At the dawn of the 21st century, wh

At the dawn of the 21st century, when the sedimentary deep past of Mars was first beginning to be appreciated, geologists realised that the aeolian landscape of Gale crater offered a pair of attributes that made it a particularly promising site for study. The sedimentary strata of lower Mount Sharp might record both the planet’s early environment and the chemical changes that accompanied its subsequent desiccation. And the adjacent crater floor offered a nice big patch of smooth terrain on which a spacecraft could land. So on August 6th 2012 a hovering “sky crane” that had parachuted down through the Martian sky lowered a rover called Curiosity onto the surface. The sediments that had been hoped for were there in abundance. “I’d expected mudstones,” said one geologist. “I hadn’t expected to spend most of the mission looking at them.” Within a couple of years Curiosity had confirmed that the inside of Gale crater really had been a lake, one with a rich history of its own. It had emptied and filled up again. There were coarser rocks closer to where the sediments had washed down from the rim; there were what seemed to be stream beds filled in by later muds, their rocks preserving ripples created by the gentle flow of water aeons ago. It was the best evidence ever seen of a habitable environment beyond Earth.