The Mars Exploration Rovers are trudging ever onward through the dead of winter on the Red Planet dreaming, perhaps, if robots dream, of a white Christmas. But Spirit and Opportunity are robots after all and come this weekend "the poor little rovers will have to keep working, even on Christmas," MER Project Scientist Joy Crisp, of the Jet Propulsion Laboratory (JPL), told The Planetary Society earlier today.
At Gusev Crater, Spirit is now closing in on Larry's Lookout, an intermediate goal on her hike up Husband Hill where she'll park for the Christmas weekend and hunker down to work. On the other side of Mars, Opportunity roved out of Endurance Crater, as planned, after six months of study, and has been heading south across the plains of Meridiani Planum. By this weekend, she'll be closing in on the heat shield she jettisoned on entry last January.
Meanwhile, on the ground, MER science team members announced at a meeting of the American Geophysical Union (AGU) in San Francisco last week, that Spirit has sent home evidence for a water-signature mineral called goethite in the bedrock she has been examining in the Columbia Hills, one of the surest indicators yet for a wet history in the Gusev Crater region of Mars, and that Opportunity has been revealing more about the changes in seasons and the weather at Mars by capturing images of frost and clouds.
Spirit from Gusev Crater
At Gusev Crater, Spirit -- now into her Sol 344 -- has been hiking a lot in the last week, continuing her trek to the top of Husband Hill. She paused last weekend to checkout an interesting, loose rock called Wishstone that turned out to be different in all aspects -- "chemistry, mineralogy, and texture," informed Crisp -- from the other rocks the rover has examined in the Hills. Now, the rover is closing in on an intermediate stop further up the hill, a ridge dubbed Larry's Lookout.
With the sand, slope, and rocks in this area, it's been tough going, but the MER team expects the rover will arrive in time to study targets there over the Christmas weekend. "Spirit will be on the rocky slopes of Husband Hill for Christmas and we're going to be looking for more rocks like Wishstone to try and figure out what exactly it is," Crisp said. "We're excited by it because of the new things we can learn."
Physically, Spirit is, by all accounts, a real trooper. The amount of electric current drawn by the motor on the right front wheel is back in the normal range. You may remember that near the end of Spirit's long series of drives from Bonneville Crater to the Columbia Hills, the right front wheel began to draw roughly twice the current of the other five wheels. That prompted engineers to limit the use of this wheel to preserve and extend its longevity. Since arriving at the Hills, Spirit, overall, has had relatively few driving days, and the long rest periods appear to have allowed the lubricant in this wheel to redistribute, causing the current draw to return to normal -- at least that's what the engineers are thinking right now.
In coming sols, periodic rest days will be included in Spirit's demanding driving plans, and the rover will alternate from driving forward to driving backward to keep the lubricant in all of the wheels more evenly distributed. Additionally, rover wranglers will continue to minimize use of that wheel by letting it drag while the other five wheels drive. "Babying that wheel seems to be helping," noted Rover Project Manager Jim Erickson, of JPL, where the rovers were designed and built, and are being overseen.
As Spirit has continued to collect evidence and data around her landing site, science team members have had their hands and schedules full trying to analyze it all and figure out what it all means. Goestar Klingelhoefer of the University of Mainz, Germany, lead scientist for the iron-mineral analyzer on each rover, the Mössbauer spectrometer, presented the new results from a rock in the West Spur part of Husband Hill at the AGU meeting -- that Spirit's examination of a rock called Clovis last August revealed the surest sign yet that water played a role in the history of Gusev Crater.
Since goethite forms only in the presence of water, whether in liquid, ice or gaseous form, "like the jarosite that Opportunity found on the other side of Mars, [it] is strong evidence for water activity," Klingelhoefer said. Hematite, a mineral that had previously been identified in Columbia Hills bedrock -- and which Opportunity has found all over Meridiani Planum -- usually, but not always, forms in the presence of water.
One remaining question is whether water was only underground or ever pooled above the surface, as it did at Opportunity's site. "As we climb Husband Hill and characterize the rock record, we'll be looking for additional evidence that the materials were modified by ground water and searching for textural, mineralogical and chemical evidence that the rocks were formed in or modified by surface water," offered Ray Arvidson of Washington University in St. Louis, deputy principal investigator for the rover instruments.
Opportunity from Meridiani Planum
After finishing six months studying various rock and soil targets on the exposed layered bedrock inside Endurance, Opportunity roved out and onward according to plan last week, to continue her exploration of the broad plains of the Meridiani region. Before leaving, she examined a transition point between dark and light rock layers about 66 feet [20 meters] from the rim of the crater, and completed her close-up examinations of targets Paikea and Wharenhui. This is the first time Opportunity -- now into her Sol 324 -- had been completely outside the stadium-sized crater since Sol 134.
On Christmas Day, Opportunity should be within a few meters of the heat shield, and will spend the holidays imaging it and examining it and the areas where it has dug into the soil on impact. "It's going to take several days to look it from different sides and get color images," said Crisp. But the prospect of checking out the heat shield and better understanding what happened to it when it came through the atmosphere is something that no other mission has ever done. "The engineers have said this is the first time they've been able to examine a heat shield on another planet," Crisp confirmed.
Communication with Mars Odyssey has been good, so the rover has been able to send home a lot of the backlog of onboard data she's been storing. That's great news, because in recent weeks, this robot field geologist has been capturing some pretty remarkable images of frost and clouds that denote the winter season on Mars. At last week's AGU conference, rover science-team member Michael Wolff, of the Brookfield, Wisconsin branch of the Boulder, Colorado-based Space Science Institute, reported on atmospheric observations. "We're seeing some spectacular clouds . . . a dramatic reminder that you have weather on Mars," Wolff said. "Some days are cloudy. Some are clear." Sound familiar?
A portion of Mars' water vapor is moving from the north pole toward the south pole during the current northern-summer and southern-winter period. The transient increase in atmospheric water at Meridiani, just south of the equator, plus low temperatures near the surface, contribute to appearance of the clouds and frost, Wolff explained. On some mornings, the frost has even formed on the rover itself. During the last couple of months, Opportunity has been experiencing significant and considerable boost in energy and that continues. MER team members are still investigating the possibility that the frost has served to clump together some of the accumulated dust on solar panels that has then fallen off, thereby 'cleaning' the solar arrays and thus helping to boost the rover's vitality.
Before leaving Endurance, Opportunity's last major scientific endeavor inside was to inspect rock layers exposed in a part of the crater wall area dubbed Burns Cliff, the furthest point around the rim of the crater she drove. From that data, science team members are now beginning to draw a better picture of the past water in this area. "In the lower portion of the cliff, the layers show very strong indications that they were last transported by wind, not by water like some layers higher up," Steve Squyres of Cornell University, principal investigator for the rover instruments, announced in an official news release. "The combination suggests that this was not a deep-water environment, but more of a salt flat, alternately wet and dry."