A.J.S. Rayl • Mar 25, 2005
Mars Exploration Rovers Update: Spirit 'Snaps' Dust-Devils while Opportunity Roves on to Viking (the Crater)
After more than a year of active-duty research, the Mars Exploration Rovers have caught 'second winds' -- in part because of their new, recently uploaded software and, in part -- however strange it may seem -- from the planet's notorious dust devils.
As winter gives way to spring on the Red Planet, both rovers have gotten a kind of "new lease on life," MER project scientist Joy Crisp, of the Jet Propulsion Laboratory (JPL), told The Planetary Society. It's showing in their work.
During the last couple of weeks or so, Spirit has managed to 'snap' the first-ever images of mini Martian dust devils in action at the Gusev Crater site, while Opportunity has uncovered some intriguing clods, and set more driving records at Meridiani Planum.
Now, following completion of a 360-degree panorama of Tennessee Valley from atop Larry's Lookout and a reexamination of the saltiest target ever found on Mars, Spirit is back to hiking up Husband Hill in the Columbia Hills region of Gusev, Crisp said.
Opportunity, meanwhile, continues the trek southward across the Meridiani plains toward the Etched Terrain, having finished an exhaustive study of Vostok Crater with all instruments, except the mini-thermal emission spectrometer (mini-TES). That instrument -- one of the mineral-identifying tools -- was turned off about three weeks ago when it started sending home incomplete data sets. The team is still troubleshooting that problem, but right now the device still functions, sometimes. "The good news is that it's not completely dead," Phil Christensen, of Arizona State University, lead scientist for the mini-TES on both rovers, said in an interview yesterday.
Although each rover has encountered minor technical problems here and there, both are remarkably active at this point and in overall good health. "Opportunity's other instruments are healthy and providing excellent science, and Spirit's entire instrument suite is working well and being kept busy by the science team," reported Jim Erickson, of JPL, project manager for the Mars Exploration Rover Project.
The global dust storm season on Mars won't start until the end of April, but both rovers have been encountering local or regional dust storms for the last several weeks. These storms are likely to continue and may well soon be replaced by bigger, planet-sized storms that are legendary at the Red Planet. Consequently, the rovers will probably continue to 'feel' the impact of the infamous whirlwinds, for better, it appears, and, in the days to come, perhaps, for worse.
Spirit recently got an energy boost to the same levels she had when she first started out on her exploration of the Gusev site some 15 months ago -- and the boost was apparently courtesy of the dust devils' gusty winds that appear to have blown off dust that had accumulated on the rover's solar arrays, an effect that Opportunity has also experienced. On the other hand, Opportunity's rear hazard-avoidance camera is still carrying around dust picked up three months ago. The dust has not really affected operations and has neither decreased nor increased perceptibly since first noticed, but it has caused a slight 'mottling' in the images sent home. To the team's good fortune, the navigation cameras or panoramic cameras on both rovers remain clear of dust contamination.
In all probability, there are likely large transient dust storm events that reduce solar energy by depositing dust on the solar arrays and by blocking some sunlight. At the same time, these storm events may occasionally raise energy levels by blowing off dust from the rovers' solar arrays. Since the MER rovers are the first solar-powered vehicles to be on the surface of Mars during the dust storm season, no one really knows what to expect from these storms or how well the rovers will hold up during these tempestuous events, so team members will be keeping very close eyes on the solar arrays, cameras, and on other rover systems in the sols to come.
Spirit from Gusev Crater
Once Spirit completed her 360-degree panorama of Tennessee Valley -- a 4-sol effort that took place earlier this month -- the rover turned attentions to a nearby rock target at Larry's Lookout called Watchtower, examining it with the various instruments in her toolkit. The rover did a three-hour grind with the rock abrasion tool (RAT), digging a little more than a quarter of an inch [about 7 millimeters] into Watchtower, then spent the ensuing couple of days, studying it up close with the microscopic imager (MI), the alpha particle X-ray spectrometer (APXS), and the Mössbauer spectrometer.
A regional dust storm kicked up on Sol 418, March 7, causing the tau -- a measure of how much sunlight cannot penetrate the atmosphere -- to rise to a high of 1.5 that afternoon, with visibility something like that in the thick darkness of a forest fire. That, in turn, slowed the solar-powered rover down. But once the storm subsided, the energy output from Spirit's solar panels went up - way up, by about 50 percent, and the rover was raring to go.
Team members speculated that Spirit's power boost, like similar ones on Opportunity in October, resulted from wind blowing off the accumulated dust from solar panels and subsequent images bolster that theory. "We have taken pictures of the solar panels and have seen that indeed they are much cleaner," confirmed Crisp. "The PanCam calibration target, the magnets are all cleaner. We definitely know that that's what happened."
The images -- particularly the one of the panoramic camera's calibration target -- were quite the buzz at the Lunar and Planetary Science Conference (LPSC) held in Houston, March 14-18, and they no doubt create something of a stir in cities all around the world. The Planetary Society's Red Rover Goes to Mars student astronauts -- an international group of 16 students representing 12 countries -- worked with the PanCam team to process images every day during the first two months of the mission.
Images from Spirit's front hazard-avoidance camera, which had showed signs of dust contamination similar to that viewed on Opportunity's rear hazard-avoidance camera, also cleared noticeably the same day the power increased. Between sols 416 and 418, the front hazard-avoidance cameras showed signs of dust contamination, but images from Sol 420 indicate that the left front hazard-avoidance camera has been mostly cleaned off.
The sudden jump in output from solar panels, meanwhile, has doubled Spirit's daily power supply, to 800+ watt hours, close to what the rover had to work with right after landing some 15 months ago. The amount of energy either of the solar-powered rovers has on any given day varies with the amount of dust in the atmosphere and the seasons, but both rovers started out with around 900 watt hours of energy a day. "We just dropped with time," said Crisp.
"As the dust built up on Spirit's solar panels and we moved into winter, the energy output came down to the high 300s to the low 400s for much of the mission, from Sol 150 to almost now," Crisp elaborated. "So only recently, when we had this dust clearing event, did the energy level come back up. It changes with time and we knew it would," she added, noting that at the beginning of the mission, the team thought Spirit might have to hibernate through part of the winter because of low energy availability during the coldest months of the Martian year. [Because Opportunity is closer to the equator, that rover's energy level did not drop as low as Spirit's. "It dropped down to about 550 for awhile, then went back up to 900 with a few cleaning events, and since has come back down to the 600s," said Crisp.]
Throughout the hike up Husband Hill, Spirit has consistently been taking the time to collect remote sensing and atmospheric data. And on Sol 421, March 10, the day after her power boost, the rover returned to the planetary exploration spotlight by sending home the first images ever taken of Martian dust devils, which she caught with her navigation and hazard avoidance cameras. Although the whirlwinds are small, and appear as faint, wispy spirals just visible to the naked eye, the science team members now have data in hand to help them determine the possible causes of these legendary phenomena. And, team members are currently reviewing more images that they believe captured other whirlwinds, Crisp said.
Dust devils often occur when the Sun heats the surface of Mars. It may be that warmed soil and rocks heat the layer of atmosphere closest to the surface, and the warm air rises in a whirling motion, stirring dust up from the surface like a miniature tornado. Another possibility is that a flow structure might develop over craters as wind speeds increase, and as they pick up, turbulence eddies and rotating columns of air form, and grow in diameter, becoming taller and gaining rotational speed. Eventually they become self-sustaining and the wind blows them down range.
What the team knows for sure now is how far away the dust devils were from the rover and how fast they were moving. By comparing the separate images from the rover's different cameras, team members have estimated that the dust devils moved about 1,640 feet [500 meters] in the 155 seconds between the navigation camera and hazard-avoidance camera frames. That translates to 7 miles per hour [about 3 meters per second]. The dust devils appear to have been almost three-quarters of a mile [about 1,100 meters] from the rover.
After completing the work on Watchtower and capturing the historic dust devil pictures, Spirit returned to Paso Robles, a soil target the rover examined earlier this month. That patch, as it turned out, contained the highest concentration of salt of any rock or soil sample ever examined on Mars, and the scientists wanted to take another look, Crisp said.
"It appears to be containing an iron-bearing sulfate, an oxidized iron, FE3+ type iron, and we have never seen that before," Crisp explained. "We have seen what looked more like magnesium sulfate and possibly some calcium, but this time we had a signature that indicated iron, and the data also suggest the iron sulfate is possibly hydrated, or in other words has bound water -- OH -- in the formula. It was so different that we wanted to go back and check it out."
Whatever else they learn, the composition of Paso Robles is more strong evidence that water played a role there on Mars, as lead rover scientist Steve Squyres, of Cornell University, pointed out in the last MER Update.
Spirit used her newfound energy to carry out a very aggressive scientific campaign at a soil patch dubbed Paso Robles 2, scuffing the surface with the wheels, and examining it with all the instruments in her toolkit. The rover then homed in on selected targets named Big Clod and Bitty Clod with the MI, and other targets called Paso Dark and Paso Light with the MI, APXS, and the Mössbauer spectrometer.
About a week ago, on Sol 429, March 18, Spirit swept the surface of a particularly intriguing, bright clump, dubbed Ben's Clod, with the brush on the rock abrasion tool (RAT) and took before-and-after shots of the area with the MI, and examined the target with the APXS.
The rover is now uncovering clods and clumps that look to science team member like harder patches of soil. "It's hard to tell whether these are chunks of rock in the soil, like the hard pan you can hit when you dig in your garden, or not," said Crisp. "We've seen that at Viking, and Pathfinder [sites]. Scooby Doo on Pathfinder looked like a hard pan kind of material and this clod might be that kind of thing, but it takes examination and work to figure out exactly."
The opacity of the atmosphere at Gusev has returned to smoggy-LA-day-like levels of visibility and since Spirit continues producing more than 800 watt-hours of energy a day, the robot geologist has ample power and a full battery at the start of each recent sol. The rover's flash memory -- which went on the overload fritz last year -- is also in good shape despite the large panorama acquired from atop Larry's Lookout because of good downlinks and data management.
"We do have a lot more solar energy now," said Crisp. In recent days the rover has been exerting around 860 watt hours, right up to where the rover was in terms of wattage in the beginning," she informed.
Spirit is climbing back up hill now toward Watchtower again, and will carry on, steady as she goes, up Husband Hill. The plan still calls for the rover to continue onward to the summit, which is another 50 meters in elevation from her current position. "We'll be looking and responding to any other new rock or soil types we discover along the way, otherwise the rover will keep trying to get further up." Crisp said.
As Mars approaches the spring season, the immediate future looks bright for Spirit. "But that 50 meters of elevation is still a chunk," reminded Crisp, "and it's a struggle."
Opportunity from Meridiani Planum
On the other side of the planet, Opportunity has continued the journey southward across the Meridiani plains to the area dubbed Etched Terrain. Atmospheric opacity there has been generally stable over the plains, with tau hovering between 0.85 and 0.90, similar to what it has been in recent days at the Gusev site, and Opportunity has withstood recent dust storms in stride.
Rover operators have continued to put Opportunity into DeepSleep mode off and on to conserve her energy during reduced-sunshine months of Mars' winter. A heater switch on the rover's instrument deployment device (IDD) or robotic arm got stuck in the 'on' position stuck on early in the mission and was continually draining power from the rover. DeepSleep mode -- turning off power to the instruments at night so the heater could not drain power -- was the fix.
The robot field geologist and her instruments have generally fared astoundingly well, although the team was concerned then the mini-TES might be damaged as a result. Turning off power to overnight heaters let the instrument get cold enough to possibly damage its beam-splitter. The mini-TES, however, exceeded all expectations and proved to be a real trooper of an instrument and kept working through the coldest winter months.
"As soon as the project said we're going to take a risk with this instrument by taking it cold for DeepSleep, in my mind, I said -- 'It's dead, and every day we might get is a bonus,'" Christensen recalled. "So we said our good-byes then and the next day it was working! And it continued to work for 140 DeepSleeps, and the engineers are amazed that it survived any of them."
Although the mini-TES is located inside the Warm Electronics Box, it uses the PanCam Mast Assembly as a periscope to get a good view of Mars, and to observe rocks and other targets from afar, measuring the infrared radiation they emit in 167 different wavelengths. That data provides scientists with information about the targets' composition. [The APXS and Mössbauer -- two other types of spectrometers -- are mounted on the rover's robotic arm or instrument deployment device (IDD) and provide additional information about composition when the rover is close enough to actually touch the target.]
The mini-TES, Christensen explained, was designed to work in temperatures as low as -40 degrees Celsius. "We designed it, tested it, inspected and we told the engineers that it had to survive to -40C, which is pretty darn rigorous. That's with the margin built in. We figured it would never get colder than -30C, because we had planned to keep it warm through the coldest nights, so we designed for -40C, and the engineers did what we told them. And even so, the mini-TES survived 15 degrees past that, which is a lot," he pointed out. "If you took your personal computer and took it down to those temperatures and brought it back warm again it would never work again. But it's been down greater to -50-55C 140 times because of the Deep Sleep mode."
It was about three weeks ago that the mini-TES started exhibiting problems. The trouble began on March 3 and 4 when Opportunity transmitted data sets for 17 successful readings by the mini-TES, but also reported that eight other attempted readings yielded incomplete data sets. The team has more or less shuttered the mini-TES until it can figure out what's going on. "We've been using it occasionally, but we haven't used it a lot in the last three weeks," said Christensen. "Rather than waste a lot of bits, we've been using it very rarely until we have the meeting with members of the original design team."
In the meantime, the team members have been considering several possible root causes for the spectrometer-data problem, but none of them appear to have anything to do with DeepSleep mode. "What we're seeing now does not appear to be any problem with the beam-splitter," Christensen confirmed. "It's behaving like an intermittent problem and that's consistent with something cracking, breaking, some intermittent component, part of solder joint maybe that literally is intermittent and it works if you vibrate the rover just right or something -- like in the old days when you would kick the TV to make it work again, for those of us old enough to remember tubes," he elaborated. "So it's behaving like an intermittent electrical short for want of a better word."
One possibility is that an optical switch that tells a mirror in the instrument when to begin moving is malfunctioning. Another possibility is that the mirror is not properly moving at a constant velocity. "If it is the optical switch, we could use a redundant one built into the instrument," explained Christensen.
The problem is there is nothing obvious about the problem. "We've run it hot and it's worked and not worked and we've run it cold and it's worked and not worked," Christensen said. "But it might be that if it's hot with the rover tilted and having just the right something- something, so we're going to pow-wow to see if we can come up with a set of tests we might do over the next few weeks that will give us more insight into what could be wrong and how we might mitigate that."
The good news, Christensen said, is that they have gotten some good data out of the mini-TES even since they started seeing this problem. "So it's not a catastrophic failure," he stressed. Not yet anyway. And, if the root cause cannot be remedied, scientists could still get useful, if incomplete, data from the instrument in its currently impaired condition. "In short, the instrument measures a spectrum and if we get the center part of that spectrum, there's useful information, but if we only get the wings of the spectrum there's not much useful information. So it all depends on what part of the partial data set we get," he explained.
"We don't have a whole lot more information right now. We're going to pow-wow first," said Christensen. "It's been a long time since the mini-TES was built," he pointed out. "The original design was actually for the [Mars] 2001 lander, and so it got put on the shelf, and then dusted off and we built a second one for the rover. But many of the guys who actually designed it have left the company or retired and sort of scattered to the winds."
Christensen, however, managed to get a hold of most of them and they will convene with the rover mini-TES team Tuesday in Santa Barbara, California. "Some of these guys are in their 70s now," he said. "One guy who is coming back is 73 and the funny story is he's down in LA visiting his mother. (Holy cow! Good genes.) Anyway, at that meeting we will try to figure out what's going on and come up with a plan to get the most out of it. "The mini-TES on Spirit is still performing nominally and so the rover team has not restricted use of the instrument on that rover while investigating the problem on Opportunity's instrument.
In the meantime, up on Mars, Opportunity has been spending a lot of time driving with the new mobility software that is enabling the rover to go for greater distances in auto navigation mode. As a result, the rover has been breaking and setting records again and again.
Earlier this month, the robot rover logged nearly .27 of a mile [about 450 meters] in a 6-sol period, and in recent days set a new record for greatest distance in one sol by roving .13 of a mile [about 220 meters] before stopping for the night. Opportunity has made a number of pit stops on the trek southward as planned, using her tools to check out targets called Russet and Jason up close. The rover also took pictures -- and a 360-degree panorama of a distinctive crater triplet. At the crater triplet, Opportunity moved right up to the edge of one of the craters, and with front wheels at the very edge of one of the craters, the rover was able to extend her robotic arm to characterize some of the mineralogy.
Following that research, the team decided to have the robot field geologist take an in situ look at a nearby rock target called Normandy. Then, the rover headed for Vostok Crater, arriving there on Sol 399, March 8, after a long, sustained series of traverses. Although Opportunity found Vostok to be a crater almost completely buried in sand, the rover was charged with taking various in-situ measurements on a soil target called Laika, and a target called Gagarin on a rock referred to as Yuri. In addition, the rover acquired a panoramic image of the crater. [Yuri Gagarin, a Russian cosmonaut, was the first human to orbit the Earth. His spacecraft was named Vostok 1.]
The rover hit the road again this week. "We're continuing south toward Etched Terrain and heading toward two little craters called Viking and Voyager," Crisp informed. "We had a record-breaking drive of 220 meters in one day last Sunday and we've got about 240 meters to go before we reach Viking." The MER team "has drawn a beeline toward Viking" and if that's interesting will probably go on to Voyager; otherwise, Opportunity might bypass it and look at it from a distance. "When people look at pictures of the plains they should understand why we're stopping at these craters," said Crisp, "because it's pretty monotonous terrain until we get to things like this."
There's been talk in the science working group of conducting some analyses of the soil out on the plains, "looking at the soil on the troughs and soil that's up on these little crests," Crisp expounded. "We can't drive everyday because we don't want to be too hard on the motors, so every now and then we want to stop to do little studies." The rover is getting closer to the seemingly elusive Etched Terrain and the team is finding only a "vague" boundary, as Crisp described it. "We're not even sure if we're in it yet or not. It's possible the so-called Etched Terrain might not be a rock unit. We're seeing these bright patches from orbit and we don't know what it looks like on the ground yet, so we'll be on the lookout.
Once Opportunity reaches Viking and Voyager, the rover has another 800 meters to go to get to the next target on the list -- Erebus, a feature 300 meters in diameter that looks like a filled-in, largely eroded crater. "It's interesting enough that we want to take a closer look," Crisp said.
Past that, the science team members have eyed other targets, but they also know that every day for the rovers from here on out is a research gift. Anything can happen at any time, especially now with the onslaught of the global dust storm season.
About a year ago, as the rovers were closing in on the end of their primary missions, team members were taking bets backstage on how long the rovers would last. No one imagined then they would still be going 15 months later, much less be bounding out of winter with as much energy as they had just after landing. But the rovers have defied the odds and the bets, roving over every deadline someone put money on.
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