Europa is the smallest of the four Galilean moons orbiting Jupiter, the sixth-closest to the planet of all the 79 known moons of Jupiter. It is the sixth-largest moon in the Solar System. Europa was discovered in 1610 by Galileo Galilei and was named after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus. Smaller than Earth's Moon, Europa is made of silicate rock and has a water-ice crust and an iron–nickel core, it has a thin atmosphere composed of oxygen. Its surface is striated by cracks and streaks, but craters are few. 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 known 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, which could conceivably harbour extraterrestrial life. The predominant model suggests that heat from tidal flexing causes the ocean to remain liquid and drives ice movement similar to plate tectonics, absorbing chemicals from the surface into the ocean below.
Sea salt from a subsurface ocean may be coating some geological features on Europa, suggesting that the ocean is interacting with the sea floor. This may be important in determining. In addition, the Hubble Space Telescope detected water vapor plumes similar to those observed on Saturn's moon Enceladus, which are thought to be caused by erupting cryogeysers. In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated critical analysis of data obtained from the Galileo space probe, which orbited Jupiter from 1995 to 2003; such plume activity could help researchers in a search for life from the subsurface Europan ocean without having to land on the moon. 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 Agency's Jupiter Icy Moon Explorer is a mission to Ganymede, due to launch in 2022, will include two flybys of Europa.
NASA's planned. Europa, along with Jupiter's three other large moons, Io, Callisto, was discovered by Galileo Galilei on 8 January 1610, independently by Simon Marius; the first reported observation of Io and Europa was made by Galileo on 7 January 1610 using a 20×-magnification refracting telescope at the University of Padua. However, in that observation, Galileo could not separate Io and Europa due to the low magnification of his telescope, so that the two were recorded as a single point of light; the following day, 8 January 1610, Io and Europa were seen for the first time as separate bodies during Galileo's 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, the Greek counterpart of Jupiter. Europa became the queen of Crete; the naming scheme was suggested by Simon Marius. The names fell out of favor for a considerable time and were not revived in general use until the mid-20th century.
In much of the earlier astronomical literature, Europa is referred to by its Roman numeral designation as Jupiter II or as the "second satellite of Jupiter". In 1892, the discovery of Amalthea, whose orbit lay closer to Jupiter than those of the Galilean moons, pushed Europa to the third position; the Voyager probes discovered three more inner satellites in 1979, so Europa is now considered Jupiter's sixth satellite, though it is still sometimes referred to as Jupiter II. Europa orbits Jupiter in just over three and a half days, with an orbital radius of about 670,900 km. With an orbital eccentricity of only 0.009, the orbit itself is nearly circular, the orbital inclination relative to Jupiter's equatorial plane is small, at 0.470°. Like its fellow Galilean satellites, Europa is tidally locked to Jupiter, with one hemisphere of Europa facing Jupiter; because of this, there is a sub-Jovian point on Europa's surface, from which Jupiter would appear to hang directly overhead. Europa's prime meridian is a line passing through this point.
Research suggests that the tidal locking may not be full, as a non-synchronous rotation has been proposed: Europa spins faster than it orbits, or at least did so in the past. This suggests an asymmetry in internal mass distribution and that a layer of subsurface liquid separates the icy crust from the rocky interior; the slight eccentricity of Europa's orbit, maintained by the gravitational disturbances from the other Galileans, causes Europa's sub-Jovian point to oscillate around a mean position. As Europa comes nearer to Jupiter, Jupiter's gravitational attraction increases, causing Europa to elongate towards and away from it; as Europa moves away from Jupiter, Jupiter's gravitational force decreases, causing Europa to relax back into a more spherical shape, creating tides in its ocean. The orbital eccentricity of Europa is continuously pumped by its mean-motion resonance with Io. Thus, the tidal flexing kneads Europa's interior and gives it a source of heat allowing its ocean to stay liquid while driving subsurface geological processes.
The ultimate source of this energy is Jupiter's rotation, tapped by Io through the tides it raises on Jupiter and is transferred to E
The Elysium quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey Astrogeology Research Program. The Elysium quadrangle is referred to as MC-15; the name Elysium refers according to Homer in the Odyssey. The Elysium quadrangle covers the area 180° to 225° west longitude and 0° to 30° north latitude on Mars. Elysium Planitia is in the Elysium quadrangle; the Elysium quadrangle includes a part of Lucus Planum. A small part of the Medusae Fossae Formation lies in this quadrangle; the largest craters in this quadrangle are Eddie and Tombaugh. Elysium contains major volcanoes named Elysium Mons and Albor Tholus and river valleys—one of which, Athabasca Valles may be one of the youngest on Mars. On the east side is an elongated depression called Orcus Patera. A large lake may once have existed in the south near Athabasca Valles. InSight lander landed in the southern part of this quadrangle in 2018; the Elysium quadrangle contains Albor Tholus. David Susko and his colleagues at Louisiana State University analyzed geochemical and surface morphology data from Elysium using instruments on board NASA's Mars Odyssey Orbiter and Mars Reconnaissance Orbiter.
Through crater counting, they found differences in age between the northwest and the southeast regions of Elysium—about 850 million years of difference. They found that the younger southeast regions are geochemically different from the older regions, that these differences related to igneous processes, not secondary processes like the interaction of water or ice with the surface of Elysium in the past. "We determined that while there might have been water in this area in the past, the geochemical properties in the top meter throughout this volcanic province are indicative of igneous processes," Susko said. "We think levels of thorium and potassium here were depleted over time because of volcanic eruptions over billions of years. The radioactive elements were the first to go in the early eruptions. We are seeing changes in the mantle chemistry over time." "Long-lived volcanic systems with changing magma compositions are common on Earth, but an emerging story on Mars," said James Wray, study co-author and associate professor in the School of Earth and Atmospheric Sciences at Georgia Tech.
Overall, these findings indicate that Mars is a much more geologically complex body than thought due to various loading effects on the mantle caused by the weight of giant volcanoes. For decades, we saw Mars, as a lifeless rock, full of craters with a number of long inactive volcanoes. We had a simple view of the red planet. Finding a variety of igneous rocks demonstrates that Mars has the potential for useful resource utilization and a capacity to sustain a human population on Mars. "It's much easier to survive on a complex planetary body bearing the mineral products of complex geology than on a simpler body like the moon or asteroids."Much of the area near the volcanoes is covered with lava flows, some can be shown approaching stopping upon reaching higher ground. Sometimes when lava flows the top cools into a solid crust. However, the lava below still flows, this action breaks up the top layer making it rough; such rough flow is called aa. Research, published in January 2010, described the discovery of a vast single lava flow, the size of the state of Oregon, that "was put in place turbulently over the span of several weeks at most."
This flow, near Athabasca Valles, is the youngest lava flow on Mars. It is thought to be of Late Amazonian Age. Other researchers disagree with this idea. Under Martian conditions lava should not stay fluid long; some areas in the Elysium quadrangle are geological young and have surfaces that are hard to explain. Some have called them Platy-Ridged-Polygonized terrain; the surface has been suggested to be from basalt lava, or a muddy flow. Using HiRISE images the heights of the ridges of the surface were measured. Most were less than 2 meters; this is far smaller than. The high resolution photos demonstrated that the material seemed to flow which would not occur with pack ice. So, the researchers concluded. So-called "Rootless cones" are caused by explosions of lava with ground ice under the flow; the ice turns into a vapor that expands in an explosion that produces a cone or ring. Featureslike these are found in Iceland; the Elysium Fossae contain layers called strata. Many places on Mars show rocks arranged in layers.
Sometimes the layers are of different colors. Light-toned rocks on Mars have been associated with hydrated minerals like sulfates; the Mars Rover Opportunity examined such layers close-up with several instruments. Pictures taken from orbiting spacecraft show that some layers of rocks seem to break up into fine dust. Other layers break up into large boulders, so they are much harder. Basalt, a volcanic rock, is thought comprise the layers. Basalt has been identified on Mars in many places. Instruments on orbiting spacecraft have detected clay in some layers. Scientists are excited about finding hydrated minerals such as sulfates and clays on Mars because they are formed in the presence of water. Places that contain clays and/or other hydrated minerals would be good places to look for evidence of life. Rock can be formed into layers in a variety of ways. Volcanoes, wind, or water can produce layers. Layers can be hardened by the action of groundwater
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different chemical composition; the minerals which compose the rocks are dissolved and new mineral formations are deposited in their place. Dissolution and deposition occur and the rock remains solid. Synonyms to the word metasomatism are metasomatic process; the word metasomatose can be used as a name for specific varieties of metasomatism. Metasomatism can occur via the action of hydrothermal fluids from an metamorphic source. In the igneous environment, metasomatism creates skarns and may affect hornfels in the contact metamorphic aureole adjacent to an intrusive rock mass. In the metamorphic environment, metasomatism is created by mass transfer from a volume of metamorphic rock at higher stress and temperature into a zone with lower stress and temperature, with metamorphic hydrothermal solutions acting as a solvent; this can be envisaged as the metamorphic rocks within the deep crust losing fluids and dissolved mineral components as hydrous minerals break down, with this fluid percolating up into the shallow levels of the crust to chemically change and alter these rocks.
This mechanism implies that metasomatism is open system behaviour, different from classical metamorphism, the in-situ mineralogical change of a rock without appreciable change in the chemistry of the rock. Because metamorphism requires water in order to facilitate metamorphic reactions, metamorphism nearly always occurs with metasomatism. Further, because metasomatism is a mass transfer process, it is not restricted to the rocks which are changed by addition of chemical elements and minerals or hydrous compounds. In all cases, to produce a metasomatic rock some other rock is metasomatised, if only by dehydration reactions with minimal chemical change; this is best illustrated by gold ore deposits which are the product of focused concentration of fluids derived from many cubic kilometres of dehydrated crust into thin highly metasomatised and altered shear zones and lodes. The source region is largely chemically unaffected compared to the hydrated, altered shear zones, but both must have undergone complementary metasomatism.
Metasomatism is more complicated in the Earth's mantle, because the composition of peridotite at high temperatures can be changed by infiltration of carbonate and silicate melts and by carbon dioxide-rich and water-rich fluids, as discussed by Luth. Metasomatism is thought to be important in changing the composition of mantle peridotite below island arcs as water is driven out of ocean lithosphere during subduction. Metasomatism has been considered critical for enriching source regions of some silica-undersaturated magmas. Carbonatite melts are considered to have been responsible for enrichment of mantle peridotite in incompatible elements. Metasomatic rocks can be varied. Metasomatised rocks are pervasively but weakly altered, such that the only evidence of alteration is bleaching, change in colour or change in the crystallinity of micaceous minerals. In such cases, characterising alteration requires microscope investigation of the mineral assemblage of the rocks to characterise the minerals, any additional mineral growth, changes in protolith minerals, so on.
In some cases, geochemical evidence can be found of metasomatic alteration processes. This is in the form of mobile, soluble elements such as barium, rubidium and some rare earth elements. However, to characterise the alteration properly, it is necessary to compare altered with unaltered samples; when the process becomes advanced, typical metasomatites can include: Chlorite or mica whole-rock replacement in shear zones, resulting in rocks in which the existing mineralogy has been recrystallised and replaced by hydrated minerals such as chlorite and serpentine. Skarn and skarnoid rock types adjacent to granite intrusions and adjacent to reactive lithologies such as limestone and banded iron formation. Greisen deposits within granite margins and cupolas. Rodingite typical of ophiolites partiuarly serpentised mafic dykes in an ultramafic sequence, containing grossular-andradite garnet and calcic pyroxene. Effects of metasomatism in mantle peridotite can be either cryptic. In cryptic metasomatism, mineral compositions are changed, or introduced elements are concentrated on grain boundaries and the peridotite mineralogy appears unchanged.
In modal metasomatism, new minerals are formed. Cryptic metasomatism may be caused as rising or percolating melts interact with surrounding peridotite, compositions of both melts and peridotite are changed. At high mantle temperatures, solid-state diffusion can be effective in changing rock compositions over tens of centimeters adjacent to melt conduits: gradients in mineral composition adjacent to pyroxenite dikes may preserve evidence of the process. Modal metasomatism may result in formation of amphibole and phlogopite, the presence of these minerals in peridotite xenoliths has been considered strong evidence of metasomatic processes in the mantle. Formation of minerals less common in peridotite, such as dolomite, ilmenite and armalcolite, is attributed to melt or fluid metasomatism. Investigation of altered rocks in hydrothermal ore deposits has highlighted several ubiquitous types of alteration assemblages which create distinct groups of metasomatic alteration effects and mineral assemblages.
Propylitic alteration is caused by ir
Aluminium or aluminum is a chemical element with symbol Al and atomic number 13. It is a silvery-white, soft and ductile metal in the boron group. By mass, aluminium makes up about 8% of the Earth's crust; the chief ore of aluminium is bauxite. Aluminium metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is found combined in over 270 different minerals. Aluminium is remarkable for its low density and its ability to resist corrosion through the phenomenon of passivation. Aluminium and its alloys are vital to the aerospace industry and important in transportation and building industries, such as building facades and window frames; the oxides and sulfates are the most useful compounds of aluminium. Despite its prevalence in the environment, no known form of life uses aluminium salts metabolically, but aluminium is well tolerated by plants and animals; because of these salts' abundance, the potential for a biological role for them is of continuing interest, studies continue.
Of aluminium isotopes, only 27Al is stable. This is consistent with aluminium having an odd atomic number, it is the only aluminium isotope that has existed on Earth in its current form since the creation of the planet. Nearly all the element on Earth is present as this isotope, which makes aluminium a mononuclidic element and means that its standard atomic weight equates to that of the isotope; the standard atomic weight of aluminium is low in comparison with many other metals, which has consequences for the element's properties. All other isotopes of aluminium are radioactive; the most stable of these is 26Al and therefore could not have survived since the formation of the planet. However, 26Al is produced from argon in the atmosphere by spallation caused by cosmic ray protons; the ratio of 26Al to 10Be has been used for radiodating of geological processes over 105 to 106 year time scales, in particular transport, sediment storage, burial times, erosion. Most meteorite scientists believe that the energy released by the decay of 26Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.
The remaining isotopes of aluminium, with mass numbers ranging from 21 to 43, all have half-lives well under an hour. Three metastable states are known, all with half-lives under a minute. An aluminium atom has 13 electrons, arranged in an electron configuration of 3s23p1, with three electrons beyond a stable noble gas configuration. Accordingly, the combined first three ionization energies of aluminium are far lower than the fourth ionization energy alone. Aluminium can easily surrender its three outermost electrons in many chemical reactions; the electronegativity of aluminium is 1.61. A free aluminium atom has a radius of 143 pm. With the three outermost electrons removed, the radius shrinks to 39 pm for a 4-coordinated atom or 53.5 pm for a 6-coordinated atom. At standard temperature and pressure, aluminium atoms form a face-centered cubic crystal system bound by metallic bonding provided by atoms' outermost electrons; this crystal system is shared by some other metals, such as copper. Aluminium metal, when in quantity, is shiny and resembles silver because it preferentially absorbs far ultraviolet radiation while reflecting all visible light so it does not impart any color to reflected light, unlike the reflectance spectra of copper and gold.
Another important characteristic of aluminium is its low density, 2.70 g/cm3. Aluminium is a soft, lightweight and malleable with appearance ranging from silvery to dull gray, depending on the surface roughness, it is nonmagnetic and does not ignite. A fresh film of aluminium serves as a good reflector of visible light and an excellent reflector of medium and far infrared radiation; the yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa. Aluminium has stiffness of steel, it is machined, cast and extruded. Aluminium atoms are arranged in a face-centered cubic structure. Aluminium has a stacking-fault energy of 200 mJ/m2. Aluminium is a good thermal and electrical conductor, having 59% the conductivity of copper, both thermal and electrical, while having only 30% of copper's density. Aluminium is capable of superconductivity, with a superconducting critical temperature of 1.2 kelvin and a critical magnetic field of about 100 gauss.
Aluminium is the most common material for the fabrication of superconducting qubits. Aluminium's corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the bare metal is exposed to air preventing further oxidation, in a process termed passivation; the strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper. This corrosion resistance is reduced by aqueous salts in the presence of dissimilar metals. In acidic solutions, aluminium reacts with water to form hydrogen, in alkaline ones to form aluminates—protective passivation under these conditions is negligible; because it is corroded by dissolved chlorides, such as common sodium chloride, household plumbing is never made from aluminium. However, because
The alkali metals are a group in the periodic table consisting of the chemical elements lithium, potassium, rubidium and francium. This group lies in the s-block of the periodic table of elements as all alkali metals have their outermost electron in an s-orbital: this shared electron configuration results in their having similar characteristic properties. Indeed, the alkali metals provide the best example of group trends in properties in the periodic table, with elements exhibiting well-characterised homologous behaviour; the alkali metals are all shiny, soft reactive metals at standard temperature and pressure and lose their outermost electron to form cations with charge +1. They can all be cut with a knife due to their softness, exposing a shiny surface that tarnishes in air due to oxidation by atmospheric moisture and oxygen; because of their high reactivity, they must be stored under oil to prevent reaction with air, are found only in salts and never as the free elements. Caesium, the fifth alkali metal, is the most reactive of all the metals.
In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, excluding hydrogen, nominally a group 1 element but not considered to be an alkali metal as it exhibits behaviour comparable to that of the alkali metals. All the alkali metals react with water, with the heavier alkali metals reacting more vigorously than the lighter ones. All of the discovered alkali metals occur in nature as their compounds: in order of abundance, sodium is the most abundant, followed by potassium, rubidium and francium, rare due to its high radioactivity. Experiments have been conducted to attempt the synthesis of ununennium, to be the next member of the group, but they have all met with failure. However, ununennium may not be an alkali metal due to relativistic effects, which are predicted to have a large influence on the chemical properties of superheavy elements. Most alkali metals have many different applications. One of the best-known applications of the pure elements is the use of rubidium and caesium in atomic clocks, of which caesium atomic clocks are the most accurate and precise representation of time.
A common application of the compounds of sodium is the sodium-vapour lamp, which emits light efficiently. Table salt, or sodium chloride, has been used since antiquity. Lithium finds use as an anode in lithium batteries. Sodium and potassium are essential elements, having major biological roles as electrolytes, although the other alkali metals are not essential, they have various effects on the body, both beneficial and harmful. Sodium compounds have been known since ancient times. While potash has been used since ancient times, it was not understood for most of its history to be a fundamentally different substance from sodium mineral salts. Georg Ernst Stahl obtained experimental evidence which led him to suggest the fundamental difference of sodium and potassium salts in 1702, Henri-Louis Duhamel du Monceau was able to prove this difference in 1736; the exact chemical composition of potassium and sodium compounds, the status as chemical element of potassium and sodium, was not known and thus Antoine Lavoisier did not include either alkali in his list of chemical elements in 1789.
Pure potassium was first isolated in 1807 in England by Sir Humphry Davy, who derived it from caustic potash by the use of electrolysis of the molten salt with the newly invented voltaic pile. Previous attempts at electrolysis of the aqueous salt were unsuccessful due to potassium's extreme reactivity. Potassium was the first metal, isolated by electrolysis; that same year, Davy reported extraction of sodium from the similar substance caustic soda by a similar technique, demonstrating the elements, thus the salts, to be different. Petalite was discovered in 1800 by the Brazilian chemist José Bonifácio de Andrada in a mine on the island of Utö, Sweden. However, it was not until 1817 that Johan August Arfwedson working in the laboratory of the chemist Jöns Jacob Berzelius, detected the presence of a new element while analysing petalite ore; this new element was noted by him to form compounds similar to those of sodium and potassium, though its carbonate and hydroxide were less soluble in water and more alkaline than the other alkali metals.
Berzelius gave the unknown material the name "lithion/lithina", from the Greek word λιθoς, to reflect its discovery in a solid mineral, as opposed to potassium, discovered in plant ashes, sodium, known for its high abundance in animal blood. He named the metal inside the material "lithium". Lithium and potassium were part of the discovery of periodicity, as they are among a series of triads of elements in the same group that were noted by Johann Wolfgang Döbereiner in 1850 as having similar properties. Rubidium and caesium were the first elements to be discovered using the spectroscope, invented in 1859 by Robert Bunsen and Gustav Kirchhoff; the next year, they discovered caesiu
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