Scientists have long assumed that Mars has changed over time. Currently dry with temperatures rarely above freezing, Mars has a very thin atmosphere that is likely incapable of retaining water, which is why scientists expect that any water the planet does have to be underground.
But according to NASA’s Mars Science Laboratory (MSL), Mars may have been very, very different billions of years ago. They think it may have had a much thicker, heavier atmosphere capable of supporting streams, lakes, and maybe even oceans. They think that because, thanks to the Mars Curiosity Rover, they’ve found distinct patterns in the sediments of the Gale Crater where Curiosity has been spending most of the last three years. The way the sediments have been deposited leads them to believe that the area must have been a lake at some point, at least intermittently.
They aren’t sure how deep that lake could have been, but they can tell that there is a point where the sediments of Mount Sharp, which borders it along one side, stop looking like they were deposited by water, and start looking like they were deposited by wind or erosion. That basically indicates a maximum depth.
The problem, though, is that current geology doesn’t really agree with this wetter ancient Mars, and the evidence Curiosity is finding is proving difficult to reconcile with other evidence we have about Mars. That may sound difficult, but in science, things like this happen. According to John Grotzinger, who was lead author on the paper that published MSL’s finding, things like this have happened in our own past.
Namely, scientists started noticing that there were a lot of similarities between the continents that implied they shared a spatial past, that they were, somehow, closer to each other. It took a while to discover plate tectonics and realize that the continents had been part of a super continent before being pushed apart. Grotzinger thinks that we need to find that missing piece to explain Mars’ transition.