Cannon acknowledged that his theory doesn’t explain the presence of minerals such as hematite on the surface, at least not yet. An iron-oxide mineral, hematite generally forms in water-wet environments, such as from groundwater. It’s been found at the landing spots for the Opportunity and Curiosity rovers.
“That might be a different story,” Cannon said. “We didn’t really address that in the paper.”
The research genesis came from previous work by Lindy Elkins-Tanton, a planetary scientist at Arizona State University who specializes in planetary formation and evolution. (Elkins-Tanton is also principal investigator of the newly selected NASA Psyche mission. After launch in 2022, the spacecraft will explore the metallic asteroid 16 Psyche to learn more about planetary cores.)
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Elkins-Tanton’s papers showed that most rocky planets had a completely global layer of molten rock when they formed. As the rock cooled, outgassing from the surface interacted with the crust. Cannon’s team connected her theories with the clays observed on Mars, he said.
“We think that this process may have happened on other planets like Earth and Venus,” he added, noting that other researchers have suggested that Earth had an early steam atmosphere as well. The old surfaces of Earth and Venus, however, have been almost completely obliterated by geological processes such as plate tectonics, erosion, and volcanism.
Mars is an environment where old surfaces are preserved, allowing scientists to make extrapolations for planets in our own solar system — or even exoplanets. But Cannon cautioned it would be a while before telescopes can look for clay minerals on small, rocky exoplanets.
“With the James Webb Space Telescope” — which launches in 2019 — “we may be able to see outgassed atmospheres,” he said.
Other evidence could come from the Mars 2020 mission, a rover which is tasked with picking up and caching samples on the Martian surface for a potential sample-return mission. If the clays formed from a steamy atmosphere, researchers would expect to see noble gases such as krypton or xenon. But given Mars 2020’s instruments, which aren’t necessarily designed to search for these gases, Cannon suggested that samples may need to come back to Earth in order to provide definitive answers.