As the Martian winter descends on Gusev Crater and Meridiani Planum, the Mars Exploration Rovers have hit the snowless slopes.
Spirit has begun to negotiate her way up into the Columbia Hills where she has encountered a strange rock called Pot of God that contains hematite, something that may well lead to the discovery of past water there. On the other side of the planet, Opportunity has continued her descent into Endurance Crater and is now investigating some intriguing rock layers that are already expanding the water story at Meridiani Planum.
"It's felt to us in the last couple of weeks like the mission has started over again -- we're into totally new stuff in both places and it's been fairly wild," Steve Squyres, principle investigator of the rovers, announced at a press conference held at the Jet Propulsion Laboratory (JPL) today.
Both rovers have nearly doubled their original three-month missions and are each still "in very good health," reported Chris Voorhees, rover mechanical systems engineer at JPL. By the end of next week, Spirit will have worked on the Red Planet for six months. To date, she has driven more than three times the design requirement of one kilometer (0.6 mile).
Opportunity will hit the half-year mark three weeks later. The only symptom of wear or aging on either rover so far is the increased friction on the right front wheel on Spirit, something that was anticipated might happen as the rovers' time on Mars wore on.
Spirit and Opportunity are also taking in less solar energy as a result of the winter season and the MER team is now beginning to consider good sites for the solar-powered robots to spend the period of Martian winter when reduced daily sunshine cuts power supply to a minimum. At the same time, both rovers are "approaching "more aggressive terrain," according to Vorhees. "So that will require that more planning and more thought go into each sol we put into the system." And, because of the reduced solar power, "planning the sols is becoming more of a challenge."
It's a challenge the team is ready, willing, and able to meet of course, especially in light of the new territories into which the two robot field geologists have roved.
Spirit From Gusev Crater
At Gusev Crater, Spirit has been examining the softball-sized Pot of Gold for some days now and the funny-looking rock has the science team stumped. "We may be dealing with something uniquely Martian here," said Squyres, of Cornell University. While the rock has some planar features -- very thin "only a couple of millimeters thick" layers, even more notably "it's got these little nuggets on the end of stalks -- I don't know any better way to describe it than that. When we first saw these little nuggets many of us noted that they looked sort of like the blueberries, concretions, we saw at Meridiani Planum, so with great anticipation, we decided to take some microscopic imager (MI) pictures of these things and see if they really were as round as they look like they might be," he continued. "They are not. These do not look to be concretions, or related to the blueberries. They really are something else. I don't know how these things formed and they're driving me nuts to be perfectly honest, but we're having a lot of fun trying to figure them out."
Pot of Gold, Squyres added, "is not typical of rocks we've seen elsewhere at Gusev . . . (and) just the texture of these things is dramatically different from anything we've seen before. It's trying to tell us something about how it formed, but I have to confess that we don't have the shape of this thing figured out yet in terms of how this morphology developed."
The key to figuring out just what kind of a rock Pot of Gold is will be found in the analyses of its composition, and the science team has been instructing Spirit to use the tools on her instrument deployment device (IDD) or 'arm' to unravel the mystery, even though it is, as Squyres put it, "a fiendishly difficult target." After examining the oddball rock with the MI and two spectrometers on her IDD, the rover backed away from the rock to re-approach at a better angle for using its rock abrasion tool to expose the rock's interior. In the rough and slippery terrain, that maneuver took several days.
So far, the team has analyzed the readings that Spirit took with her Mössbauer spectrometer -- the instrument that reveals the presence of iron -- and has found hematite. This is the same mineral that was found to exist at Meridiani Planum, first from the thermal emission spectrometer (TES) onboard Mars Global Surveyor (MGS), and more recently from her twin, Opportunity. Hematite, which on Earth is usually formed in a process involving water, was NASA's primary reason for choosing Opportunity's landing site halfway around Mars from these hills within Gusev Crater. The discovery of hematite in Gusev Crater is "an exciting discovery," enthused Doug Ming, a science team member from the Johnson Space Center.
"I'll be honest -- right after Opportunity landed Steve and I were sitting here talking about hematite and what the importance and implications that had," Ming recalled. "The hematite was discovered by the thermal emission spectrometer in Meridiani Planum, but we didn't see that here in Gusev. So this is really a spectacular finding. What does it mean? Could it mean water? Yes." And it could mean something else.
Hematite can form in environments where there's no water, such as in igneous environments. "For example, in volcanoes where we have the primary minerals there in the material such as magnetite, which is a primary iron oxide in igneous materials actually transforms under thermal oxidation processes -- heat and oxidizing conditions -- to form hematite," Ming explained. But at the moment, the team just can't say what story the hematite has to tell. "I think it's safe to say that right now the science team doesn't have a real clue on how this hematite formed," Ming said. "What we hope to do is continue integrating this rock and get under its skin and hopefully provide some information about the chemistry that's underneath and then unravel if there was water there."
In coming sols, Spirit may investigate other nearby rocks before she resumes longer drives exploring the Columbia Hills area, but for now, she's going to hunker down and take her time examining Pot of Gold to try and determine unequivocally if past environments in Gusev Crater featured the presence of water, Squyres said. "We may have a water story here. . . the key is that you can tell the difference between a watery origin for hematite and a non-watery origin if you can measure what other elements are present," he elaborated. "So we are going to take our time. We are going to go very methodically through the chemistry and the mineralogy, just like we did with Opportunity back at Eagle Crater when we first landed there. We're going to try to take one of these rocks apart and we're going to use the RAT, the wheels, whatever we have we have to do to get inside these things to figure out what's going on . . .so we've got some very exciting times ahead of us with Spirit."
The rover's 3.3-kilometer trek across the plains represents three times the driving that the vehicle was designed to do”, noted Vorhees, and, not surprisingly, the team has begun to see the first signs of aging in her right front drive actuator. "Each of the drive wheels is independently driven and we're seeing an increase in the drive current for the motor on that drive actuator," he explained. "It's drawing about 2.5 times as much as the others and it's been steadily increasing over the last kilometer of traverse as we approached the Columbia Hills."
There are "lots of reasons" why that might be occurring, Vorhees continued. "The most probable one at this time is a lubrication issue in the actuator's drive train and gear box. Each of these gearboxes is lightly lubricated with aerospace grease so they can perform well at the very low temperatures to -70 degrees Centigrade on Mars. Over time, that lubrication can either be lost or can migrate away from the critical areas of the actuators." To mitigate the problem, one option is to try to get the lubrication back into those critical spots, so over the next few sols the team will have Spirit move away from Pot of Gold, and orient herself in a "preferential" north-facing location for heating up the right front wheel, "then kicking on the heaters at the appropriate time." By leaving those on over a certain period of time over the day and driving the actuators, they hope to cause an intermixing of the lubrication inside, hopefully working it back into to the tooth bed of the gears and back into the raceways of the bearings, with the goal being to bring that current back down into the area we expect to see," he explained.
That may or may not work. So, as a contingency, Vorhees said, they're going to send their engineering team and rover planners back to driving school to start learning how to drive with five wheels. No real worries yet though. "It's not even close to being the end of the world for our traversability, but it does present new challenges and the vehicle will behave differently and slip differently and we have to get used to that."
Opportunity from Meridiani Planum
Over at Meridiani Planum, Opportunity has slowly, methodically continued her descent into Endurance Crater and has driven far enough into the stadium-sized hole in the ground to put her within arm's reach of three layers of rock beneath a sulfate-rich layer. As it turns out, the rover has found unexpected similarities between lower layers of rock she has been examining for the first time and an overlying layer at Eagle Crater where, months ago, the rover discovered evidence that water once soaked the area.
"Once the science team took a look at [the images of the layers] it became clear that what we are viewing is a section of stratigraphy or layers of rock and some of this stratigraphy looks to be in place, and if that's the case, we may be going back in time in a sense," explained Nicholas Tosca, a science-team affiliate, of the State University of New York, Stony Brook. "If we can systematically characterize these layers in terms of chemistry and mineralogy, then we can start to view through a window back in time [the] different depositional environments or environments in general."
One by one, Opportunity is sampling each layer -- which the team has labeled simply Unit A, Unit B, Unit C, and Unit D -- as well as in-between each layer. "In a sense, what we're looking at is really pages out of a book and they almost look [like pages in a book] too," said Tosca. "The layers, as we look closer, seem to be finer, so we refer to those as being laminated. Basically, what we've been seeing is the same or similar material that saw back at Eagle Crater in that it has a large proportion of sulfate chemically. We're seeing jarosite in roughly the same proportions as we did back at Eagle Crater and we're also seeing hematite blueberries or nodules." That in and of itself is pretty interesting -- that the first two newly accessed layers resemble the upper layer by having sulfate salts and spherical concretions or blueberries is an indication that the rocks were formed under wet conditions.
"There are really two take-away messages from our explorations down in Endurance Crater so far," Squyres offered. "One is that there's an awful lot of salt down there, considerably more than many of us initially suspected. A lot of us expected we'd see more of the stuff we saw at Eagle Crater and we wouldn't go too far before we'd start to get into material that looked more like a basaltic sand. We still have very good reason to believe that basaltic sand is down there, but it's further down. What that means is there's a lot more salt here than we originally thought, and that means that there was a lot more water involved in doing this than we originally thought. What we hope to do here is work our way down the stack and really try to understand just how much sulfate is in this place. Once we've got that number, then by doing the kind of geochemical calculations in which Nick is an expert, we can actually start to attach some numbers as to how much water and we're very excited about that prospect."
When Opportunity crossed the 'boundary' between Unit A and Unit B, she crossed into "unknown territory," Squyres said. "We were seeing rocks we haven't seen before. As we've gotten down into the lower units, while the chemistry has been just about the same, what we're seeing here is in some respects and, in important ways, different from the stuff we saw further up the stack. In the highest level stuff, we saw lots of these little ripples that indicate deposition in flowing water. We're not seeing those here. We haven't since we crossed the border between A and B and we haven't seen any ripples [created by winds]. In this higher unit, we saw lots of these little tabular holes we call vugs -- once we've gotten down into this unit, we've seen a lot less of that."
Additionally, they rover had detected widely varying amounts of bromine in the upper level by orders of magnitude within the rock and that is not the case as Opportunity descend lower into Endurance. "We're starting to think that the materials we're seeing deeper down into the crater may have been stirred, mixed, blown around by the wind," offered Squyres. "We think that at some point they may have actually been ground up into fine particles and got mixed around by the wind, so it wiped out any ripples that might have been there and a lot of vugs that might have been there went away. If there was a big variation in the amount of bromine, it all got stirred up so it now all looks the same geochemically."
That's a hypothesis the science team can test if Opportunity can work her way down the stack of layers. Not only would they get a better sense of how much sulfate is there, but they could begin to look for the telltale signature of wind action. "For example, wind will make great big crossbeds where you have layers that cross-cut one another but at high angles and much larger features and a much larger scale of crossbedding than we saw in the ripples over at Eagle Crater," Squyres explained. "So we think we're into something that is subtly different. It's telling us something fundamental we believe about what took place here -- both about how much water was involved and it's pointing in a direction of there having been more water than we originally thought."
So how much water? "I don't think we have evidence for an ocean . . . I don't see anything that points yet to a deep body of water . . . but periods of wet and dry, where it's wet for while, then gets dried out and the wind blows things around and it gets wet again and dries out and so forth," said Squyres.
In the coming sols, Opportunity will, no doubt, find more pages in the geological history book of Meridiani Planum as she continues to explore the stratigraphy at Karatepe. Like her twin Spirit though, she is facing some tricky roads ahead. "Right now, Opportunity is sitting at a 23-degree slope, but [she's] on the precipice of an even steeper slope, which could be between 35 and 40-degrees, right in front of where [the rover] is at right now," informed Vorhees. "There's an extremely large advantage to being able to continue down that slope, but we're right at the [limit of the] design capabilities of the vehicle, so when we get into conditions like that we like to see a test."
In fact, they conducted that test just two days ago, using a version of the rover that has an appropriate Mars weight sitting on a tilt platform near JPL's Mars Yard. "The concern is not that we would be able to get down it," said Vorhees. "The concern is that we wouldn't be able to get back up and out."