Where is Mars’s water?

A delta at the edge of Jazero Crater on Mars; definite evidence that water once flowed into the crater. Colours show different minerals in the delta sediments (credit: Brown University)

Early in the exploration of Mars using orbiting imaging systems it was easy to be sceptical about evidence for water being present at or near the surface of the Red Planet. Resolution was poor and some claims seemed to be wishful thinking or a sort of astronautical agitprop. For instance, gullies on steep slopes appeared so sharp that they must be forming continually, otherwise Mars’s periodic huge dust storms would have muted them. Some scientists claimed that they were signs of flowing water and even presented pictures from different overpasses that showed changes in them, such as darkening and small shifts in microtopography, which may have resulted from flowing water. Because Mars has a mean surface temperature of about -50°C that seems unlikely; at such extremes in Antarctica spit at the ground and it lands as ice. Nonetheless a bit of special pleading that deeply buried ice in Martian sediments might melt because of pressure gave the idea some traction.

A far more plausible explanation for the active gulley formation is that loose fine sediment can flow in the manner of a liquid, as it does in sand dunes on Earth (see: First signs of liquid water on Mars? June 2000). Yet as remotely sensed image coverage expanded and its resolution improved (currently about 50 cm) masses of evidence for drainage networks, signs of catastrophic floods and even glaciers (The glaciers of Mars, July 2003) emerged. Huge areas of the planet bore witness to a period in its past history – 4.1 to 3.8 billion years (Ga) ago – when it was a warm and wet planet. It has even been suggested that the flat, low-elevation northern hemisphere was the bed of a former ocean, covering about a third of Mars to a depth of about a kilometre. Now the planet has a hyperarid surface and a very thin atmosphere dominated by CO2, a little nitrogen and argon but almost no water vapour (~0.03%). Its poles are covered by ice caps whose extents fluctuate seasonally. They each have a core of permanent water ice, and seasonally expand and contract due to formation and sublimation of dry ice made of solid CO2. So what happened to Mars’s once abundant water?

One long-held theory is that water and most of Mars’s original atmosphere escaped to space. A suggested mechanism is the photo-dissociation of water to hydrogen and oxygen. Mars’s gravity cannot prevent hydrogen escape, which would leave an excess of atmospheric oxygen. One thing in abundance on the Martian surface is oxygen combined in iron oxides (Fe2O3); hence its red coloration. This hematite may have formed during chemical weathering of surface rocks and sediments during the wet phase, which released Fe2+ ions that were immediately oxidised by the hyper-oxygenated atmosphere that resulted from photo-dissociation. But there is another plausible explanation …

The lake-bed sediments of Gale Crater on Mars from NASA’s Curiosity rover (credit: NASA/JPL, California Institute of Technology)

The much publicised successful landing of NASA’s Perseverance rover on 18 February 2021 was aimed at the small Jezero Crater, near the Martian equator. This contains an indisputable delta of a large drainage system that must once have filled the crater with a circular lake; a good place to seek out signs of early life, for which Perseverance is impressively equipped. Shortly afterwards there appeared a Research Article in Science (Scheller, E.L. et al. 2021. Long-term drying of Mars by sequestration of ocean-scale volumes of water in the crust. Science, Online research article eabc7717; DOI: 10.1126/science.abc7717) that examines the fate of the planet’s water. The authors estimate that by 3.0 Ga Mars’s surface had reached its current dry state. They model three processes – supply of water by volcanic degassing and its loss by atmospheric escape and chemical weathering of the Martian surface. The modelling was constrained by the ratio of deuterium (2H) to hydrogen inferred from meteorites believed to come from Mars and estimates by orbiting spacecraft of the current escape of hydrogen from the atmosphere. The latter is too slow to explain the huge loss of water between 4 and 3 Ga and subsequently. Addition of water from Mars’s mantle by volcanoes, even from the gigantic Olympus Mons, was far slower than on Earth because continuous plate tectonics was never achieved on Mars. Chemical weathering of the surface during Mars’s warm-wet phase formed abundant hydrated minerals as well as the hematite that gives the planet its characteristic hue. Water transport before 3 Ga moved clays and hydroxides etc to sedimentary basins, where they have remained undisturbed. On Earth, tectonics recycles sediments and their content of hydrated minerals into the mantle, eventually to regurgitate their water content through volcanism. On Mars, weathering and deposition has irreversibly locked-up between 30 and 99% of Mars’s original endowment of water in its ancient sedimentary crust.

That seems to be a ‘bit of a downer’ for ambitious prospects of terraforming Mars and making it a human escape destination. There are, however, some locations where water may be available in sufficient quantities to support some kind of permanent presence of small colonies, in the form of buried layers of ice, similar to permafrost (see: Ice cliffs on Mars, January 2018)

See also:  Carr, M.H. 2012. The fluvial history of Mars. Philosophical Transaction of the Royal Society (A), v. 370, p. 2193-2215; DOI: 10.1098/rsta.2011.0500.

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