The Mars Exploration Rovers are still going strong, and both robot field geologists will be working through the July 4th holiday.
Spirit is hiking ever upward toward the summit of Husband Hill in the Columbia Hills area of Gusev Crater and has been making considerable progress of late. In recent weeks, the rover found an unusual, loose basaltic rock that may hold important new clues about past environments there, and this weekend will hunker down at a new chunk of bedrock dubbed, appropriately enough, Independence.
On the other side of the planet, Opportunity has completed the examination of Purgatory Dune that trapped her for six weeks, and is heading southward now toward the next major science target, Erebus, a particularly bright spot on the dark, hematite plains the rover has been crossing for some time now.
Both rovers are in "excellent" health, according to Steve Squyres, of Cornell University, principal investigator for the Mars rovers' science instruments. Spirit has "ample" battery power, he added, and although "Opportunity is a little tighter than we'd like, we're doing fine."
In addition, Mars continues to be fairly welcoming of the two robot field geologists. "We've been very fortunate with the weather," noted Jeff Favretto, mission manager at the Jet Propulsion Laboratory, where the rovers were designed and built. "This is high dust storm season, and [neither rover has] been hit yet with a huge dust storm. We've also been very fortunate with cleaning events and that's given [the rovers] more power. As long as we can get funding," he added, "we're going to drive these things for -- ever."
Spirit from Gusev Crater
After completing work at Larry's Outcrop at the beginning of the month, Spirit headed back toward Methuselah. Throughout their journeys, the rovers have continued to stop to 'smell the roses' so to speak, looking around and taking snapshots of their surroundings. Oftentimes, these pictures are breathtaking images that also happen to convey important scientific information.
Late in May, for example, Spirit snapped a picture of a transcendent Martian sunset that the MER team recently released (presented below). Sunset and twilight images are occasionally acquired intentionally to enable the science team to determine how high into the atmosphere the dust extends, or to look for dust or ice clouds. Other images have shown that the twilight glow remains visible, but increasingly fainter, for up to two hours before sunrise or after sunset, longer than on Earth. The Martian twilight is caused by sunlight scattered around to the nightside of the planet by abundant high-altitude dust. [Similar long twilights or particularly colorful sunrises and sunsets sometimes occur on Earth when tiny dust grains erupted from powerful volcanoes scatter light high into the atmosphere.]
Along the way back to Methuselah, Spirit conducted some remote sensing observations with the mini-thermal emission spectrometer (mini-TES) and in the process spotted an intriguing rock the team called Backstay.
As it turned out, Backstay is a kind of basaltic rock, but "a flavor of basalt we haven't seen before," Squyres said in an interview yesterday. "It's a loose rock, a piece of impact ejecta from some place, but we don't know where it came from so you can't put it into geologic context. But what we're discovering is that there's an enormous range, an enormous variety of basalt types of Mars, and this adds a new one to the zoo."
The rover's investigation of Backstay proved to be "a really nice example of the way this whole thing is supposed to work," Squyres added. "We took a look around with PanCam, found a bunch of rocks, used those images to target mini-TES coverage. The mini-TES measurements showed that Backstay looked like a spectrum that we'd never seen before. We thought that was interesting, and drove over to it, used the RAT to clean the rock off, and then made measurements with microscopic imager, the alpha particle x-ray spectrometer (APXS) and Mössbauer [spectrometer] -- and indeed showed it to be a different kind of basalt than we've seen. Every piece of the payload on the rover contributed to the discovery -- it was pretty cool."
Once Spirit completed the examination of Backstay, the rover moved on back toward Methuselah, stopping at a scenic overlook to take some high-resolution images of the outcrop. "From Backstay, we went back toward Methuselah and did some very quick imaging there that was aimed at determining the strike and dip -- the attitude -- of the bedding there," Squyres explained. "Since then we've been mountaineering."
During the last week or so, Spirit has really been making tracks up toward the summit of Husband Hill and is, Squyres said, sort of spiraling upward. The rover even accomplished three straight drives gaining 3 meters on each drive. "It has been just excellent driving," he said. "The terrain here is very firm, and our wheels are hardly sinking in at all, so we've been actually climbing."
Spirit is no longer using her RAT to grind, because one of the grinding bits is low, but the rover will continue to use the tool for brushing. "Other than that, Spirit is healthy as a horse -- and has a super amount of power right now," said Favretto. The rover is making an amazing amount of progress now. In fact, every time I'm working on Spirit I have the song "The Bear Went Over the Mountain" playing in the back of my head -- because this is what we're doing -- we're going to see what we can see and nobody knows what's on the other side of that mountain and Spirit is going to be the first to see it."
Heading into the July 4th weekend, Spirit has "fortuitously" parked herself right on top of "a nice little outcrop of bedrock," Squyres said, "which we've named Independence." The rover is just beginning her intensive study of Independence now and will be hunkered down there until after the holiday is over.
Opportunity from Meridiani Planum
Once Opportunity escaped from Purgatory Dune on June 4, she turned around and drove back -- very carefully -- to check it out with her suite of instruments. That had been the plan all along as both Squyres and project manager Jim Erickson have noted several times. The rover's wheels dug some nice deep trenches in working a way out of the sand trap, and science team members were anxious to get a look at the materials below the surface in hopes they might be able to figure out what this stuff really is.
By studying the rippled dune -- which is about one foot [1/3 meter] tall and about 8 feet [2.5 meters] wide -- the team hopes to figure out what makes it so different from the dozens of similar ones the rover easily crossed before entering Purgatory, so the rover doesn't make the mistake of driving into another such sand trap.
There's no new information on Purgatory yet. "We're collecting data and still haven't gotten it all down yet, but [we're] planning our final drive away from Purgatory [today]," Squyres elaborated.
While the team had every confidence that Opportunity would get out of Purgatory Dune, they are all very happy now that the rover is free, Favretto confirmed. No matter what anyone on the outside thought, the team was adamant in the belief its rover would be back out on the Meridiani Plains at some point -- it was only a matter of time. "Anything can happen -- we're on another planet, but the thing is, if it has the ability to put itself in that position it should have the ability to get itself out of that position," he said, speaking like a true engineer.
Although Opportunity got a good, long, six-week break from roving during her days in Purgatory, the rover's operations team got some restless nights of sleep instead. Nevertheless, there was never any doubt Favretto said. "We have a great amount of confidence in our vehicle. We worked long and hard in order to put our review together and do testing we needed to do to understand this new terrain we're in. It was a lot of work, but we got through it and we're now ready to move again." Both team and rover have gained second winds and Opportunity is ready to rove on as if nothing happened. The team, however, is cooling her 'jets.'
The next stop -- Erebus. Although this interesting bright patch of what may be bedrock is only about .24 of a mile [400 meters] away, Squyres was not going to even 'guesstimate' how long it would take for the rover to get there. "We're going to be using some very different driving techniques, and how long it takes us to get there, I'm not prepared to say," he said.
The reason they're using different driving techniques is the obvious -- "because we got stuck in a dune," Squyres confirmed. "Before we got stuck, we were traveling at very high speed, doing more than 200 meters a sol, and we were accomplishing that by having almost all of our safety checks turned off. Now that works fine in benign terrain, and the terrain had been very benign, and we saw these ripples and we'd go over one and go up one side and down the other. Then we hit Purgatory and got stuck." That told them in no uncertain terms that the terrain was not as benign as it appeared. "So we're going to start treating it with a lot more respect," said Squyres.
"I think we did underestimate the terrain here, because we were boogying along and doing 200-meter drives and that was a fabulously fun time," agreed Favretto, who still holds the land distance record for driving on Mars. "But those days are gone at least for the foreseeable future and we're going to have to take it easy and really try to approach this terrain with a lot more caution."
The new, more cautious driving may mean the next major science target of Erebus is some weeks away, but before that destination, Opportunity will hit on something the team members are calling the Erebus Highway, Favretto said. "From the orbital images, this area seems to be different -- that's all we can really tell you -- we're hoping that it's an area of a little more outcrop, and a little bit more rocky, so we can get a little better footing on the soil there, but until we actually get over there we won't really know." The general plan now, Favretto added, is "to kind of hit it between 10 and 30 meters a day." Since the highway is 150 to 180 meters away, Opportunity could be on the highway, if the driving goes well, in about two weeks, he added.