailing hemisphere of the moon. Tethys is subject to constant bombardment by the energetic particles present in the magnetosphere. Tethys has two co-orbital moons and Calypso orbiting near Tethys's trojan points L4 and L5 respectively.
Tethys is the 16th-largest moon in the Solar System, with a radius of 531 km. Its mass is 6.17×1020 kg, less than 1% of the Moon. The density of Tethys is 0.98 g/cm³, indicating that it is composed entirely of water-ice. It is not known whether Tethys is differentiated into a rocky ice mantle. However, if it is differentiated, the radius of the core does not exceed 145 km, its mass is below 6% of the total mass. Due to the action of tidal and rotational forces, Tethys has the shape of triaxial ellipsoid; the dimensions of this ellipsoid are consistent with it having a homogeneous interior. The existence of a subsurface ocean—a layer of liquid salt water in the interior of Tethys—is considered unlikely; the surface of Tethys is one of the most reflective in the Solar System, with a visual albedo of 1.229. This high albedo is the result of the sandblasting of particles from Saturn's E-ring, a faint ring composed of small, water-ice particles generated by Enceladus's south polar geysers; the radar albedo of the Tethyan surface is very high.
The leading hemisphere of Tethys is brighter by 10–15% than the trailing one. The high albedo indicates that the surface of Tethys is composed of pure water ice with only a small amount of darker materials; the visible spectrum of Tethys is flat and featureless, whereas in the near-infrared strong water ice absorption bands at 1.25, 1.5, 2.0 and 3.0 μm wavelengths are visible. No compound other than crystalline water ice has been unambiguously identified on Tethys; the dark material in the ice has the same spectral properties as seen on the surfaces of the dark Saturnian moons—Iapetus and Hyperion. The most probable candidate is nanophase hematite. Measurements of the thermal emission as well as radar observations by the Cassini spacecraft show that the icy regolith on the surface of Tethys is structurally complex and has a large porosity exceeding 95%; the surface of Tethys has a number of large-scale features distinguished by their color and sometimes brightness. The trailing hemisphere gets red and dark as the anti-apex of motion is approached.
This darkening is responsible for the hemispheric albedo asymmetry mentioned above. The leading hemisphere reddens slig
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System after Mercury. In English, Mars carries a name of the Roman god of war, is referred to as the "Red Planet" because the reddish iron oxide prevalent on its surface gives it a reddish appearance, distinctive among the astronomical bodies visible to the naked eye. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys and polar ice caps of Earth; the days and seasons are comparable to those of Earth, because the rotational period as well as the tilt of the rotational axis relative to the ecliptic plane are similar. Mars is the site of Olympus Mons, the largest volcano and second-highest known mountain in the Solar System, of Valles Marineris, one of the largest canyons in the Solar System; the smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature. Mars has two moons and Deimos, which are small and irregularly shaped.
These may be captured asteroids, similar to a Mars trojan. There are ongoing investigations assessing the past habitability potential of Mars, as well as the possibility of extant life. Future astrobiology missions are planned, including the Mars 2020 and ExoMars rovers. Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, less than 1% of the Earth's, except at the lowest elevations for short periods; the two polar ice caps appear to be made of water. The volume of water ice in the south polar ice cap, if melted, would be sufficient to cover the entire planetary surface to a depth of 11 meters. In November 2016, NASA reported finding a large amount of underground ice in the Utopia Planitia region of Mars; the volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior. Mars can be seen from Earth with the naked eye, as can its reddish coloring, its apparent magnitude reaches −2.94, surpassed only by Jupiter, the Moon, the Sun.
Optical ground-based telescopes are limited to resolving features about 300 kilometers across when Earth and Mars are closest because of Earth's atmosphere. Mars is half the diameter of Earth with a surface area only less than the total area of Earth's dry land. Mars is less dense than Earth, having about 15% of Earth's volume and 11% of Earth's mass, resulting in about 38% of Earth's surface gravity; the red-orange appearance of the Martian surface is caused by rust. It can look like butterscotch. Like Earth, Mars has differentiated into a dense metallic core overlaid by less dense materials. Current models of its interior imply a core with a radius of about 1,794 ± 65 kilometers, consisting of iron and nickel with about 16–17% sulfur; this iron sulfide core is thought to be twice as rich in lighter elements as Earth's. The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet, but it appears to be dormant. Besides silicon and oxygen, the most abundant elements in the Martian crust are iron, aluminum and potassium.
The average thickness of the planet's crust is about 50 km, with a maximum thickness of 125 km. Earth's crust averages 40 km. Mars is a terrestrial planet that consists of minerals containing silicon and oxygen and other elements that make up rock; the surface of Mars is composed of tholeiitic basalt, although parts are more silica-rich than typical basalt and may be similar to andesitic rocks on Earth or silica glass. Regions of low albedo suggest concentrations of plagioclase feldspar, with northern low albedo regions displaying higher than normal concentrations of sheet silicates and high-silicon glass. Parts of the southern highlands include detectable amounts of high-calcium pyroxenes. Localized concentrations of hematite and olivine have been found. Much of the surface is covered by finely grained iron oxide dust. Although Mars has no evidence of a structured global magnetic field, observations show that parts of the planet's crust have been magnetized, suggesting that alternating polarity reversals of its dipole field have occurred in the past.
This paleomagnetism of magnetically susceptible minerals is similar to the alternating bands found on Earth's ocean floors. One theory, published in 1999 and re-examined in October 2005, is that these bands suggest plate tectonic activity on Mars four billion years ago, before the planetary dynamo ceased to function and the planet's magnetic field faded, it is thought that, during the Solar System's formation, Mars was created as the result of a stochastic process of run-away accretion of material from the protoplanetary disk that orbited the Sun. Mars has many distinctive chemical features caused by its position in the Solar System. Elements with comparatively low boiling points, such as chlorine and sulphur, are much more common on Mars than Earth. After the formation of the planets, all were subjected to the so-called "Late Heavy Bombardment". About 60% of the surface of Mars shows a record of impacts from that era, whereas much of the remaining surface is underlain by immense impact basins caused by those events.
There is evidence of an enormous impact basin in the northern hemisphere of Mars, spanning 10,600 by 8,500 km, or four times the size of the Moon's South Pole – Aitk
Cassini (Martian crater)
For the lunar crater, see Cassini. For other things named Cassini, see Cassini. Cassini is a crater on Mars named in honour of the Italian astronomer Giovanni Cassini; the name was approved in 1973, by the International Astronomical Union Working Group for Planetary System Nomenclature. The crater measures 415 kilometers in diameter and can be found at 327.9°W and 23.4°N. It is in the Arabia quadrangle of Mars. Pictures of small craters on the floor of Cassini reveal multiple layers; some of these layers can be seen in the pictures below. Many places on Mars show rocks arranged in layers. Rock can form layers in a variety of ways. Volcanoes, wind, or water can produce layers. A detailed discussion of layering with many Martian examples can be found in Sedimentary Geology of Mars. Recent research leads scientists to believe that some of the craters in Arabia may have held huge lakes. Cassini Crater once was full of water since its rim seems to have been breached by the waters. Both inflow and outflow channels have been observed on its rim.
The lake would have contained more water than Earth's Lake Baikal, our largest freshwater lake by volume. Many craters once contained lakes. List of craters on Mars