The Aeolis quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The Aeolis quadrangle is also referred to as MC-23 (Mars Chart-23).^[1] The Aeolis quadrangle covers 180° to 225° W and 0° to 30° south on Mars, and contains parts of the regions Elysium Planitia and Terra Cimmeria. A small part of the Medusae Fossae Formation lies in this quadrangle.

The name refers to the name of a floating western island of Aeolus, the ruler of the winds. In Homer's account, Odysseus received the west wind Zephyr here and kept it in bags, but the wind got out.^{[2] [3]}

It is famous as the site of two spacecraft landings: the Spirit rover landing site (14.5718°S 175.4785°E / -14.5718; 175.4785) in Gusev crater (January 4, 2004), and the Curiosity rover in Gale Crater (4.591817°S 137.440247°E / -4.591817; 137.440247) (August 6, 2012).^[4]

A large, ancient river valley, called Ma'adim Vallis, enters at the south rim of Gusev Crater, so Gusev Crater was believed to be an ancient lake bed. However, it seems that a volcanic flow covered up the lakebed sediments.^[5] Apollinaris Patera, a large volcano, lies directly north of Gusev Crater.^[6]

Gale Crater, in the northwestern part of the Aeolis quadrangle, is of special interest to geologists because it contains a 2–4 km (1.2–2.5 mi) high mound of layered sedimentary rocks, named "Mount Sharp" by NASA in honor of Robert P. Sharp (1911–2004), a planetary scientist of early Mars missions.^[7][8][9] More recently, on 16 May 2012, "Mount Sharp" was officially named Aeolis Mons by the USGS and IAU.^[10]

Some regions in the Aeolis quadrangle show inverted relief.^[11] In these locations, a stream bed may be a raised feature, instead of a valley. The inverted former stream channels may be caused by the deposition of large rocks or due to cementation. In either case erosion would erode the surrounding land but leave the old channel as a raised ridge because the ridge will be more resistant to erosion

Yardangs are another feature found in this quadrangle. They are generally visible as a series of parallel linear ridges, caused by the direction of the prevailing wind.

The rocks on the plains of Gusev are a type of basalt. They contain the minerals olivine, pyroxene, plagioclase, and magnetite, and they look like volcanic basalt as they are fine-grained with irregular holes (geologists would say they have vesicles and vugs).^[12][13] Much of the soil on the plains came from the breakdown of the local rocks. Fairly high levels of nickel were found in some soils; probably from meteorites.^[14] 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, maybe bromine compounds. All the rocks contain a fine coating of dust and one or more harder kinds of material. One type can be brushed off, while another needed to be ground off by the Rock Abrasion Tool (RAT).^[15]

There are a variety of rocks in the Columbia Hills, 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 the magnetism was caused by the mineral magnetite, especially magnetite that contained the element titanium. One magnet was able to completely divert all dust hence all Martian dust is thought to be magnetic.^[16] The spectra of the dust was similar to spectra of bright, low thermal inertia regions like Tharsis 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.^[17][18]

Recent studies lead scientists to believe that the water that formed Ma'adim Vallis originated in a complex of lakes.^[32][33][34] The largest lake is located at the source of the Ma'adim Vallis outflow channel and extends into Eridania quadrangle and the Phaethontis quadrangle.^[35] When the largest lake spilled over the low point in its boundary, a torrential flood would have moved north, carving the sinuous Ma'adim Vallis. At the north end of Ma'adim Vallis, the flood waters would have run into Gusev Crater.^[36]

There is enormous evidence that water once flowed in river valleys on Mars. Images of curved channels have been seen in images from Mars spacecraft dating back to the early 1970s with the Mariner 9 orbiter.^{[37][38][39][40]}

Vallis (plural valles) is the Latin word for "valley". It is used in planetary geology for the naming of landform features on other planets, including what could be old river valleys that were discovered on Mars, when probes were first sent to Mars. The Viking Orbiters caused a revolution in our ideas about water on Mars; huge river valleys were found in many areas. Space craft cameras showed that floods of water broke through dams, carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers.^[41][42][43] Some valles on Mars (Mangala Vallis, Athabasca Vallis, Granicus Vallis, and Tinjar Valles) clearly begin at graben. On the other hand, some of the large outflow channels begin in rubble-filled low areas called chaos or chaotic terrain. It has been suggested that massive amounts of water were trapped under pressure beneath a thick cryosphere (layer of frozen ground), then the water was suddenly released, perhaps when the cryosphere was broken by a fault.^[44][45]

Gale Crater, in the northwestern part of the Aeolis quadrangle, is of special interest to geologists because it contains a 2–4 km (1.2–2.5 mi) high mound of layered sedimentary rocks. On 28 March 2012 this mound was named "Mount Sharp" by NASA in honor of Robert P. Sharp (1911–2004), a planetary scientist of early Mars missions.^[7][8][9] More recently, on 16 May 2012, Mount Sharp was officially named Aeolis Mons by the USGS and IAU.^[10] The mound extends higher than the rim of the crater, so perhaps the layering covered an area much larger than the crater.^[46] These layers are a complex record of the past. The rock layers probably took millions of years to be laid down within the crater, then more time to be eroded to make them visible.^[47] The 5 km high mound is probably the thickest single succession of sedimentary rocks on Mars.^[48] The lower formation may date from near the Noachian age, while the upper layer, separated by an erosional unconformity, may be as young as the Amazonian period.^[49] The lower formation may have formed the same time as parts of Sinus Meridiani and Mawrth Vallis. The mound that lies in the center of Gale Crater was created by winds. Because the winds eroded the mound on one side more than another, the mound is skewed to one side, rather than being symmetrical.^[50][51] The upper layer may be similar to layers in Arabia Terra. Sulfates and Iron oxides have been detected in the lower formation and anhydrous phases in the upper layer.^[52] There is evidence that the first phase of erosion was followed by more cratering and more rock formation.^[53] Also of interest in Gale Crater is Peace Vallis, officially named by the IAU on September 26, 2012,^[54] which 'flows' down out of the Gale Crater hills to the Aeolis Palus below, and which seems to have been carved by flowing water.^[55][56][57] On December 9, 2013, NASA reported that, based on evidence from Curiosity studying Aeolis Palus, Gale Crater contained an ancient freshwater lake which could have been a hospitable environment for microbial life.^[58][59] Gale Crater contains a number of fans and deltas that provide information about lake levels in the past. These formations are: Pancake Delta, Western Delta, Farah Vallis delta and the Peace Vallis Fan.^[60]

• Gale crater - surface materials (false colors; THEMIS; 2001 Mars Odyssey)
• Gale Crater with Aeolis Mons rising from the center. The noted Curiosity rover landing area is near Peace Vallis in Aeolis Palus.
• Ancient Lake fills Gale Crater on Mars (simulated view).
• Ancient Lake on Aeolis Palus in Gale Crater - possible size (December 9, 2013)^[58][59]
• Peace Vallis and alluvial fan near the Curiosity rover landing ellipse and site (noted by +)

• Curiosity rover landing site (green dot) - Blue dot marks Glenelg Intrigue - Blue spot marks the Base of Mount Sharp - a planned area of study.
• Curiosity rover landing site - "Quad Map" includes "Yellowknife" Quad 51 of Aeolis Palus in Gale Crater.
• Gale Crater Grand Canyon, as seen by HiRISE. Scale bar is 500 meters long.
• Gale Crater sediment layers may have formed by lake or windblown particle deposition.
• Gale Crater rim about 18 km (11 mi) North of the Curiosity rover on August 9, 2012
• Aeolis Mons and Aeolis Palus in Gale Crater as viewed by the Curiosity rover on August 6, 2012
• Aeolis Mons may have formed from the erosion of sediment layers that once filled Gale Crater.
• Aeolis Mons as viewed from the Curiosity rover (August 9, 2012) (white balanced image)
• Layers at the base of Aeolis Mons - dark rock in inset is same size as the Curiosity rover (white balanced image).
• Wheels on the Curiosity rover. "Mount Sharp" is in the background (MAHLI, September 9, 2012).
• First-Year and First-Mile Traverse Map of the Curiosity rover on Mars (August 1, 2013) (3-D)

• Slip Face on Downwind Side of 'Namib' Sand Dune on Mars, as seen by Curiosity. Dune stands about 13 feet (4.0 meters) high. Picture taken with Navcam.
• This evenly layered rock photographed by the Mast Camera (Mastcam) on NASA's Curiosity Mars rover shows a pattern typical of a lake-floor sedimentary deposit not far from where flowing water entered a lake.
• View from the "Kimberley" formation on Mars taken by NASA's Curiosity rover
• View from Mastcam on Curiosity showing sloping buttes and layered outcrops on lower Mount Sharp

Impact craters generally have a rim with ejecta around them, in contrast volcanic craters usually do not have a rim or ejecta deposits. As craters get larger (greater than 10 km in diameter) they usually have a central peak.^[61] The peak is caused by a rebound of the crater floor following the impact.^[41] Sometimes craters will display layers. Since the collision that produces a crater is like a powerful explosion, rocks from deep underground are tossed onto the surface. Hence, craters can show us what lies deep under the surface.

• Boeddicker Crater Floor, as seen by HiRISE
• Central uplift of an Unnamed crater on the floor of Molesworth Crater, as seen by HiRISE. Dark sand dunes are on left side of image. The scale bar is 500 meters long.
• Reuyl Crater Central Peak, as seen by HiRISE
• Galdakao Crater, as seen by HiRISE. Click on image to see dark slope streaks.
• Layers in crater wall, as seen by HiRISE under HiWish program. Area in box is enlarged in the next image.
• Enlargement from previous image, showing many thin layers. Note that the layers do not seem to be formed from rocks. They may be all that is left of a deposit that once filled the crater. Image was taken with HiRISE, under HiWish program.
• Gullies on wall of impact crater, as seen by HiRISE under HiWish program. Curved ridges on the floor are remains of old glaciers.
• Graff Crater (Martian Crater), as seen by CTX camera (on Mars Reconnaissance Orbiter)

The aim of the Mars Science Laboratory mission, and its surface robotic payload Curiosity rover, is to search for signs of ancient life. It is hoped that a later mission could then return samples that the laboratory identified as probably containing remains of life. To safely bring the craft down, a 12 mile wide, smooth, flat circle was needed. Geologists hoped to examine places where water once ponded^[62] and to examine sedimentary layers.

On August 6, 2012, the Mars Science Laboratory landed on Aeolis Palus near Aeolis Mons in Gale Crater.^{[7][8][9][10][63][64]} The landing was 2.279 km (1.416 mi) from the target (4.591817°S 137.440247°E / -4.591817; 137.440247), closer than any previous rover landing and well within the target area.

On September 27, 2012, NASA scientists announced that Curiosity found evidence for an ancient streambed suggesting a "vigorous flow" of water on Mars.^[55][56][57]

^[65][66]

On October 17, 2012, at Rocknest, the first X-ray diffraction analysis of Martian soil was performed. The results revealed the presence of several minerals, including feldspar, pyroxenes and olivine, and suggested that the Martian soil in the sample was similar to the weathered basaltic soils of Hawaiian volcanoes. The sample used is composed of dust distributed from global dust storms and local fine sand. So far, the materials Curiosity has analyzed are consistent with the initial ideas of deposits in Gale Crater recording a transition through time from a wet to dry environment.^[67]

On December 3, 2012, NASA reported that Curiosity performed its first extensive soil analysis, revealing the presence of water molecules, sulfur and chlorine in the Martian soil.^[68][69] The presence of perchlorates in the sample seems highly likely. The presence of sulfate and sulfide is also likely because sulfur dioxide and hydrogen sulfide were detected. Small amounts of chloromethane, dichloromethane and trichloromethane were detected. The source of the carbon in these molecules is unclear. Possible sources include contamination of the instrument, organics in the sample and inorganic carbonates.^[68][69]

On March 18, 2013, NASA reported evidence of mineral hydration, likely hydrated calcium sulfate, in several rock samples including the broken fragments of "Tintina" rock and "Sutton Inlier" rock as well as in veins and nodules in other rocks like "Knorr" rock and "Wernicke" rock.^[70][71][72] Analysis using the rover's DAN 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 the Bradbury Landing site to the Yellowknife Bay area in the Glenelg terrain.^[70]

In March 2013, NASA reported Curiosity found evidence that geochemical conditions in Gale Crater were once suitable for microbial life after analyzing the first drilled sample of Martian rock, "John Klein" rock at Yellowknife Bay in Gale Crater. The rover detected water, carbon dioxide, oxygen, sulfur dioxide and hydrogen sulfide.^[73][74][75] Chloromethane and dichloromethane were also detected. Related tests found results consistent with the presence of smectite clay minerals.^{[73][74][75][76][77]}

In the journal Science from September 2013, researchers described a different type of rock called Jake M (or Jake Matijevic) It was the first rock analyzed by the Alpha Particle X-ray Spectrometer (APXS) instrument on the Curiosity 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 that Curiosity could encounter even more fractionated alkaline rocks (for example, phonolites and trachytes).^[78]

On December 9, 2013, NASA researchers described, in a series of six articles in the journal Science, many new discoveries from the Curiosity rover. Possible organics were found that could not be explained by contamination.^[79][80] Although the organic carbon was probably from Mars, it can all be explained by dust and meteorites that have landed on the planet.^[81][82][83] Because much of the carbon was released at a relatively low temperature in Curiosity's Sample 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 called Yellowknife Bay into a rock called “Sheepbed mudstone”. The samples were named John Klein and Cumberland. Microbes could be living on Mars by obtaining energy from chemical imbalances between minerals in a process called chemolithotrophy which means “eating rock.”^[84] However, in this process only a very tiny amount of carbon is involved—much less than was found at Yellowknife Bay.^[85][86]

Using SAM's mass spectrometer, scientists measured isotopes of helium, neon, and argon that cosmic 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 four-billion-year-old lakebed rock drilled by Curiosity was uncovered between 30 million and 110 million years ago by winds which sandblasted away two meters of overlying rock. Next, they hope to find a site tens of millions of years younger by drilling close to an overhanging outcrop.^[87]

The absorbed dose and dose equivalent from galactic cosmic rays and solar 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 organic biosignatures can be preserved. This study estimates that a one-meter depth drill is necessary to access possible viable radioresistant microbe cells. The actual absorbed dose measured by the Radiation 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 ~1.01 sievert. Exposure to one sievert is associated with a five 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 three percent.^[88] Maximum shielding from galactic cosmic rays can be obtained with about 3 meters of Martian soil.^[89]

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 neutral pH, low salinity, and variable redox states of both iron and sulfur species.^{[81][90][91][92]} These types of iron and sulfur could have been used by living organisms.^[93] Carbon, hydrogen, oxygen, sulfur, nitrogen, and phosphorus were measured directly as key biogenic elements, and by inference, phosphorus is assumed to have been available.^[84][86] The two samples, John Klein and Cumberland, contain basaltic minerals, Ca-sulfates, Fe oxide/hydroxides, Fe-sulfides, amorphous material, and trioctahedral smectites (a type of clay). Basaltic minerals in the mudstone are similar to those in nearby aeolian deposits. However, the mudstone has far less Fe-forsterite plus magnetite, so Fe-forsterite (type of olivine) was probably altered to form smectite (a type of clay) and magnetite.^[94] A Late Noachian/Early Hesperian 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.^[90]

In a press conference on December 8, 2014, Mars scientists discussed observations by Curiosity rover that show Mars' Mount Sharp was built by sediments deposited in a large lake bed over tens of millions of years. This finding suggests the climate of ancient Mars could have produced long-lasting lakes at many places on the Planet. Rock layers indicate that a huge lake was filled and evaporated many times. The evidence was many deltas that were stacked upon each other.^{[95][96][97][98][99]}

Also in December 2014, it was announced that Curiosity had detected sharp increases in methane four times out of twelve during a 20-month period with the Tunable Laser Spectrometer (TLS) of the Sample Analysis at Mars instrument (SAM). Methane levels were ten times the usual amount. Due to the temporary nature of the methane spike, researchers believe the source is localized. The source may be biological or non-biological.^{[100][101][102]}

On December 16, 2014, a team of researchers described how they have concluded that organic compounds have been found on Mars by Curiosity. The compounds were found in samples from drilling into Sheepbed mudstone. Chlorobenzene and several dichloroalkanes, such as dichloroethane, dichloropropane and dichlorobutane were discovered in the samples.^[103][104]

On March 24, 2015, a paper was released describing the discovery of nitrates in three samples analyzed by Curiosity. The nitrates are believed to have been created from diatomic nitrogen in the atmosphere during meteorite impacts.^[105][106] Nitrogen is needed for all forms of life because it is used in the building blocks of larger molecules like DNA and RNA. Nitrates contain nitrogen in a form that can be used by living organisms; nitrogen in the air can not be used by organisms. This discovery of nitrates adds to the evidence that Mars once had life.^[107][108]

The Jet Propulsion Laboratory (JPL) announced in April 2015 the discovery of a network of two-tone mineral veins at an area called "Garden City" on lower Mount Sharp. The veins stand about 2.5 inches above the surface and are composed of two different minerals formed from at least two different fluid flows.^[109] In Pahrump Hills, an area about 39 feet lower, the minerals clay, hematite, jarosite, quartz, and cristobalite were found.^[110][111]

Measurements made by Curiosity allowed researchers to determine that Mars has liquid water at times. Because the humidity goes to 100% at night, salts, like calcium perchlorate, will absorb water from the air and form a brine in the soil. This process in which a salt absorbs water from the air is called deliquescence. Liquid water results even though the temperature is very low, as salts lower the freezing point of water. This principle is used when salt is spread on roads to melt snow/ice. The liquid brine produced in the night evaporates after sunrise. Much more liquid water is expected in higher latitudes where the colder temperature and more water vapor can result in higher levels of humidity more often.^[112][113] The researchers cautioned that the amount of water was not enough to support life, but it could allow salts to move around in the soil.^[114] The brines would occur mostly in the upper 5 cm of the surface; however, there is evidence that the effects of liquid water can be detected down to 15 cm. Chlorine-bearing brines are corrosive; therefore design changes may need to be made for future landers.^[115]

French and U.S. scientists found a type of granite by studying images and chemical results of 22 rock fragments. The composition of the rocks was determined with the ChemCam instrument. These pale rocks are rich in feldspar and may contain some quartz. The rocks are similar to Earth's granitic continental crust. They are like rocks called TTG (Tonalite-Trondhjemite-Granodiorite). On the Earth, TTG was common in the terrestrial continental crust in the Archean era (more than 2.5 billion years ago). By landing in Gale crater, Curiosity was able to sample a variety of rocks because the crater dug deep into the crust, thus exposing old rocks, some of which may be about 3.6 billion years old. For many years, Mars was thought to be composed of the dark, igneous rock basalt, so this is a significant discovery.^{[116][117][118]}

On October 8, 2015, a large team of scientists confirmed the existence of long-lasting lakes in Gale Crater. The conclusion of Gale having lakes was based on evidence of old streams with coarser gravel in addition to places where streams appear to have emptied out into bodies of standing water. If lakes were once present, Curiosity would start seeing water-deposited, fine-grained rocks closer to Mount Sharp. That is exactly what happened.

Finely laminated mudstones were discovered by Curiosity; this lamination represents the settling of plumes of fine sediment through a standing body of water. Sediment deposited in a lake formed the lower portion of Mount Sharp, the mountain in Gale crater.^{[119][120][121]}

At a press conference in San Francisco at the American Geophysical Union meeting, a group of scientists told of a discovery of very high concentrations of silica at some sites, along with the first ever discovery of a silica mineral called tridymite. The scientific team believes that water was involved with putting the silica in place. Acidic water would tend transport other ingredients away and leave silica behind, whereas alkaline or neutral water could carry in dissolved silica that would be deposited. This finding used measurements from ChemCam, the Alpha Particle X-ray Spectrometer (APXS), and the Chemistry and Mineralogy (CheMin) instrument inside the rover. Tridymite was found in a rock named "Buckskin".^[122] ChemCam and APXS measurements displayed high silica in pale zones along fractures in the bedrock beyond Marias Pass; hence silica may have been deposited by fluids that flowed through the fractures. CheMin found high silica levels in drilled material from a target called "Big Sky" and in another rock called "Greenhorn".^[123]

As of the beginning of 2016, Curiosity had discovered seven hydrous minerals. The minerals are actinolite, montmorillonite, saponite, jarosite, halloysite, szomolnokite and magnesite. In some places the total concentration of all hydrous minerals was 40 vol%. Hydrous minerals help us understand the early water environment and possible biology on Mars.^[124]

By using Curiosity's laser-firing device (ChemCam), scientists found manganese oxides in mineral veins in the "Kimberley" region of Gale Crater. These minerals need lots of water and oxidizing conditions to form; hence this discovery points to a water-rich, oxygen-rich past.^{[125][126][127]}

A study of the kinds of minerals in veins examined with Curiosity found that evaporating lakes were present in the past in Gale crater. The Sheepbed Member mudstones of Yellowknife Bay (YKB) were examined in this research.^[128][129]

Frost probably has formed in three locations in the first 1000 sols of the mission of the Curiosity exploration according to research published in Icarus in 2016.^[130] This frost can cause weathering. Frost formation can explain the widespread detection of hydrated materials from orbit with the OMEGA instrument; it also can explain the hydrated component measured by Curiosity in Martian soil.^{[131][132][133]}

Researchers in December 2016 announced the discovery of the element boron on Mars by Curiosity in mineral veins. For boron to be present there must have been a temperature between 0–60 degrees Celsius and a neutral-to-alkaline pH." The temperature, pH, and dissolved minerals of the groundwater support a habitable environment.^[134] Moreover, boron has been suggested to be necessary for life to form. Its presence stabilizes the sugar ribose which is an ingredient in RNA.^{[135][136][137]} Details of the discovery of Boron on Mars were given in a paper written by a large number of researchers and published in Geophysical Research Letters.^{[138][139][140]}

Researchers have concluded that Gale Crater has experienced many episodes of groundwater with changes in the groundwater chemistry. These chemical changes would support life.^{[141][142][143][144][145][146]}

In January 2017, JPL scientists announced the discovery of mud cracks on Mars. This find adds more evidence that Gale Crater was wet in the past.^{[147][148][149][150]}

Studies of the wind around the Curiosity rover over a period of 3 billion years has shown that the Mount Sharp, the mound inside Gale Crater was created when winds removed material over billions of years and left material in the middle that is Mount Sharp. The researchers calculated that about 15,000 cubic miles (64,000 cubic kilometers) of material was removed from the crater. Curiosity has seen dust devils in action in the distance. Also, changes were visible as a dust devil passed close to the rover. Ripples in the sand below Curiosity were observed to move about one inch (2.5 cm) in just one day.^[151][152]

CheMin found feldspar, mafic igneous minerals, iron oxides, crystalline silica, phyllosilicates, sulfate minerals in mudstone of Gale Crater. Some of the trends in these minerals at different levels suggested that at least part of the time the lake had near-neutral pH.^[153][154]

An analysis of a large amount of data from ChemCam and APXS showed that most of the material encountered by Curiosity consists of just two major igneous rock types and traces of three others. One chief type is classified as a subalkaline, Mg-rich basalt (similar to MER Spirit basalt) and the other was a more evolved, higher Si, Al, lower Mg basalt.^[155]

A large group of researchers discovered halos around fractures that they water existed in the ground long after water disappeared from Gale crater. Groundwater, carrying dissolved silica, moved in fractures and deposited silica there. This silica enrichment went across young and old rocks.^[156][157]

Research of chemicals in layers in Gale Crater, published in 2017, suggest that the lake in Gale Crater had a neutral pH for much of the time. The mudstone in the Murray formation at the base of Mount Sharp indicated deposition in a lake environment. After the layers were deposited, an acid solution may have moved through the rock, which contained olivine and pyroxene, dissolving some minerals like magnetite and forming new ones like hematite and jarosite. The elements magnesium (Mg), iron (Fe), manganese (Mn), nickel (Ni), and zinc (Zn) were carried down. Eventually, Ni, Zn, and Mn coated (adsorbed onto) clay particles. Iron-oxides, Mg, and sulfur produced sulfates. The Murray formation was sampled at several locations for this research: Confidence Hills, Mojave 2, Telegraph peak, and Buckskin.^[158][159]

Research presented in a June 2018 press conference described the detection of more organic molecules in a drill sample analyzed by Curiosity.^[160][161] Some of the organic molecules found were thiophenes, benzene, toluene, and small carbon chains, such as propane or butane.^[162] At least 50 nanomoles of organic carbon are still in the sample, but were not specifically determined. The remaining organic material probably exists as macromolecules organic sulfur molecules. Organic matter was from lacustrine mudstones at the base of the ~3.5-billion-year-old Murray formation at Pahrump Hills, by the Sample Analysis at Mars instrument suite.^[163]

With two full Martian years (five Earth years) of measurements, scientists found that the annual average concentration of methane in Mars' atmosphere is 0.41 ppb. However, methane levels rise and fall with the seasons, going from 0.24 ppb in winter to 0.65 ppb in summer. The researchers also saw relatively large methane spikes, up to about 7 ppb, at random intervals.^[160][164] The existence of methane in the Martian atmosphere is exciting because on Earth, most methane is produced by living organisms. Methane on Mars does not prove that life exists there, but it is consistent with life. Ultraviolet radiation from the sun destroys methane does not last long; consequently, something must have been creating or releasing it.^[164]

Using date gathered with Mastcam, a team of researchers have found what they believe to be iron meteorites. These meteorites stand out in multispectral observations as not possessing the usual ferrous or ferric absorption features as the surrounding surface.^[165]

Emily Lakdaealla wrote a detailed 2018 book about the Curiosity rover's instruments and history. She listed the minerals that Curiosity's CheMin has discovered. CheMin has discovered olivine, pyroxene, feldspar, quartz, magnetite, iron sulfides (pyrite and pyrrhotite), akaganeite, jarosite, and calcium sulfates (gypsum, anhydrite, basanite) ^[166]

Research presented in 2018 at the Geological Society of America Annual Meeting in Indianapolis, Indiana described evidence for huge floods in Gale Crater. One rock unit examined by Curiosity contains the rock conglomerate with particles up to 20 centimeters across. To create such a type of rock water must have been 10 to 20 meters in depth. Between two million years to 12,000 years ago, Earth experienced these type of floods.^{[167][168][169]}

Using various gravity measurements, a team of scientists concluded that Mount Sharp may have formed right where it is, as it is. The authors stated, "Mount Sharp formed largely in its current form as a free-standing mound within Gale."^[170] One idea was that it was part of material that covered a wide region and then eroded, leaving Mount Sharp. However, if that were the case, the layers on the bottom would be fairly dense. This gravity data show that the bottom layers are quite porous. Had they been under many layers of rock they would be compressed and be more dense. Intensity of the gravity was obtained by using data from Curiosity's accelerometers.^{[171][172][173]}

Researched published in Nature Geoscience in October 2019 described how Gale crater underwent many wet and dry cycles as its lake waters disappeared.^[174] Sulfate salts from evaporated water showed that pools of salty water once existed in Gale Crater. These ponds could have supported organisms. Basalts could have produced the calcium and magnesium sulfates that were found. Because of its low solubility, calcium sulfate is deposited early on as a lake dries up. However, the discovery of magnesium sulfate salts means that the lake must have almost totally evaporated. The remaining pools of water would have been very salty—such lakes on Earth contain organisms that are salt tolerant or "halotolerant". These minerals were found along the edges of what were lakes in the younger parts of Gale Crater.^[175] When Curiosity was exploring deeper in the crater, clays found there showed that a lake existed for a long time, these new findings of sulfates the lake dried up and then get wetter over and over.

Sulfate salts have been detected in other places in Gale as white veins caused by groundwater moving through cracks in the rocks.^[176]

Curiosity has found oxygen going into the air in Gale Crater. Measurements over three Martian years (almost six Earth years) by an instrument in the Sample Analysis at Mars (SAM) portable chemistry lab revealed that the level of oxygen went up throughout spring and summer by as much as 30%, and then dropped back to normal levels by fall. Each spring this occurred. These oxygen seasonal variations suggest some unknown process in the atmosphere or the surface is occurring.^{[177][178][179]}

Evidence of life on Mars was published on January 19, 2022. Rover's Tunable Laser Spectrometer (TLS) determined the abundance of carbon isotopes in 24 samples. In many of the samples the relative amount of carbon-12 compared to carbon-13 suggested organisms altered the isotopes.^[180]

Some places on Mars show inverted relief. In these locations, a stream bed may be a raised feature, instead of a valley. The inverted former stream channels may be caused by the deposition of large rocks or due to cementation. In either case erosion would erode the surrounding land but leave the old channel as a raised ridge because the ridge will be more resistant to erosion.^[181] An image below, taken with HiRISE shows sinuous ridges that may be old channels that have become inverted.^[182]

• Meandering Ridges that are probably inverted stream channels. Image taken with HiRISE.
• CTX image of craters with black box showing location of next image
• Image from previous photo of a curved ridge that may be an old stream that has become inverted. Image taken with HiRISE under the HiWish program.
• Sinuous Ridges within a branching fan in lower member of Medusae Fossae Formation, as seen by HiRISE
• Inverted channels in Aleolis Planum NASA comments on these thusly: "These likely represent ancient, meandering river channels that flowed across the surface and buried themselves over time. The channels have subsequently been exposed to the surface by the wind, forming the cross-cutting ridges."
• Close view of inverted channels. Note that some are flat on top.

Yardangs are common on Mars.^[183] They are generally visible as a series of parallel linear ridges. Their parallel nature is thought to be caused by the direction of the prevailing wind. Two HiRISE images below show a good view of yardangs in the Aeolis quadrangle.^[182] Yardangs are common in the Medusae Fossae Formation on Mars.

• Stream channels in inverted relief and yardangs, as seen by HiRISE
• Aeolis Mensae Yardangs, as seen by HiRISE. Scale bar is 500 meters long. Click on image for better view of yardangs.
• Medusae Fossae Formation southeast of Apollinaris Patera, as seen by HiRISE
• Yardangs in Medusae Fossae Formation with caprock labeled, as seen by HiRISE
• Yardangs, as seen by HiRISE

Parts of the Aeolis quadrangle contain fretted terrain which is characterized by cliffs, mesas, buttes, and straight-walled canyons. It contains scarps or cliffs that are 1 to 2 km in height.^[184][185]

Researchers, writing in Icarus, have described layered units in the Aeolis quadrangle at Aeolis Dorsa. A deposit that contains yardang was formed after several other deposits. The yardangs contain a layered deposit called "rhythmite" which was thought to be formed with regular changes in the climate. Because the layers appear harden, a damp or wet environment probably existed at the time. The authors correlate these layered deposits to the upper layers of Gale crater's mound (Mt. Sharp).^[186]

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. Some layers are probably made up of fine particles because they seem to break up into find dust. Other layers break up into large boulders so they are probably much harder. Basalt, a volcanic rock, is thought to in the layers that form boulders. Basalt has been identified on Mars in many places. Instruments on orbiting spacecraft have detected clay (also called phyllosilicate) in some layers. Recent research with an orbiting near-infrared spectrometer, which reveals the types of minerals present based on the wavelengths of light they absorb, found evidence of layers of both clay and sulfates in Columbus crater.^[187] This is exactly what would appear if a large lake had slowly evaporated.^[188] Moreover, because some layers contained gypsum, a sulfate which forms in relatively fresh water, life could have formed in the crater.^[189]

Scientists were excited about finding hydrated minerals such as sulfates and clays on Mars because they are usually formed in the presence of water.^[190] Places that contain clays and/or other hydrated minerals would be good places to look for evidence of life.^[191]

Rock can form layers in a variety of ways. Volcanoes, wind, or water can produce layers.^[192] Layers can be hardened by the action of groundwater. Martian ground water probably moved hundreds of kilometers, and in the process it dissolved many minerals from the rock it passed through. When ground water surfaces in low areas containing sediments, water evaporates in the thin atmosphere and leaves behind minerals as deposits and/or cementing agents. Consequently, layers of dust could not later easily erode away since they were cemented together. On Earth, mineral-rich waters often evaporate forming large deposits of various types of salts and other minerals. Sometimes water flows through Earth's aquifers, and then evaporates at the surface just as is hypothesized for Mars. One location this occurs on Earth is the Great Artesian Basin of Australia.^[193] On Earth the hardness of many sedimentary rocks, like sandstone, is largely due to the cement that was put in place as water passed through.

• Layers in lower member of Medusae Fossae Formation, as seen by HiRISE
• Buttes and layers in Aeolis, as seen by Mars Global Surveyor
• Layers, as seen by HiRISE
• Layers along crater rim in Terra Sirenum, as seen by HiRIS under the HiWish program
• Layered terrain, as seen by HiRISE under HiWish program. Location is East of Gale Crater in the Aeolis quadrangle.
• Layers and mounds in Medusae Fossae Formation, as seen by HiRISE under HiWish program. Location is East of Gale Crater in the Aeolis quadrangle.
• Layers and a field of small mounds Medusae Fossae Formation, as seen by HiRISE under HiWish program. Location is east of Gale Crater in the Aeolis quadrangle.
• Mound showing layers at the base, as seen by HiRISE under HiWish program. Location is east of Gale Crater in the Aeolis quadrangle.
• Layered structure, as seen by HiRISE under HiWish program
• Layered features northeast of Gale Crater, as seen by HiRISE under HiWish program. Layers may be similar to the many layers that are being examined by the Curiosity rover.
• Wide view of layered terrain, as seen by HiRISE under HiWish program. Location is northeast of Gale Crater.
• Close view of mound with layers, as seen by HiRISE under HiWish program. Note: this is an enlargement from the previous image.
• Close view of mound with layers, as seen by HiRISE under HiWish program. Note: this is an enlargement from a previous image.
• Wide view of layered terrain, as seen by HiRISE under HiWish program. Note: parts of this image are enlarged in the next three images.
• Close view of layers in a mound, from previous image, as seen by HiRISE under HiWish program
• Close view of layers in a mound, from a previous image, as seen by HiRISE under HiWish program
• Close view of layers in a mound, from a previous image, as seen by HiRISE under HiWish program

• Wide view of layered buttes and small mesas, as seen by HiRISE under HiWish program. Some dark slope streaks are visible.
• Layered mesa and mounds with dark slope streaks, as seen by HiRISE under HiWish program
• Close view of layered small mesa with dark slope streak, as seen by HiRISE under HiWish program. Box shows the size of a football field.
• Close view of dark slope streak with strange breaks, as seen by HiRISE under HiWish program
• Very close view of individual blocks breaking off layer in a butte, as seen by HiRISE under HiWish program. Blocks have angular shapes. Box shows size of football field.
• Close view of blocks from a mesa, as seen by HiRISE under HiWish program. Arrow show a cube-shaped block.

• Layered mesas, as seen by HiRISE under HiWish program
• Layered mesas, as seen by HiRISE under HiWish program. Dark slope streaks are also visible.
• Mesas, as seen by HiRISE under HiWish program. Top layer, the cap rock is breaking up into boulders.
• Close view of cap rock breaking up into boulders, as seen by HiRISE under HiWish program

Linear ridge networks are found in various places on Mars in and around craters.^[194] Ridges often appear as mostly straight segments. They are hundreds of meters long, tens of meters high, and several meters wide. It is thought that impacts created fractures in the surface, these fractures later acted as channels for fluids. Fluids cemented the structures. With the passage of time, surrounding material was eroded away, thereby leaving hard ridges behind. Since the ridges occur in locations with clay, these formations could serve as a marker for clay which requires water for its formation.^{[195][196][197]}

• Wide view of ridges, as seen by HiRISE under HiWish program
• Color view of ridges, as seen by HiRISE under HiWish program
• Ridges, as seen by HiRISE under HiWish program
• Ridges, as seen by HiRISE under HiWish program
• Ridges, as seen by HiRISE under HiWish program
• Ridges, as seen by HiRISE under HiWish program
• Ridges, as seen by HiRISE under HiWish program
• Ridges, as seen by HiRISE under HiWish program
• Ridges, as seen by HiRISE under HiWish program

• Landslide
• Possible fan or delta, as seen by HiRISE under HiWish program
• Channel, as seen by HiRISE under HiWish program
• Channels (indicated with arrows), as seen by HiRISE under HiWish program
• Channel, as seen by HiRISE under HiWish program
• Tilted blocks as seen by HiRISE under HiWish program These blockls were formed horizonality, but have been tilted. Perhaps ice left the ground on one side.