A.J.S. RaylJun 05, 2005

Mars Exploration Rovers Update: Opportunity Roves Out of Sand Dune as Spirit Uncovers Clues of Past Explosions

Nearly a year and a half after landing on the Red Planet, the Mars Exploration Rovers are continuing to collect important science and impress the team with their resiliency.

Opportunity roved her way out of the rippled sand dune that had her stuck for five weeks and is now back out on the plains. The rover had been making painfully slow progress inching her way out of the sand dune into which she drove on April 26 while cruising across the Meridiani plains to the Etched Terrain -- until this past week, when forward progress improved dramatically. The data confirming that Opportunity was indeed unstuck arrived at JPL yesterday morning, and a test drive may be scheduled as early as tomorrow or Tuesday.

On the other side of the planet, Spirit has uncovered new clues that Gusev Crater once had a wet and violent history, During the last month, the rover continued her work on and around the flanks of Husband Hill in the Columbia Hills region and has finally found "the kind of geology you can really sink your teeth into," according to Steve Squyres of Cornell University, principal investigator for the Mars rovers' science instruments. The multiple layers of rock the rover has been examining in this general location suggest, he said, successive deposits of water-altered explosive debris.

Both Spirit and Opportunity continue to be in remarkably good health with good power supplies.

Spirit from Gusev Crater

In the first days of May, Spirit completed her study of the outcrop dubbed Methuselah, which included an extensive examination of a specific rock target there called Keystone, and another target the team calls Pittsburgh.

On Sol 476/May 6, Spirit stowed the robotic arm, bumped backwards and finished imaging Pittsburgh, then began a two-sol drive toward Jibsheet, another outcrop in the nearby area that the team found intriguing. Along the way, the rover has been reaching out and sampling rocks and analyzing soil targets, as well as collecting panoramic camera (PanCam) images and making observations with her remote-sensing instruments.

By Sol 481/May 11, the rover arrived at Jibsheet, and immediately got to work on a target there called Keel. Then, the rover carried out a more extensive examination of the target dubbed Reef.

Roving into the weekend of Sol 484/May 14 and Sol 484/May15, Spirit performed another detailed study using the instruments on her robotic arm on another Jibsheet target called Davis. Then, she packed up and began driving to Larry's Outcrop, which is actually the other side of Larry's Lookout. "Larry's Lookout is one big massive rock, and Larry's Outcrop is what we've designated the other eastern side, but it's all the same hunk of rock," explained Squyres.

At Larry's Outcrop, Spirit took a series of navigation camera and PanCam pictures, and then made detailed examinations of various rock and soil targets. Over the weekend of Sols 491-491/May 21-22, the rover moved to a location the team nicknamed Paros, where the rover used her used the microscopic imager, the alpha particle X-ray spectrometer (APXS) and Mössbauer spectrometer to examine surface details. [On Earth, Paros is one of the Cyclades Islands and lies 96 miles southeast from Athens, Greece.]

Since Spirit has been working in such a rocky area, the rover has been typically taking a set of images with the hazard-identification cameras as she makes the final approach to selected targets. The team has been using these images to determine whether or not it is safe to deploy the robotic arm. Fortunately, the scientists and engineers have discovered far more often than not that it was safe to deploy the robotic arm. In the mix of her imaging, the rover sent home a series of pictures of yet another lone dust devil passing briskly through the rover's general work area.

Although it may seem that Spirit has just been trudging along, returning bits of interesting science here and there, patience -- on the part of both team and rover -- has proved to be the better part of valor. "In the last few weeks, we have gone from a state of confusion about the geology of the Columbia Hills to having real stratigraphic sequence and a powerful working hypothesis for the history of these layers," Squyres said.

For several months, Spirit hiked a flank of Husband Hill, the tallest in the Columbia Hills range she reached nearly one year ago. The slope closely matched the angle of underlying rock layers, and that made the layering difficult to detect. But the rover kept on trucking and reached an intermediate destination dubbed Larry's Lookout in February, then continued on uphill until she reached a point where she could look back. "That was the critical moment, when it all began falling into place," Squyres said. "Looking back downhill, you can see the layering, and it suddenly starts to makes sense."

Intriguingly, the team has found that some of the rocks Spirit has been checking out in Larry's Lookout, Methuselah, Jibsheet, and Larry's Outcrop contain ilmenite, a titanium-iron oxide formed during crystallization of magma, and a mineral that the rover had not found previously in her exploration of Gusev Crater. The presence of ilmenite is, according to Dick Morris, a rover science-team member at the Johnson Space Center, "evidence for diversity in the volcanic rocks in the Gusev region."

These rocks -- although they are in different layers within the outcrops share compositional traits, and are high in titanium, and low in chromium, which suggests a shared origin. However, the degree to which minerals in rocks have been chemically altered by exposure to water or other processes varies greatly from outcrop to outcrop, and their textures also vary. Close-up pictures from Spirit's microscopic imager (MI) reveal that the rocks at Methuselah have thin laminations, while those at Jibsheet are built of bulbous grains packed together, and the ones at Larry's Lookout are massive, with little fine-scale structure.

"Our best hypothesis is we're looking at a stack of ash or debris that was explosively erupted from volcanoes and settled down in different ways," Squyres said. "We can't fully rule out the possibility the debris was generated in impact explosions instead of volcanic ones. But we can say, once upon a time, Gusev was a pretty violent place. Big, explosive events were happening, and there was a lot of water around."

Once Spirit finished her work on Paros, she stowed the robotic arm, and moved on. This week, the rover has been examining a rock called Ahab, another target, not far from Paros, on another part of Larry's Outcrop. Yesterday, she completed work at Ahab and headed out. The plan now calls for Spirit to return to Methuselah "to do a little more imaging" before moving on to her next target, which, said Squyres, has yet to be determined.

Opportunity from Meridiani Planum

The MER team members working with Opportunity spent the first days of May scrutinizing images the rover returned in the hours after getting roving into the rippled dune on April 26. The analysis moved quickly to JPL's In-situ Instrument Laboratory sandbox, where the engineers, scientists, and even the project manager began mixing sandy and powdery materials.

They used common sand, clay, and diatomaceous earth (a silica-rich powder of mostly microscopic plant shells, for its texture) to concoct a mixture they believe closely simulates the stuff that has the rover stuck up to her hubcaps on Mars. After filling the sandbox, they dug holes and built dunes with the sandy material and then began tests with one of the ground models of the MER rovers.

Meanwhile, up at Meridiani Planum, Opportunity hasn't been loafing. Rather, the rover has been collecting as much science data as possible with instruments and cameras, including numerous PanCam images of the rippled dunes, her own far tracks, and a 360-degree panorama of the area called Rub al Khali, named after a region of the Arabian Peninsula. In addition, the rover has been taking daily measurements of the sky's opacity, as well as observations for clouds, and Sun surveys.

On Sol 461/May 11, the MER team put Opportunity's egress plan into action commanding the rover to straighten her wheels. Two sols later, she successfully moved her wheels about two-and-a-half rotations. The images the rover returned of that effort showed results that the team had anticipated from the ground tests at JPL -- and that meant Opportunity was ready to go. The team settled on an exit plan in which the rover would move forward and arc slightly to the left.

Opportunity began making progress, incredibly slowly at first, centimeter by centimeter, inch by inch. From the time the rover began the egress on Sol 463/May 13 to Sol 476/May 26, she had executed some 213 feet [64.8 meters] worth of wheel rotations, but had only moved forward about one foot [34.8 centimeters]. Considering that the rover had been actually slipping at a rate of roughly 99.5 percent, progress, no matter how small, is progress.

In the midst of this setback, the Martian forces did give Opportunity a break of sorts when winds once again whipped up and cleared some dust from her solar panels, increasing the rover's daily energy supply from 620 to about 650 watt-hours, allowing the team to add a horizon survey to her daily agenda of science tasks.

This past week, the outlook brightened for Opportunity and the rover made much better progress in the primary task of executing her way out of the sand dune, according to Squyres. "We had been doing 12 meter increments, commanding 12 meters of motion and typically seeing 5 centimeters of progress. [Wednesday] we commanded 20 meters and saw almost 13 centimeters of movement."

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