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Let's go to Mars

Gale Crater held a lake you could have drunk from

Curiosity's first drill hole, 6.4 cm into a mudstone called John Klein, produced clay, sulfur and nitrogen — and an ancient lake of fresh, neutral water.

A greyscale wide-angle view of flat martian ground, projected onto a black surround with a faint grid. At the left, a low ledge of flat-lying rock stands above a sandy slope scattered with loose angular blocks. Across the centre and right, a pale rock pavement broken by a network of cracks into broad, irregular slabs runs to a level horizon. The rover's arm and deck hardware fill the bottom right corner, with the rover's shadow on the ground beside them, and seams between the frames of the mosaic show along the sky.
The edge of Yellowknife Bay, photographed by Curiosity's Navigation Camera on sol 130, 17 December 2012. The rover drove to an edge of the shallow depression to record the ledge at the margin and this view across the bay. · Image: NASA/JPL-Caltech

Curiosity first drilled into Mars for a sample on 8 February 2013, sol 182, into pale mudstone named John Klein after a deputy project manager who had died two years earlier. The hole was 1.6 centimetres across and 6.4 deep; the powder was grey, not red, its iron never thoroughly oxidised.

NASA announced its contents on 12 March 2013. The CheMin X-ray diffractometer found smectite clay, which forms in water that is not acidic. APXS, ChemCam, CheMin and SAM found sulfur, nitrogen, hydrogen, oxygen, phosphorus and carbon, the sulfur in both oxidised and reduced forms — a chemical gradient terrestrial microbes make a living from. Calcium sulfate veins showed water had moved through the rock after the mud was laid down. The water had been fresh: near-neutral pH, low salinity, not harshly oxidising. The project scientist, John Grotzinger, told the press conference you could have drunk it.

Mars had once been habitable: not inhabited — no fossils, no life, at that point not even organic molecules — but a place life as we understand it could have lived.

In January 2014 Ken Farley's team reported the first radiometric age measured on another planet's surface: potassium-argon on Cumberland, a sample from the same mudstone, gave 4.21 billion years give or take 0.35. That dates the rock the grains eroded from, not the lake — a ceiling on the mudstone, not a birthday. Cosmic-ray exposure showed the rock had been shielded until roughly 78 million years ago, which is why organic molecules might survive in it, and why future missions should drill where erosion has recently stripped the surface. The sedimentology showed rivers running into a lake that stayed wet through the interval examined, with drying and refilling inferred over a stretch later argued to be thousands to millions of years.

After Mariner 9's dry channels, Viking's ambiguous soil and Opportunity's acidic brines, Yellowknife Bay was the first martian environment describable in the same ordinary terms as an Earth lake bed.