The Mars Exploration Rovers are roving ever on into new territories and deeper into the history books as they close in on the end of one full Earth year of active duty at their respective sites on Red Planet.
It is the dead of winter in the southern hemisphere of Mars, and the twin rovers, which are each investigating an equatorial region, though on opposite ends of the planet, are looking good and functioning well. Having held up through the winter solstice, Spirit and Opportunity are healthier now than anyone really expected them to be by this stage, and there is little doubt that the rovers have plenty of 'miles to go before they sleep.'
A little luck, it seems, is also on their side. There are no Martian groundhogs to indicate for sure, but spring may come early this year on Mars. This much is for certain -- the local environments in both Gusev Crater and Meridiani Planum are beginning to warm up. "I'm no weather forecaster for Mars . . . [but] the temperature on both vehicles has started to go up by a few degrees and there has been a noticeable increase in the environmental temperature," Rick Welch, one of the MER mission managers at JPL, told The Planetary Society today. Every day from now on should only improve in terms of solar power and temperate weather conditions as winter gives way to spring.
Spirit, now into her Sol 333, continues the long hike up Husband Hill in the Columbia Hills area of Gusev Crater. The science team hopes she will uncover more clues the higher she goes about the water they now know was once there.
At Meridiani Planum, Opportunity, now into Sol 313, has finished her work at the base of Burn's Cliff and is backtracking to Karatepe to exit Endurance Crater the same way she came in, maybe as early as this weekend. Then she'll hit the plains and head south, to the heatshield she jettisoned during her entry last January 24.
Spirit has been more challenged by aging 'aches' and 'pains' that are affecting the steering and brakes on her right front and left rear wheels, and has had to contend with much rougher terrain than her twin. But Opportunity has not been without challenges. The journey to the base of Burn's Cliff was more difficult than anticipated and at times the rover slipped as much as 100% in crumbly Martian soil, making little if any progress on some days.
All things considered, the difficulties have been minor. Both rovers are drawing plenty of power from the Sun and are accessing every one of their scientific tools nearly a year after landing. In fact, Opportunity, which landed just 20 days after Spirit, continues to experience increased power from her solar arrays, which somehow were 'cleaned.' Currently, this rover has as much energy as when she first landed - and that is leading many team members to believe these two intrepid explorers have a lot more 'life' in them. Spirit and Opportunity have already survived more than four times longer than their 90-day 'warranties.'
Spirit from Gusev Crater
During the last several weeks, Spirit has continued her climb in the Columbia Hills within the Gusev Crater region, and generally headed toward Husband Hill. The rover conducted systematic atmospheric observations along the way, using her miniature thermal emission spectrometer (mini-TES) and panoramic camera (PanCam), and examined the layered bedrock in the hills with the tools on her instrument deployment device (IDD) or robotic arm, homing in on rocks dubbed Uchben, Palenque, and Lutefisk. By the end of November, beginning of December, Spirit reached a position northeast of a ridge called Machu Picchu and crossed a 656-foot-wide [200-meter-wide] flat saddle area that better positioned her for the hike up Husband Hill.
With her rock abrasion tool (RAT), Spirit dug shallow holes in two locations on Uchben -- identified as Koolik and Chijkbes, then used her microscopic imager (MI) to take pictures of the abraded holes and her alpha particle X-ray spectrometer (APXS) and Mössbauer spectrometer to determine the chemical composition deeper inside the rock. The Mössbauer spectrometer radiation source has weakened significantly since landing through normal decay, so longer integration times are now required to get acceptable data. The spectrometers' identification of minerals and elements in the rocks is integral to the mission -- it's what is providing much of the evidence that these rocks in the Columbia Hills were altered by water in the past.
The MER science team is now analyzing these observations and measurements and trying to figure out how, exactly, water played a role in the creation of what look to be volcanic rocks - if, indeed, they are volcanic rocks. Right now, that is the leading hypothesis, according to Ray Arvidson of Washington University, St. Louis, deputy principal investigator for the mission. The idea is that these rocks originated as volcanic ash that fell from the air or moved in ground-hugging ash flows, and that minerals in them were altered by water. "This is still a working hypothesis, not a firm conclusion, but all the instruments have contributed clues that fit . . . [h]owever, [w]e are just beginning to put together the big picture," he cautioned.
The rocks could have originated, team members agree, by other means. "They could be sedimentary -- laid down by water, or there's still a possibility they could be impact deposits," Joy Crisp of JPL, MER project scientist told The Planetary Society earlier this week.
As the team continues its investigation, more surprises have popped up in the images Spirit has been sending home. In the MI pictures of the targets on Uchben, for example, the scientists have found some intriguing granular, sand-size particles. Many of the particles are sharply angular in shape and some are rounded. The angularity is consistent, according to Arvidson, with transport by an eruption. Particles carried across the surface by wind or water usually tumble together and become more rounded. Uchben's rounded particles may be volcanic clumps, or they may be concretions similar to what Opportunity has found, or they may be particles tumbled in a water environment.
Following her study of Uchben, Spirit drove west, heading to Palenque, a target rock that allured the scientists with its strangely positioned layers. "This was one of those targets where some of the layering doesn't even look parallel and it's hard to tell if it's crossbedding," said Crisp. Although the robot field geologist was unable to get her IDD tools down on this particular target, she took enough images to make a monochrome mosaic with her PanCam before roving on to her next target rock, Lutefisk. Named for the fishy Norwegian fish dish, Lutefisk featured some prominent 'nodules' that the rover examined in detail.
Although these rocks may appear to be different by virtual of their outward appearance, the one thing the scientists can say for sure about the rocks Spirit has examined in the Columbia Hills is that, so far, they are "essentially the same kind of rock," Crisp offered. "What all the rocks we've examined show are signs they have been changed by water. We're still searching for the evidence that will tell us how the rocks were deposited, and what was going on when these rocks were formed."
The team is hoping more clues to the past water story at Gusev Crater will be found higher up Husband Hill, named for Space Shuttle Columbia's final commander Rick Husband. The further up Spirit goes, "the better chance we have," Crisp confirmed. "When you're a geologist looking for answers to questions like this, sometimes it takes finding a slightly different version of the rock, or a rock that's a little bit younger or a little bit older. If this is a sequence and these are layers, maybe we'll see a change and maybe we'll see something diagnostic as we go up the hill. Or as we move laterally, suddenly we might see a texture or come to a whole new unit, and go 'A-ha! Now we've figured it out."
Between Sols 306 and 325, Spirit finished shooting a Thanksgiving panorama with her PanCam, investigated new rock targets Corn, Cocomama, and Butter with her science instruments, and continued to drive eastward between the West Spur and an area where the terrain slopes back upward toward Husband Hill. The trek took her to a targeted position northeast of a ridge called Machu Picchu, where she crossed a 656-foot-wide [200-meter-wide] flat saddle area, roving down a bit to ultimately climb higher up Husband Hill. "Spirit went into the 'saddle' just to get further up into the hills," Crisp explained. "The path was chosen because it offered good solar energy and good slopes. Now we're heading back higher up into the hills, for the top of Husband Hill."
At the same time Spirit has been steadily uncovering clues for the science team, she has been suffering from 'aches and pains' on her right front and left rear wheels. The engineering team at JPL, where the rovers were designed and built, have continued to monitor her right wheel, which has drawn more electric current than it's supposed to since last June and which continues to be a concern. As a result of that, Spirit has been driving backwards most of the time. However, during a recent drive, the draw was closer to normal than it had been in preceding weeks. Even so, the engineers will continue to limit use of this wheel by driving backwards when terrain allows, and by dragging it 90 percent of the time.
The engineering team has also continued to diagnose and study work-arounds for the problem Spirit has been having with her steering brake release system that appeared to fail to release on a couple of previous sols. The rover has intermittently sent information in recent weeks that the electrical brakes on her right front and left rear wheel steering actuators (or motors) had not released properly when the rover received commands to set a new course. In other words, Spirit's right-front and left-rear wheel brakes indicated they failed to disengage during drive attempts.
On Spirit and Opportunity, the brakes are designed to keep the rover wheels from being bumped off course while driving. Dynamic braking is accomplished via a relay switch that places a short across the motor windings of an actuator [motor] that is not being used. If that actuator starts to move unexpectedly, it acts as a generator and the short provides an electrical load that slows the motor down. The same principle is used to generate electrical energy for hybrid cars when the brakes are applied.
As it turned out, when engineers finished their analyses, they concluded that the mechanism for detecting whether the brakes are released was probably sending a false indication. The most likely cause of this anomaly, they determined, was a buildup of insulating material on the electronic relay contacts that indicate that the brakes are disengaged. Their fix? Simply disregard the 'brake-disengaged' indicator and presume the brakes have actually released properly when commanded to do so. If the engineers' theory is correct, the brake will actually be disengaged despite the 'failure-to-disengage' indication. If they are wrong, a fuse that is in line with the brake relay circuit would have deliberately and safely blown the next time they attempted to move the steering actuators. This capability to disable the dynamic braking function from ground commands was a feature built into both Spirit and Opportunity by the rover design team.
"After we analyzed the limited data we had, our best guess was that the problem was something that was just affecting the status indicator and not actual functionality," explained Welch. "Flight software had a contingency in it that allowed us to tell it to ignore that status indication. Unfortunately, when we tell the rover to ignore it, it no longer reports on it. And so had it stuck or not displayed appropriately again, flight software would have just ignored it and gone on and used it, and we would not get an indication one way or another whether software had seen [the indicator] or not." As a result, the engineers cannot be certain if the 'failure-to-disengage' indicator is giving a false reading or if the fuse has blown. In either scenario, the incident will not adversely affect the accuracy of Spirit's drive, engineers maintain, nor will it threaten the rover's safety when stopped.
"We've have gone back to using the full steering capabilities of Spirit," Jim Erickson, rover project manager at JPL, announced a couple of weeks ago. And, for the time being, the MER team is continuing to disregard that signal. "So far, we see no problem with it," Welch updated today. "All our drives are going just fine and we believe it was just a problem with the status indicator." [This anomaly has not been observed on Opportunity.]
Spirit suffered another anomaly last month that was traced to electric-circuit grounding. The engineering team regularly receives telemetry that tells them the voltage difference between 'rover chassis' and 'power bus return.' The rover chassis is the conductive structure of the rover, akin to an automobile chassis. The power bus return is a collection of wires designed to carry current back to the rover power source [battery or solar array]. Ideally, all rover current flows in a loop from the battery or solar array, returning by way of the power bus return wires. No current is supposed to flow in the rover chassis, though in reality some leakage paths exist that allow current to return by way of the rover chassis.
When these currents flow across the circuitry that separates the rover chassis and power bus return, they create a small voltage that is measured and reported in telemetry. Until Sol 287, the reported voltage was typically in the range of 0.6 to 0.8 volts, then the voltage dropped to 0 volts. The 0 volt reading could indicate there is a problem with the measurement circuit, or it could indicate that power bus return and rover chassis are now shorted or making direct contact. The rover can operate when the chassis and power bus return are shorted together, as well as when they are separated from each other by electrical circuitry. In the shorted scenario, however, the rover is more susceptible to permanent damage if another short occurs somewhere else.
This anomaly occurred at the same time the Spirit team was performing an inspection of her IDD or robotic arm. The sequence commanded one of the arm joints to a position beyond where it had previously been. That particular joint, number 5, is the rover arm turret, which rotates the four rover arm instruments into position. It may be that movement of joint 5 somehow created the electrical short, or it may be that the short and the joint 5 movement had nothing to do with each other and the short could be the result of a failed measurement circuit.
Although the mechanical team members have not found any reason to suspect a failure in the joint 5 cabling, to be safe, they quickly constrained the use of joint 5 on both rovers to avoid this extreme position. It is a constraint, according to rover officials, that should not significantly impact normal operations, at least not in the near future. It does remove, however, one layer of protection against effects of future shorts should they occur.
"We reviewed all the [appropriate] data for the robotic arm, including the cabling runs, how the cabling was done and did tests and there is no indication that we would be anywhere near the end of life, that something would be wearing out and actually exposing some electrical contacts there," Welch said. "So there's no likely cause from the mechanical team's [perspective] as to what might be causing a short in the arm. Right now, we have had no change in that signal. It certainly could still be a coincidence and we certainly haven't narrowed it down to just the arm. It's one of those things with the limited data we have, it's very hard to narrow down. So, we're just continuing on, and restricting the operation of joint 5 of the arm, just as a safety measure."
Charged now with conquering Husband Hill, Spirit will keep looking for other rocks she can grind into and examine up close with the other instruments on her IDD. She has been on the move for the past week or so, logging enough meters roving around Gusev to boost her odometer to nearly 2.5 miles [about 4 kilometers]. The rover, though, has a long way to go. "We're probably about 197 feet [60 meters] in elevation change from the top of Husband Hill and the drive is probably at least 500 meters away," [which is 3/10 of a mile]," explained Welch. "That's a lot. But we definitely want to get to the top and look for interesting rock outcroppings along the way."
Opportunity from Meridiani Planum
Opportunity spent the last month or so finishing work at the rock Wopmay, slipping and sliding her way to the base of Burn's Cliff, turning away from a potential shortcut exit, and backtracking to Karatepe, where she entered Endurance Crater last June 8. Rover engineers are now preparing the rover for her grand exit.
Throughout her journey around the interior wall of Endurance, the terrain tested the little robot field geologist's mettle. The slope of the ground and loose surface material around the Wopmay, for example, prevented the robot field geologist from getting firm enough footing to RAT it and peer inside with her IDD instruments, so she did the best she could and examined the exterior.
As Opportunity took off for Burn's Cliff, the rover began slipping as much as 100% during her drives. At a buried rock, dubbed Son of Banes, she wound up digging herself into a hole with her wheels, and experienced a 'drive and mobility goal error' due to high current draw in the steering motors attempting to deal with her predicament. The plan to place the APXS on the filter magnet for extended integrations, therefore, was cancelled because the ground team thought that maybe the rover's position against Son of Bane, and the churning up of sandy terrain meant that there might be an unseen rock in the IDD's work volume. Instead, Opportunity collected a bunch of remote-sensing observations, then roved out of the hole at Son of Banes with relatively little effort.
Solar exposure proved favorable enough for Opportunity to begin a routine of forfeiting DeepSleep, which meant she could once again support early morning Mars Odyssey communication sessions. The timing couldn't have been better, because the rover had a backlog of onboard data that not been downlinked.
For a while, the team thought Opportunity might be able to exit Endurance Crater by a proposed route eastward, but by mid-November, rover operators announced it wasn't advisable to attempt it. The rover would have had to cross terrain sloping 28 degrees. The opening of the chute is about 4 feet [1.14 meters] wide, and there appears to be tall rock outcropping very close to the opening.
By comparison, the exit path at Karatepe, where Opportunity entered the crater six months ago, has slopes that average only 22 degrees and there are no large obstacles to avoid. It didn't take too long before the decision was made -- skip the shortcut and continue back to Karatepe. "We've done a careful analysis of the ground in front of Opportunity and decided to turn around," Jim Erickson, rover project manager at JPL, stated in an official release Nov. 11."To the right, the slope is too steep -- more than 30 degrees. To the left, there are sandy areas we can't be sure we could get across."
Opportunity's new orders put her on the backtrack path to exit the crater by the southward route, essentially retracing her path. Before turning around, the rover hunkered down for a few days of studying her destination at the easternmost point of her drive inside Endurance Crater. There, from the western edge of the base of Burns Cliff, she conducted intensive remote-sensing campaigns of the rock layers in scarp that stands some 33 feet [10 meters] high.
The rover was unable to reach an area about 50 feet [15 meters] farther east where two layers at different angles meet at the base of the cliff. As Opportunity worked her way around the interior rim of the crater, she experienced drive slippage of up to 100 percent and tilts as high as 31.05 degrees. "We have pushed the vehicle right to the edge of its capabilities . . . Going any farther could cut off our line of retreat from the crater, and that's not something anybody on the team wants to do," Squyres reported.
"Basically, we made our way up to where we were, at about a 30-degree slope at the base of the cliff, which is as steep as we'd like the rover to be," Crisp recounted. "We brought the rover back down a few degrees where it was safer and then started approaching this place where we wanted to look at the crossbedding up close with the MI and other IDD instruments. We took our time, doing the best we could from there. We took as many high-resolution images with the PanCam as possible, and characterized the cliff. We even took a big 360-degree panorama and a lot of mini-TES observations of selected targets in the rock layers." From this data, scientists hope to determine whether some of the layers were deposited by wind, rather than by water.
As Americans celebrated Thanksgiving down on Earth and officially entered the holiday season, Opportunity continued her remote sensing campaigns from inside Endurance -- of dunes and dust with the PanCam and mini-TES, super-resolution imaging of Whatanga, a contact boundary between two distinct layers of rocks, and long-dwell mini-TES observations of Burns Cliff targets, as well as conducting atmospheric imaging of clouds and taking pictures of skyscapes.
The remote sensing campaigns generated large volumes of data at a time when, due to the rover's orientation, there was less bandwidth available for downlinking the data. As a consequence, Opportunity briefly operated with limited memory headroom, though she was still within planning guidelines. To improve the situation, the team took advantage of the Deep Space Network's 70-meter antenna availability and the rover's good energy state to plan a one-hour, direct-to-Earth session in the middle of the day on Sol 291, resulting in the downlink of an extra 15 megabits of data. Since then, Opportunity has applied some of her excess energy to engage in several more direct-to-Earth communications sessions.
Given that data storage volume is finite, there is always, in the minds of some, 'limited memory headroom.' Indeed, it is almost a daily issue in terms of putting a graph on the board during the planning meetings -- what the scientists want and what they can have. "The data are stored in flash memory, and when the scientists come up with big observations, like 'Let's take a huge panorama here,' it's very easy, taking low compression images, to fill that memory space very quickly," Welch pointed out. [As anyone with a computer knows from the first time they tried to download a large GIF image on a dial-up connection.] Dealing with limited memory is, therefore, a persistent condition. That noted, to date nothing of great scientific significance, as far as the team knows, has been passed up or sacrificed because of memory limitations, he assured.
The fact that Opportunity has had such a good energy supply that she can afford to 'squander' some of it on the direct-to-Earth communications sessions -- which take a lot more watts per megabit -- bodes well for the rover. The direct sessions return on average 10 megabits per session; whereas the relays that send data through the orbiters -- Odyssey or Mars Global Surveyor -- are often 40, 50, 60 and even 100 megabits per session; hence, the direct-to-Earth sessions are not a substitute for the relay transfers, but they have taken a little of the overflow edge off.
"We try to balance what [the scientists want] with what we predict our upcoming relay communications sessions will be," added Welch. "But those are heavily affected by the rover attitude and exactly where our antennas pointed, which is a little non-deterministic when we drive. We do get close to the edge sometimes . . . but it's not something that causes a problem per se. We know where we are, how much space we have left, and we haven't yet overflowed memory."
When Opportunity finished her work at the base of Burns Cliff, she turned around and began the trek back to Karatepe. On Sol 295, for the first time in 10 sols, she was on the move, performing post-drive observations as she went. "We just back-tracked, kind of stayed at the same elevation and came back toward our egress," offered Crisp. Once other potential shortcut exits were scratched from the list in November, Opportunity "pretty much finished up at the base of the cliff, then drove heading for this spot where there is a slab of rock that features a light-dark boundary" -- Ivory and Ebony, Crisp informed. "We've been studying this slab with the robotic arm, taking a few rock abrasion tool characterization spots on either side of the light-dark boundary."
Opportunity's first RAT grind on the Ebony & Ivory slab was on a spot identified as Wharenhui. "It's the same kind of layered rock we've been seeing, with hematite concretions in it, but it's on the darker side of this boundary," said Crisp. "The area we'll RAT next is called Paikea, on the lighter side. By characterizing it here, we hope to better correlate the layers that we're seeing around the crater and how these layers correlate with those we saw where we came down Karatepe. We wanted to do a careful assessment right here, and then we'll head out of Endurance Crater."
It's been six Earth months that Opportunity has been inside Endurance examining various different rock and soil targets and digging up clues that point to past water there. During that time, she has found and examined multiple layers of rocks that are providing further evidence that the Meridiani region featured a body of salty water in the past. Although the evidence is "still not conclusive," data from the spectrometers and MI is "consistent with scientists' earlier hypothesis that rocks near the bottom of the crater were affected by water both before and after the crater formed," Steve Squyres, of Cornell, the principal investigator for rover science, announced recently.
Despite her advanced age, Opportunity is showing no signs of slowing down and JPL mission engineers report that her health is "excellent." Solar exposure is extremely good inside the crater -- "we think the environment inside the crater is just a little warmer," Welch said -- so Opportunity's power and battery state of charge have continued to increase. Presently, the rover's power state is nearly as high as at the beginning of the mission. In fact, the rover's 'vitality' is so good, she is now being put into DeepSleep mode only occasionally and has gone for more than a week at a time without it.
Opportunity apparently "had some kind of cleaning event" on the solar arrays, Crisp reminded. "Either dust devil winds cleaned it off, or it could be that frost has caused the dust to clump and clean off large fractions of dust from the solar panel," she added. "No matter what -- it's just gotten a lot better. We're back up to 800 to 900 watt hours per day of energy, which is what it was when we landed. Neither Spirit nor Opportunity have managed to capture any of those dust devils on video yet, Crisp said, but that remains a goal.
Currently, Opportunity is in her final approach to Karatepe, the location along the southern rim Endurance Crater where she entered last June 8. She will leave the crater for good, "perhaps as early as this weekend," Welch offered. "We should know next week if we're out."
As the warmth of the spring slowly begins to heat things up on Mars, most team members are optimistically anticipating that the rovers' energy and output will continue at their present clips, at least for the immediate future. Things are heating up back down on Earth, too, as the data -- and reports -- continue to stream in as fast as they ever have for a planetary exploration mission. Last week, in a special issue of the journal Science, 11 papers by 122 authors reported the scientific findings from Opportunity's 90-day primary mission, during which she discovered that Meridiani Planum once boasted a salty sea. The MER team reported Spirit's primary mission findings in the August issue of the journal.