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Mars had two wet eras, Curiosity rover tells us

At the European Planetary Science Congress, researchers summed up the flood of evidence that Mars was once a wet planet.

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"It’s amazing that we found a mudstone," said presenter Aileen Yingst, a Curiosity science team member from the Planetary Science Institute in Tucson, Ariz. "Mudstones mean that you have very fine grains inside the rock — meaning that these grains settle down slowly. On Earth, that usually means that it happened because of wind or water. And we think that it was probably water."

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Researchers think the mudstone formed in a place where water was calm, such as a lake — perhaps an ideal place for microbes to survive and reproduce. [Curiosity's 7 Biggest Achievements (So Far)]

"If you’re a microbe that is trying to get a hold on a place to live, you don’t necessarily want to be living in churning water; it’s not good for you to start growing and thriving," Yingst said. "[Calm lake] water is a better place to live."

Another rock that received a lot of attention at the conference is Tintina, a tiny pebble that Curiosity rolled over and broke apart.

The small piece of rock revealed a snowy-white interior, strongly hinting at the presence of hydrated minerals that formed when water flowed through Gale Crater billions of years ago.

Flowing rivers

More strong evidence of Mars' wet past comes from Curiosity’s discovery of calcium sulfate veins — fissures in the surface rock that, once sampled with a laser-firing instrument called ChemCam, were shown to contain sulfate.

"If you have veins, then you have had water that has some sort of rock-forming mineral solution that has dissolved in the water, transported somewhere else and then deposited again. So it’s just another indicator that you’ve had about water activity," Yingst said.

And then there are the ancient river deposits. 

One such feature of the Martian landscape, studied by Curiosity while it was at a small depression near its landing site called Yellowknife Bay, is an outcrop researchers dubbed Shaler.

Shaler is an example of cross-stratification, which comprises thin, inclined layers of sediment. Features resembling Shaler are commonly formed by rivers here on Earth; turbulent water creates "dunes" on the riverbed, which slowly migrate in the direction of the current.

What Curiosity has seen are the remnants of that migration process, researchers said.

"The grain size there are small pebbles and coarse sand grains, too large to be lifted up and transported by wind, so the only way we can produce these dunes is by water flow," said presenter Sanjeev Gupta from Imperial College London, a member of the Curiosity team.

"And these look exactly like the sort of features I’ve looked at on Earth formed by ancient rivers," Gupta added. "So we can tell that these outcrops are clear evidence for sustained water transport and dune migration. When they are preserved, they are recording minutes to hours of motion, and they’ve been preserved for millions to billions of years."

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