During the last four weeks, the Mars Exploration Rovers have braved the Martian winter to continue their geologic field work, sending home more evidence of past liquid water on the Red Planet and images of bizarre geologic formations the likes of which no one has seen before.
As Spirit has continued her climb up into the Columbia Hills, she roved onto "the best" bedrock exposure seen to date on Mars, and discovered the most "compelling" evidence so far for past liquid water at Gusev Crater, lead scientist Steve Squyres, of Cornell University, announced yesterday at a news conference held at the Jet Propulsion Laboratory (JPL), where the rovers were designed and built.
While Opportunity originally landed right beside exposed bedrock and promptly found evidence at Meridiani for an ancient body of saltwater, it has taken Spirit half a year of driving across a sometimes treacherous Martian plain just to reach bedrock in Gusev Crater. Along the way, she 'strained' her right front wheel and in recent weeks has had to change her driving approach, from forward to backward. The long and winding rove, suddenly, has proved worth it. "We have now arrived at one of the best exposures of bedrock we have found on the whole mission at either site -- a place called Clovis, a rock about nine meters [30 feet] up in the Columbia Hills," Squyres told reporters.
Spirit has been perched on Clovis for more than a week, taking a "vista view of the scene" and conducting a "very intensive survey," he added. The initial review of that survey holds that water thoroughly altered some rocks at this location. "We have evidence that interaction with liquid water changed the composition of this rock," Squyres confirmed yesterday. "This [Clovis] is different from the rocks out on the plain, where we saw coatings and veins apparently due to effects of a small amount of water. Here, we have a more thorough, deeper alteration, suggesting much more water."
On the other side of Mars, at Meridiani Planum, Opportunity has spent the last month descending deeper into Endurance Crater, examining layers of rock exposed in the southern inner slope of the stadium-sized Endurance Crater and traveling further back in Martian time. From the data returned, the science team has learned that the composition of the layered rocks changes the farther down she goes, revealing more evidence that water played an important role there at some point in the distant past.
About a week and a half ago, as Opportunity neared the deepest it looks like she'll go into the crater -- about 10 meters down, she uncovered a bunch of small, irregularly-shaped, red colored stuff, which the team has taken to calling 'popcorn,' mixed in with the grey hematite spherules or 'blueberries' that the rover has found everywhere around Meridiani Planum. "This is something unlike anything we have ever seen at Meridiani," said Zoe Learner, a science team collaborator and graduate student at Cornell, who is working with the team to unravel the origins of mysterious stuff.
Opportunity has also spotted what Squyres described as "one of the strangest looking rocks we have seen on the surface of Mars" that is within reachable distance on a proposed egress path out of the big hole.
Spirit and her twin, Opportunity, completed successful three-month primary missions on Mars in April and are now roving on extended missions.
Although both rovers are continuing to show signs of age, both are managing to perform "marvelously" and "soldier on," Chris Salvo, a mission manager at JPL, informed. Both rovers have now passed the 200-sol mark, more than double their 90-day 'warranties.'
Spirit was the second rover built, but the first to arrive at the Red Planet; therefore, she is generally considered the older of the twins, and she is showing it. She has traveled a total of some 3.5 kilometers now, much further and over rougher terrain than Opportunity. As a result, Salvo said, there's no surprise that she's exhibiting more signs of age -- the anomaly with her right front wheel being one, and dust accumulation on her solar arrays being another.
Spirit is also farther to the south than Opportunity, where the winter is harsher and the overnight temperatures are lower. While she is tilted north now and trying to point at the Sun, Spirit is generating "something on the order of 400 watt hours everyday," Salvo said. "Opportunity, on the other hand, which is also tilted roughly northward, is generating something around 550 or 600 watt hours a day, so is faring better being further north, and, being young, has less of a dust deposition on the solar array," he noted.
Despite those physical aches and technological pains, however, the rovers "really aren't showing any sign they're going to stop anytime soon," Salvo added.
Spirit from Gusev Crater
In the last month, Spirit has traveled from Engineering Flats up into the higher ground of the West Spur in the Columbia Hills. She is now perched on Clovis outcrop of bedrock, near the crest of the West Spur, about 30 feet [9 meters] above the plains of Gusev, and about 2.6 meters, as the crow flies, from her landing site.
"We spent the first 160 Martian days of this mission traipsing across the plains of Gusev Crater to the Columbia Hills, and since roughly Sol 160 we've been doing Martian mountaineering, working our way up the hills in search of bedrock and rock that can tell us stories about what's happened on Mars in the past," Squyres recapped.
Since arriving at the Columbia Hills, Spirit has roved upon a number of intriguing rocks and outcrops as she made her way up the hill to Clovis. On one, dubbed Wooly Patch, she used her rock abrasion tool (RAT) to dig holes in targets called Sabre and Mastodon. Then, after examining those with her microscopic imager (MI), Mössbauer spectrometer, and alpha particle x-ray spectrometer (APXS), she moved on to Clovis.
The road up was rough and steep, and at one point along the way Spirit wound up in a sandy hollow. "We had to climb a circuitous route, sort of like switch-backing up a mountainside," explained Squyres. "We did that for a couple of reasons. One is that obviously you want to drive on fairly gentle slopes. The other is that it is deep into winter in the southern hemisphere of Mars now at Gusev Crater and the Sun tends to be low in the northern sky, so we chose a route that would keep the solar array pointed to the north to give us a little more energy, a little more solar power and help us have the energy to make that climb."
Some parts of the climb were particularly difficult and Spirit slipped frequently. "We went through some very tough gyrations as we went up that hill to Clovis," Squyres said. "One of the things we keep track of on the rover is something called the tilt high watermark -- it's the maximum tilt you get over the course of a drive, and we had one sol where we had a tilt high watermark of 34 degrees on Spirit -- this thing was really, really tilted up. We had one segment of our drive up to Clovis where we slipped 125%. In other words, in trying to go forwards, we actually went backwards, so we're doing some serious rock climbing with this vehicle."
Most of the time now, Spirit is driving backwards, because of her gimpy right front wheel, and as odd as that is to even the team members, it's nothing, they say, to the rover. Although the wheel has improved as a result of rest and lubricating activities the team conducted a little more than a month ago, it is still drawing two times the amount of current as any other wheel; therefore, the rover crew has kept to the plan they announced last month of driving mainly on five wheels backwards, having Spirit drag her gimpy wheel while on easily negotiable terrain. By using this strategy the team hopes to extend the lifetime of the wheel's drive motor for use when it's most needed.
"Where we're not trying to do precision maneuvering we're using five wheels -- that will get us across the countryside pretty well without beating on the wheel too severely," said Squyres. "We're using that sixth wheel in the normal fashion when we're doing really steep rugged climbs or high precision maneuvering. For example, during that final maneuvering onto Clovis, which was a very tough move, very high precision maneuvering on a steep slope, we used all six wheels."
In just the last week or so, the struggle and undaunted effort appear to be paying off big. The discovery of water -- past of present -- is the prime science objective of this mission. While Spirit found traces of past water that probably trickled to some small extent underground last spring, first in the rock Humphrey, and then at Mazatzal, the data from recent weeks, hints of a much bigger water story. "We had evidence when we were down on the plains that there's been a little bit of water percolating through the soil and depositing salts -- the APXS discovered some wonderful magnesium sulfate salt deposits out in the plains in the soil," recounted Squyres. "This [discovery at Clovis] is the most compelling, the most powerful evidence for liquid water that we have seen in the rocks at Gusev. But we have a ways to go on this. It's compelling evidence but it's incomplete at this time," he cautioned.
One of the most enticing aspects of Clovis is that there are altered and unaltered rocks near each other that may hold secrets to what happened there so many eons ago. "What we think we may have here is relatively altered and relatively unaltered rock side by side," Squyres said. "If you can examine both, you kind of get a before and after of the alteration process. By looking at the change, you can really get a handle on what took place, the nature of the alteration. Clovis is one of these rocks we think was altered. So it's the after -- we still need the before. If we're lucky, there maybe rocks nearby that will give us that."
There is a possibility, however, that the less altered rocks came around long after the process that altered the altered rocks occurred, in which case no comparison could be made. But unraveling the relationships between rocks is something the team is focused on at both sites.
Indications of water impact at Clovis come from analyzing the rock's surface and interior with Spirit's APXS and finding relatively high levels of bromine, sulfur, and chlorine, Doug Ming, a rover science team member from the Johnson Space Center, explained. "This material is light-toned, a much different color than we saw on the basalts we saw back on the floor of Gusev Crater, back out on the plains," he elaborated. "This is also a very soft rock, and it appears to be highly altered. This is really significant. This is going to help us understand a lot better about the formation of the Columbia Hills. This is why we came here. This is what we were looking for."
Spirit used her rock abrasion tool (RAT) to "interrogate this rock," as Ming put it. "The rock abrasion tool did a grind into this that was incredible -- we went 8.9 millimeters into this rock. The key things we have here is we have gone into this 8 to 9 millimeters -- and we see compositions deep into this rock that suggest that potentially there is some migration of these elements into it or potentially this high chlorine, sulfur, bromine might have been somehow involved with the original deposition of this rock. What we are looking at now is trying to get a composition that would give us a baseline to compare the outcrop material to, such that we can see enrichments in elements, depletions in elements, so that we can get a better idea of how these things formed," Ming elaborated.
"The bottom line here is that so far we have intriguing clues, hinting that this rock Clovis has interacted with liquid water," Squyres summed up. "We still need to understand the nature of that interaction -- was it hot water or cold water, was it water in a vapor phase or in the liquid phase? We're going to be making Mössbauer spectrometer measurements in this RAT hole, which could be extremely revealing and we hope to have data on that really soon." Then, Squyres said, they will have Spirit examine one of the other unaltered-looking rocks to do a side-by-side comparison. "That should tell us a lot about the story of alteration by water at Gusev Crater."
From there, Spirit will continue up the hill, working her way up another few meters to the crest of Husband Hill and "some nice layered rock," Squyres said. In fact, it's "the first layered rock" seen at Gusev. "I don't want to get too excited about this -- there are a lot of ways to make layered rock -- lots of volcanic rocks are layered, for example. But this is something new, something different . . . and that lies ahead once we've finished our business in the area of Clovis and Longhorn. The good news is that the steepest parts of the climb are now behind us. We've completed the crux move of the climb and the hardest stuff is behind us and I'm hopeful we'll get to that layered stuff without too many gyrations."
Opportunity from Meridiani Planum
As Opportunity has continued her descent into Endurance Crater, she has been stopping along the way to examine exposed outcrops of bedrock. From outcrops near the rim down through progressively older and older layers that have been given such names as Kentucky, Dahlia, Knossos, Razorback, Flatland, Millstone, Diamond Jenness, and Tuktoyuktuk, to the lowest accessible outcrop, dubbed Axel Heiberg (after a Canadian Arctic island), the rover has been, effectively, traveling "back in Martian time," as Squyres put it.
"What we have at Meridiani is a wonderful layered stack of sedimentary rocks [which] are great in that they record a history," Squyres explained. "If you can go downhill from younger rocks to older rocks, you're traveling in time through Martian history, going deeper and deeper into older and older rocks and looking farther and farther into the past. So what we have been doing with Opportunity is marching our way slowly, very systematically, down this stack of layered rock using Endurance Carter as a natural borehole drilled into the rocks for us in Mars and going backwards in time."
On her journey, Opportunity has been boring a number of her own holes to peer even further back into time. The goods she's returned show laminations, and some of the little holes they call vugs, and ripples, different signs of the action of flowing water preserved in the rocks. Once she got to Diamond Jenness, though, "things started to get different," said Squyres. "When we look in that RAT hole -- it looks completely different -- you don't see any laminations, any of the stratigraphy, and the same thing here with Mackenzie [a target on Diamond Jenness]. It's lumpy and peculiar looking. Once we have gotten deep down into the stack, the appearance of the rocks has changed. It's still fundamentally the same kind of rock. It's got the same general sort of chemistry. It looks like it was laid down in liquid water, but it's telling us something about changing conditions as we look further into the Martian past."
What exactly it's saying about those changing conditions is still to be determined, but they have found notable differences among the layers. "We found different compositions in different layers," Ralf Gellert, of Max-Planck-Institut fur Chemie, Mainz, Germany, told reporters. Chlorine concentration increased up to threefold in middle layers. Magnesium and sulfur declined nearly in parallel with each other in older layers, suggesting those two elements may have been dissolved and removed by water.
As Opportunity roved onto Axel Heiberg, she came across something they MER team had never before seen, 'popcorn' spread out all over a reddish-tan slab called Bylot in the outcrop. "They look like little balls, but they were light and red," recalled Learner. "And as we were examining this color picture, we thought we saw things that look like they might be mixtures, like they had some light reddish tones and also some gray-bluish tones, and things that looked half-toned, half blue and half red. Opportunity zoomed in with her MI to take some close-up images of this strange looking 'popcorn.'
"Up close, we saw these things don't look at all like blueberries, but are very coarse, and rough. Blueberries tend to be very smooth. Also these vary a lot more in size -- there are really tiny ones and big ones and some that are irregularly shaped. We've noticed that in some cases where these are eroding, you can see a regular blueberry or a berry fragment inside." One possibility is that a water-related process has added a coarser outer layer to the blueberries, she said, adding, "It's still really a mystery."
The 'popcorn' may well turn out to be a variation of the blueberries, Squyres proffered. "I think it's really kind of variations on a theme. I think we're talking about rocks that are fundamentally similar, but you've got geologic processes that are expressing themselves in different ways."
The most recent data suggests "these things they may be slightly different than the outcrop but those differences might turn out to be pretty significant," Learner added. "We are not done with this rock. We are definitely going to be looking at this data a lot more carefully."
On Sol 199 - last Sunday, Opportunity jammed her RAT, but the rover team is convinced they'll be able to un-jam it in fairly short order. "It appears that we have chanced upon a known vulnerability in the RAT design," said Salvo, as he pulled out a model and a pebble to demonstrate the jam. "It looks like there's a pebble trapped between the cutting heads of the rock abrasion tool. We think we can treat it by turning the heads in reverse, but we are still evaluating the best approach to remedy the situation and I have confidence that we know what's going on here and we're going to clear it up before these guys want to use the RAT again."
Like her twin, Opportunity has also suffered some slippage and a few technical glitches, and the crew has been putting her into Deep Sleep mode regularly to save power from the heater on the IDD that is stuck in the 'on' position, but even though the sheer quantity of data has slowed with the onset of the Martian winter, there is still an awful lot of science coming in.
In addition to geology, the rovers are also conducting atmospheric science experiments, and taking hundreds of images of the sky. On one fine winter's day just last week, Opportunity took a short video of clouds moving across the Martian sky. "These are waterized clouds," said Squyres as he rolled the clip at the press conference. "Clouds that tend to form at this latitude in the winter months. It's great for scientific value, but for me [it's] also a very evocative image."
At 10 meters in and counting, it appears the Opportunity has gone about as far as she's going to go into Endurance Crater. The reason? In a word -- sand. "If you look beyond Axel Heiberg there's nothing but sand and blueberries," said Squyres. A view all the way into the crater reveals the artistic ripples of a dune field on the floor. "This is enticing," Squyres admitted. "We are seeing clues that this dune field has been recently active. You can see little ripples along some of the dunes that indicate these things may have been stirred by the winds really quite recently. But this is really a dangerous place. We built a wonderful rover, but we didn't build a dune buggy and I think if we were to wander into that in search of science, we might not ever wander out again."
Nevertheless, the dune field is really alluring. "There is a little tendril of the dune field that sticks up toward a level very close to where we are now, and there is so much interesting science we could do on the sands of the dune field that we are investigating the possibility of making a safe approach to that most extreme tip of the tendril of the dune field," Squyres informed. "But we value this vehicle and if there's any chance we would get trapped, we're not going to get close to this thing." Even if they do venture to touch the tendril, Opportunity will not be going any deeper into Endurance. "Topographically, I do not expect us to go any deeper into this crater because we don't need to."