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As NASA’s Curiosity rover treks up a three-mile-high mountain on Mars, the rocks are changing. That says something about how the planet’s climate and environment changed more than three billion years ago — but scientists are not sure what.

Since it landed more than three years ago in a 96-mile-wide depression known as Gale Crater, Curiosity has made a number of discoveries, notably that the crater once held lakes of fresh water. For most of that time, the rocks it encountered were generally basaltic, a volcanic composition typical on Mars.

“Now in the recent few months, that has changed,” Ashwin R. Vasavada, the project scientist for the mission, said at a news conference on Thursday at a meeting of the American Geophysical Union in San Francisco, where researchers were presenting some of their newest results.

They have surprising clues but no definitive story, yet.

The attraction of Gale Crater to planetary scientists is the mountain at the center. A space rock slamming into Mars created the crater about 3.6 billion years ago. It then filled with sediment, which was subsequently carved away by the wind, leaving behind the formation known as Mount Sharp.

Each layer of sedimentary rock tells something about the geological conditions at the time the rock formed, meaning that Curiosity, which arrived at the base of the mountain in September 2014, is in a sense moving forward through the geological history of Mars as it climbs.

What has caught the attention of Dr. Vasavada and his colleagues lately is silica, a class of minerals made of silicon and oxygen. The evidence points to the action of liquid water even after the lakes disappeared.

“Groundwater passed through the rock multiple times, leaving different chemical signatures behind,” Dr. Vasavada said.

Basalt is generally half silica. Curiosity has been examining two rock units: one a mudstone of lake bed deposits, among the oldest rocks the rover will examine, and the other a sandstone of coarse grains that were blown in and draped onto the mountain. “It probably is among the youngest rocks we’ll encounter on the mission,” Dr. Vasavada said.

In the mudstone and the sandstone, Curiosity found much higher levels of silica, up to 90 percent more than it had observed previously in basaltic rocks.

“All of this we’re just beginning to piece together and understand,” Dr. Vasavada said.

After arriving at a spot the scientists named Marias Pass, an intersection between the older mudstone and younger sandstone near the base of the mountain, Curiosity spied a patch of light-toned bedrock, part of the mudstone. It fired a laser to vaporize the rock in several places; the instrument identifies the constituent elements from the colors of light given off. Then Curiosity drove off to do more science elsewhere.

Back on Earth, scientists analyzing the data realized this was something different: It turned out to be the first of the high-silica rocks.

“The science team decided to make the rare decision to go back and investigate this more,” said Jens Frydenvang, a member of the science team from Los Alamos National Laboratory and from the University of Copenhagen.


Curiosity drilled into a rock target called Buckskin and scooped up the bits for more detailed examination with its onboard chemistry laboratory. Silica comes in many different forms, and the particular form can tell the conditions when the rock became a rock.

“So we all placed friendly bets on what sort of silica phase we would find,” said Elizabeth Rampe, another member of the science team for Curiosity. “But we could never have predicted this result.”

It was tridymite, a mineral that is rare on Earth and has never seen before on Mars. “And we actually found a lot of it,” Dr. Rampe said.

On Earth, tridymite generally forms at high temperatures in volcanic or metamorphic rocks, not a finely layered sedimentary rock like Buckskin. That may tell something about the origin of the sediments, or it is possible that tridymite forms through a different process on Mars.

In the younger sandstone, the scientists found a different type of silica known as Opal-A along fractures in the rocks.

The scientists hypothesize two possibilities: acidic water washed away the other elements, or neutral water washed in silica that accumulated in the sandstone. “They both involve liquid water,” said Albert Yen of NASA’s Jet Propulsion Laboratory, another science team member. “We’re just trying to figure out the flavor of the water.”

The scientists did not present new findings on organics, the carbon-based molecules that could serve as the building blocks for life. Dr. Vasavada said that signs of organics had been spotted, but the scientists were still analyzing them.

“Stay tuned,” he said. “There are organics in several of these samples we’ve been seeing lately.”

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