After imaging during the holidays, NASA's Mars rover Curiosity resumed driving Jan. 3 and pulled within arm's reach of a sinuous rock feature called "Snake River," a thin curving line of darker rock cutting through flatter rocks and jutting above sand. Curiosity's science team plans to get a closer look at it before proceeding to other nearby rocks.
"It's one piece of the puzzle," said the mission's project scientist, John Grotzinger of the California Institute of Technology in Pasadena. "It has a crosscutting relationship to the surrounding rock and appears to have formed after the deposition of the layer that it transects."
The drive during the mission's 147th Martian day, or sol, on the Red Planet took Curiosity about 10 feet (3 meters) northwestward and brought the mission's total driving distance to 2,303 feet (702 meters). The rover is within a shallow depression called "Yellowknife Bay," which is a flatter and lighter-toned type of terrain from what the mission crossed during its first four months inside Gale Crater.
During a holiday break for the rover team, Curiosity stayed at a location within Yellowknife Bay from which the rover took images of its surroundings. The team is evaluating possible first targets for use of Curiosity's hammering drill in coming weeks. The drill will collect powdered samples from the interior of rocks for analysis by instruments inside the rover.
"We had no surprises over the holidays," said the mission's project manager, Richard Cook of NASA's Jet Propulsion Laboratory, Pasadena. "Now, Curiosity is back on the move. The area the rover is in looks good for our first drilling target."
NASA's Mars Science Laboratory Project is using Curiosity to assess whether areas inside Gale Crater ever offered a habitable environment for microbes. JPL, a division of Caltech, manages the project for NASA's Science Mission Directorate in Washington.
Near the center of Gale Crater (about the size of Connecticut), hundreds of exposed rock layers form a tower as tall as the Rockies and reveal a record of major environmental changes on Mars billions of years ago.
Gale crater, like the Gusev crater, sits near the boundary between Mars' southern highlands and northern lowlands. Its interior is filled with layer upon layer of rocks standing 5.5 kilometers (3.4 miles) higher than the northern crater floor, but it is unknown whether these are sediments came in with ancient floods, or lava flows, or windblown ash deposits.
The geological history told by the rock layers matches what has been proposed in recent years as the dominant planet-wide pattern for early Mars, according to a new report by geologists using instruments on NASA's Mars Reconnaissance Orbiter.
"Looking at the layers from the bottom to the top, from the oldest to the youngest, you see a sequence of changing rocks that resulted from changes in environmental conditions through time," said Ralph Milliken of NASA's Jet Propulsion Laboratory, Pasadena, Calif. "This thick sequence of rocks appears to be showing different steps in the drying-out of Mars."
Using geological layers to understand stages in the evolution of a planet's climate has a precedent on Earth. A change about 1.8 billion years ago in the types of rock layers formed on Earth became a key to understanding a dramatic change in Earth's ancient atmosphere.
Milliken and his colleagues found that clay minerals, which form under very wet conditions, are concentrated in layers near the bottom of the Gale stack. Above that, sulfate minerals are intermixed with the clays. Sulfates form in wet conditions and can be deposited when the water in which they are dissolved evaporates. Higher still are sulfate-containing layers without detectable clays. And at the top is a thick formation of regularly spaced layers bearing no detectable water-related minerals.
Rock exposures with compositions like various layers of the Gale stack have been mapped elsewhere on Mars, and researchers, including Jean-Pierre Bibring of the University of Paris, have proposed a Martian planetary chronology of clay-producing conditions followed by sulfate-producing conditions followed by dry conditions. However, Gale is the first location where a single series of layers has been found to contain these clues in a clearly defined sequence from older rocks to younger rocks.
"If you could stand there, you would see this beautiful formation of Martian sediments laid down in the past, a stratigraphic section that's more than twice the height of the Grand Canyon, though not as steep," said Bradley Thomson of the Johns Hopkins University Applied Physics Laboratory.
NASA selected Gale Crater in 2008 as one of four finalist sites for the Mars Science Laboratory rover, Curiosity, which has a planned launch in 2011. The finalist sites all have exposures of water-related minerals, and each has attributes that distinguish it from the others.
Casey Kazan via NASA
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