A.J.S. Rayl • Oct 08, 2004
Mars Exploration Rovers Update: Rovers Expand the Water Story as Spirit Gets Back to Climbing and Opportunity Stops at Wopmay
The Mars Exploration Rovers are returning more and more evidence that there was liquid water on Mars at some point in the distant past, team members reported at a telecom news briefing yesterday.
After releasing a dynamic brake on the steering motors affecting two of her six wheels, Spirit got back to climbing up Husband Hill, a peak in the Columbia Hills where scientists believe layered rocks may hold more clues to the water story at Gusev Crater. On the other side of Mars, Opportunity has stopped at the odd-looking, lumpy rock known as Wopmay on her way to Burn's Cliff, the final target in Endurance Crater, which may harbor evidence of a second "water event" occurring sometime after the impact that excavated the huge hole in the ground the rover has been studying.
Finding geological evidence about watery periods in Mars' past is the rover project's primary goal, because such persistently wet environments may have been hospitable to life -- the Holy Grail for any Mars scientist.
Like those battery-powered bunnies in TV commercials, Spirit and Opportunity, now into their second six-month mission extension, just keep on going and going on their search for past water. Their vitality has surprised and impacted the members of the science team, each of whom has returned to his/her home institutions to teach classes, in addition to analyzing the data coming from Mars.
"The rovers are lasting so long and performing so well that we realized if we were going to make rover operations sustainable over the long term, we were going to have to let people go back and live at their homes again," said Steve Squyres, of Cornell University, the principal investigator for the science payload on both rovers. With telecoms and videocams and remote networking, he said, "we're doing exactly the same things we've been doing for the last 9 months and it's working really well."
As Spirit enters her 272nd sol and Opportunity begins her 251st sol, the cameras on board are also continuing to work well. "We have now obtained more than 50,000 images -- more than 6 gigabytes of data -- from the cameras on both rovers," informed Jim Bell, the principal investigator for the panorama camera (PanCam), of Cornell University. Of those images, 37,000 have come from the PanCam. Among the latest images is the first 360-degree panorama that Spirit has taken since she arrived at the hills, which the team dubbed the Cahokia Panorama. This mosaiced image is, suggests Bell, the "defining" image of the mission. [See image above, right.]
Spirit from Gusev Crater
Since leaving her landing spot, Spirit has driven about two miles [more than 3 kilometers] across the rocky plain of Gusev to reach the Columbia Hills, and is now carefully heading up the West Spur to the peak of Husband Hill.
During the past few weeks, the science team has been instructing the rover -- which found bedrock extensively altered by water last June -- to look for relatively unchanged rocks as a comparison to the heavily altered rocks for understanding the area's full range of environmental changes. Instead, the rover has found that even the freshest-looking rocks she has examined in the Columbia Hills have shown signs of pervasive water alteration.
Spirit examined a rock called Ebenezer up close, using all of her instruments, including her rock abrasion tool (RAT) with which she ground a "big deep hole," as Squyres put it, to determine the chemistry. But what the rover found was that Ebenezer, which looked visually much different than Clovis -- the last major rock target in the hills that the rover examined up close -- was in fact very similar to that rock in chemical composition.
"I was not expecting that," Squyres said. "I thought we were going to see that these rocks were rather different in their composition because they were strikingly different in their visual appearance, but when you get down into the details and the chemistry what you find out is that they are very similar -- when one element goes up in one rock, the same one goes up in the other rock and vice versa. We're beginning to suspect that essentially all of the rocks in the Columbia Hills have undergone this alteration process."
The elements that appear enriched from measurements that Spirit took with her alpha particle x-ray spectrometer (APXS) are potassium, phosphorous, sulfur, chlorine, bromine. "These are elements that are easily transported, easily mobilized and moved around by water, so what we think we're seeing here is the chemical signature of interaction with the rock with liquid water, and it appears to have happened on Ebenezer just as it did on Clovis," explained Squyres.
There are still a lot of different rocks to look at in the Columbia Hills, but, Squyres said, "the fact of the matter is we haven't seen a single unaltered volcanic rock since we crossed the boundary from the plains into the hills, and I'm beginning to suspect we never will. All the rocks in the hills have been altered significantly by water."
More clues to deciphering the environmental history of the hills could lie in layered rock outcrops farther upslope, Spirit's targets. "Just as we worked our way deeper into the Endurance crater with Opportunity, we'll work our way higher and higher into the hills with Spirit, looking at layered rocks and constructing a plausible geologic history," Squyres said.
Spirit's next target will probably be a rock called Tetl, which lies just a few meters from the rover's current location. "This is one of the nicest layered rocks we have seen at the Gusev site," Squyres said. "We have been searching 9 months for good layered rocks at Gusev Crater and we have now finally found them. You should not infer from the layering that this is necessarily a sedimentary rock. Many kinds of volcanic rocks are layered as well and a very likely scenario here is that this is some kind of layered volcanic rock which has then been changed by interacting with water." Time and more measurements and observations will tell.
Opportunity from Meridiani Planum
Over at Meridiani Planum, Opportunity may have stumbled onto evidence of a second water event happening following the impact that excavated the stadium sized Endurance Crater. About six months ago, this rover established that its exploration area was very wet a long, long time ago, and that the area was wet before it dried and eroded into a wide plain. The rover's new findings now suggest some rocks there may have gotten wet a second time.
"One of the things I have come to realize after 9 months of operating these vehicles is that some discoveries we make we can take credit for as a result of very careful planning by the science team's thoughtful observations, and remote sensing, and other discoveries are just purely serendipitous, just plain good luck," mused Squyres. "We had one of those over the last several weeks with Opportunity."
A couple of weeks ago, Opportunity set out for one of the 'tendrils' reaching up the crater wall from the artful and alluring sand dunes rippling at the bottom of the crater. The team knew the rover couldn't get all the way to the bottom of the crater and the dunes without getting stuck in the sand, but reaching one of the tendrils looked possible. "With high hopes, we headed off, and spent a couple days driving towards [the tendril] . . . and then we reached the conclusion it was not safe," said Squyres.
"We decided to bail out and our bailout took us off in a different direction," Squyres continued. "We were sort of clawing our way back onto solid ground as effectively as we could, with no intention of doing science when we got there. We just drove to safest piece of terrain that we could find. We turned onto our cameras to take a look around and directly in front of us was a fantastic rock which we had now named Escher -- and we got to it very, very well."
Opportunity spent nearly 10 days at Escher, drawn in by the remarkable patterns of fractures on it and on its neighboring rocks near the bottom of the crater. These plate-like rocks bear networks of cracks dividing the surface into patterns of polygons, somewhat similar in appearance to cracked mud after water has dried up here on Earth.
"Generally, when we see all the fractured rocks and the rocks that are still rock but have the fractures running through them on the mission we have thought these are mostly the results of the impacts that have hit the plains at Meridiani and created a lot of ejecta rocks -- the rocks are fractured but not broken apart," said rover team geologist John Grotzinger, of the Massachusetts Institute of Technology (MIT). "The thing that makes this [rock] distinct is that these polygons are so well developed."
The fractures not only cross-cut the original depositional bedding plains, Grotzinger said, but the fractures' three dimensions probably occur at a high angle to the bedding -- "and that means they might have been associated with the final form of drying." In other words, "the fact that they have this angle to the bedding suggests possibly -- and we consider it to be very remote -- that after the crater formed, there was another episode that made the crater possibly transiently wet and then after that dried out for the second time, [the process] formed these cracks."
The Opportunity science team has a number of multiple working hypotheses as to how that second wet period occurred, what process might explain the polygonal cracks, and whether or not the cracks formed along after the crater. "When we saw these polygonal crack patterns, right away we thought of a secondary water event significantly later than the episode that created the rocks," Grotzinger noted. "Did these cracks form after the crater was created? We don't really know yet."
One possible source of moisture could be accumulations of frost partially melting during climate changes, as Mars wobbled on its axis of rotation, in cycles of tens of thousands of years. "One possibility is that due to climate oscillations that are documented from Mars' history there was phase where there might have been frost that accumulated inside the crater," Grotzinger offered. "Most of that frost would have sublimated, but some of that may have melted. And whatever little melt might have occurred would have penetrated into these older rocks that were still very porous. They would have soaked up this water, possibly causing expansion and then when the water contracted, maybe it went through these cracks again."
Another possibility could be the melting of underground ice or release of underground water in large enough quantity to pool a little lake within the crater, according to Grotzinger. "It might have existed for only the briefest of time. But if it did get the rocks wet [in that manner], then they would have potentially fractured in this way," he explained.
The team is also seeking to determine how these altered rocks compare to the rocks up above the crater and back at Eagle Crater where Opportunity bounced to a stop last January. So the team proposed to do a chemical analysis at a target on Escher called Kirchner relative to rock called Virginia, father up the crater and what the team considered to be a relatively unaltered rock.
"In just the same way Steve described the differences then between the unaltered rocks in Gusev and the altered rocks, we see the differences between the composition of the material on the surface of the rock versus the composition in the interior of the rock," said Grotzinger. "On the surface of the rock, we see an enrichment of elements like chlorine, and a corresponding depletion in elements like sulfur, and then again we see enrichment in sodium. This is really intriguing . . . we don't yet understand how to interpret this data and there is a lot more work to do, but what it clearly shows is that the differences between the surface of Escher and the core of Escher, deeper inside, is substantial and that is consistent with alteration of that rock due to contained interaction with water -- or possibly a secondary water event."
"That is not a definitive result," Squyres cautioned. "It is something we're working on and something we plan to test. In the days ahead, our plan is aimed in large part to make more observations on more rocks and to test this hypothesis about a possible second water episode in the crater. We have absolutely compelling bullet proof evidence for a long-ago water episode that preceded the crater and we're wrestling with the idea that there might have been a little bit of water episode that followed the forming of the crater as well."
The rover scientists hope Opportunity's next target for inspection, a large and lumpy boulder nicknamed Wopmay, may help narrow the list of possible explanations. "Our hope is that when we look at that rock it will reveal more to us about the possibility of second water event in the crater after its formation," said Squyres. The team plans to spend about a week studying this strange looking rock. One type of evidence Wopmay could add to the case for wet conditions after the crater formed would be a crust of water-soluble minerals.
"The beautiful thing about Wopmay is that it shows some similar sorts of textures but we can measure it on surfaces at different orientations and at different angles and that's the reason we expect to be [there] for a week," Squyres explained. "We'll go at it with our RAT and spectrometers and help test the hypothesis there. If you're going to reach a conclusion as far reaching and important as the actions of liquid water on Mars - you want to be darn sure you're reaching that conclusion on the basis of sufficient data. Right now our data are not sufficient to say there was a second water episode and we're working hard to collect more data."
In coming sols, after examining Wopmay, Opportunity will continue toward Burn's Cliff, a tall stack of layered rock, and examine the rocks from the base of the cliff. Following that examination, the rover will then climb out of Endurance Crater -- hopefully from a route right next to Burn's Cliff. If that route proves too steep and treacherous, the team has decided they will head over to the spot where they first entered Endurance and exit the way they came in.
Once back out on the plains, the plans calls for Opportunity to head south to the spacecraft's original heat shield and nearby rugged terrain, where deeper rock layers may be exposed. "The heatshield was dropped off as the spacecraft was plummeting toward the Martian surface back in January," Squyres reminded. "It hit the surface going well over 100 miles per hour -- and it's probably the deepest, fresh hole we're going to find around here; moreover, we can study heatshield and learn more about how to build better heatshields for future missions to Mars."
Once she has checked out her heatshield, Opportunity will then "set sail," said Squyres, and rove her way clear across the plains to the Etched Terrain, about 2 to 2.5 kilometers to the south of where the rover is now positioned. There, they hope to find the same kind of rocks as they found in Endurance, but without the fracturing or rupturing as those in the crater. "The Meridiani plains are very flat, very smooth, and very good for driving. Etched Terrain is the place where we believe these same rocks have been exposed, not by a violent fracturing impact event but gentle erosion by the wind over many millions of years."
Rover Health Status
No one can say for certain how long the rovers will hold up, though some projections maintain that this dynamic duo could rove through next year. In any case, the plan is "to use these rovers as long as we can, get the most out of them that we can and try to make the effort that people put into these rovers pay off as best we can," said Jim Erickson, rover project manager for the rovers at the Jet Propulsion Laboratory (JPL), where Spirit and Opportunity were designed and built.
One thing that is certain is that the power situation will now begin to improve, because with the days will be getting longer as the winter season comes to an end and Mars' orbit getting closer to the Sun, the rovers will be able to take in more sunlight on their solar arrays. "As the Martian day increases and we have more energy available, we expect to ramp up the amount of activity on both vehicles," said Erickson.
Although MER engineers have not yet determined why Spirit temporarily failed to release her steering brake when first commanded to do so last week, that incident was, they now believe, a one-time occurrence and the rover seems to be operating as if it never happened. At the same time, rover drivers have not experienced any more trouble with Spirit's left front wheel, which caused problems last summer and which is why she's driving backward more often these days than forward. That problem, "has stabilized," said Erickson, "with the effort we're taking in using care in making sure that we're not using that wheel any more than we have to."
While no one actually expected either rover to last this long, both Spirit and Opportunity have "only minor problems," Erickson said. "There is really no way of knowing how much longer they will keep operating, but we are optimistic about their conditions," he added. "As the vehicles age, we may very well see aches and pains begin to happen here and there, and we'll have to learn to deal with the problems as we go."
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