The Mars Reconnaissance Orbiter's HiRISE instrument has taken many images that strongly suggest that Mars has had a rich history of water-related processes. Many features of Mars appear to be created by large amounts of water. That Mars once possessed large amounts of water was confirmed by isotope studies in a study published in March 2015, by a team of scientists showing that the ice caps were highly enriched with deuterium, heavy hydrogen, by around seven times as much as the Earth. This means that Mars had lost a volume of water 6.5 times what is stored in today's polar caps. The water for a time would have formed an ocean in the low-lying Mare Boreum. The amount of water could have covered the planet about 140 meters, but was probably in an ocean that in places would be almost 1 mile deep.
A major discovery by HiRISE was finding evidence of hot springs. These may have contained life and may now contain well-preserved fossils of life.
Recurring slope lineae Seasonal flows on warm Martian slopes (also called recurring slope lineae, recurrent slope lineae and RSL) are thought to be salty water flows occurring during the warmest months on Mars. Indeed, there is much photographic and spectroscopic evidence that water does today flow on parts of Mars.
Some researchers have proposed that the flow is aided by the water boiling in thin Martian atmosphere. Boiling water would cause soil particles to bounce and help them to flow down slopes.
In a study published in Icarus in 2016, A team, headed by David Stillman, identified 239 locations for recurring slope lineae within Valles Marineris. About half of lineae locations on Mars are in Valles Marineris. It was found that the lengthening of the lineae depends on how the slope is orientated. Lengthening was observed in all seasons inside Valles Marineris. This long active season suggests that there must be an appreciable source of water that supplies the lineae. Calculations show that the lineae are active between minus 27 degrees C and minus 9 degrees C to display the activity observed. Water would do this if it was in a brine having of tens of wt% salt.
Although these features were initially thought to be evidence of current water flow, some studies show that little or no water may be involved. An analysis of data from the Mars Odyssey neutron spectrometer revealed that the RSL sites do not contain any more water than found at anywhere else at similar latitudes. The authors concluded that RSL are not supplied by large, near-surface briny aquifers. It is still possible with this data that water vapor from deeply buried ice, from the atmosphere, or from small deeply buried aquifers.Wilson, J. et al. 2018. Equatorial locations of water on Mars: Improved resolution maps based on Mars Odyssey Neutron Spectrometer data. Icarus: 299, 148-160.
A study published in Nature Geoscience, suggest that very little water is involved in creating recurrent slope lineae because these streaks only occur on steep slopes—slopes that would allow dry sand and dust to move down. If water were involved, at least some of the lineae would move onto lower slopes. Small amounts of water may start the process however. A study spanning 4 Martian years concluded that a dry origin was the most likely cause of the streaks. In the study region, Tivat Crater, rockfall markings faded at the same time as recurrent slope lineae (RSL), suggesting a fading mechanism that acts across the whole crater.
Eridania Lake
Eridania Lake is a theorized ancient lake with a surface area of roughly 1.1 million square kilometers.de Pablo, M., M. Druet. 2002. XXXIII LPSC. Abstract #1032.de Pablo, M. 2003. VI Mars Conference, Abstract #3037. Its maximum depth is 2,400 meters and its volume is 562,000km. It was larger than the largest landlocked sea on Earth, the Caspian Sea and contained more water than all the other martian lakes together. The Eridania sea held more than 9 times as much water as all of America's Great Lakes. The upper surface of the lake was assumed to be at the elevation of valley networks that surround the lake; they all end at the same elevation, suggesting that they emptied into a lake.Baker, D., J. Head. 2014. 44th LPSC, abstract #1252
Research with CRISM found thick deposits, greater than 400 meters thick, that contained the minerals saponite, talc-saponite, Fe-rich mica (for example, glauconite-nontronite), Fe- and Mg-serpentine, Mg-Fe-Ca-carbonate and probable Fe-sulphide. The Fe-sulphide probably formed in deep water from water heated by volcanoes. Such a process, classified as hydrothermal may have been a place where life began. Saponite, talc, talc-saponite, nontronite, glauconite, and serpentine are all common on the seafloors on Earth.Cuadros, J. et al. 2013. Crystal-chemistry of interstratified Mg/Fe-clay minerals from seafloor hydrothermal sites. Chemical Geology 360–361, 142–158. The earliest evidence of life on Earth appear in seafloor deposits that are similar to those found in the Eridania basin. So, samples of material from the Eridania may give us insight into the environment of the early Earth. Chloride deposits were found where a shoreline existed. They were deposited as water evaporated from the sea. These chloride deposits are thought to be thin (less than 30 meters), because some craters do not display the chemical in their ejecta. A crater's ejecta contains material from under the surface, therefore if the chloride deposits were very deep they would have appeared in the ejecta.
Branched channels Research, in the January 2010 issue of Icarus, described strong evidence for sustained precipitation in the area around Valles Marineris. The types of minerals there are associated with water. Also, the high density of small branching channels indicates a great deal of precipitation because they are similar to stream channels on the Earth.
Inverted relief
Some places on Mars show inverted relief. In these locations, a stream bed appears as a raised feature, instead of a depression. The inverted former stream channels may be caused by the deposition of large rocks or due to cementation of loose materials. In either case erosion would erode the surrounding land and consequently leave the old channel as a raised ridge because the ridge will be more resistant to erosion. Images below, taken with HiRISE show sinuous ridges that are old channels that have become inverted.
In an article published in January 2010, a large group of scientists endorsed the idea of searching for life in Miyamoto Crater because of inverted stream channels and minerals that indicated the past presence of water.
Chloride deposits Using data from Mars Global Surveyor, Mars Odyssey and the Mars Reconnaissance Orbiter, scientists have found widespread deposits of chloride minerals. Some of these chloride deposits have been identified to be a mixture of anhydrous chloride salt and regional basaltic regolith. These are located in the southern highlands of Mars. Usually chlorides are the last minerals to come out of solution. A picture below shows some deposits within the Phaethontis quadrangle. Evidence suggests that the deposits were formed from the evaporation of mineral-enriched waters. Lakes may have been scattered over large areas of the Martian surface. Carbonates, sulfates, and silica should precipitate out ahead of them. Sulfates and silica have been discovered by the Mars rovers. Places with chloride minerals may have once held various life forms. Furthermore, such areas should preserve traces of ancient life.
Layers
Rocks on Mars have been found to frequently occur as layers, called strata, in many different places. Columbus Crater is one of many craters that contain layers.Grotzinger, J. and R. Milliken (eds.) 2012. Sedimentary Geology of Mars. SEPM Rock can form layers in a variety of ways. Volcanoes, wind, or water can produce layers. Many places on Mars show rocks arranged in layers. Scientists are happy about finding layers on Mars since layers may have formed under large bodies of water. Layers may be formed by groundwater rising up depositing minerals and cementing sediments. The hardened layers are consequently more protected from erosion. This process may occur instead of layers forming under lakes.
Sometimes the layers display 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 fine dust. In contrast, other layers break up into large boulders so they are probably much harder. Basalt, a volcanic rock, is thought to form layers composed of boulders. Basalt has been identified all over Mars. Instruments on orbiting spacecraft have detected clay (also called phyllosilicates) in some layers. Scientists are excited about finding hydrated minerals such as sulfates and clays on Mars because they are usually 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.
More research using data collected by the SHARAD (SHAllow RAdar Detector) on the MRO has found layers of ice-water interspersed with dirt at the northern pole under the Planum Boreum. The layering is believed to have come from the global warming and cooling cycle on Mars; during cooling periods, water migrated to the poles and formed the ice-water layers, while on subsequent warming, the unthawed ice water was covered by layers of dust and dirt from windstorms on the surface, helping to preserve the ice water.
Below are a few of the many examples of layers that have been studied with HiRISE.
Mantle
Much of the surface of Mars is covered by a thick smooth mantle that is thought to be a mixture of ice and dust. This ice-rich mantle, a few yards thick, smooths the land. But in places it displays a bumpy texture, resembling the surface of a basketball. Because there are few craters on this mantle, the mantle is relatively young.
It's generally accepted that mantle is ice-rich dust that fell from the sky as snow and ice-coated dust grains during a different climate One evidence of its ice-rich nature is the presence of gullies which form when some of the ice melts.
The images below, all taken with HiRISE, show a variety of views of this smooth mantle.
Ring mold craters Ring mold craters are kind of Martian craters that look like the ring molds used in baking. They are thought to be caused by an impact into ice. The ice is covered by a layer of debris. They are found in parts of Mars that have buried ice. Laboratory experiments confirm that impacts into ice result in a "ring mold shape". Impacts into ice warm the ice and cause it to flow into the ring mold shape.
Gullies
Changes in Mars's orbit and tilt cause significant changes in the distribution of water ice from polar regions down to latitudes equivalent to Texas. During certain climate periods water vapor leaves polar ice and enters the atmosphere. The water returns to the ground at lower latitudes as deposits of frost or snow mixed generously with dust. The atmosphere of Mars contains a great deal of fine dust particles.Head, J. et al. 2008. Formation of gullies on Mars: Link to recent climate history and insolation microenvironments implicate surface water flow origin. Proceedings of the National Academy of Sciences of the United States of America: 105. 13258-13263. Water vapor condenses on the particles, then they fall down to the ground due to the additional weight of the water coating. When ice at the top of the mantling layer goes back into the atmosphere, it leaves behind dust, which insulates the remaining ice.