Mars used to be wet. That’s the overwhelming conclusion of the last two decades of Martian geological research based on data recorded by multiple probes and landers. It took us a lot longer to confirm the planet’s past state than its present one — we’ve had strong evidence suggesting arid conditions on Mars dating back to 1894, with additional confirming evidence gathered over the next 30 years. The current model for Mars’ transformation from a wet planet to a dry one relies on sublimation and the long-term damage our sun did to the Martian atmosphere after the planet’s global magnetic field shut down. But what if that’s not true?
That’s the argument presented in a new paper, which argues that the waters of Mars may not have technically gone anywhere.
The evidence we’ve gathered from rovers like Perseverance, Curiosity, Opportunity, and Spirit, together with planetary observations from orbiting satellites, collectively suggests that Mars enjoyed a relatively warm, wet era from 4.1B to 3.7B years ago. Some of the evidence for the so-called Late Heavy Bombardment comes from the large number of craters on Mars and the Moon that appear to have been created during this period. Craters formed during this era have eroded edges similar to what we would expect from flowing water. Craters created later, during the Hesperian period (~3.7 – 3 billion years ago), are much less eroded.
Several factors allowed Mars to hold an atmosphere during this period. The immense series of volcanos known as the Tharsis Bulge were actively under construction. The Tharsis Bulge is a volcanic province approximately the size of North America. The total amount of CO2 released during the Tharsis eruptions is thought to be sufficient to form a 1.5-bar atmosphere on Mars, with a global ocean up to 120m provided solely by this source. Tharsis is large enough that its formation may have caused Mars to tilt over to one side and changed the location of the poles in the process. Massive impacts could have delivered additional water of their own, and early Mars might have held enough surface water to cover the surface of the planet to a depth of 1,500 meters. These uncertainties are why the Global Equivalent Layer (GEL) estimates are so variable.
One reason scientists think Mars’ water evaporated is that the Martian atmosphere and water samples taken by Curiosity both show a surplus of deuterium relative to hydrogen compared with what we’d find on Earth. This suggests that lighter ordinary hydrogen was preferentially lost to space, while the heavier deuterium isotope remained.
The problem with the sublimation/atmospheric loss model is that current rates of loss are not high enough to account for the scope of Mars’ transformation over the past few billion years. The solar wind is known to have played a long-term role, but how do we account for the rest? One theory is that mass loss rates were much higher in the past. These researchers are suggesting that much of Mars’ water stayed right where it was and became bound up in surface minerals instead.
We’re not talking about the idea of a layer of liquid preserved beneath the surface. The research report discusses “crustal hydration through irreversible chemical weathering, in which water and/or hydroxyl are incorporated into minerals.” The water is not available for other purposes on Mars; it’s locked directly within the crystalline structure of the minerals themselves.
Herein lies a critical difference between Mars and Earth. Mars has what’s known as a stagnant lid tectonic system, meaning that there are no plate tectonics and there is no system of recycling rock — or, critically, water. On Earth, plate tectonics carries water deep into the mantle while mid-oceanic volcanic vents return it to the oceans. This is called the deep water cycle.
So long as Mars’ volcanoes kept erupting, it maintained a deep water cycle of its own. Once that process began to slow, water began a one-way sequestration trip into the planet’s crust. Impacts and fading eruptions would have maintained a colder climate with at least intermittent liquid water for a long period of time — Mars dried out over several hundred million years — but the end of volcanism may have allowed between a third and almost all of Mars’ water to flow into the ground and form hydrous minerals. This means, by extension, that Mars’ water reserves are much higher than previously thought, though locked in a form we wouldn’t find all that useful.
We may not know if the report is accurate until if and when humans are able to conduct widescale geological surveys of subsurface rock, but it’s an alternate model for how Mars lost its atmosphere that explains current conditions well. It would also further imply that features of Earth such as plate tectonics may be vital to the long-term preservation of a biosphere capable of supporting intelligent life.
Image by Ittiz, CC BY-SA 3.0