WASHINGTON U.-ST. LOUIS (US) — The Mars rover Opportunity is poised on the edge of the vast Endeavour Crater, sampling a rock unlike any other examined during its mission.
“Chester Lake” is the second rock the rover has sampled with a microscopic imager and a spectrometer since reaching its long-term destination, the rim of the Endeavour crater, in August. Unlike the first rock, which was a boulder tossed by excavation of a small crater on Endeavour’s rim, Chester Lake is an outcrop of bedrock.
The rocks at Endeavour apparently come from an earlier period of Martian history than the rocks that Opportunity examined during its first seven-and-a-half years on Mars. The sample could yield clues to the planet’s early environment, when running rivers coursed the surface.
An outcrop informally named “Chester Lake” is the second rock on the rim of Endeavour crater to be approached by NASA’s Mars Exploration Rover Opportunity for close inspection with instruments on the rover’s robotic arm. This view of Chester Lake combines images taken through three different filters by Opportunity’s panoramic camera (Pancam) and is presented in false color to emphasize differences among materials in the rock and soil. The component images were taken during the 2,709th Martian day, or sol, of Opportunity’s work on Mars (Sept. 7, 2011). (Credit: NASA/JPL-Caltech/Cornell/ASU)
A portion of the west rim of Endeavour crater sweeps southward in this color view from NASA’s Mars Exploration Rover Opportunity. This crater—with a diameter of about 14 miles (22 kilometers)—is more than 25 times wider than any that Opportunity has previously approached during the rover’s 90 months on Mars. (Credit: NASA/JPL-Caltech/Cornell/ASU)
The yellow line on this map shows where NASA’s Mars Rover Opportunity has driven from the place where it landed in January 2004—inside Eagle crater, at the upper left end of the track—to a point about 2.2 miles (3.5 kilometers) away from reaching the rim of Endeavour crater. (Credit: NASA/JPL-Caltech/MSSS)
“In late August, we looked at a rock named Tisdale, with a composition unlike any we’ve seen before. It has an enormous amount of zinc, bromine, phosphorus, chlorine, and sulfur, all elements that are mobile in the presence of water.
“The ancient rim of Endeavour represents a period when there was probably a lot more water on the surface,” Arvidson says. “So, we’re trying to get the chemical, mineralogical, and geological setting to ‘back out’ those ancient conditions to reconstruct environmental conditions during this earlier time period.”
The conditions that formed the sandstones Opportunity has sampled over the past seven years represent a kind of drying-out period of Mars. Occasionally wet but usually dry and wind-blown, the sulphur-rich mineral grains formed vast dune fields that were cemented into sandstone over millions of years by occasional seeping groundwater.
But the terrain Opportunity now is sampling—largely buried by lakebed sediments—pops up in places like the Endeavour rim and is much older, going back to the earliest days of the planet. That’s some 3.5 to 4 billion years ago in the last stages of heavy bombardment, when Mars was sweeping up the last planetessimals—cosmic dust grains that collided and stuck to each other to form larger bodies. Endeavour is an impact crater produced during that heavy bombardment period.
Clay on Mars
Arvidson and colleagues also are part of the Compact Imaging Spectrometer for Mars (CRISM) team of the Mars Reconnaissance Orbiter, a craft orbiting Mars with sophisticated instrumentation that helps Arvidson and other NASA scientists determine where Opportunity should try to go next. The CRISM instrument provides spectral information on martian rocks at a super-high resolution that is processed and analyzed at Washington University.
CRISM spectral data and other orbital data on the part of the Endeavour crater rim named Cape York indicated a good possibility that clay minerals would be found there.
“Clays form in more neutral, less acidic conditions than the sulfate-rich sandstones we’ve been looking at,” Arvidson says. “Our hypothesis is that if there are clay minerals, the water was less acidic and therefore more conducive to life. The presence of zinc, bromine, phosphorus, chlorine and sulfur in the Tisdale rock is exciting in that it points toward water moving through and altering the rock, although we have no evidence for clay minerals in Tisdale.
“Further, the Tisdale surfaces were very rough and we were unable to make use of the Rock Abrasion Tool (RAT) to clean off the surfaces to expose fresh materials.
“Thus, we moved on and are now in the thick of the hunt to find these elusive clay minerals by traversing to other rock targets and making detailed measurements, including making chemical and mineralogical observations of natural, brushed and ground surfaces. The rock Opportunity is sitting over currently is called Chester Lake and will be our first target in which we will use the RAT to progressively expose deeper and deeper surfaces,” Arvidson says.
‘Way, way beyond warranty’
The key Opportunity instrument to verify the presence of what Arvidson thinks are iron-bearing clays is the Mössbauer spectrometer. This instrument measures the abundance of iron-bearing minerals in martian rocks and soil by probing tiny changes in the energy levels of the iron atomic nucleus in response to its environment. The Mössbauer spectrometer illuminates a rock with a radioactive source of gamma rays tuned to the energy levels in the iron nucleus and then uses the energies of the returning gamma rays to determine the abundance of iron-bearing minerals in the surface rocks.
Because Opportunity’s arm cannot move side to side due to a broken actuator, Arvidson and team members drive the vehicle to a spot where the arm can still place the RAT and in-situ instruments onto the surface.
Opportunity’s mast-mounted emission spectrometer is no longer working, and the steering on one of the wheels is rotated in a bit. But Arvidson says the engineering and science teams have met every challenge through diligence and guarded optimism.
“Opportunity is just a really well-made vehicle,” Arvidson says. “It’s way, way beyond warranty. It was supposed to drive about 600 meters and so far it’s gone 33,500 meters in round numbers, and has taken maybe 150,000 pictures by now.”
What was supposed to have been a 90- to 180-day exploration of two distinct regions of the red planet has turned into a saga that has become one of science’s most compelling and long-lasting adventures (now into its eighth year), enthralling the public and the science communities alike.
Launched the summer of 2003 and landing in January 2004, the solar-powered Spirit and Opportunity rovers completed their intended basic missions in April 2004. Each continued roving until March 2010, when Spirit, mired in unexpected but scientifically interesting martian sand and pointed in an unfavorable direction to survive the winter dark, gave up the ghost.
Opportunity reached the Endeavour Crater on August 9, after a journey that took almost three years.
“It’s well-made and has a dedicated team that has eked out every ounce of science that we can get from it. Plus, we’ve been lucky and landed on a terrain that has rocks that contain detailed clues on past environmental conditions,” says Arvidson.
“Stay tuned, we have a new mission and expect to make yet more exciting discoveries about the red planet.”
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