Opportunity Uncovers Chlorine Clues Deeper in Endurance
"Eureka! We have found it!"
An ebullient Matt Golombek cheered Spirit's discovery of a rock outcrop in the Columbia Hills at a Mars Exploration Rovers press conference held yesterday at the Jet Propulsion Laboratory (JPL).
"For the first time in Spirit history, we have outcrop underneath the rover wheels," announced Golombek, a science team member from JPL, where the rovers were designed and built. "Outcrop is the currency that geologists use. It's contiguous bodies of rock that really contain the secrets we want to unveil and for the entire mission now, 190 sols, we've been seeking it. This is what we came to the Columbia Hills for -- to look at what the older rocks will tell us about the early history of Mars."
Six months after arriving on the Red Planet -- and three months after their primary missions came to a hugely successful end -- the Mars Exploration Rovers are steadily roving on in their extended missions, surprising scientists and the public alike with the packages of data they're sending home.
Spirit's discovery of outcrop is clearly the biggest rover news in weeks and comes on the eve of her embarking on a climb up into the foothills of the Columbia Hills, to a highpoint atop West Spur where there appears to be a massive slab of the rock just awaiting exploration. This rover's find may well lead to a much greater scientific understanding of how water might have played a role in the environmental history of Gusev Crater -- exactly what the little rover and her team were hoping to uncover. It has also allowed Spirit to rove back into the spotlight commandeered for so long by her twin Opportunity -- and in coming sols she just may level the playing field.
While Spirit has found, at long last, what she's been looking for, Opportunity, on the other side of the planet, has been continuing her downward drive deeper into the stadium-sized Endurance Crater, uncovering some intriguing chlorine clues in the layers she has been examining along the way.
Although both Spirit and Opportunity are "healthy right now," according to MER Project Manager Jim Erickson, they are continuing to show signs of aging. Opportunity is still losing power from the faulty heater that is stuck 'on,' on her instrument deployment device (IDD) and so the team has instituted the DeepSleep mode as a "general practice," Erickson noted. This mode shuts down power overnight night, preventing the errant heater from drawing power and leaving Opportunity listless the next morning.
Spirit is feeling her age in her 'bones" so to speak by experiencing increased internal resistance on her right front wheel -- in other words, she's been dragging that wheel. "We're starting to see signs of wear in the actuator to the right front drive," elaborated Joe Melko, rover engineer at JPL. Recent efforts to mitigate the problem by redistributing the wheel's lubricant through rest and heating have been only partially successful, but the intrepid little rover roves on -- and now is driving backward more than forward as part of a creative plan to accommodate her 'arthritic' front wheel.
Meanwhile, the Martian winter -- which peaks around September 20 -- is now fast approaching and the MER team is focused now on insuring the rovers are prepared. "The flight team is busy reviewing the fault protection both for power and thermal conditions to make sure the rovers will be ready when winter sets in," said Erickson.
The rovers will have to endure temperatures ranging from -20 Centigrade [-4 degrees Fahrenheit] to -100 degrees Centigrade [-148 degrees Fahrenheit] during the middle of winter. "That sounds pretty bad, but it's actually only a few degrees colder both daytime and nighttime to what we're seeing now," Erickson noted.
Even so, the coming of winter means Spirit and Opportunity will have less solar power and take longer to recharge because of the dwindling daily sunshine. So the flight team will be ordering up long periods of rest and DeepSleep for both rovers so that they can keep working, albeit at lower activity levels, through the winter. The winterizing plan also calls for orienting the rovers' solar panels toward the north as much as possible to help elevate power supplies. "We'll gradually adjust the amount of activities we're doing everyday -- from very little to more, and we also have the ability to tuck in a few recharge days on the batteries where we don't do much in the way of any activities and spend the time charging the batteries," Erickson said.
In addition, there is some concern about the solar conjunction -- an astronomical event where Mars will be on the other side of the Sun from Earth -- which is also on tap for mid-September -- if only because it will cause a communications blackout "for a period of 11 days centered around September 16," said Erickson. "During those days, the rovers will be very quiescent and basically recharging, downloading data to orbiters overhead and in some cases doing some benign remote sensing observations."
Spirit from Gusev Crater
During the past month, Spirit has been parked near several hematite-containing rocks, including Pot of Gold, conducting science studies and undergoing a long-distance tune-up for her right front wheel. The rover has also returned her fourth 360-degree, stereo, color panorama -- dubbed the Santa Anita Pan -- that "will represent a treasure trove for scientists to study for many, many years to come," Golombek predicted.
The Spirit science team, however, is homing in on the outcrop at the base of West Spur, her present location, and isn't too worried about the creative, backward approach she'll have to use to get to her target atop this foothill. "This is what we came to the Columbia Hills for -- to look at what the older rocks tell us about the early history of Mars and we're in a target rich environment that doesn't require a lot of driving at this point," Golombek said.
The discovery came just last Thursday -- July 15, when Spirit successfully drove backwards 26 feet [8 meters] north along the base of the Columbia Hills. For the most part, she dragged her gimpy wheel, which was activated only about 10 percent of the time to surmount obstacles and to pull her out of trenches dug by the immobile wheel. But in the process of that drive, she rolled right over what scientists had been hoping to find in the Columbia Hills -- a slab of rock outcrop that may represent some of the oldest rocks observed in the mission so far. "The outcrop is intriguing looking -- it almost looks layers that are thin zones there that are layered one on top of the other and if those are layers, it's very steeply dipping," said Golombek. "The cruel twist of fate of course is that we did not stop at this outcrop but continued to a place where the energy situation is slightly better."
It wasn't that cruel of a twist, however -- looking through the eyes of her NavCam, it seems the outcrop is all around Spirit. "The good news is that it's trending -- or striking off in the distance we are going," Golombek continued. "From NavCams that we've seen around us, we believe there are broad areas here that have this outcrop and we will be stopping to look at it in more detail quite soon."
The mini-thermal emission spectrometer spectra of the outcrop slab on top of West Spur are indicating a basaltic composition, Golombek said, "not unlike the basalts we've been driving across on the crater plains. The outcrop we drove over and [that] we can see on both sides of the rover in images looking off to either side is this pavement, we think, that's extending for some area." More intriguing that that, the outcrop looks "fairly different" than the outcrops Opportunity has roved upon. "We don't know whether that difference is due to the rock itself or subsequent activity," said Golombek. "There are zones in it that look like they're finely layered. They don't in any first cut look similar to what we saw in Meridiani. In fact, we haven't seen any rocks that really look very similar to what we've seen at Meridiani. So the question -- is this outcrop layered sedimentary rock or is it layered volcanic rock or is it layered at all or does it appear that way due to erosion by winds or something else . . . until we really get a fresh and good look at it, I refuse to speculate because I'd be wrong."
As expected, the Spirit science team has found that, from all indications so far, the Columbia Hills are replete with geologic materials that are older than the area over which Spirit has been roving to get to the hills -- meaning the rock outcrop should lead them a little further into the environmental history there. The photo geology analyses of Mars Orbital Camera (MOC) images taken from orbit by Mars Global Surveyor indicate that the hills, of which West Spur is a part, "are in fact coming up from beneath the crater plains we've been on," said Golombek. "The crater plains are basaltic materials that have been broken up into an impact regolith of some sort and there are actually edges to that unit. And as it comes up and hits the edge of the Columbia Hills, it actually looks like those units stop. So we are interpreting the Columbia Hills as older materials. How much older we don't know." The team is currently estimating that the crater plains are about 3 billion years old, while the material underneath may be about 3.5 to 4 billion years in age. "That's about as precise as we can be," Golombek said.
With an odometer reading of more than 2.2 miles [3.5 kilometers], Spirit has already traveled six times her designed capacity. "Right now, [she] is still capable of performing all tasks that we ask [her] to do," said Melko. "But of course, it is a concern for us. We attempted to improve the situation through rest and heating. We'd hoped this would allow any lubricant to migrate to the high resistant zones . We had about a 25% improvement. It wasn't as much as was hoped for but it is a slight improvement." To better cope with the condition, rover engineers devised what they're calling a 'roundabout' strategy -- driving the rover backward on five wheels, rotating the sixth wheel only sparingly to ensure its availability for demanding terrain, Melko explained. "The rover was actually designed to work just as well in forward and reverse," he added. "To drive in reverse is not that big of a deal."
To make sure of that, the flight team hit the rover test facility at JPL to conduct a variety of tests with Spirit's Earth double on different surfaces to figure out how well they could drive with only 5 wheels. "We found the results were very encouraging and we were able to do pretty much everything we wanted to do except in a few different types of terrains," said Melko. The drivers will now only use Spirit's right front wheel when the terrain demands it and when the terrain doesn't demand it, she'll just rove on 5 wheels, backwards.
"We are efficient at doing this and it doesn't significantly effect how far we get," Melko assured. "This approach allows us to have all 6 wheels available whenever we do get to an area where we need all 6 -- if we have a steep slope or an area with a lot of pitched jagged rock this [strategy] will allow us to keep the right front wheel available for many more kilometers of rover travel. Driving may take a little bit longer than it used to," he admitted. "It's a little bit like dragging an anchor on one side, but we think we've got a good handle on the situation -- we know when to drag and when not to drag -- and everything looks like we should be able to get several more kilometers out of this rover and get to many of the other science targets the scientists want to see."
Driving with the wheel disabled means that corrections might have to be made to Spirit's steering if she veers off its planned path. This limits her accuracy, but rover planners working at JPL's rover test facility have also come up with some creative commands that allow the rover to auto-correct itself to a limited degree.
Perhaps the greatest adjustment will have to be made the scientists. "It's pretty exciting for the scientists who are driving -- we're always referring to the front of the rover and now the front is the back," noted Golombek, with a chuckle.
Given everything Spirit has been through so far, it's hard to imagine this little robot field geologist not keeping on track in coming sols as she examines the rock outcrop and heads up to the top of West Spur. "We are traversing north and then we will be traversing south along a north-facing slope," explained Golombek. "That's important for two reasons. . . we're skirting high slope areas to get up there, and we're on a north-facing slope which gives us substantially more energy than we would if we were to choose any other path up to the top. And as soon as we get onto that north-facing slope tomorrow or the next sol we'll probably choose an appropriate target in that outcrop and begin investigating those rocks right away."
Opportunity from Meridiani Planum
As Opportunity has been inching her way further down Endurance Crater at Meridiani Planum, she has been examining increasingly deep layers of bedrock lining the walls of around her, as discussed in our last update. In recent sols, this rover has observed a puzzling increase in the amount of chlorine the deeper she goes. Data from her alpha particle X-ray spectrometer (APXS) show that chlorine is the only element that dramatically rises with deepening layers, leaving the scientists to wonder how it got there.
"We do not know yet which element is bound to the chlorine," said Jutta Zipfel, a rover science-team member from the Max Planck Institute for Chemistry, Mainz, Germany. Whatever element that may be, these are "the highest concentrations of chlorine in any of the rocks we've analyzed so far," she added. In fact, the last measurements, taken on Sol 162, of Millstone, in Layer F, revealed chlorine concentrations that were "a factor of 3 times higher" than in the Layer A.
"None of the other elements that we analyzed showed a similar increase; therefore at this moment we cannot tell what is the exact mineral that is the carrier of this chlorine," Zipfel continued. "It could be sodium chloride -- table salt on Earth -- or it could also be a more complex sulfur-chlorine compound. One possibility is that the residual fluids that actually led to the precipitation and formation of these evaporate rocks changed in composition over time." In order to solve this chlorine mystery, Opportunity will roll down even farther into the crater in the next few days to see if this trend continues.
This rover also will investigate a series of ridges featuring a row of sharp, teeth-like features the science team is calling Razorback. These tiny structures may have formed when fluid flowed through cracks, depositing hard minerals. Scientists hope the findings from this study will help pull together more pieces to the puzzle of Meridiani's mysterious and watery past. "Razorback may tell us more about the history of water at Endurance Crater," confirmed Jack Farmer, a rover science-team member from Arizona State University.
What they have already determined is that the impact that created Endurance Crater came after the rock outcrops were in place -- meaning the impact is younger than the outcrops. "You have this impact that creates Endurance Crater -- and the outcrop rock units we've been examining at Eagle [Crater] and as we've driven over are sitting there and they get completely fractured by this impact," Farmer explained. "Those fractures then become conduits for fluids that can migrate through later, though how much later is uncertain. The point is that terrestrial examples like this typically involve water moving through those fractures depositing minerals that are in solution, forming essentially veins and the minerals are oftentimes more resistant than the host rock containing them and so when you start to erode them they stand up in relief." Of course, they do not yet know the composition of these ridges -- that's what the forthcoming examination will reveal, but, he suggested, "if they are comprised of a more pure in number aqueous mineral, it could be really exciting."
The good news, Farmer continued, is that "all of the hypotheses that we put on the table for these things point back to a possibility for water, so that's what's driving us to want to look at these things. We're following the water story. This would allow us to do it maybe in the context of younger geological events than we've been seeing in the outcrops rocks themselves. What we can say for sure is that that event had to be younger than the deposition of the outcrops themselves and the actual impact that excavated that material."
For Opportunity, one thing is certain -- the further down she goes into Endurance, the farther back in time she goes, and, now, the risk that once so concerned NASA seems all but absent. "We actually don't think we're putting ourselves at greater risk so far," offered Erickson. "The terrain has varied. There's been little short sections over 30 degrees, but they're small compared to the size of the rover so it's sort of been like driving down a short step. In general, we're staying above 25 degrees for a lot of it, and more recently we've gone down terrain that's gentler, more like 17 degrees."