Opportunity Gets Eyeful of Endurance
"It has been a great week on Mars," Mars Exploration Rovers principal investigator Steve Squyres reported at a press conference yesterday at the Jet Propulsion Laboratory (JPL).
While Spirit kept on truckin' to the Columbia Hills, her distant locale in Gusev Crater, Opportunity completed a six-week trek across Martian flatlands of Meridiani last Friday, to arrive at the stadium-sized Endurance Crater. The rover promptly returned a color panorama that Squyres presented at the conference and deemed "the most spectacular image yet" from the MER mission. "It's spectacular for the sheer scenic grandeur of it, and it's spectacular for the scientific potential that it offers," he said.
Meanwhile, both Spirit and Opportunity remain in good health, and so far are drawing plenty of power from the Sun to keep their batteries powered up. In fact, the battery on Opportunity -- which has been losing power because of a heater on her arm that was stuck on -- is now near a full state of charge.
The RSVP software -- which was written in-house at JPL and was uploaded to the rovers about a month ago -- features new driving modes and is continuing to prove its worth with every passing day. On Wednesday, "Spirit reached 92 meters" and "during our long traverse leading up to our approach to Endurance, we had a 140-meter record day on Opportunity," noted Brian Cooper, leader of JPL's squad of rover drivers for both rovers. "That's pretty ambitious compared to our original plans of how far we'd want to drive everyday."
With the new software, the rover drivers have been using a combination of modes, Cooper said, including a human-controlled mode "where we're basically giving [the rovers] blind orders," and an autonomous navigation mode "where we tell the rover[s] it's okay to determine whether it's [confronting] a hazard, and if so, either stop or drive around," he explained.
While rocks pose most of the hazards for Spirit at Gusev Crater, Opportunity has been dodging the numerous small craters or depressions in Meridiani Planum, but "still getting in close enough to get images of them," Cooper said.
Although Spirit has had, by far, the tougher road to travel, the rover has nevertheless "exceeded all of expectations in terms of durability, mobility, and health," noted Amy Knudson, a science team collaborator and grad student from Arizona State University. "Because of that, we're able to actually think about going to places that we wouldn't have dreamed we would have been able to make it to at the beginning of the mission."
Spirit From Gusev Crater
After conducting a number of close-up investigations in the trench called Big Hole that she dug at the MayFly soil target about a week and a half ago, Spirit suffered a slight glitch in communications last Friday and wound up taking an unplanned day of rest. As it turned out, a command load was sent up at the wrong uplink rate, according to Squyres. "Spirit was listening just like she was supposed to be for a signal at a certain bit rate and got something different from what she was expecting, but it was our fault down here," he explained.
On Saturday - May 1, Spirit's Sol 116, the rover repeated the microscopic imaging of a target in the trench, stowed her arm, backed away from the Big Hole trench, and took some panoramic camera (PanCam) images of her work. Then, she continued her trek toward the Columbia Hills, driving some 298 feet [90.8 meters], establishing a new, personal best drive record at Gusev.
On sol 117, Spirit drove another 121 feet [37 meters] to a small ridge, where she experienced a pitch up of 12.2 degrees. Engineers believe that the change in tilt caused the rover to recompute her 'goodness map,' which helps her autonomously drive over the Martian terrain; consequently, she declared that it was not safe to continue the drive. As luck would have it, the end-of-drive tilt positioned the solar arrays to maximize afternoon solar exposure, boosting her battery charge.
On Monday, May 3 - Spirit's Sol 118, the rover revved up for another long sol of driving, during which she roved another 303 feet [92.4 meters] down the road, a distance that broke the personal best record she'd set just last Saturday. (Opportunity still holds the one-sol driving record of 459.3 feet [140 meters].)
Sol 119 proved to be another down sol for Spirit. An uplink configuration error prevented the sequence load from successfully getting on board the rover, so rover controllers deleted the afternoon communication sessions, enabling the rover to charge her battery during a long afternoon nap.
After imaging a rock called Tulula with her PanCam, Spirit was back on the road Wednesday - Sol 120. She successfully executed a human-directed blind drive before using her autonomous navigation system into uncharted territory. As she neared the time-of-day driving limit, Spirit turned and performed penultimate (next to last stop) imaging. Although the final move of the day would have taken her 33.5 inches [85 centimeters] to her ultimate stopping point, the rover did not execute the short drive because she was facing a small sand ridge that she perceived as a hazard. Without a penultimate/ultimate pair of images, rover controllers couldn't be sure the area underneath the rover was clear of hazards. As a result, Spirit spent Thursday - Sol 121 maneuvering herself into a suitable location to use the instruments on her instrument deployment device (IDD).
Meanwhile, at JPL, the scientists and engineers working with Spirit were examining the latest images that the rover has returned from the hills. Even though it will take the rover several more weeks to reach the Columbia Hills, the team members are already identifying possible targets of study and beginning to assess how well the rover might get to them. In addition, the crew is also studying images and surface-temperature data taken by cameras and instruments onboard the Mars Global Surveyor and Mars Odyssey orbiters, which are supplementing Spirit's own increasingly detailed pictures of the hills.
On Thursday - Spirit's Sol 121, the rover scooted past Lahontan Station and Lahontan Crater, said Knudson, and to within approximately 1 mile [1.7 kilometers] of her target - "a little spur that sticks out on the west side of the hills," located at base of the Columbia Hills, said Knudson. "We hope to get to those hills in the middle of June [and] we're targeting Sol 160 as our approach date on that spur," she added.
"The hills represent a different rock unit, likely older than the plains we're on," Knudson reminded. "There are intriguing features in the hills and we want to investigate the processes that formed them. We're especially interested to see if water played any role. We're basically looking for our own Endurance Crater. We're looking for in place rocks with a history that [tell] the geologic history of all the things that have happened at Gusev Crater."
Today - Sol 122, Spirit logged another 213 feet [65 meters] on a touch'n go drive, pushing her odometer past the 1-mile [1734 meters] mark, and putting her ever closer to the Midway point, according to flight director Chris Lewicki.
Opportunity From Meridiani Planum
Following a 164-foot [50-meter] drive on Thursday, April 29, Opportunity completed her final approach of 56 feet [17 meters] and pulled up to the western rim Endurance Crater last Friday, April 30 - Sol 95. As usual for such final approaches, the rover drivers sent a series of sequences up to the rover. "The next day we came in and actually waited to see how close it got, and we're very happy to see that it got right up to the edge, to about 40 centimeters from the lip where it started to tip down," informed Cooper.
Opportunity had come to a stop at the edge of the crater with a positive pitch of 4.7 degrees -- meaning the rover was slightly tilted with her head up, with western side of the crater rim sloping down in front of her with an angle of about 18 degrees for about 56 feet [17 meters]. On her arrival, the rover began surveying her new domain.
Over the weekend and into Monday - Sols 96 and 97, which ended at 12:08 a.m. May 3 Pacific Daylight Time, Opportunity focused on conducting remote sensing of Endurance Crater and some of the features in and around it.
The color panorama of Endurance Crater that she returned reveals that multiple layers of exposed bedrock line much of the inner slope. To really understand how important the Endurance panorama is scientifically, a 'journey' back to Eagle Crater -- where Opportunity landed and spent her first eight weeks on the Red Planet -- is, said Squyres, in order. "When we landed, we looked out across a crater and saw a wall of bedrock we nicknamed the Great Wall, because it looked like this massive wall of bedrock," he recounted. The science team quickly realized, however, that the outcrop wasn't so massive at all. In fact, it was barely a foot high. That, however, didn't preclude them from discovering what they went to find -- evidence of past water.
"What we found in the outcrop that was exposed there, [was] a wonderful record of water in the past -- the last evaporation of a salty sea on Mars . . . the last dying gasp of a body of water," said Squyres. "But what we didn't know -- what we couldn't know there -- was what came before that. So the question that has intrigued us since we left Eagle Crater is what preceded that? Was there a deep body of water for a long time? Was there a shallow, short-lived playa?"
Although the answer to that question is "literally just a few meters beneath our wheels," Squyres continued, "we didn't bring dynamite and we didn't bring a drilling rig, or any means of getting [to it]." So in order to get to the answer, the team had to seek rocks from deeper down beneath the rocks at Eagle Crater. "We've got to rely on what geologists call the Principle of Stratigraphy - that is, the idea that when you have a set of layered rocks the youngest ones are at the top and the older ones are below that," Squyres explained. "By looking deeper in the crust, you look back in time."
So, Opportunity was dispatched on drives across the wilderness of sand and gravel totaling a half-mile [about 800 meters] to reach Endurance, the deepest crater nearby. Presumably excavated by the impact of an asteroid or a piece of a comet, Endurance Crater is about 430 feet [130 meters] wide and, from the highest point on the rim, more than 66 feet [20 meters] deep. That's 10 times as deep as Eagle.
Putting it another way, Squyres said: "This is a big hole in the ground." And that, he said, "is good news in the sense that it exposes a lot of rock -- this is meters of rocks and that means lots of history. But it's bad news in the sense that this is a dangerous place. At Eagle, we could rove with impunity over whatever there was. Here, there are cliffs the rover could fall off and die."
While the outcrop at Eagle was but "a tiny amount of rock," the outcrops at Endurance are substantial. "We see enormous outcrops, much bigger than anything we've seen before of layered rock," Squyres informed. Some of these rocks, they believe, lie lower than the rocks seen at Eagle Crater, and thus preserve a record of what the environment on Mars was like before the salty standing body of water evaporated to produce the telltale rocks that were discovered in the tiny Eagle Crater.
An exposure of outcrop in a cliff high on the inner wall across from the rover's current position, for example, reveals a stack of layers 16 to 33 feet [5 to 10 meters] tall. Other exposures around the inner slope of the crater may be more accessible than that cliff, and chunks from the same layers may have been thrown out onto surrounding ground by the crater-forming impact. "At the tops of these cliffs --- and they are cliffs -- you'll see busted up rock, rubble, the ejecta that was thrown out of the impact, but below it is intact rock and that intact rock is the stuff that contains the record that we hope to be able to read," said Squyres.
"This is fundamentally different from anything we've seen before," Squyres added. "It's big. It's massive. It has a story to tell us."
The bands of rock or layerings in the outcrop visible in Endurance Crater, Squyres elaborated, not only look different to human eyes, they look different to PanCam and to mini-TES. "They are telling us a story about a different environment. When we look at the outcrop with mini-TES, we don't see any trace of sulfates that we saw at Eagle. That doesn't mean there isn't any there, but it means there is not enough to show up nicely in the mini-TES spectrum." And, although it looks more like a basaltic composition, that doesn't necessarily point to a lava flow. "Mars is a basalt planet, and if you take the salt and you grind it up to make gravel or sand or silt, you're going to get something that looks like basalt to mini-TES," he explained. "This is probably a sedimentary rock made up of particles that have accumulated on a fairly flat surface. Did they accumulate in air? Did they accumulate in water? That's the kind of thing we're going to try to settle."
One of the things the scientists will be looking for in Endurance Crater is crossbeds. "Back at Eagle Crater, we saw tiny crossbeds that were indicators of current flow and flowing water," Squyres reminded. "Here, in some of the PanCam images, we're thinking that we're seeing from a distance -- and this is preliminary -- what look like great big, honkin' massive crossbeds, the kind of stuff that forms when you have dunes. We may be looking at a dune environment. We're going to be looking for evidence of a beach environment. I don't know what it's going to be, but it ain't what we saw back at Eagle. It's something different."
Still, Endurance Crater features some familiar objects. "If you look within the crater, you'll see the hematite bearing spherules -- the blueberries as we call them -- are all over the ground along the upper reaches of the crater, but when you get deeper down in the crater what we're seeing is mostly sand with a composition like that of basalt, which is what lies under the blueberries, the hematite we've been driving across," Squyres pointed out.
"There's a lot for us to figure out here, but the most appealing, the most attractive, the most scientifically important part of all is this lovely exposure of bedrock that is going to tell us much about what happened in Meridiani Planum before the rocks at Eagle Crater were laid down."
On Tuesday and Wednesday - Opportunity's Sols 98 and 99, the MER team took advantage of the rover's vista point at the western rim of Endurance Crater by recording some high-resolution mini-TES readings of the far wall. Once her work at that spot on the rim is complete, the plan calls for Opportunity to circle the rim of Endurance, making make a counterclockwise loop around the crater, observing the interior from various angles with all of her remote sensing instruments.
When NASA sent astronauts to the lunar surface more than 30 years ago, agency officials decided against sending them into craters as fresh and steep as Endurance, but Opportunity may be able to do what no human has done before on another planet. Scientists and engineers have already begun to identify interesting science targets, assessing how difficult it would be for the rover to descend partway into the crater and climb back out, but no decisions have been made yet with regard to going into Endurance Crater or even part way in.
"The focus of our attention now [is] looking to see what it's going to take to drive around [the rim] safely, and then what it would take to drive in," said Cooper. To that end, they are analyzing the slopes and slippage, using previous data garnered from Eagle Crater, and data from experiments in the JPL rover test bed with various at soil samples -- "all of which will feed in to a final decision whether we'll go in or not," he explained.
"The slope right in front of us averages 18 to 20 degrees, so getting into the crater is no problem, but we have a lot more work to do to assess whether we could get back out," Cooper continued. "Unlike the hazards we faced coming out of Eagle Crater, here we are very, very aware that if we go into the crater from the wrong side it could be the end of the mission, so the tension, the amount of excitement everyday to make sure that we're doing this right is building."
Even the planned 360-degree traverse around the rim, however, requires careful navigation. "If you don't go close enough to the lip, you can't look in, but if you go too far, you could fall in," Cooper pointed out. "We're going to have a very interesting few weeks."
"We're going to proceed cautiously and we are going to proceed slowly, very, very methodically here," Squyres added. "What we seek to do, first and foremost, is to characterize what this crater is like" by conducting "an initial reconnaissance -- the same thing you would do if you were there and you were a human geologist. You walk around the thing. You look at it [and] get a sense of what you're dealing with. Then what we hope to do is find places along the rim of the crater where we may actually have safe access, traversable access to the rocks that form the outcrop. That's the plan of attack we're choosing."
The initial 360-degree traverse around the rim will "take weeks," said Squyres. "This is not something we're going to go about in a hasty fashion. It took four sols just to do the remote sensing that we've done from this first spot on the rim and we're going to two more spots on the rim and do a very similar sort of thing. That will give the rover drivers enough time to figure out how they feel about going down into this thing or not. Then when the time comes, when all of that safety assessment has been done, we're going to have to make some decisions about what we're going to do with this vehicle," he said.
"If the decision -- as we balance the risk against the benefit -- is to go in, then we're going to have some other things to do first," Squyres informed. "We went blasting across the plains from Eagle to Endurance very quickly, shootin' by all sorts of good stuff along the way. We were eager to see what Endurance looked like, but we're not done with science out on the plains yet. There's a lot of stuff out there," as well as questions that beg for answers.
As Opportunity cruised across the Meridiani plains, "there were three big events -- one was to stop at the Anatolia fractures, another was the trench that we dug [at the Anatolia fracture site], and a third was to stop at Fram Crater," said Squyres.
"At Anatolia, the fracture system, we took a quick survey look and then moved on, but there are some big questions associated with that," he continued. "How did those cracks form? It looks like you've got some kind of crack in the rock and then you've got sand and blueberries draining down into it, but what is responsible for the fracture? Is this made of a rock that is somehow soluble in water and the cracks being widen by the action of water? Is it is just a physical fracture that's opening up across the crust that's stretched? We don't know. The other thing I find really intriguing about these fractures is that it's kind of the one place we can go and look at exposed bedrock that was exposed by a process other than impact, and that means it's not going to be as busted up, as fragmented, fractured, jumbled. You might actually see bedrock that has not been significantly damaged, and that's an intriguing possibility."
At the trench, the science team members saw a single layer of blueberries on top of a sandy surface. "How deep does the sand go before you get down to the rock - the cracks might actually be the best place to answer that," Squyres suggested. "We saw wind ripples all over the place, but there's a lot we haven't studied about them. And sprinkled all over the plains, we see these cobbles, rocks that are a few inches across We never had one right in front of us, so we haven't looked at those and we don't know what those are," he said.
"At Fram Crater - mini TES [found] one rock across the crater -- a difficult part to get to so we didn't bother with it at the time -- that looked kind of different from all the other sulfate-bearing stuff we've seen, and don't know what that is," he said.
Then there's the heatshield, which lies some 150-200 meters to south. "I'm intrigued by the heatshield, because this thing came screaming down through the Martian atmosphere and it underwent some ablation," said Squyres. "But how much ablation did it undergo? We could take some pictures of the heatshield that might help people design better heatshields for Mars at some point in the future. The other thing is that it hit the ground at 200-300 miles an hour. It was really screaming when it hit the ground and that probably dug the deepest fresh hole we're going to find anywhere - certainly deeper than we can dig with our wheels."
Although the heatshield is tempting, the agenda of the moment calls for Opportunity to complete her initial survey around the Endurance rim before taking any side trips. "The focus of the next few weeks is going to be a good tight traverse around the crater looking carefully at ejecta. The heatshield is pretty far out to logically make it part of that traverse, but the direction we're expecting to head when we finish up at Endurance is south anyway, so the heatshield would be on the way once we finish up and scoot off to the south," he offered.
Although Squyres said that he could envision circumstances wherein they might consider sending the rover into Endurance on a one-way trip, it is not the most likely scenario. "This is a priceless resource and it's not something you take risks with in a thoughtless way. If we decided that going in was the right thing to do - even though there was some risk we would never get out again -- we would look very, very hard at that decision before we would make it -- and it would be reviewed at much higher levels at JPL and at NASA headquarters. I could imagine doing that if we really concluded the stuff in there was important enough scientifically and we had no other way to do it."
In any event, if the team did decide to go into Endurance and thought there was some chance that they might never get out again, then, said Squyres, "we would go down and hit the heatshield, among other things, before we go in."
Right now, the most likely point of entry, at least for a 'toe-dip' into Endurance is an area they've dubbed Karatepe, after an archeological site in Turkey where a bi-lingual document was found dating around 8th century BC, reminiscent to the Rosetta Stone. Visually, from the most recent panorama it looks like it's most easily accessed. On Thursday, Cooper and his cohorts were at work putting together a sequence to drive over to Karatepe, about a 16-meter drive from the rover's position at the rim.
Karatepe features the banded rock, which, Squyres said, appears to be the same banded rock that shows up in other areas around the rim, which they have named Kalahari, Namib, and Burns Cliff. "We think all of that is exposures of the very same material." And of all of those, "Karatepe is the place we're going to go driving by first is the spot where we might have the best chance of getting in, because it involves a traverse over rock."
No one has forgotten the 100% slip that happened the first time Opportunity tried to rove out of Eagle Crater, and the similar "fluffy soil" as Cooper described it should be recognizable remote sensing. Still, there is no guarantee.
"We certainly know what we saw over at Eagle -- we know what it looked like to PanCam and we know what it looked like to mini-TES," Squyres added. "But this is Mars -- and you could have two things that sort of look similar that are in fact very different. One of the keys to success here may be to try to stay off of loose soil as much as we can and try to stay mostly on kind of rough irregular bedrock surfaces where the wheels can get some significant [traction]. Assessing how well we could do on that kind of stuff is going to be a big part of the assessment that's going to be coming up."
Although the science team members have been talking about a toe dip, "there's no evidence that it's necessarily safer than just going in all the way," proffered Cooper. "Most of the enthusiasm to go in is from the science team because there are obvious benefits to getting closer, and applying all the instruments on the arm directly and getting close to all this other stuff. The engineering team tends to be more cautious and our enthusiasm is to live to rove another day. The most important thing we do is we assess what is safe. If the decision to go in is made with the understanding that there is high risk, then I agree with Steve's assessment that we want to do a lot more in the plains and get a lot more under our belt."
All that said, Squyres added: "In order to really work out this puzzle -- to determine if [the outcrop] is really sandstone or something different from that, to determine what kind of environment it was laid down in, what the stuff is glued together with, and what the nature of the transition is between what we see in the cliff and what we see further up -- we've got to get rover into play so we can use the arm, with all of its powerful instruments. We've got to use all of the tools in our toolkit to really nail this stuff." Although the outcrop in Endurance "looks for all the world to look like a big mass of sandstone, looks can be deceiving. We're a long way from understanding this. This is like the mission has started over again. Give us a few weeks."
Yesterday, Opportunity continued to gaze into and around Endurance Crater from her vantage point on the western rim, conducting remote sensing, including gathering of imagery of two potential traverse targets just inside the northern edge and southwestern edge of the crater.
Today, the rover team tested the Deep Sleep mode for the first time. In this mode, all power to the rover is turned off over night -- a 'fix' written into the new RSVP flight software that allows the team to get around the stuck heater problem. "Now that we know this works, we can opt to use it to save energy," said Lewicki, who was serving as Opportunity's flight director for the day.