Curiosity heading to Mars’s major geochemical boundary
Gale Crater, the landing site for the Curiosity Rover, was chosen as the target for several reasons. First and foremost, the goal of this rover was to verify that Mars could contain a “Habitable environment”, by which scientists mean that there was fairly fresh water present for long times. The rover accomplished that mission in its first few months on Mars by verifying that it was sitting on lake sediments that contain clay minerals, both of which require the presence of substantial amounts of water for thousands of years or more.
The presence of clay minerals in Gale Crater was expected based on analyses from the CRISM spectrometer on the orbiting Mars Reconnaissance Orbiter spacecraft, but verifying their presence on the ground and seeing the type of rocks that held them was vital to verifying that Gale could have once held a habitable environment. But it turns out that other analyses from the CRISM spectrometer told us something else about Gale Crater, and we might get to see those rocks later this year.
Gale Crater contains a gigantic mound of layered sediments at its center. The official name of that 5-kilometer-high mound of sediments is Aeolis Mons, but it is also famously named Mount Sharp after Caltech Geologist Bob Sharp (who also contributed a lot to early planetary geology by working with Apollo mission astronauts). One of the rules from geology for how you interpret layered sedimentary rocks is the law of superposition, which basically says that in layered sedimentary rocks, the oldest layers are at the bottom and the youngest layers are on the top.
The Curiosity Rover landed at the bottom of Gale Crater, and it has been gradually starting to ascend Mount Sharp through a valley on the slope. That means the rocks at the bottom of the mound were the oldest, and the rover is now marching through Mars’s geologic history.
We know, in part thanks to the Curiosity Rover, that Mars once had a wet enough surface to support the presence of lakes. However, today Mars does not have lakes at its surface; the surface is dry, frozen, and salty. That means at some point, Mars underwent a permanent climate transition; it went from being wet and fairly temperate to dry, frozen, and salty. The water that was there must have gradually disappeared and dried up.
This type of transition is one that should be preserved in the rocks. Rocks that formed with substantial water should be clay-rich, while later rocks should be richer in salts. It turns out that Mount Sharp seems to have this boundary preserved right in the middle of its layers. The lowermost layers on Mount Sharp are clay rich, but higher up on the slopes there are layers richer in sulfates, which indicate higher salt contents.
In other words, as the rover climbs Mount Sharp, it should be able to see geologic evidence of the planet drying. This year, the Curiosity Rover is heading to that boundary, between the older clay-bearing unit and the younger sulfate-bearing, salty unit.
How will the layers differ across this boundary? Is the transition sharp or gradual? What other types of structures might we see in the salty unit that are different from the clay-rich unit? We might get to answer those questions this year.
-JBB
Image credits: Caltech/JPL/NASA/MSSS
https://mars.nasa.gov/resources/25097/curiositys-path-to-the-sulfate-bearing-unit/?site=msl
https://www.wikiwand.com/en/Mount_Sharp
