Cassini at Saturn may have grabbed the limelight, but don't forget Spirit and Opportunity. NASA's twin Mars rovers continue to roll along despite the impending onset of Martian winter. The vehicles are revealing new information about the role of water in Martian history, particularly at Opportunity's site in Meridiani Planum.
In early June, Opportunity ventured into Endurance Crater down a moderately sloped rock face that scientists named Karatepe (kare-uh-TEP-ay). As of mid-July the rover had crept down exposed sedimentary layers several meters thick, acquiring images and spectral data en route. Chemical, mineralogical, and textural evidence all point to liquid water here in the distant past. Karatepe's rocks are rich in sulfates, chemicals typically left behind when water evaporates. The spectrometers also found abundant jarosite, an iron-bearing mineral that forms in water on Earth. "The clincher for some of the water being on the surface is crossbeds and ripple marks in the rocks," says Mars rover project scientist Joy A. Crisp (NASA/Jet Propulsion Laboratory).
The layers extend vertically many meters below the outcrop 740 meters away in Eagle Crater, where Opportunity first found evidence for surface water. This means that water must have persisted on Meridiani's surface for thousands of years and perhaps much longer. Lacking absolute ages of Martian rocks, however, scientists can only speculate on the time frame.
Opportunity's cameras have also imaged thin ridges poking a few centimeters above some of Karatepe's rock slabs. These ridges, nicknamed "razorbacks," probably consist of erosion-resistant minerals deposited in rock fractures by flowing water. "Maybe water was migrating around rocks in the crater long after the outcrop had formed," says science team member Jack Farmer (Arizona State University), noting that the impact that excavated Endurance likely caused the fractures. "We are very keen to get the rover's instruments on the razorbacks to figure out what they are."
Meanwhile, on the other side of Mars in Gusev Crater, Spirit's spectrometers have identified the iron-rich mineral hematite in Martian rocks at West Spur, a small knob at the base of the Columbia Hills. On Earth, hematite often forms in liquid water, and Opportunity has found water-derived hematite nodules in droves at Meridiani. Spectral analysis has yet to reveal how the Gusev hematite formed, but Crisp notes that the hematite-bearing rocks have been eroded in a manner consistent with liquid water.
In mid-July, Spirit rolled onto an extensive outcrop of bedrock. "For 190 sols we have been seeking it, and now we have finally found it," said science team member Matthew Golombek (NASA/JPL). Since Spirit landed in early January, it has been crawling atop loose volcanic debris that has been transported to Gusev. The bedrock will give scientists a chance to examine older, indigenous material that might date back to an epoch when Gusev was a lake.
Both rovers are performing remarkably well despite having doubled their minimum design lifetime. Spirit's right front wheel motor is drawing too much current, however, perhaps because of excessive friction. The rover can drive backward reasonably efficiently on five wheels (dragging the right front wheel), though ground controllers will use the problem wheel when ascending sandy slopes. A shoulder-joint heater in Opportunity's instrument arm continues to draw excess power, forcing ground controllers to put the rover into deep sleep on a semiregular basis at night to conserve energy.
Although the vehicles' solar electrical power is decreasing as winter approaches, project manager Jim Erickson (NASA/JPL) expects that they will be able to continue limited science operations throughout the winter, with the worst day for solar power being September 20th. "Winter will limit the science activities because we will need extra days to sit and soak in the Sun," says Erickson. The rovers will take these occasional days off to recharge their batteries, and ground controllers will have to ensure that the solar panels tilt north toward the Sun.
In the likely event that the rovers survive winter, Spirit will climb to the top of West Spur and eventually to a high lookout point, a one-kilometer trek that might take several months. From that vantage point, Spirit should see what appear in orbital images to be layered outcrops in a basin below.
Opportunity might remain in Endurance for a few more weeks, but it will eventually ascend Karatepe onto the surrounding plains to examine its own heat shield, which excavated a fresh "mini-crater" upon impact last January 25th. Scientists are also anxious to determine the composition and origin of small rounded pebbles known as cobbles that are freshly exposed here.
Spirit and Opportunity have traversed a combined five kilometers, beamed back more than 45,000 images, and sent 6.8 gigabytes of data. That's not bad for missions that have fallen off most of the media's radar screens.