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Sediment yields clues on Martian lake effect

More than 3.5 billion years ago, a meteor slammed into Mars near its equator, carving a 96-mile depression now known as Gale Crater. That was unremarkable. Back then, Mars, Earth and other bodies in the inner solar system were regularly pummeled by space rocks, leaving crater scars large and small.

What was remarkable was what happened after the impact.

Even though planetary scientists disagree on exactly what that was, they can clearly see the result: a mountain rising more than 3 miles from the floor of Gale. More remarkable still, the mountain is layer upon layer of sedimentary rock.

The layered rock drew the attention of the scientists who chose Gale as the destination for NASA’s Curiosity rover, a mobile laboratory the size of a Mini Cooper. Now, more than two years after arriving on Mars, Curiosity is climbing the mountain.

In sedimentary rock, each layer encases the geological conditions of the time it formed, each a page from the book of Mars’ history. As Curiosity traverses the layers, scientists working on the $2.5 billion mission hope to read the story of how young Mars, apparently once much warmer and wetter, turned dry and cold in what John P. Grotzinger, the project scientist, calls “the great desiccation event.”

Grotzinger remembers the first time he heard about Gale. “I looked at it, and immediately I’m like, ‘This is a fantastic site,’ ” he said. “What’s that mountain in the middle?”

Officially, the name is Aeolis Mons, but mission scientists call it Mount Sharp in homage to Robert P. Sharp, a prominent geologist and Mars expert who died in 2004.

On Earth, mountains rise out of volcanic eruptions or are pushed upward by plate tectonics, the collision of pieces of the planet’s crust.

Mars lacks plate tectonics, and volcanoes do not spew out of sedimentary rock. So how did this 18,000-foot mountain form?

In the late 1990s, NASA’s Mars Global Surveyor spacecraft was sending back images of the Martian surface far sharper than those from earlier missions, like Mariner and Viking.

Kenneth S. Edgett and Michael C. Malin of Malin Space Science Systems, the San Diego company that built Global Surveyor’s camera, saw fine layered deposits at many places on Mars, including Gale. In 2000, they offered the hypothesis that they were sedimentary, cemented into rock.

Indeed, Edgett said, it appeared that Gale Crater had been fully buried with sediment and that later winds excavated most of it, leaving the mountain in the middle.

Grotzinger asked Ralph E. Milliken, then a postdoc in his research group at Caltech, to take a closer look at Gale. With data from an instrument on NASA’s Mars Reconnaissance Orbiter that can identify minerals in the rocks below, Milliken showed the presence of clays at the base of Mount Sharp as well as other minerals that most likely formed in the presence of water.

“The fact we have this mountain, and it’s not all the same stuff – the mineralogy is changing from one layer to the next – that gives us the hope that maybe those minerals are recording the interaction of the water and the atmosphere and the rocks,” said Milliken, now a geologist at Brown.

Were water conditions there becoming more acidic? Was there oxygen in the water? “That’s something we can assess with the rover on the ground,” Milliken said.

Since its landing on Mars in August 2012, Curiosity took a detour to explore a section named Yellowknife Bay and discovered geological signs that Gale was once habitable, perhaps a freshwater lake.

After that, the rover drove to Mount Sharp, with only brief stops for science. To date, the rover, operated by NASA’s Jet Propulsion Laboratory in Pasadena, California, has driven more than 6 miles, taken more than 104,000 pictures and fired more than 188,000 shots from a laser instrument that vaporizes rock and dirt to identify what they are made of.

In September, Curiosity drilled its first hole in an outcrop of Mount Sharp and identified the iron mineral hematite in a rock. That was the first confirmation on the ground for a Gale mineral that had been first identified from orbit.

When Curiosity reaches rocks containing clays, which form in waters with a neutral pH, that will be the most promising place to look for organic molecules, the carbon compounds that could serve as the building blocks of life, particularly if the rover can maneuver into a spot shielded from radiation. The orbiter also detected magnesium sulfate salts, which Milliken described as possibly similar to Epsom salts.