Thecomposition of Mars covers the branch of thegeology of Mars that describes the make-up of the planetMars.
Mars isdifferentiated, which—for aterrestrial planet—implies that it has a centralcore made up of high density matter (mainly metalliciron andnickel) surrounded by a less dense, silicatemantle andcrust.[4] Like Earth, Mars appears to have a molten iron core, or at least a molten outer core.[5] However, there does not appear to be convection in the mantle. Presently, Mars shows little geological activity.
The elemental composition of Mars is different from Earth's in several significant ways. First, Martian meteorite analysis suggests that the planet's mantle is about twice as rich in iron as the Earth's mantle.[6][7] The planet's distinctive red color is due toiron oxides on its surface. Second, its core is richer in sulphur.[8] Third, the Martian mantle is richer in potassium and phosphorus than Earth's and fourth, the Martian crust contains a higher percentage ofvolatile elements such as sulphur and chlorine than the Earth's crust does. Many of these conclusions are supported byin situ analyses of rocks and soils on the Martian surface.[9]
Much of what we know about the elemental composition of Mars comes from orbitingspacecraft and landers. (SeeExploration of Mars for list.) Most of these spacecraft carryspectrometers and other instruments to measure the surface composition of Mars by eitherremote sensing from orbit orin situ analyses on the surface. We also have many actual samples of Mars in the form ofmeteorites that have made their way to Earth.Martian meteorites (often called SNC's, forShergottites,Nakhlites, andChassignites[10]—the groups of meteorites first shown to have a martian origin) provide data on the chemical composition of Mars's crust and interior that would not otherwise be available except through asample return mission.
Based on these data sources, scientists think that the most abundant chemical elements in the Martian crust aresilicon,oxygen,iron,magnesium,aluminium,calcium, andpotassium. These elements are major components of the minerals comprisingigneous rocks.[11] The elementstitanium,chromium,manganese,sulfur,phosphorus,sodium, andchlorine are less abundant[12][13] but are still important components of many accessory minerals[14] in rocks and of secondary minerals (weathering products) in the dust and soils (theregolith). On September 5, 2017, scientists reported that theCuriosity rover detectedboron, an essential ingredient forlife onEarth,on the planet Mars. Such a finding, along with previous discoveries that water may have been present on ancient Mars, further supports the possible early habitability ofGale Crater on Mars.[15][16]
Hydrogen is present as water (H2O) ice and inhydrated minerals.Carbon occurs ascarbon dioxide (CO2) in the atmosphere and sometimes asdry ice at the poles. An unknown amount of carbon is also stored incarbonates. Molecularnitrogen (N2) makes up 2.7 percent of the atmosphere. As far as we know,organic compounds are absent[17] except for a trace ofmethane detected in theatmosphere.[18][19] On 16 December 2014, NASA reported theCuriosity rover detected a "tenfold spike", likely localized, in the amount ofmethane in theMartian atmosphere. Sample measurements taken "a dozen times over 20 months" showed increases in late 2013 and early 2014, averaging "7 parts of methane per billion in the atmosphere." Before and after that, readings averaged around one-tenth that level.[20][21]
On 25 October 2023, scientists, helped by information from theInSight lander, reported that the planet Mars has aradioactivemagma ocean under its crust.[22]
Mars is fundamentally anigneous planet. Rocks on the surface and in the crust consist predominantly of minerals that crystallize frommagma. Most of our current knowledge about themineral composition of Mars comes from spectroscopic data from orbiting spacecraft,in situ analyses of rocks and soils from six landing sites, and study of the Martian meteorites.[23] Spectrometers currently in orbit includeTHEMIS (Mars Odyssey), OMEGA (Mars Express), andCRISM (Mars Reconnaissance Orbiter). The twoMars exploration rovers each carry an Alpha Particle X-ray Spectrometer (APXS), a thermal emission spectrometer (Mini-TES), andMössbauer spectrometer to identify minerals on the surface.
On October 17, 2012, theCuriosity rover on theplanet Mars at "Rocknest" performed the firstX-ray diffraction analysis ofMartian soil. The results from the rover'sCheMin analyzer revealed the presence of several minerals, includingfeldspar,pyroxenes andolivine, and suggested that the Martian soil in the sample was similar to the "weatheredbasaltic soils" ofHawaiian volcanoes.[24]
The dark areas of Mars are characterised by themafic rock-forming mineralsolivine,pyroxene, andplagioclasefeldspar. These minerals are the primary constituents ofbasalt, a dark volcanic rock that also makes up the Earth's oceanic crust and thelunar maria.
The mineral olivine occurs all over the planet, but some of the largest concentrations are inNili Fossae, an area containingNoachian-aged rocks. Another large olivine-rich outcrop is inGanges Chasma, an eastern side chasm ofValles Marineris (pictured).[25] Olivine weathers rapidly into clay minerals in the presence of liquid water. Therefore, areas with large outcroppings of olivine-bearing rock indicate that liquid water has not been abundant since the rocks formed.[10]
Pyroxene minerals are also widespread across the surface. Both low-calcium (ortho-) and high-calcium (clino-) pyroxenes are present, with the high-calcium varieties associated with youngervolcanic shields and the low-calcium forms (enstatite) more common in the old highland terrain. Because enstatite melts at a higher temperature than its high-calcium cousin, some researchers have argued that its presence in the highlands indicates that older magmas on Mars had higher temperatures than younger ones.[26]
Between 1997 and 2006, theThermal Emission Spectrometer (TES) on theMars Global Surveyor (MGS) spacecraft mapped the global mineral composition of the planet.[27] TES identified two global-scale volcanic units on Mars. Surface Type 1 (ST1) characterises the Noachian-aged highlands and consists of unaltered plagioclase- andclinopyroxene-rich basalts. Surface Type 2 (ST2) is common in the younger plains north of the dichotomy boundary and is more silica rich than ST1.
The lavas of ST2 have been interpreted asandesites orbasaltic andesites, indicating the lavas in the northern plains originated from more chemically evolved, volatile-rich magmas.[28] (SeeIgneous differentiation andFractional crystallization.) However, other researchers have suggested that ST2 represents weathered basalts with thin coatings of silica glass or other secondary minerals that formed through interaction with water- or ice-bearing materials.[29]
True intermediate andfelsic rocks are present on Mars, but exposures are uncommon. Both TES and theThermal Emission Imaging System (THEMIS) on theMars Odyssey spacecraft have identified high-silica rocks in Syrtis Major and near the southwestern rim of the craterAntoniadi. The rocks have spectra resembling quartz-richdacites andgranitoids, suggesting that at least some parts of the Martian crust may have a diversity of igneous rocks similar to Earth's.[30] Some geophysical evidence suggests that the bulk of the Martian crust may actually consist ofbasaltic andesite or andesite. The andesitic crust is hidden by overlying basaltic lavas that dominate the surface composition but are volumetrically minor.[4]
Rocks studied bySpirit rover in Gusev crater can be classified in different ways. The amounts and types of minerals make the rocks primitive basalts—also called picritic basalts. The rocks are similar to ancient terrestrial rocks called basaltickomatiites. Rocks of the plains also resemble the basalticshergottites, meteorites which came from Mars. One classification system compares the amount of alkali elements to the amount of silica on a graph; in this system, Gusev plains rocks lie near the junction of basalt,picrobasalt, and tephrite. The Irvine-Barager classification calls them basalts.[31]
On March 18, 2013, NASA reported evidence from instruments on theCuriosity rover ofmineral hydration, likely hydratedcalcium sulfate, in severalrock samples including the broken fragments of"Tintina" rock and"Sutton Inlier" rock as well as inveins andnodules in other rocks like"Knorr" rock and"Wernicke" rock.[32][33][34] Analysis using the rover'sDAN instrument provided evidence of subsurface water, amounting to as much as 4% water content, down to a depth of 60 cm (2.0 ft), in the rover's traverse from theBradbury Landing site to the Yellowknife Bay area in theGlenelg terrain.[32]
In the journalScience from September 2013, researchers described a different type of rock called "Jake M" or "Jake Matijevic (rock),” It was the first rock analyzed by the Alpha Particle X-ray Spectrometer instrument on theCuriosity rover, and it was different from other known Martian igneous rocks as it is alkaline (>15% normative nepheline) and relatively fractionated.Jake M is similar to terrestrial mugearites, a rock type typically found at ocean islands and continental rifts.Jake M's discovery may mean that alkaline magmas may be more common on Mars than on Earth and thatCuriosity could encounter even more fractionated alkaline rocks (for example, phonolites andtrachytes).[35]
On December 9, 2013, NASA researchers described, in a series of six articles in the journalScience, many new discoveries from theCuriosity rover. Possible organics were found that could not be explained by contamination.[36][37] Although the organic carbon was probably from Mars, it can all be explained by dust and meteorites that have landed on the planet.[38][39][40] Because much of the carbon was released at a relatively low temperature inCuriosity'sSample Analysis at Mars (SAM) instrument package, it probably did not come from carbonates in the sample. The carbon could be from organisms, but this has not been proven. This organic-bearing material was obtained by drilling 5 centimeters deep in a site calledYellowknife Bay into a rock called “Sheepbed mudstone”. The samples were namedJohn Klein andCumberland. Microbes could be living on Mars by obtaining energy from chemical imbalances between minerals in a process calledchemolithotrophy which means “eating rock.”[41] However, in this process only a very tiny amount of carbon is involved — much less than was found at Yellowknife Bay.[42][43]
Using SAM'smass spectrometer, scientists measuredisotopes ofhelium,neon, andargon thatcosmic rays produce as they go through rock. The fewer of these isotopes they find, the more recently the rock has been exposed near the surface. The 4-billion-year-old lakebed rock drilled byCuriosity was uncovered between 30 million and 110 million years ago by winds which sandblasted away 2 meters of overlying rock. Next, they hope to find a site tens of millions of years younger by drilling close to an overhanging outcrop.[44]
The absorbed dose and dose equivalent from galactic cosmic rays andsolar energetic particles on the Martian surface for ~300 days of observations during the current solar maximum was measured. These measurements are necessary for human missions to the surface of Mars, to provide microbial survival times of any possible extant or past life, and to determine how long potential organicbiosignatures can be preserved. This study estimates that a few meters drill is necessary to access possiblebiomolecules.[45] The actual absorbed dose measured by theRadiation Assessment Detector (RAD) is 76 mGy/yr at the surface. Based on these measurements, for a round-trip Mars surface mission with 180 days (each way) cruise, and 500 days on the Martian surface for this current solar cycle, an astronaut would be exposed to a total mission dose equivalent of ~1.01sievert. Exposure to 1 sievert is associated with a 5 percent increase in risk for developing fatal cancer. NASA's current lifetime limit for increased risk for its astronauts operating in low-Earth orbit is 3 percent.[46] Maximum shielding from galactic cosmic rays can be obtained with about 3 meters ofMartian soil.[45]
The samples examined were probably once mud that for millions to tens of millions of years could have hosted living organisms. This wet environment had neutralpH, lowsalinity, and variableredox states of bothiron andsulfur species.[38][47][48][49] These types of iron and sulfur could have been used by living organisms.[50]C,H,O,S,N, andP were measured directly as key biogenic elements, and by inference, P is assumed to have been there as well.[41][43] The two samples,John Klein andCumberland, contain basaltic minerals, Ca-sulfates, Fe oxide/hydroxides, Fe-sulfides, amorphous material, and trioctahedralsmectites (a type of clay). Basaltic minerals in themudstone are similar to those in nearbyaeolian deposits. However, the mudstone has far less Fe-forsterite plusmagnetite, so Fe-forsterite (type ofolivine) was probably altered to form smectite (a type of clay) andmagnetite.[51] A LateNoachian/EarlyHesperian or younger age indicates that clay mineral formation on Mars extended beyond Noachian time; therefore, in this location neutral pH lasted longer than previously thought.[47]
Much of the Martian surface is deeply covered by dust as fine as talcum powder. The global predominance of dust obscures the underlying bedrock, making spectroscopic identification of primary minerals impossible from orbit over many areas of the planet. The red/orange appearance of the dust is caused byiron(III) oxide (nanophase Fe2O3) and theiron(III) oxide-hydroxide mineralgoethite.[54]
TheMars Exploration Rovers identifiedmagnetite as the mineral responsible for making the dust magnetic. It probably also contains sometitanium.[55]
The global dust cover and the presence of other wind-blown sediments has made soil compositions remarkably uniform across the Martian surface. Analysis of soil samples from the Viking landers in 1976 shows that the soils consist of finely broken up basaltic rock fragments and are highly enriched in sulphur and chlorine, probably derived from volcanic gas emissions.[56]
Minerals produced throughhydrothermal alteration andweathering of primary basaltic minerals are also present on Mars. Secondary minerals includehematite,phyllosilicates (clay minerals),goethite,jarosite, ironsulfate minerals,opaline silica, andgypsum. Many of these secondary minerals require liquid water to form (aqueous minerals).
Opaline silica and iron sulphate minerals form in acidic (low pH) solutions. Sulphates have been found in a variety of locations, including nearJuventae Chasma,Ius Chasma,Melas Chasma,Candor Chasma, andGanges Chasma. These sites all containfluvial landforms indicating that abundant water was once present.[57] Spirit rover discovered sulfates and goethite in the Columbia Hills.[58][59]
Some of the mineral classes detected may have formed in environments suitable (i.e., enough water and the proper pH) for life. The mineral smectite (a phyllosilicate) forms in near-neutral waters. Phyllosilicates and carbonates are good for preserving organic matter, so they may contain evidence of past life.[60][61] Sulfate deposits preserve chemical and morphological fossils, and fossils of microorganisms form in iron oxides like hematite.[62] The presence of opaline silica points toward a hydrothermal environment that could support life. Silica is also excellent for preserving evidence of microbes.[63]
Layered sedimentary deposits are widespread on Mars. These deposits probably consist of bothsedimentary rock and poorlyindurated or unconsolidated sediments. Thick sedimentary deposits occur in the interior of several canyons in Valles Marineris, within large craters in Arabia andMeridiani Planum (seeHenry Crater for example), and probably comprise much of the deposits in the northern lowlands (e.g.,Vastitas Borealis Formation). The Mars Exploration RoverOpportunity landed in an area containing cross-bedded (mainlyeolian)sandstones (Burns formation[64]). Fluvial-deltaic deposits are present inEberswalde Crater and elsewhere, and photogeologic evidence suggests that many craters and low lying intercrater areas in the southern highlands contain Noachian-aged lake sediments.
While the possibility ofcarbonates on Mars has been of great interest to astrobiologists and geochemists alike, there was little evidence for significant quantities of carbonate deposits on the surface. In the summer of 2008, the TEGA and WCL experiments on the 2007Phoenix Mars lander found between 3–5wt% (percent by weight) calcite (CaCO3) and an alkaline soil.[65] In 2010, analyses by the Mars Exploration RoverSpirit identified outcrops rich in magnesium-iron carbonate (16–34 wt%) in the Columbia Hills of Gusev crater. The magnesium-iron carbonate most likely precipitated from carbonate-bearing solutions under hydrothermal conditions at near-neutral pH in association with volcanic activity during the Noachian Period.[66]
Carbonates (calcium or iron carbonates) were discovered in a crater on the rim of Huygens Crater, located in theIapygia quadrangle. The impact on the rim exposed material that had been dug up from the impact that created Huygens. These minerals represent evidence that Mars once had a thicker carbon dioxide atmosphere with abundant moisture, since these kind of carbonates only form when there is a lot of water. They were found with theCompact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument on theMars Reconnaissance Orbiter. Earlier, the instrument had detected clay minerals. The carbonates were found near the clay minerals. Both of these minerals form in wet environments. It is supposed that billions of years ago Mars was much warmer and wetter. At that time, carbonates would have formed from water and the carbon dioxide-rich atmosphere. Later the deposits of carbonate would have been buried. The double impact has now exposed the minerals. Earth has vast carbonate deposits in the form oflimestone.[67]
The rocks on the plains of Gusev are a type ofbasalt. They contain themineralsolivine,pyroxene,plagioclase, and magnetite, and they look like volcanic basalt as they are fine-grained with irregular holes (geologists would say they have vesicles andvugs).[68][69]Much of the soil on the plains came from the breakdown of the local rocks. Fairly high levels ofnickel were found in some soils; probably frommeteorites.[70]Analysis shows that the rocks have been slightly altered by tiny amounts of water. Outside coatings and cracks inside the rocks suggest water deposited minerals, maybebromine compounds. All the rocks contain a fine coating of dust and one or more harder rinds of material. One type can be brushed off, while another needed to be ground off by theRock Abrasion Tool (RAT).[71]
There are a variety of rocks in theColumbia Hills (Mars), some of which have been altered by water, but not by very much water.
The dust in Gusev Crater is the same as dust all around the planet. All the dust was found to be magnetic. Moreover,Spirit found themagnetism was caused by the mineralmagnetite, especially magnetite that contained the elementtitanium. One magnet was able to completely divert all dust hence all Martian dust is thought to be magnetic.[55] The spectra of the dust was similar to spectra of bright, low thermal inertia regions likeTharsis and Arabia that have been detected by orbiting satellites. A thin layer of dust, maybe less than one millimeter thick covers all surfaces. Something in it contains a small amount of chemically bound water.[72][73]
  Above: An approximatetrue-color view of Adirondack, taken bySpirit's pancam. Right: Digital camera image (fromSpirit'sPancam) of Adirondack after aRAT grind (Spirit's rock grinding tool) | |
| Feature type | Rock |
|---|---|
| Coordinates | 14°36′S175°30′E / 14.6°S 175.5°E /-14.6; 175.5 |
Observations of rocks on the plains show they contain the minerals pyroxene, olivine, plagioclase, and magnetite. These rocks can be classified in different ways. The amounts and types of minerals make the rocks primitive basalts—also called picritic basalts. The rocks are similar to ancient terrestrial rocks called basaltickomatiites. Rocks of the plains also resemble the basalticshergottites, meteorites which came from Mars. One classification system compares the amount of alkali elements to the amount of silica on a graph; in this system, Gusev plains rocks lie near the junction of basalt,picrobasalt, and tephrite. The Irvine-Barager classification calls them basalts.[31]Plain's rocks have been very slightly altered, probably by thin films of water because they are softer and contain veins of light colored material that may be bromine compounds, as well as coatings or rinds. It is thought that small amounts of water may have gotten into cracks inducing mineralization processes).[31][69]Coatings on the rocks may have occurred when rocks were buried and interacted with thin films of water and dust.One sign that they were altered was that it was easier to grind these rocks compared to the same types of rocks found on Earth.
The first rock thatSpirit studied was Adirondack. It turned out to be typical of the other rocks on the plains.
Scientists found a variety of rock types in the Columbia Hills, and they placed them into six different categories. The six are: Adirondack, Clovis, Wishstone, Peace, Watchtower, Backstay, and Independence. They are named after a prominent rock in each group. Their chemical compositions, as measured by APXS, are significantly different from each other.[74] Most importantly, all of the rocks in Columbia Hills show various degrees of alteration due to aqueous fluids.[75]They are enriched in the elements phosphorus, sulfur, chlorine, and bromine—all of which can be carried around in water solutions. The Columbia Hills' rocks contain basaltic glass, along with varying amounts of olivine andsulfates.[76][58]The olivine abundance varies inversely with the amount of sulfates. This is exactly what is expected because water destroys olivine but helps to produce sulfates.
The Clovis group is especially interesting because theMossbauer spectrometer (MB) detectedgoethite in it.[59] Goethite forms only in the presence of water, so its discovery is the first direct evidence of past water in the Columbia Hills's rocks. In addition, the MB spectra of rocks and outcrops displayed a strong decline in olivine presence, although the rocks probably once contained much olivine.[77] Olivine is a marker for the lack of water because it easily decomposes in the presence of water. Sulfate was found, and it needs water to form.
Wishstone contained a great deal of plagioclase, some olivine, andanhydrate (a sulfate). Peace rocks showedsulfur and strong evidence for bound water, so hydrated sulfates are suspected. Watchtower class rocks lack olivine consequently they may have been altered by water. The Independence class showed some signs of clay (perhaps montmorillonite a member of the smectite group). Clays require fairly long term exposure to water to form.
One type of soil, called Paso Robles, from the Columbia Hills, may be an evaporate deposit because it contains large amounts of sulfur,phosphorus,calcium, andiron.[78] Also, MB found that much of the iron in Paso Robles soil was of the oxidized, Fe+++ form, which would happen if water had been present.[72]
Towards the middle of the six-year mission (a mission that was supposed to last only 90 days), large amounts of puresilica were found in the soil. The silica could have come from the interaction of soil with acid vapors produced by volcanic activity in the presence of water or from water in a hot spring environment.[79]
AfterSpirit stopped working scientists studied old data from the Miniature Thermal Emission Spectrometer, orMini-TES and confirmed the presence of large amounts ofcarbonate-rich rocks, which means that regions of the planet may have once harbored water. The carbonates were discovered in an outcrop of rocks called "Comanche."[80][81]
In summary,Spirit found evidence of slight weathering on the plains of Gusev, but no evidence that a lake was there. However, in the Columbia Hills there was clear evidence for a moderate amount of aqueous weathering. The evidence included sulfates and the minerals goethite and carbonates which only form in the presence of water. It is believed that Gusev crater may have held a lake long ago, but it has since been covered by igneous materials. All the dust contains a magnetic component which was identified as magnetite with some titanium. Furthermore, the thin coating of dust that covers everything on Mars is the same in all parts of the planet.
Opportunity rover found that the soil atMeridiani Planum was very similar to the soil at Gusev crater and Ares Vallis; however in many places at Meridiani the soil was covered with round, hard, gray spherules that were named "blueberries."[82] These blueberries were found to be composed almost entirely of the mineralhematite. It was decided that the spectra signal spotted from orbit by Mars Odyssey was produced by these spherules. After further study it was decided that the blueberries were concretions formed in the ground by water.[72] Over time, these concretions weathered from what was overlying rock, and then became concentrated on the surface as a lag deposit. The concentration of spherules in bedrock could have produced the observed blueberry covering from the weathering of as little as one meter of rock.[83][84] Most of the soil consisted of olivine basalt sands that did not come from the local rocks. The sand may have been transported from somewhere else.[85]
AMössbauer spectrograph was made of the dust that gathered onOpportunity's capture magnet. The results suggested that the magnetic component of the dust wastitanomagnetite, rather than just plainmagnetite, as was once thought. A small amount ofolivine was also detected which was interpreted as indicating a long arid period on the planet. On the other hand, a small amount of hematite that was present meant that there may have been liquid water for a short time in the early history of the planet.[86]
Because theRock Abrasion Tool (RAT) found it easy to grind into the bedrocks, it is thought that the rocks are much softer than the rocks at Gusev crater.[citation needed]
Few rocks were visible on the surface whereOpportunity landed, but bedrock that was exposed in craters was examined by the suite of instruments on the Rover.[87] Bedrock rocks were found to be sedimentary rocks with a high concentration ofsulfur in the form of calcium andmagnesium sulfates. Some of the sulfates that may be present in bedrocks arekieserite,sulfate anhydrate,bassanite,hexahydrite,epsomite, andgypsum.Salts, such ashalite,bischofite,antarcticite,bloedite,vanthoffite, orglauberite may also be present.[88][89]
The rocks contained the sulfates had a light tone compared to isolated rocks and rocks examined by landers/rovers at other locations on Mars. The spectra of these light toned rocks, containing hydrated sulfates, were similar to spectra taken by theThermal Emission Spectrometer on board theMars Global Surveyor. The same spectrum is found over a large area, so it is believed that water once appeared over a wide region, not just in the area explored byOpportunity.[90]
TheAlpha Particle X-ray Spectrometer (APXS) found rather high levels ofphosphorus in the rocks. Similar high levels were found by other rovers atAres Vallis andGusev Crater, so it has been hypothesized that the mantle of Mars may be phosphorus-rich.[91] The minerals in the rocks could have originated byacid weathering ofbasalt. Because the solubility of phosphorus is related to the solubility ofuranium,thorium, andrare earth elements, they are all also expected to be enriched in rocks.[92]
WhenOpportunity rover traveled to the rim ofEndeavour crater, it soon found a white vein that was later identified as being pure gypsum.[93][94] It was formed when water carrying gypsum in solution deposited the mineral in a crack in the rock. A picture of this vein, called "Homestake" formation, is shown below.
Examination in 2004 of Meridiani rocks, showed the first strongin situ evidence for past water by detecting the mineraljarosite, which only forms in water. This discovery proved that water once existed inMeridiani Planum.[95] In addition, some rocks showed small laminations (layers) with shapes that are only made by gently flowing water.[96] The first such laminations were found in a rock called "The Dells." Geologists would say that the cross-stratification showed festoon geometry from transport in subaqueous ripples.[89] A picture of cross-stratification, also called cross-bedding, is shown on the left.
Box-shaped holes in some rocks were caused by sulfates forming large crystals, and then when the crystals later dissolved, holes, called vugs, were left behind.[96] The concentration of the elementbromine in rocks was highly variable probably because it is very soluble. Water may have concentrated it in places before it evaporated. Another mechanism for concentrating highly soluble bromine compounds is frost deposition at night that would form very thin films of water that would concentrate bromine in certain spots.[82]
One rock, "Bounce Rock," found sitting on the sandy plains was found to be ejecta from an impact crater. Its chemistry was different from the bedrocks. Containing mostly pyroxene and plagioclase and no olivine, it closely resembled a part, Lithology B, of the shergottite meteorite EETA 79001, a meteorite known to have come from Mars. Bounce rock received its name by being near an airbag bounce mark.[83]
Opportunity rover found meteorites just sitting on the plains. The first one analyzed withOpportunity's instruments was called "Heatshield Rock," as it was found near whereOpportunity's heatshield landed. Examination with the Miniature Thermal Emission Spectrometer (Mini-TES),Mossbauer spectrometer, and APXS lead researchers to classify it as anIAB meteorite. The APXS determined it was composed of 93%iron and 7%nickel. The cobble named "Fig Tree Barberton" is thought to be a stony or stony-iron meteorite (mesosiderite silicate),[97] while "Allan Hills," and "Zhong Shan" may be iron meteorites.
Observations at the site have led scientists to believe that the area was flooded with water a number of times and was subjected to evaporation and desiccation.[83] In the process sulfates were deposited. After sulfates cemented the sediments, hematite concretions grew by precipitation from groundwater. Some sulfates formed into large crystals which later dissolved to leave vugs. Several lines of evidence point toward an arid climate in the past billion years or so, but a climate supporting water, at least for a time, in the distant past.[98]
TheCuriosity rover encounteredrocks of special interest on the surface ofAeolis Palus nearAeolis Mons ("Mount Sharp") inGale Crater. In the autumn of 2012, rocks studied, on the way fromBradbury Landing toGlenelg Intrigue, included"Coronation" rock (August 19, 2012),"Jake Matijevic" rock (September 19, 2012),"Bathurst Inlet" rock (September 30, 2012).
On September 27, 2012,NASA scientists announced that theCuriosity rover found evidence for an ancientstreambed suggesting a "vigorous flow" of water on Mars.[1][2][3]
On December 3, 2012, NASA reported thatCuriosity performed its first extensivesoil analysis, revealing the presence ofwater molecules,sulfur andchlorine in theMartian soil.[52][53] On December 9, 2013, NASA reported that, based on evidence fromCuriosity rover studyingAeolis Palus,Gale Crater contained an ancientfreshwater lake which could have been a hospitable environment formicrobial life.[99][100]
In March 2013, NASA reportedCuriosity found evidence thatgeochemical conditions inGale Crater were once suitable formicrobial life after analyzing the first drilled sample ofMartian rock,"John Klein" rock atYellowknife Bay inGale Crater. The rover detectedwater,carbon dioxide,oxygen,sulfur dioxide andhydrogen sulfide.[101][102][103]Chloromethane anddichloromethane were also detected. Related tests found results consistent with the presence ofsmectite clay minerals.[101][102][103][104][105]
On 16 December 2014, NASA reported theCuriosity rover detected a "tenfold spike", likely localized, in the amount ofmethane in theMartian atmosphere. Sample measurements taken "a dozen times over 20 months" showed increases in late 2013 and early 2014, averaging "7 parts of methane per billion in the atmosphere." Before and after that, readings averaged around one-tenth that level.[20][21]
In addition, high levels oforganic chemicals, particularlychlorobenzene, were detected in powder drilled from one of the rocks, named "Cumberland", analyzed by theCuriosity rover.[20][21]
In 2024,Curiosity discovered a rock containing abundant elemental sulfur.[106]
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