The steam atmosphere associated with a magma ocean could have survived for as long as 10 million years or more, Cannon and his colleagues say. That would have been long enough, they estimate, to create as much as three kilometers of clay on the primordial Martian surface.
To get an idea what the fate of that clay might be as the planet evolved, the researchers created a computer model to simulate a slab of Martian crust with a three-kilometer clay layer on top. Then they simulated the first billion years of Martian geologic history — the period when volcanic activity and asteroid bombardment were most prevalent. The model showed that the burial, excavation and scattering of clays over time created distribution of exposed deposits similar to what’s seen on Mars today.
“To put some numbers on it, clays cover about 3 percent of the oldest crust exposures on Mars,” Cannon said. “We’re finding about that same order of magnitude in these models.”
The lab experiments and simulations can’t say for certain that this scenario occurred, the researchers say, but they do suggest a strong hypothesis that could be tested during future Mars exploration.
“One of the things I like about this is that it’s truly testable,” said Steve Parman, a geology professor at Brown and co-author of the study. “With a returned sample, or maybe even with the analytical equipment on a rover, I’m optimistic that you could distinguish this primordial process from some other alteration process.”
If the process did indeed occur, it could have some interesting implications for early Martian history. In addition to providing a mechanism for clay formation even if Mars was as cold and icy as climate models suggest, the scenario suggests that vast deposits of clay were — and might still be — present beneath the surface. Those deposits could explain why the Martian crust is less dense than expected for a basaltic crust, the researchers say. The deposits would also serve as large underground storage reservoirs for water.
“There potentially would have been quite a lot of water locked up in these buried clays,” Parman said. “You could imagine that if those deposits were heated up by magmatism or some other process they would have released that water, perhaps providing a transient water supply to the surface. That could have implications for past habitability.”
Mustard, who chaired the committee that laid out the science goals for NASA’s Mars 2020 rover, hopes this new hypothesis could inform future Martian exploration.
“This would be a really interesting hypothesis to test,” he said. “Depending on where the rover ultimately lands, I think we could get the right samples to illuminate these questions.”