A.J.S. Rayl • Feb 28, 2006
Mars Exploration Rovers Update: Spirit Slides into Home Plate as Opportunity Finishes Work at Erebus
As early autumn descends on the southern hemisphere of the Red Planet, the Mars Exploration Rovers are on the move and picking up the pace as they rove toward their next major destinations.
Spirit slid into Home Plate earlier this month and is finishing its study of the unusual geologic feature in the Inner Basin of Gusev Crater before heading on to McCool Hill, the next peak in the Columbia Hills that this rover will check out.
On the other side of the planet, Opportunity is nearing Payson, “the tasty promontory,” as rover principal investigator Steve Squyres, of Cornell University, has described it, located on the Mogollon Rim at the western end of Erebus crater. Soon, this rover will head back onto the plains and begin driving as quickly as possible toward Victoria crater, which lies 2.5 kilometers (about 1.5 miles) to the south.
Both Spirit and Opportunity remain in good health and at the present time have adequate power supplies. “Opportunity still has the issue with the instrument deployment device (IDD), but it still works,” noted rover deputy principal investigator Ray Arvidson, of the University of Washington in St. Louis. “We just have to be very careful about how we use it.”
With the onset of autumn on the Red Planet, the Sun is moving north in Mars’ southern hemisphere and there is an increasing sense of urgency to keep the rovers roving onward – and to get Spirit up on the north-facing slope of McCool Hill – since they will be receiving less solar input for power from now on until the end of winter.
Spirit from Gusev Crater
After the MER team detected a dynamic brake fault associated with the left-front and right-rear steering actuators on Spirit in late January, the rover completed two diagnostic tests and no dynamic brake warnings flashed. The intermittent behavior of the relay status that controls the dynamic brakes and the results of the diagnostic tests are consistent with an anomaly that had occurred earlier in January, and so the team instructed Spirit to ignore the dynamic brake error warning and the rover continued driving with normal steering function throughout this month.
In fact, Spirit literally drove into February bent on getting to Home Plate, a roughly circular feature about 80 meters (260 feet) in diameter that when seen from orbit looks like the home plate of a baseball diamond. The rover – which logged its 6,430th meter to push its odometer to the 4-mile mark on Sol 741 (February 2) -- spent the first half of the month getting to the alluring geologic feature in the Inner Basin and the second half studying it, while conducting atmospheric observations and collecting images of nearby and distant targets with its cameras along the way.
In honor of Black History month, Arvidson and Squyres decided to name the features Spirit studied this month after star players and managers of the Negro Leagues of Baseball from the first half of the 20th Century. The two huddled with noted essayist and baseball fan Gerald L. Early, the Merle Kling Professor of Modern Letters at Washington University in St. Louis and a consultant on the Ken Burns documentary Baseball for the Public Broadcasting Service (PBS). “Steve Squyres was here and the three of us sat down and came up with a list,” said Arvidson in an interview earlier today. Meanwhile, the Baseball Hall of Fame officials announced this week that they had elected more players to it roster from the Negro Leagues.
As Spirit closed in Home Plate, it stopped and positioned itself to examine a rock dubbed Barnhill with the instruments on its instrument deployment device (IDD). Situated just to the north of the large flattened geologic feature, the rock was named for David Barnhill, the ace of the New York Cubans' pitching staff during the early 1940s. He compiled an 18-3 record in 1941 and defeated Satchel Paige in the 1942 East-West all-star game. Other rocks in the area were named for Paige, Josh Gibson, "Bullet Joe" Rogan, and Cumberland Posey, among others. The rover also collected images with the panorama camera (PanCam) that will be used to create a large mosaic of Barnhill, and spent its restricted sols this month engaged in untargeted remote sensing and atmospheric science.
On Sol 750 (February 11), the rover checked out Pitcher with its microscopic imager (MI), then completed and overnight study of the rock Fastball with the alpha particle X-ray spectrometer (APXS) and moved onto Posey the following sol. On Valentine’s Day, Spirit acquired some geologically romantic images of a rock called Gray using the MI, then brushed a surface target dubbed Manager using the rock abrasion tool (RAT), and completed an 18-hour observation of it using the Mössbauer spectrometer. Since the data showed that Manager was similar in composition to the unbrushed Barnhill, the team instructed the rover to resume driving after collecting the APXS data, and taking after-brushing images with the MI.
Spirit rolled back from Manager and collected general data on the site with its mini-thermal emission spectrometer (mini-TES), then roved on 11.5 meters (about 37 feet) to slide into Home Plate on Sol 755 (February 16). The rover immediately began collecting images of the surrounding terrain with its PanCam, and since then has been working hard to collect as much scientific data as possible before embarking for McCool Hill.
The Spirit team, which had been enjoying distant views of Home Plate and the curious "bathtub ring" of light-colored materials along its edges taken by the Mars Orbital Camera (MOC) onboard Mars Global Surveyor, as well as from Spirit atop Husband Hill, had been eager to get to this destination for a long time. “We’ve seen Home Plate from orbit since we’ve landed and so it’s always been a target,” Arvidson said. Its unusual characteristics were particularly enticing, so once the rover completed scientific research at the Husband Hill summit, it took off on the ambitious drive of 848 meters (2,782 feet or a little more than half a mile) to Home Plate and managed to complete the trip in 94 sols.
When the rover used its PanCam to image Barnhill and surrounding rocks just to the north of Home Plate, the science team was surprised by the most spectacular layering that this rover has found. “There’s a lot of fine layering and crossbedding in and around Home Plate, and the general consensus is that it is the best layering that Spirit has come across so far,” said Joy Crisp, MER project scientist at the Jet Propulsion Laboratory (JPL), where the rovers were designed and built.
Moreover, the PanCam and MI views of the layers in the rocks at Home Plate reveal a range of grain sizes and textures that change from the lower to the upper part of the outcrop that may help scientists figure out how the material was emplaced.
Ever since they saw Home Plate from orbit, science team members have had many theories about what the feature could be and what they might see when Spirit got there. Now they have winnowed those theories down and one primary working hypothesis seems to have emerged. “It’s kind of conical, with the top of the cone sliced off, and it makes this bright round elliptical feature that looks to me to be -- and most of the team agrees -- an eroded down volcanic construct,” Arvidson said. “Exactly what that means is a subject of debate and requires some additional data and some more analysis. The sediments we see, which are only about 10 centimeters or so thick in each case, in part seem to be coarse at the bottom to fine grained at the top. Above that is a very finely laminated set of relatively bright deposits that weather into this kind of block-like texture. So when we saw it, we all thought, ‘This is an ash cone or what is called a tuff cone,’” he said. A tuff cone or maar is a low-relief, broad volcanic crater formed by shallow explosive eruptions usually caused on Earth by the heating and boiling of groundwater when magma invades the groundwater table.
While science team members hypothesize that Home Plate may be an old impact crater or volcanic feature, they are working now with the data from the Mössbauer spectrometer and APXS to better determine the chemistry and mineralogy of the rocks and exactly what it is. “The rocks in the Columbia Hills are a lot older than those we traversed over getting to the hills, but they’re still basaltic in composition,” Arvison noted. “And they’re granular – made of individual grains – and they seem to have been invariably altered in water a long time ago. The expectation I think is that Home Plate is part of a dissected volcanic cone that formed maybe as part of the volcanic plains, but with the magma coming up through the Columbia Hills, which may have had a little bit more moisture available, so the deposits rather than being emplaced as lavas were explosive ash deposits.” The hypothesis is still a working hypothesis and so, he said, “it remains to be seen as we collect the data.”
During the past week and a half or so, Spirit acquired images of rocks dubbed Wilmington and James ‘Cool Papa’ Bell, and conducted more atmospheric observations. The rover also used its MI to image a rock dubbed Stars, which it brushed with its RAT and then took observations with the Mössbauer spectrometer. Last weekend, the rover examined another rock called Crawfords, taking MI images, spectrometer integrations, brushing the rock with the RAT, and re-imaging the brushed area.
Spirit’s objective now is to pack in the science as much as possible while focusing on a drive campaign that will take the rover to the north-facing slopes of McCool Hill, named for the Columbia’s last pilot. There, Spirit will attempt to survive a second winter on Mars with its solar panels tilted toward the Sun. “What we need to do soon is get off of Home Plate and drive to McCool Hill, so we can have the vehicle’s solar arrays pointed to the north, because we’re moving into the fall and winter seasons, and since we’re in the southern hemisphere the Sun is moving north,” explained Arvidson.
Currently, Spirit is continuing its investigation of Home Plate. “We want to leave Home Plate no later than Sol 780 [about two weeks from today], so we’re going to use these precious sols to look at Home Plate some more,” said Crisp. “If you look down from orbit at Home Plate, we’re going to go clockwise, or basically around the east side of Home Plate as we head to McCool Hill. We made that decision last week. The idea is to look at these different places around Home Plate as Spirit drives the 100 meters (328 feet) or so around the feature. Looking on the side is very helpful because you can then see the layering and the changes in the way the layers appear,” she added. The team is hopeful that Spirit will arrive at McCool Hill by Sol 820 or so (around mid-April).
Opportunity from Meridiani Planum
Opportunity finally left the Olympia area last week and has made progress to Payson, the richly layered outcrop that lies along the western end of Erebus crater in section dubbed the Mogollon Rim. Although Payson caught the eyes of the science team members as the rover first approached the crater last year, Opportunity may not spend much time examining it up close, because of the science team’s desire to get the rover back on the road for the long journey to Victoria Crater.
Opportunity began the month conducting remote sensing and finishing its investigation of the feature called Overgaard in the Olympia outcrop area where the rover had been stuck in place since last Thanksgiving. When the rover’s IDD stalled just before the holiday, the team realized that a wire in its shoulder joint’s azimuth motor had apparently broken. “There was a long time spent deciding on how to overcome that -- on experiments and analyses on how to drive,” said Arvidson. “It took a long time because it’s a difficult problem. The engineers did a lot of analysis and then testing with the engineering vehicles at JPL.”
Despite the fact that Opportunity is still suffering from stalls, which stop all work commands when they happen, once enough experimenting had taken place with the test bed rovers at JPL, the rover was instructed to stow its IDD in the “hover” position, with the arm “hovering” over the solar arrays. The big concern was that if the rover hit a bump with the IDD in the “hover” stow position it could hurt the arm and/or the instruments on it. “If you go over a rock – boom – you shake the arm in a way it was not built to withstand and back drive the motors,” said Crisp. “The issue is we have to visualize the terrain in front of us and make sure we’re not driving over any bumps, but we can only see out so far with our cameras and know that for sure, so we can only use the “hover” stow position for short drives.”
There were also issues with how much current to apply to the actuators or motors to get the rover’s IDD to move. “We spent almost half the time we were at Olympia on engineering anomalies and deciding on how to overcome the IDD issues,” Arvidson said.
Using the “hover” stow configuration, the rover roved onward on Sol 724 (February 5) beginning the short drive to move onto Roosevelt, an interesting outcrop rock that the team had been looking at since Opportunity “broke” her arm and was ordered to stay at Olympia. At Roosevelt, the rover acquired high-resolution images of the surrounding outcrops with its PanCam, then used the MI to take images for a mosaic. It zeroed in on targets there dubbed Rough Rider and Fala, which the rover later examined with the APXS and Mössbauer spectrometer.
Roosevelt lines a fracture in the local pavement and the MER scientists theorize that it is a fracture fill, formed by water that percolated through the fracture. This would mean the feature is younger than surrounding rocks and, therefore, might provide evidence of water that was present some time after the formation of Meridiani Planum sedimentary rocks.
From Roosevelt, Opportunity took a short drive to Bellemont where it conducted an in-depth study, which included taking MI mosaics of 4 targets there, named Vicos, Tara, Chaco, and Verdun. Despite a couple of technical glitches with the IDD, Opportunity completed the work and, said Arvidson, “we decided it was a wild success so we closed up shop and got out of there.”
Although the “hover” stow position proved to work just fine for those two very short drives, it would not do for longer drives – and Opportunity has many long drives on its agenda once it leaves Erebus and heads for Victoria crater some 2.5 kilometers (1.5 miles) farther to the south. So, after performing more diagnostic tests on Sol 735 (February 17), the MER engineers decided to increase rotor resistance for stowing and unstowing the IDD. Voila! The rover successfully stowed and unstowed its arm into its original stow position on both Sols 740 and 741 (February 22 and 23). Apparently, as along as the IDD remains in calibration, the higher resistance value provides no additional risk. “When we do a long drive we are going to be completely tucking the IDD under like we always have, but when we only do a one meter bump or drive just a short distance then we can put the arm and “hover” it over the solar panel,” summarized Crisp.
Finally, Opportunity was ready to blow the Olympia popsicle stand, and head for an outcrop between there and the western wall of Erebus dubbed Zane Grey, but it didn’t happen seamlessly. After a brief delay caused by missing a data uplink because of a Deep Space Network (DSN) problem, the rover began its journey only to stop after moving only 21 centimeters (8 inches) when the right middle wheel reached the maximum current allowed. Motor current on all the other wheels remained nominal. The team had reduced the current limits after leaving Purgatory Dune last summer in order to help prevent another imbedding event. Once the drivers returned the limits back to nominal levels, Opportunity roved onward and right up to Zane Grey, completing a 35-meter (115-foot) drive with its arm in the original stowed position.
“Zane Grey is this little small patch of flat rock sitting amongst the sand dunes inside Erebus crater, a kind of waypoint just inside the crater and on our way to Payson,” explained Crisp. “The pictures [of it] are stunning. We had a very nice – on the order of half a meter -- of relief sitting in front of us. Instead of looking down on a flat slab of rock, we can now see a kind of like Burn’s Cliff exposure where we’re looking at the layering in an exposure of a vertical mini cliff.”
From its position at Zane Grey, the rover imaged possible exit paths from Erebus, then wrapped up its work and headed onward last weekend. “We could have gone up the rim and out to the west, but we decided to follow the outcrop just on the eastern portion on the floor where the dunes kind of die out and the outcrop is present, and there’s this little half-tube that we can zoom down,” said Arvidson. “Now we’re poised to shoot down due south, right next to the outcrop north of Payson.” Since Opportunity is currently in restricted sols, the rover cannot drive every day. Coming up this weekend, however, the rover will drive a total distance “of perhaps another 30 to 40 meters,” he said.
From this point on, Opportunity’s objective is to put her pedal to the metal. “We won’t stop for IDD work unless there is something really new and exciting,” said Arvidson. That includes Payson, in spite of the anticipation early on about getting to this target. “We’re very close to the outcrop on the western wall of Erebus to the north of Payson on the Mogollon Rim and we should be in the position to get to Payson next week. But it’s going to take something extraordinary for us to stop and do IDD work for two reasons: first, we are losing energy each sol and we want to get onto the plains so that we can drive as much as we can before we get to the dead of winter because we really want to get to Victoria; second, the IDD is a touchy system to use successfully, because we’re still experimenting with how much current to apply to the various actuators and because it’s likely that we’ll attempt deployment more than once to make it successfully, so we only want to do deployment for key soils and rocks. So we’re going to drive as much as we can and as for the outcrops at Payson and the rocks we’re looking at right now, we’ll just do PanCam and imaging, and mini-TES unless something extraordinary turns up.”
Although Opportunity won’t be hanging at Payson for long if at all, the images that the rover has already returned of the outcrop are more than satisfying. “It’s nice that we could see such defined layering exposed in the cliff at Payson, because we may not have that kind of thing for a while,” said Crisp. “I still never cease to be amazed.”
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