Ample evidence—from lake-, river-, and delta-like geological features to aqueous mineralogy—supports the inference that Mars once had vast quantities of water pooling and flowing across its surface. A longstanding question has centered around what happened to that water, as the Red Planet is now a barren, frozen desert. Prevailing theory has held that Mars permanently lost a great deal of its water to space. Now a new study is arguing against this convention, instead positing that much of Mars' original water remains trapped in the planet's crust. The findings have implications for the continuing search for life on Mars by the recently landed Perseverance rover and other probes, as well as future human exploration, where visitors might retrieve this water to support long-term habitation of the Red Planet. See also: Mars; Robot rover Perseverance lands on Mars; Water
Scientists estimate that early Mars possessed sufficient water to have covered the entire world in an ocean approximately 100–1,500 meters (330–4,920 feet) deep. For comparison, that volume is equal to about half of the Atlantic Ocean. Mars could have plausibly lost its water to space as sunlight progressively broke apart atmospheric molecules of water vapor, made up of hydrogen and oxygen. This phenomenon, known as photodissociation, likewise occurs in Earth's atmosphere. Photodissociation separates the heavier oxygen from the lighter hydrogen. Because Mars' gravity is so low compared to Earth's gravity (about 38% as strong), however, that freed hydrogen could have more easily escaped into space than on Earth. Over time, photodissociation would have left a skewed ratio of “common” hydrogen, whose nucleus is composed of only a single proton, to “heavy” hydrogen, or deuterium, whose nucleus contains a single neutron in addition to a proton, as the heavier hydrogen disproportionately accumulated in the atmosphere. This so-called D/H ratio has been measured by probes; although a skewed ratio is clearly present, its magnitude only supports the very low end of the estimates for the water budget Mars initially possessed during the first eon or so of its 4.5-billion-year existence. Likewise, currently measured rates of hydrogen escape into space, when extrapolated back in time, cannot account for the middle- and higher-end water budget estimates, suggesting that a large reservoir of water has remained unaccounted for over Martian geological history. See also: Atlantic Ocean; Atmosphere; Deuterium; Gravity; Hydrogen; Oxygen
In the new study, researchers took a fresh, comprehensive look at the various kinds of ever-accumulating data collected by spacecraft, rovers, landers, and Martian meteoritic studies—all stored in NASA’s Planetary Data System (PDS)—in reevaluating the fate of Mars' water. The results offer a model in which chemical weathering of water with rocks on Mars' surface (crust) ultimately sequestered a substantial—or even overwhelming—amount of Mars' water. On Mars, as on Earth, chemical reactions between water and exposed rock trap water inside the rock. This water has remained trapped in Mars’ crust, however, because the Red Planet does not have tectonic plates like Earth does. Earth's lithosphere is fragmented into over a dozen plates. Throughout Earth's history, some of those plates have continued to grind against and dive under each other at subduction zones. Where they are driven down lower into Earth's mantle, the plates subsequently melt and release the water stored in their crustal rocks. Through volcanism, that water is then returned to Earth’s atmosphere and overall hydrological cycle. This recycling of water does not happen on Mars, though; instead, weathering is a one-way process. Accordingly, the minerals of the Red Planet's static crust are likely loaded with Mars’ original water. The model comprising this theory—and which also accounts for expected atmospheric loss of hydrogen—estimates that at least 30% and as much as 99% of the original Martian water budget has been locked away in Mars’ crust. See also: Lithosphere; Plate tectonics; Rock; Subduction zone; Volcano; Weathering processes
Understanding the history of Martian water is critical for gauging the planet's climate and habitability, given the central role water plays in moderating Earth's climate and in the chemistry necessary for life. By continuing to study hydrated minerals on the Martian surface, the Perseverance rover, which landed in February 2021, will put old and new theories about the fate of Martian water to the test. Perseverance will also further the search for signs of past life, as well as evidence of active habitation by microbes. See also: Astrobiology; Climatology