Mars rover plots escape from giant crater

By Maggie McKee (Image: NASA/JPL/Cornell) NASA’s Opportunity rover is days away from clambering out of Victoria Crater, the gaping basin it has been exploring for nearly a year. The decision to leave came after one of the rover’s six wheels seemed to show an early sign of failure – a scenario that could have trapped the rover inside the crater forever. Opportunity descended into the 800-metre-wide crater in September 2007, after skirting its rim for a year before that. It found that the layers of rocks exposed in the crater’s walls are like those it had seen elsewhere in the Meridiani Planum region of Mars – sulphate sandstones. These rocks are thought to have formed when sand dunes came into contact with water and cemented into solid rocks. The crater was deeper than any the rover had explored previously. So scientists hoped to study the exposed rock layers to trace how the geological conditions in which the layers formed – such as the level or chemistry of the subsurface water table – changed over time. “What we were really interested in seeing in Victoria is whether as you went down deeper – which takes you back farther in time – there could be a systematic change in the trace chemistry of the rocks,” says Bruce Banerdt, rover project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. Over the last year, the rover had studied layers of rock about 6 metres thick at close range, and that data is still being analysed. But in July, as Opportunity was struggling down a steep, sandy slope to try to reach rocks at the deepest level it had yet seen, on a cliff called Cape Verde, it experienced a spike in the electric current in its left front wheel. A similar current spike had occurred in the right front wheel of Opportunity’s twin, Spirit, a day before the wheel stopped working in 2006. The idea that one of Opportunity’s wheels could fail while it was inside the crater rattled the team. “If we did have a wheel failure, it would be difficult to impossible to get back out of the crater,” Banerdt told New Scientist. The wheel remained in good health, suggesting the current spike had been caused by a piece of detritus that temporarily got stuck in the wheel. But the rover team saw it as a wake-up call. “We were having a hard time making progress anyway, and the scientific goal was interesting but not something worth risking the mission over . . . so we decided it was time to pack our bags,” says Banerdt. They plotted an escape route, deciding to send the rover out of the crater the way it had come in. Opportunity is now just 8 metres or so from the crater’s rim, and will likely emerge onto the plains in the next few days. “We’ll probably be out by the end of the week,” says Banerdt. Once there, the rover will study fist-sized rocks, called cobbles, that litter the plains. These rocks seem to be different in colour and composition from the sandstone at Meridiani, which tends to break apart into dust rather than forming such chunks of rock, says Banerdt. “These are rocks that came from somewhere else,” he says. “Some are meteorites and others are ejecta from impacts into the local Meridiani plains.” The rocks may have originated hundreds of kilometres away from where they now lie – thrown by the violent impacts that gouged them out of the Martian surface. Or they may have originated below the sandstone at Meridiani, which is estimated to form a layer several hundred metres thick. “This will give us insights into other places than we’re sitting on right now – either deeper or off in the distance somewhere,” says Banerdt. Opportunity is running on nearly 500 watt-hours of energy per day (compared to about 900 when the mission started), which should be enough to get it safely out of the crater and send it on a mission to study cobbles. But Spirit, on the other side of the planet, is subsisting on just 230 or so watt-hours of energy. Because it lies farther south than Opportunity and therefore receives less sunlight on its solar panels during wintertime in the southern hemisphere, it has had to hunker down in one place to conserve power over the winter season. But the coldest and darkest part of the winter is now over, says Banerdt, and Spirit has started to resume taking photographs of its surroundings, which will eventually be assembled into a 360° panorama. In October, as the Sun moves higher in the sky, Spirit might move a few tens of centimetres to try to angle its solar panels to catch more sunlight. But it will likely not get enough power to begin driving again until late December, says Banerdt. Mars Rovers – Mars is full of surprises; learn more in our continually updated special report. More on these topics:
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