What were some of the most recent discoveries as Spirit and Opportunity completed their main missions?
By looking at the rocks in the craters we have discovered that a shallow salty lake existed there for some time. We are not sure how long it was present. It is believed (but not proven) that the water has all evaporated and escaped from the gravity field of Mars. What is exciting is that the presence of water for some time may mean that primitive bacteria-like life may have been present on Mars. We need to evaluate more rocks from different periods of time to get a better picture of the history of the water. Some of the other, deeper craters that are near Spirit and Opportunity seem to have older rocks exposed that may give more information. The risk of climbing down into those craters is being evaluated.