Washington, 14 Apr 2006
NASA's Mars rover Spirit has reached a safe site for the Martian winter; its twin, Opportunity, is making fast progress toward a safe destination of its own.
The two rovers recently set out on important â€" but very different â€" drives after earlier weeks inspecting sites with layers of Mars history, according to an April 12 NASA press release.
Opportunity finished examining sedimentary evidence of ancient water at a crater called Erebus, and is now rapidly crossing flat ground toward a much larger crater, Victoria.
Spirit studied signs of a long-ago explosion at a bright, low plateau called â€œHome Plateâ€ in February and March. Then one of its six wheels quit working, and Spirit struggled to complete a short advance to a north-facing slope for the winter.
"For Spirit, the priority has been to reach a safe winter haven," said Steve Squyres of Cornell University in New York, principal investigator for the Mars Exploration Rover project.
The rovers have operated more than eight times as long as their originally planned three-month explorations on Mars. Each has driven more than 6.8 kilometers, about 11 times as far as planned. Together, they have returned more than 150,000 images.
Two years ago, the project already had confirmed that at least one place on Mars had a wet and possibly habitable environment long ago. The scientific findings continue as the craft exceed their expected performance life.
Opportunity spent most of the past four months at Erebus, a highly eroded impact crater about 300 meters in diameter, where the rover found extensive exposures of thin, rippled layering interpreted as a fingerprint of flowing water.
â€œWhat we see at Erebus is a thicker interval of wetted sediment than we've seen anywhere else,â€ said John Grotzinger of the California Institute of Technology. â€œThe same outcrops also have cracks that may have formed from wetting and drying.â€
In mid-March, Opportunity began a 2-kilometer trek from Erebus to Victoria, a crater about 800 meters across, where a thick sequence of sedimentary rocks is exposed. In the past three weeks, Opportunity already has driven more than a fourth of that distance.
At Home Plate, Spirit found coarse layering overlain by finer layering in a pattern that fits accumulation of material falling to the ground after a volcanic or impact explosion. In one place, the layers are deformed where a golf-ball-size rock seems to have fallen on them while they were soft.
â€œGeologists call that a â€˜bomb sag,â€™â€ Squyres said, â€œand it is strong evidence for some kind of explosive origin.
"We would like to have had time to study Home Plate longer,â€ he added, â€œbut we needed to head for a north-facing slope before winter got too bad."
MARS IN WINTER
Spirit is in Marsâ€™ southern hemisphere, where the sun is crossing lower in the northern sky each day.
The rovers rely on solar power. The amount of sunlight available to the rovers will keep dropping until the shortest days of the Mars winter, four months from now.
To keep producing enough electricity to run overnight heaters that protect vital electronics, Spirit must be driven onto north-facing slopes so its solar panels can be tilted toward the winter sun.
On March 13 the right-front wheel's drive motor gave out. Spirit has now driven about 80 meters using five wheels and dragging the sixth. An initial route toward a large hill proved impassable due to soft ground, so the team chose a smaller nearby ridge, called Low Ridge Haven, as the winter destination. Spirit reached the ridge April 9 and has a favorable 11-degree tilt toward the north.
â€œWe have to use care choosing the type of terrain we drive over,â€ said Ashitey Trebi-Ollennu, a rover planner at NASAâ€™s Jet Propulsion Laboratory (JPL) in California, about the challenge of five-wheel driving.
In tests at JPL, the team has been practicing a maneuver to gain more tilt by perching the left-front wheel on a basketball-size rock.
Spending eight months or so at Low Ridge Haven will offer time for many long-duration studies that science team members have been considering since early in the mission, said Ray Arvidson of Washington University in Missouri, deputy principal investigator.
These include detailed mapping of rocks and soils, in-depth determination of rock and soil composition, and monitoring of clouds and other atmospheric changes.
In other studies, the researchers will watch for subtle surface changes due to winds and learn properties of the shallow subsurface by tracking surface-temperature changes over a span of months.
Images and more information about the rovers are available at the NASA Web site.
Text of the press release is available at the NASA Web site.