Today, NASA announced that its Curiosity rover might have stumbled upon the most compelling evidence for water seen yet.

Richard Cook, project manager at Mars Science Laboratory, said in a press conference today that 158 days into its mission on the surface of Mars, Curiosity is healthy and finally ready to drill—a process he admits NASA has been promising for a while. The engineering tasks required before drilling commences are all complete, he says. And more importantly, Curiosity has landed in a better drilling spot than anyone had hoped for. In the Yellowknife Bay area, which is at the bottom of the depression next to where Curiosity landed, scientists have noticed a certain set of conditions that highly suggest the presence of flowing water at some time in Mars' history.

Photos Curiosity has taken so far show that the rocks are littered with linear cracks filled with minerals. These veins, which the rover's ChemCam tool suggests are probably calcium sulfate, are similar in composition to gypsum veins in rocks on earth, according to John Grotzinger, the project scientist. "What these veins tell us is that water percolated through these rocks, and then minerals precipitated to fill them," he said today.

The minerals are what make the find truly exciting, Grotzinger said. Mineral precipitation indicates "rock has been dissolved somewhere else on the planet, so that the ions were carried by the fluid. They become geochemical recorders of the past composition of that fluid." In other words, further investigation of these mineral deposits could tell scientists what the water bodies Mars formerly were like, both chemically and geologically. The area, Grotzinger says, is "literally shot through" with these fractures. He and his colleagues call the find a "jackpot."

When asked if researchers knew how long the water had been flowing, Grotzinger said it was "too soon to tell" and stressed that NASA scientists were excited, but that their work in the bay had only just begun.

"Yellowknife Bay" inside Gale Crater on Mars. In the zoomed-in image on the right, white arrows point to veins (including some under the overhang), and black arrows point to concretions (small spherical concentrations of minerals). Both veins and concretions strongly suggest precipitation of minerals from water. Credit: NASA/JPL-Caltech/MSSS.

The mineral veins aren't the only indication of water in the area. Higher up the depression, rocks show linear grain marks that seem to be too course to have been made by wind, Grotzinger said. While nothing is certain yet, he estimates that a stream flow of at least 10 cm per second would have been necessary to produce the marks.

R. Aileen Yingst, who works with the Mars Hand Lens Imager, said that the grains of sediment, which have rounded edges, suggest the same idea. "There's a different between round and rounded," she says, "and these grains have been knocked around, they've been rounded. That indicates that we're looking at the effects of water."

Yellowknife Bay wasn't one of Curiosity's original destinations. Scientists steered the rover to the area because of a change in ground temperature they had noticed. As the rover traveled from Bradbury Landing to Yellowknife Bay, where terrain is lighter in tone, the daily temperature range became significantly smaller. Hoping to examine this anomaly, the Curiosity team ended up a region ripe for exploration. Grotzinger called the incident "a great example of the serendipity of scientific discovery." The area will be named after John Kline, who was deputy project manager for Curiosity until his death in 2011.

Grotzinger said these indications of water take off some of the pressure the team might have felt to move on quickly. Curiosity will eventually make its way to Mount Sharp, its intended destination. But it may stay in Yellowknife Bay longer than expected. There's a lot to discover there, he said, and NASA wants to be sure to take the time to do it right.

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