The Moon and Mars give up more secrets

THIS PICTURENASA of a set of sedimentary rocks, just published in , could have come from any geology textbook. Its illustration of the bottomset-foreset-topset transition found in river deltas is a classic of stratigraphy. Except that, technically, it is not a geological feature at all. The word “geology” derives from the Greek for “Earth discourse”. The rocks in question, however, are on Mars.Specifically, they are in Jezero crater, which was once, when Mars had liquid water, a lake that had rivers flowing into it. The photograph was taken by , ’s latest Mars rover, which landed in February, and adds to understanding of Mars’s watery past. The three groups of rocks—the gently inclined bottomset, with a more steeply sloping foreset above and a horizontal topset capping the lot—show, as might be expected, that Martian river deltas were similar to terrestrial ones.