NASA’s Curiosity rover is currently conducting science operations on the surface of the Red Planet in Gale Crater. Scientists have long believed that this crater was once an ancient lake, and many of the rocks Curiosity is rolling over were once sediments at the bottom of that lake. A new study was published this week that helps to add to the understanding of where the rock record preserved or destroyed evidence of a moist past on the surface of Mars and potential signs of ancient life.
Tom Bristow, CheMin principal investigator and the lead author of the new research paper, says that scientists used to think that the layers of clay minerals formed at the bottom of a lake in the crater and stayed that way, preserving the moment in time where they were created. However, the team has now found that later salty brine broke down the clay minerals in some areas and reset the rock record. The significant amount of undisturbed layers of rock is one specific reason that Gale Crater was chosen as the landing site for Curiosity.
The team was surprised to find that in one area, about half the clay minerals expected to be discovered were missing. Rather than those clay minerals, researchers discovered mudstone rich in iron oxide. The iron oxide is what gives Mars its iconic red color. Researchers knew the mudstone sampled was about the same age and started at about the same time, and was loaded with clay.
As Curiosity explored the sedimentary clay deposits along the crater, scientists wondered why patches of those clay minerals and the evidence they held disappeared. Scientists knew that diagenesis occurred on Mars, which is a process where some mudstone pockets experience different conditions and processes due to interaction with water changing their mineralogy. Diagenesis can erase the previous history recorded in the soil and write a new one.
The upside to the disappearance of the rock record is that the process of diagenesis on Earth can create unique habitats where microbes thrive, known as deep biospheres. If scientists can find similar deep biospheres on Mars, they would potentially hold signs of past microbial life.