On Mars, phyllosilicate (clay) minerals are widespread in terrains that date back to the Noachian period, 4.1 to 3.7 billion years ago. Phyllosilicates are formed by the interaction of water with volcanic rock, leading many planetary researchers to conclude that there must have been sustained surface water, groundwater or active hydrothermal systems at some point in Martian history. But new research from Brown University suggests that the clays may have formed during the creation of the Martian crust itself, long before any water flowed on the Red Planet.
“In the very early Solar System, Mars and other rocky planets are thought to have been covered by oceans of molten magma,” said lead author Dr. Kevin Cannon and colleagues.
“As the Mars magma ocean began to cool and solidify, water and other dissolved volatiles would be outgassed to the surface, forming a thick, steamy atmosphere surrounding the planet.”
“The moisture and heat from that high-pressure steam bath would have converted vast swaths of the newly solidified surface to clay.”
“As the planet then evolved over billions of years, volcanic activity and asteroid bombardments would have covered the clays in some places and excavated them in others, leading to the widespread but patchy distribution seen on the surface today.”
“The basic recipe for making clay is you take rock and you add heat and water. This primordial atmosphere created by a magma ocean would have been the hottest and wettest Mars ever was. It’s a situation where you could pervasively alter the crust and then just shuffle those materials around afterward,” Dr. Cannon said.
This scenario offers a means of creating widespread clay deposits that doesn’t require a warm and wet climate or a sustained hydrothermal system on early Mars, according to the team.
Climate models suggest an early Mars where the temperature rarely crept above freezing and where water flow on the surface was sporadic and isolated.
“One of the complications that comes up in Mars evolution is that we can’t create a scenario where surface weathering had the capacity to produce the extent of mineral alteration that we see,” said Professor Jack Mustard.
“We’re certainly not trying to discount other alteration mechanisms entirely. Surface weathering and other types of alteration surely occurred at different points in Martian history, but we think this is a plausible way to explain much of the widespread clay we see in the oldest Martian terrains.”
To demonstrate that the mechanism they propose is plausible, the authors synthesized rock samples matching the composition of Martian basalt.
They then used a high-pressure device to recreate temperature and pressure conditions the may have been present amid the steam atmosphere created by a magma ocean.
After cooking samples for two weeks, the scientists checked to see if they had been altered and to what extent.
“The steam atmosphere associated with a magma ocean could have survived for as long as 10 million years or more,” they said.
“That would have been long enough, they estimate, to create as much as 2 miles (3 km) 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 2-mile 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% of the oldest crust exposures on Mars,” Dr. Cannon said.
“We’re finding about that same order of magnitude in these models.”
The research appears in the journal Nature.
Kevin M. Cannon et al. 2017. Primordial clays on Mars formed beneath a steam or supercritical atmosphere. Nature 552: 88-91; doi: 10.1038/nature24657