Martian surface: icy, cold, and dry for 4 billion years

Maybe the warm and wet environment on early Mars that scientists have long proposed wasn’t at the surface, but rather buried in the crust. That’s one of the conclusions of a new review that looks at Martian clay minerals and the environments where they likely formed.

MOSTLY COLD AND DRY UP TOP, but warmer below? Mapping outcrops and exposures of clay minerals reveals they occur across nearly all of Mars, but their character differs. Crustal clays formed underground, while sedimentary clays were made at the surface, which has been warm and wet only rarely. (Image is Figure 1 from the paper.)

Scientists have now logged the detailed mineralogy at more than 350 places on Mars where clay minerals appear. By far the majority of these date from the earliest known period in Martian history, the Noachian, about 4 billion years ago to around 3.7 billion years ago. More important, the chemical nature of the clays suggest that they formed when warm groundwater came in contact with subsurface crustal rocks for long periods.

The review, by a team of scientists with lead author Bethany Ehlmann  (Caltech/Jet Propulsion Laboratory), was published in Nature November 3, 2011.

The team writes, “Evidence from the Martian rock record indicates that most clay minerals — specifically, those comprising Fe,Mg clay mineral units deep in the crust — formed in the subsurface in closed systems at temperatures ranging from ambient to low-grade hydrothermal (less than 400°C).”

The warmth, they note, could have come from any of several sources: volcanism, residual heat after large meteorite impacts, and a geothermal heat flow stronger than today’s.

Not all clays are likely to have formed underground, they say. Among the many clay deposits mapped across Mars are relatively rare ones rich in sulfates and chlorides. These appear to have been produced by weathering at the Martian surface, which washed the minerals into hollows, pockets, and craters. Also, a few locations with aluminum-rich clays at the top of the layers suggest that localized sources of water near the surface leached elements from basalts rich in iron and magnesium.

The new view of subsurface clay formation might ease a growing problem, namely that it has been getting difficult for scientists to argue for a Mars that was warm and wet at the surface for more than brief periods. The reason is that most of the Martian crust is not chemically leached, and evidence for a thick early atmosphere that would have sustained a warm climate has been hard to come by.

As the team writes, “Cold, arid conditions with only transient surface water may have characterized Mars’s surface for over 4 billion years, since the early-Noachian period, and the longest-duration aqueous, potentially habitable environments may have been in the subsurface.”

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