To assess Mars’ potential for both harboring life and providing useable resources for future human exploration, it is of paramount importance to comprehend the water situation on the planet. Therefore, studies have been conducted to determine any evidence of past or present water existence on Mars. While the presence of abundant water on Mars very early in its history is widely accepted, on its modern form, only a fraction of this water can be found, as either ice or locked into the structure of Mars’ plentiful water-rich materials. Water on the planet is evaluated through various evidence such as rocks and minerals, Martian achondrites, low volume transient briny outflows (e.g., dune flows, reactivated gullies, slope streaks, etc.), diurnal shallow soil moisture (e.g., measurements by Curiosity and Phoenix Lander), geomorphic representation (possibly from lakes and river valleys), and groundwater, along with further evidence obtained by probe and rover discoveries. One of the most significant lines of evidence is for an ancient streambed in Gale Crater, implying ancient amounts of “vigorous” water on Mars. Long ago, hospitable conditions for microbial life existed on the surface of Mars, as it was likely periodically wet. However, its current dry surface makes it almost impossible as an appropriate environment for living organisms; therefore, scientists have recognized the planet’s subsurface environments as the best potential locations for exploring life on Mars. As a result, modern research has aimed towards discovering underground water, leading to the discovery of a large amount of underground ice in 2016 by NASA, and a subglacial lake in 2018 by Italian scientists. Nevertheless, the presence of life in Mars’ history is still an open question. In this unifying context, the current review summarizes results from a wide variety of studies and reports related to the history of water on Mars, as well as any related discussions on the possibility of living organisms' existence on the planet.
While a substantial portion of the existing water on Mars today is as ice, tiny water amounts are present as vapor in the atmosphere or as low-volume liquid brine that can be found in shallow soil areas. Bright material inferred to be ice can also be seen visually in exposures within new impact craters at high latitudes imaged by HiRISE (high-resolution imaging science experiment). On the surface of Mars, water can only be seen at the northern polar ice caps. Other places on Mars that contain significant amounts of water are at the south pole, where there is a permanent carbon dioxide ice cap, as well as in the shallow subsurface, where more moderate conditions exist. Detection of water on Mars’ surface, or close to it, shows the existence of more than 21 million km3 of ice, which would be able to cover the planet with water 35 m (115 ft.) deep. Much more ice is probably frozen into the profound subsurface.
Although some liquid water can be found now on the Martian surface, as a challenging environment for known life, it is confined to thin layers or dissolved atmospheric moisture. Since the average atmospheric pressure on the planet’s surface is approximately 600 pascals (0.087 psi), lower than the melting point of water’s vapor pressure, no significant amount of liquid water exists on the surface; typically, if pure water on the Martian surface were heated to more than its melting point, it would become vapor; otherwise, it would freeze. In addition, brines have a significant impact on lowering water activity/vapor pressure over solution, and hence reducing the evaporation rate and prolonging the lifetime of liquid water/brine exposed to the Martian atmosphere. Mars probably had higher surface temperatures, as well as a denser atmosphere, 3.8 billion years ago, which resulted in large amounts of surface water that may have contained a vast ocean, possibly covering one-third of the planet. It seems that there was water flowing across the surface for short amounts of time at different periods in Mars’ history. According to NASA’s report on December 9, 2013, obtained from the Curiosity rover, which is studying Aeolis Palus, an aged freshwater lake used to exist in Gale Crater, where there may have been an environment for microbial life. Much evidence shows the notable role of water ice in Mars’ geologic history, due to a considerable amount of it on the planet.
Additionally, Martian achondrites provide clear evidence for the water presence over time. These rocks can be considered the key to get clues about the primary geologic processes at work in Mars, identify its ancient hydrothermal environments, search for traces of life forms, and study the interaction of water in sample returned rocks. In addition, as they can be accurately dated, they can provide a message in a bottle: unique information of the key regions in which future missions should focus to extract the maximum scientific information on the key questions to be answered in future robotic and manned sample-return missions to the red planet.
Figure 1. Mars’ hydrated minerals map.
Figure 2. Kasei Valles—a major outflow channel—seen in Mars orbiter laser altimeter (MOLA) elevation data. The flow was from the bottom left to right. The image is approx. 1600 km across. The channel system extends another 1200 km south of this image to Echus Chasma.
Figure 4. Map showing estimated water depth in different parts of the Eridania Sea. This map is about 530 miles across.
Figure 5. Deep-basin deposits from the floor of the Eridania Sea. The mesas on the floor are there because they were protected against intense erosion by deep water/ice cover. CRISM measurements show minerals may be from seafloor hydrothermal deposits.
Figure 6. Delta in Eberswalde crater.
Figure 7. The preservation and cementation of aeolian dune stratigraphy in Burns Cliff in Endurance Crater are thought to have been controlled by the flow of shallow groundwater.
Figure 8. The blue region of low topography in the Martian northern hemisphere is hypothesized to be the site of a primordial ocean of liquid water.
Figure 9. Subsurface water ice consistency map for the northern hemisphere of Mars.
Figure 10. The Mars Global Surveyor acquired this image of the Martian north polar ice cap in early northern summer.
Figure 12. A cross-section of underground water ice is exposed at the steep slope that appears bright blue in this enhanced-color view from the Mars Reconnaissance Orbiter (MRO). The scene is about 500 m wide. The scarp drops about 128 m from the level ground. The ice sheets extend from just below the surface to a depth of 100 m or more [177,178].
Figure 20. Thin rock layers, not all parallel to each other.
Figure 21. Close-up of a rock outcrop.
Figure 22. Springs in Vernal Crater, as seen by HiRISE. These springs may be good places to look for evidence of past life, because hot springs can preserve evidence of life forms for a long time. Location is the Oxia Palus quadrangle.
Figure 23. Rock outcrop on Mars—compared with a terrestrial fluvial conglomerate—suggesting water “vigorously” flowing in a stream.
This entry is adapted from the peer-reviewed paper 10.3390/galaxies8020040