During the last three weeks, the Mars Exploration Rovers have been continuing to rove and explore their respective regions of Mars. Spirit has continued her trek to the Columbia Hills in the Gusev Crater area of Mars, while on the other side of the planet, at Meridiani Planum; Opportunity has been investigating Endurance Crater.
Spirit has been making "breathtaking" progress over tough terrain, as lead scientist Steve Squyres put it last week -- despite having been out of commission for five days during the last couple of weeks. As of Wednesday, June 2, the rover had traveled 1.85 miles [2.98 kilometers].
She had .35 km to go to get to the base, and just .6 miles [1 km] to go to get to Lookout Point, where she is scheduled to stop and take it all in, with images and data from the various other instruments in her toolkit. "We’re starting to see some great features jumping out already," science team member James Rice announced at a press conference yesterday at the Jet Propulsion Laboratory (JPL).
Now less than a half-mile away from the base of the Columbia Hills, Spirit is on schedule to arrive at the base by mid-June. Even though software errors caused her to experience a few unexpected reboots and lose five valuable sols, Spirit remains in good health and is continuing on in fine form, according to software engineer Joe Snyder. Spirit is headed now for an area dubbed West Spur, azimuth 120-122-degrees. "That's where our first landfall will take place in the Columbia Hills, probably in a week to 10 days from now," Rice said.
Meanwhile, Opportunity has been traversing the southern rim of "picturesque" Endurance Crater, acquiring two panoramas and investigating several interesting rocks that blasted out from this stadium-sized impact bowl long, long ago, reported Matt Wallace, Opportunity mission manager. The ultimate goal now is to extend the MER team’s newfound discovery about the region's wet past. The notion that all of Meridiani Planum may once have been covered by an ocean is still on the table, science team member Wendy Calvin confirmed yesterday. And so are a lot of other hypotheses.
Although the initial plan called for Opportunity to make a counterclockwise traverse around the entire rim, the MER team has decided to turn around and head back to Karatepe. This area has appealingly gentle slopes, and may offer the science team the best hopes of getting inside the crater to get at a thick dark band of what they presume is basaltic rock, and then getting back out again.
The decision of whether to send the rover down inside the big hole has not yet been made, Wallace said. In fact, that's one of the reasons they're going back to Karatepe -- so the rover can collect the data needed for NASA to make that decision. It's basically a science versus engineering debate: the scientists want to go in and the engineers want to be sure they can get back out. The decision, therefore, must balance the allure of rock layers in the crater and the operational safety of the rover.
SPIRIT FROM GUSEV CRATER
Sols 123 - 148
Since breaking her own one-sol distance record with a 318-foot [96.8 meter] drive that pushed her odometer over the one-mile mark -- to 1.04 miles [1,669 meters] -- on Sol 121, Thursday, May 6, Spirit has continued her trek to the base of the Columbia Hills with long daily drives. This landscape -- unlike the "parking lot" her twin in Meridiani Planum is crossing "is hilly, rock-strewn terrain," says Squyres of Cornell University. "This kind of pace bodes well for having lots of rover capability left when we get to the hills."
Named after the seven Columbia astronauts who lost their lives in February 2003, this complex of hills appears to hold older rocks and geologic materials than the ones on the plain that the rover has been crossing. These older rocks and layers, the scientists, believe, probably hold valuable clues to figuring out what past environments existed in Gusev Crater, and determining whether or not water played a significant role in those environments.
Might all those boulders cause Spirit trouble? "There are some whoppers out there, but I think it's not going too be a problem for trafficability for the rover," said Rice at the press conference. Once Spirit does get to the hills, however, she probably won't be scaling any serious hills. "There's probably a lot of that stuff we'll never get to because of the slopes, but there's no reason to go up a hillside if you got [what you want] it sitting at your feet."
Spirit drove nearly every day in the month of May, and will now reach the Hills well before Sol 160. On most days the engineers have been directing half of the day’s course, with the rover relying on her autonomous navigation system for the other half. Along the way, the robot field geologist has been conducting touch-and-go's on various rocks and patches of soil, as well as conducting cloud observations, atmospheric sky and ground remote sensing, and imaging with the panoramic camera (PanCam) and navigation camera (NavCam), in addition to her standard post-drive observations.
After completing the Sol 124 drive, Spirit took a 360-degree afternoon panorama of her surroundings with the navigation camera, and then drove on, setting a new one-sol driving record of 405.8 feet [123.7 meters] on the following sol. On sol 126, the rover used her alpha particle X-ray spectrometer (APXS) and Mössbauer spectrometers, as well as her microscopic imager to examine a target called Lead Foot (in honor of the big drive on Sol 125). The Mössbauer was used as the feeler for all these activities, and as it turned out, the instrument touched down on rocks rather than soil at the Lead Foot location, causing the Mössbauer and microscopic images to be out of focus.
Spirit spent most of her Sol 127, Wednesday May 12, driving, logging another 229.7 feet [about 70 meters] and then finishing her day with some camera and mini-TES observations with her PanCam, NavCam and mini-TES. Following a PanCam observation of a rock target called Flat Head on Sol 128, the rover took a rest, then embarked on another long drive -- 295 feet [90-meters] -- toward her destination.
After waking on Sol 129, Friday May 14, Spirit performed 45 minutes of science observations and then settled down for a morning nap. With plenty of energy stored, the rover roved on, another 102 feet [31 meters] across the surface. After an engineer-directed drive, the rover spent 45 minutes using her autonomous navigation system to try to drive down the side of a small ridge. The backside slope of the ridge, however, was too steep, and the autonomous navigation system had Spirit turn in an attempt to find another way down. Unfortunately, a couple of large rocks close to the ridge prevented the robot field geologist from finding a safe path down. At the end of the drive sequence, Spirit was supposed to complete a stutter step to get in proper position to do work with the instrument deployment device on Sol 130, but the rover was unable to complete this final positioning or the ultimate post-drive imaging.
On Sol 131, the rover encountered more trouble -- a software glitch -- and wound up taking an unplanned break that sol and the following sol. "It happened about 15 minutes after Spirit woke up to continue some driving," said Snyder. "The rover correctly responded to the unexpected reboot by halting further activities, recharging its batteries and then performing the next scheduled communications session. When the telemetry from this session was received, the flight software team quickly determined that the reboot occurred because an update to an area in memory was attempted, while access to that area was restricted. Flight software treats an attempt to update write-protected unrecoverable error and forces a reboot."
In order to update protected memory like this the flight software follows a 3-step process: first the flight software disables the write protection then it actually performs the update to the memory and then it reestablishes write protection, Snyder explained. "The specific update that failed happens to occur every time the rover wakes up -- and for all the 130 previous sols on Spirit and 109 sols on Opportunity -- this particular update occurred several times each sol without incident. However, this time that 3-step update process was interrupted, thus when the memory was attempted to be updated it failed leading to the reboot." Although this flight software handles such interruptions routinely, "in this case, the software that ran during the interruption changed that write protection access and that is the flaw in the software that caused the problem," Snyder explained.
Since the team was confident this is an extremely low probability event, it has not adjusted the planning process to avoid the miniscule period of vulnerability. (Opportunity has the same vulnerability to the fault.) "Other than changing the software there is no guaranteed method to prevent this thing from happening again," Snyder said. "However, because the window of vulnerability is small, this 3-step process must be interrupted precisely between steps 1 and 2, and it must be the flawed software that does the interruption. The operations team has decided to accept the risk that the problem could reoccur and not patch the flight software for the time being."
The recovery from the anomaly occurred quickly and, Snyder said, "the health and safety of the rover was maintained throughout the event." The software error left rover planners with some uncertainty about Spirit's final position and attitude, however, so Sol 132 was spent re-establishing that knowledge with imaging of the rover's surroundings with the Pan Cam, NavCam, and hazard avoidance camera. The unplanned break did have a silver lining though -- it resulted in fully charged batteries, paving the way for yet another long drive.
On Sol 133, Spirit roved for 370.7 feet [113 meters], setting a record for autonomous navigation drives of 256 feet [78-meters], and putting her roughly a half mile [780 meters] from the base of the Hills. And on Sol 134, she kept on truckin', to put another 359.3 feet [109.5 meters] on her tires, and with .42 miles to go to get to the base of the hills.
The rover actually got to do some in situ geologic research of her surrounding area with her APXS and Mössbauer spectrometers and MI on Sol 135 -Thursday May 20. After stowing her instrument deployment device (IDD), she spun one of her front wheels to dig a trench, and then imaged that trench with the cameras on her mast. But the following day, Sol 136 - last Friday, Spirit encountered another, different computer error. "It occurred during another routine event, stopping imaging operations about eight minutes before a scheduled communications sessions," Snyder confirmed. "The rover again correctly responded to the reboot by halting further activities, but now since it was so later in the afternoon, the scheduled communications could not be performed and the rover shut down for the evening."
The team spent Sol 137 reestablishing contact with the rover and acquiring telemetry. "After we did acquire telemetry from 2 UHF passes -- one with Mars Global Surveyor (MGS) and one with Odyssey -- the flight software team quickly determined that this reboot occurred because a command to the camera interface was attempted while imaging operations were being terminated and the interface did not respond. The software treats this bus error as another unrecoverable error and forces a reboot," said Snyder.
“For the other 135 previous sols on Spirit, and 114 previous sols on Opportunity, imaging operation have been halted many times, often multiple times per sol with no incident, "however, this time we caught a bus error," Snyder said. "After review of the camera interface power software, it was discovered that it was indeed possible to cue up command to the camera interface, and then turn off the interface power as part of stopping imaging. The cued command was not cleared, so the result was a bus error. This too was a flaw."
As with first anomaly, the "window of vulnerability" to such an error is very small, but -- unlike the first one -- there was a workaround operationally so that the team members could allow any kind of cued commands to clear out before turning off the interface power. Spirit recovered from this anomaly relatively quickly, though two sols were spent recovering communication, reestablishing position and attitude knowledge, and acquiring additional telemetry, which means she lost about three sols. Normal operations resumed on Sol 139 and Spirit continued on her journey.
Are these two errors a sign of the dark days that will come for both these rovers? "No," Snyder said. "We don't believe it's a suggestive of deterioration. We think it's just unfortunate that we had two low probability events happen in the same week. These two problems could have happened in the beginning of the mission or middle or at any time." Even though these specific errors were not due to age or duration of mission, "the longer we operate the rovers and the more we do with them, it is conceivable we could trip across problems of this sort of other problems," Snyder added.
Refreshed and recharged, Spirit ventured on, homing in on a region of the Columbia Hills called Husband Hill [after Rick Husband, commander of the Columbia]. The rover's specific target there is an area called West Spur -- azimuth around 120-122 degrees. "That's where our first landfall will take place in the hills, probably roughly one week to 10 days from now," Rice announced.
Overall, the Columbia Hills by themselves "look pretty battered and beat up," Rice continued. "There are some old impact crater rims superimposed on the hills, so this is a really pulverized, beaten-up area of terrain. One thing we're seeing even at this distance are these talus slopes, which are slopes covered with boulders that are being shed from layers above rolling down hill," he explained. Although Pathfinder and Sojourner landed in an area on Mars where there were mountains, dubbed Twin Peaks, they were, at one kilometer, too far away. "Now with our more capable rover we will get to the Columbia Hills shortly and for the first time we will investigate what the Martian hills are composed of, and get to the question of how they formed. We're still several hundred meters away, but we are seeing large rocks there, and right now they beg the questions -- where were those rocks, and how did they get there?"
The science team has also noted that there may be an outcrop embedded in one of the slopes and they have seen evidence of old, buried craters as well as relatively fresh new ones, and now also have "tantalizing" hints of terracing and flow fronts. The hints of terracing are intriguing, Rice said, "because this is telling us there is some layered material in there . . . [implying] that there are cliff and forming rock type members [there]." The flow fronts, meanwhile, appear to be fairly young, and are "lapping up onto these older crater plains material and all of this stuff is surrounding the hills," Rice offered. "If think about it, the Columbia Hills have been swimming in the plains we’ve been driving across for the last five months."
As Spirit closes in on her destination, the excitement among the team members is building. "This is a brand new mission starting right now," said Rice. "We’re fixin’ to go on another great voyage of exploration here."
OPPORTUNITY FROM MERIDIANI PLANUM
Sols 103 - 128
After waking from her first DeepSleep on Sol 102 - Saturday May 8, Opportunity continued on her counterclockwise traverse around Endurance Crater, with stops planned along the way to acquire panoramas and remote sensing of the big hole, and to investigate a few of the more interesting rocks that were blasted out from this stadium-sized impact depression long, long ago.
Since that inflight checkout, the Opportunity crew has been using DeepSleep "in earnest," according to Wallace. By putting Opportunity into DeepSleep, energy needed for other tasks can be conserved. DeepSleep, as was reported here in an earlier update, is, in essence, a 'fix' that was written into the new flight software that was uploaded a couple of months back in response to the heater on the IDD shoulder that has a switch that has stuck in 'on' position. "At night, when it's cold enough, this heater is energized and dumping heat, and therefore energy, into an arm that we are typically not using and don't need to be spending that energy there," Wallace said at the press conference yesterday.
"The advantage to Deep Sleep is that is fully de-energizes the power bus on the vehicle and de-energizes that heater overnight and allows us to save that energy," he continued. "The energy consumption that we'll be saving is something on the order of 170 watt hours, which is a large percentage of our daily energy allocation of about 500 watt hours. A lot of that 500 watt hours is used for housekeeping and so the percentage left over for science and the activities we're interested in doing is even smaller, and, as you can imagine, the heater takes a big bite out of that "
There are also a couple of disadvantages. "When we're de-energized overnight, we can't wake up until the Sun wakes us up the next morning [when the Sun strikes the solar panel array] so we’re unable to take advantage of overnight UHF passes," Wallace explained. More significantly, however, since DeepSleep mode effectively turns off power to all the main electronics at night, so they cannot turn on another heater to keep the mini-TES electronics box warm. As the environment becomes increasing, bitterly colder -- winter is coming in the south hemisphere of Mars -- the overnight temperature of that box, which sits at base of the mast of the vehicle just underneath the deck, has been dropping. "It's now regularly getting down below -50 degrees Celsius, and into a regime where we know we have susceptibility with this instrument," said Wallace. This is not entirely unexpected, though. Even before they knew they had a failed switch, they knew the cost of the energy overnight to keep the mini-TES electronics box warm may eventually be something they could not afford. "It's just coming a little sooner than we might otherwise expected," noted Wallace.
To everyone’s relief -- especially Phil Christensen's, the lead scientist for the instrument -- the mini-TES instrument has been "surviving through the night and operating just fine," Wallace reported. Data returned from the inflight test on Sol 102 showed the temperature reached -50.8 degrees Fahrenheit [-46 degrees Celsius], a bit warmer than the spectrometer's lowest proven temperature for functionality, -58 F/ -50 C. "But at some point it will get colder enough that that susceptibility will damage the instrument and in all likelihood make that instrument unavailable to us."
When that happens "we'll be restricted to what we can do with PanCam 13 filters and to the extent that we can correlate the mini-TES we have already with the PanCam color data," explained Calvin. "It would be a serious loss and I would hate to see us lose the instrument," she added. "At the same time, we do have a tremendous wealth of mini-TES already to analyze and work with, so I think we have a very rich data set and we're in a bonus situation. I think it’s just one of those risks you have to accept."
On Sol 103 - Sunday May 9, Opportunity traversed approximately 43 feet [13 meters] farther south along the eastern rim of Endurance Crater, reaching the beginning of the Karatepe area. On the following sol, the rover approached Lion Stone, an intriguing, dark rock at the crater's edge that stands about 4 inches [10 centimeters] tall and is about 12 inches [30 centimeters] long, which they believe came from below the area's current surface level, tossed up by the impact that excavated Endurance Crater. It was "a rock that looked like nothing we'd ever seen before," Squyres said at a press conference last week in Montreal, Canada, where he and Christensen, of Arizona State University, presented previews of rover-science reports at a joint meeting of the American Geophysical Union and the Canadian Geophysical Union.
On Sol 105, Opportunity acquired a series of microscopic images (MI) of Lion Stone and the surrounding soil. During the next couple of sols, she did some remote sensing, conducted integrations on the rock with her Mössbauer and APXS spectrometers, and also performed a successful 'bump' maneuver to reposition herself so the top of Lion Stone was in position to use her rock abrasion tool (RAT) on Sol 107 - Thursday, May 13. One of the remote sensing targets for the Pan Cam was the heatshield that protected Opportunity during its fiery trip through the Martian atmosphere. The heatshield impacted approximately 820 feet [250 meters] south of Endurance Crater.
On Sol 107, Opportunity successfully RATed a hole into Lion Stone. Since the surface of the rock was fairly uneven, the tool had to work through some high spots before getting a good bite on an area of the rock where it could produce a full circular hole. That night, the rover conducted a nighttime integration with her APXS to uncover the elemental composition of the inner part of the rock. The following sol, Opportunity finished her work at Lion Stone by analyzing the RAT hole with the Mössbauer spectrometer and taking MI pictures to create a mosaic of the hole, then moved away from the odd-looking stone, and continued her traverse, stopping for the night on the top of a small ridge so she could get a better view of where to drive on Sol 109.
Meanwhile, back at JPL, the Lion Stone data was being reviewed. In some ways, this rock resembles rocks that provided evidence of past water at the smaller crater, Eagle Crater, in which Opportunity landed. Like them, it has a sulfur-rich composition, fine layering and spherical concretions, and likely formed under wet conditions. "However," Squyres said, "it is different in subtle ways from what we saw at Eagle Crater -- a little different in mineralogy, a little different in color. It may give us the first hint of what the environment was like before the conditions that produced the Eagle Crater rocks."
Inside Endurance Crater are multiple layers of exposed rocks that the scientists are confident will provide information about a much longer period of environmental history and from the viewpoints around the rim, Opportunity's mini-TES is returning data for mapping the mineral composition of the rocks exposed in the crater's interior. "We see the coarse hematite grains on the upper slopes and basaltic sand at the bottom," said Christensen "Most exciting is the basalt signature in the layered cliffs." Basalt is volcanic in origin, but the thinness of the layers visible in the cliffs suggests they were emplaced some way other than as flows of lava, he said. "Our working hypothesis is that volcanically erupted rock was broken down into particles that were then transported and redeposited by wind or by liquid water," Christensen said.
Although the stack of rock layers at Endurance is more than 10 times thicker than the bedrock exposure at Eagle Crater, it is still only a small fraction of the 650-foot [200-meter-thick] stack seen from orbit at some other locations in Mars' Meridian Planum region. A close-up look at the Endurance Crater rocks could help with interpreting the other exposures seen from orbit. "It's possible that the whole stack was deposited in water -- some particles washed in by flowing water and others chemically precipitated out of the water," Christensen said. "An alternative is that wind blew sand in."
After finishing her study of Lion Stone, Opportunity headed on toward Pan Position 2. On Sol 109 and sol 110, May 15 and May 16, respectively, Opportunity drove about 135 feet [41 meters] each sol. She ended her drive on Sol 110 with a 'scuff' of the soil and rocks under the front wheel. This scuff action produced an interesting dislodged plate of some kind, which the scientists have not yet explained.
At this point, however, Opportunity was driving along the south edge of Endurance Crater, with a southward tilt of about 8 degrees. Since the Sun is now at higher latitudes with the onset of winter in the southern hemisphere, a southward tilt limited the amount of direct sunlight that could reach the solar panels, making it more difficult for the rover to perform many activities. Although rover planners had built a Sol 112 plan that deleted two of the three UHF windows to help the battery recover to its normal state of charge, a Deep Space Network configuration error prevented the command load from reaching Opportunity on Sol 112 and, as expected in such cases, the rover executed the onboard run-out sequence, which included an hour of remote sensing and the three on-board UHF communication windows. The rover's 131-foot [40-meter] traverse along the southern edge of Endurance Crater on Sol 111, and an error with a Deep Space Network command transmission the next sol left the rover in a low battery state of charge. Although still perfectly healthy, she was "sluggish." So, Opportunity got to take it easy on Sol 113, and she spent the sol sleeping and recharging.
On Sol 115 - Friday May 21, drove 38.4 feet [11.7 meters] and pulled up to rest just 10 feet [3 meters] from the edge of Endurance's rim as planned for the second panorama. The rover controllers had commanded the rover to go another 3.9 inches [10 centimeters], but Opportunity decided to stop when she saw the edge of the crater in her NavCam images. As it turned out, she was being over cautious -- she could have completed the drive and now the team is considering changing the way they send commands to prevent her from making such overly conservative calls. The next morning, Opportunity turned slightly to the right and crept a little closer to the edge to within just 1 meter of the edge and maneuvered herself into just the right position to set up camp for a few sols to complete the panorama. The rover performed this 4.9-foot [1.5 meter] drive as planned and wound up facing northwest with a total tilt of about 8 degrees pitched nose up. Dead ahead, she faced a sloping drop-off of about 40 degrees.
"Most of the panoramas that we do are with four filters just to limit the data volume we have, then we go back and look with all 13 filters of the PanCam in order to get spectroscopic information and more compositional information," explained Calvin. With those 13-filter PanCam observations and the mini-TES data, they have gleaned that the rocks in the outcrops appear to be basaltic. "We see a basalt character down in the rocks in the crater and also halfway down the slope. This is consistent with what we saw from mini-TES, basaltic signatures at the base of this upper wall-forming unit and then down into the interior. We may actually have a source of this basalt material in the will rock itself that's been shedding into the interior of the crater, so one of the things we really wanted to do was chase down this basalt story both with 13-filter PanCam observations as well as mini-TES observations when we moved to the second panorama position."
The rover managed to acquire "good coverage" of both the upper wall rock and the interior materials with mini-TES, Calvin noted, although the data is still trickling in so all the analyses have not yet been done. In the meantime, she said, "we've been stepping back and looking at what’s directly under our feet." In addition, they have spotted some distinct flat stones nearby. "Between the Lion Stone outlook area and around the southern end of the crater there are very few places where we've seen a lot of these flat, flagstone-type, outcrop materials and most of the shoulder is covered with blueberries and with sand," noted Calvin. “So we've been taking a high-resolution look with PanCam and with the MI [of those] and there are some more materials that show potential ripple marks and actually some braided ripple structures, as well as new vugs and new vug textures near a target called Pyrro.
Once Opportunity completed the Pan Position 2 panorama, the team decided that they were going to instruct the rover to turn around and head back to Karatepe to collect more close-up and remote sensing data needed to determine whether she can drive into the crater and get back out. "We’ve been focusing a lot of energy on trying to make a decision on whether or not we’re going to enter Endurance Crater and that effort is ongoing," Wallace told reporters. "We'll be using a technique called long baseline stereo, which improves the precision even more."
Although the decision to go in or not to go in has not yet been made, the team is preparing for it as if it had. "We've been doing a lot of driving in test facility here at JPL, both in sand and on rocky surfaces, clean rocks, sandy rocks, rocks with these little hematite blueberries on them, trying to characterize the vehicle performance," Wallace informed. The science and engineering teams have also been spending a lot of time looking at the rock characteristics, rock distributions, the surface roughness, and the hardness of the rocks. "There’s a lot of information we need to bring together," he added. "It's clear that the vehicle can climb much higher slopes on rocks and we're in process of nailing down really what [Opportunity’s] capability is, finishing some target selection thoughts relative to where we might want to enter the crater and try to come around to this conclusion this very important decision as to whether or not we want to go in and try to approach one of those targets."
If NASA does decide to give the MER team the green light for Opportunity to go into Endurance Crater, they all know that there is a possibility no matter how much testing they do that Opportunity might not come out. "The risk-benefit equation is still being worked," Wallace said. "We're spending a lot of time talking about this and one of the reasons we are driving back around the south again to Karatepe is to better understand the rock distributions and densities and slope characteristics. What those images tell us will dictate which was we go next."
From all the data acquired to date, Karatepe clearly looks to be the easiest way in and out. Nevertheless, "Our understanding is that we're not going to risk anything that we're going to get stuck on and we're not going to go anywhere we can't get back out of," Calvin told The Planetary Society later. "Really the hope is to [determine] if we are really seeing a really distinct stratigraphy at Karatepe -- is this basalt unit a coherent unit that underlays the outcrop materials that we see at the top, and can we get at it at Karatepe? There may be places where it's exposed at the rim up on the northwest but we're not certain of that. And we have some pretty good views inside the crater. So we want to chase the stuff we know we can see before we go traversing back the other direction and little bit farther around the crater to see if we can see it exposed on the rim. There's a hope that if we decide not to go into Karatepe, that we could potentially get this unit on the outside, but I think we really think our best bet is to get it within the crater or within the upper reaches of the crater wall."