Mars used to be wet. That’s the overwhelming conclusion of the last two decades of geological research on Mars based on data recorded by multiple probes and landers. It took us much longer to confirm the state of the planet in the past than it does today – we’ve had strong evidence pointing to arid conditions on Mars dating back to 1894, with additional confirmatory evidence gathered over the next 30 years . The current model for Mars’s transformation from a wet to a dry planet is based on sublimation and the long-term damage our Sun caused to the Martian atmosphere after the planet’s global magnetic field was turned off. But what if that’s not true?
That’s the argument presented in a new paper, which states that the waters of Mars may have technically gone nowhere.
The evidence we’ve gathered from robbers, such as Persistence Curiosity Opportunity, and Spiritalong with planetary observations from satellites orbiting Earth, suggests that Mars experienced a relatively warm, wet era from 4.1 billion to 3.7 billion 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 formed during this period. Craters formed in this era have eroded edges, similar to what we would expect from running 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 volcanoes known as the Tharsis Bulge were actively under construction. The Tharsis Bulge is a volcanic province roughly the size of North America. The total amount of CO2 released during the Tharsis eruptions is believed to be enough to form a 1.5 bar atmosphere on Mars, with a global ocean of up to 120 meters supplied solely by this source. Tharsis is large enough that its formation tilted Mars to one side, changing the location of the poles. Huge impacts could have provided additional water on their own, and early Mars would have retained enough surface water to cover the planet’s surface to a depth of 1,500 meters. These uncertainties are why Global Equivalent Layer (GEL) estimates are so variable.
One reason scientists think the water from Mars has evaporated is that the atmosphere of Mars and water samples have been taken Curiosity both show a surplus of deuterium over hydrogen compared to what we would find on Earth. This suggests that lighter ordinary hydrogen was preferentially lost in space, while the heavier deuterium isotope remained.
The problem with the sublimation / atmospheric loss model is that the current loss rates are not high enough to account for the magnitude of Mars’s transformation over the past few billion years. The solar wind is known to have played a long-term role, but how do we explain the rest? One theory is that the percentage of mass loss was much higher in the past. These researchers suggest that much of Mars’ water stayed exactly where it was and was instead trapped in surface minerals.
As time went on, the water poured down and froze at the surface, while other water sublimated into space. Image from ScienceMag
We are not talking about the idea of a layer of liquid being kept under the surface. The research report discusses “crustal hydration through irreversible chemical weathering, in which water and / or hydroxyl is incorporated into minerals.” The water is not available for other purposes on Mars; it is directly confined within the crystalline structure of the minerals themselves.
Therein lies a crucial difference between Mars and Earth. Mars has what’s called a stagnant lid tectonic system, which means there is no plate tectonics and no system for recycling rock – or, critically, water. On Earth, plate tectonics carries water deep into the mantle, while volcanic vents in the middle of the ocean return it to the oceans. This is called the deep water cycle.
As long as Mars’ volcanoes continued to erupt, it maintained its own deep water cycle. Once that process began to slow, water began a one-way journey to the Earth’s crust. Influences and fading eruptions would have maintained a colder climate over a long period of time with at least intermittent liquid water – Mars dried up for several hundred million years – but the end of volcanism may have enabled between a third and nearly all of Mars’ water. to flow into the ground and form water-bearing minerals. This means, by extension, that Mars’ water resources are much higher than previously thought, although locked in a form we wouldn’t find as useful.
We may not know if the report is accurate until humans are able to conduct large-scale geological surveys of underground rocks, but it is an alternative model for how Mars lost its atmosphere that explains current conditions well. It would further imply that features of the Earth, such as plate tectonics, could be vital to the long-term maintenance of a biosphere that can support intelligent life.
Image by Ittiz, CC BY-SA 3.0
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