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Spirit's roving chances bleak, team cheered by discoveries
Posted: November 16, 2009

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The Jet Propulsion Laboratory on Monday will begin commanding the Mars rover Spirit to start backing out of the sand pit in the same tracks it left going into the scientifically rich location where it became stuck in April.

Engineers have been conducting tests in a "sandbox" at JPL. Credit: NASA/JPL
"We have been at one of the most scientifically exciting places that we have found on Mars and we still could have a lot of work to do at that site [even if we remain stuck,]" says Ashley Stroupe, a JPL Spirit rover driver.

If Spirit is declared stuck it will remain an operational science station, just no longer mobile. Its operations would involve:

  • Weather observations: The rover would use its cameras for long term documentation of cloud formation and atmospheric opacity caused by dust.
  • Surface changes: Rover cameras will also undertake long term observation of the surface to watch for changes caused by Martian winds.
  • Panoramas: The cameras will be used to make detailed stereo panoramas of the Columbia hills area. Panoramas are time consuming so done less often when the rovers are on the move.
  • Geodynamics measurements: Use of radio transmissions to and from a fixed rover can over time build up new geodynamic information on Mars, experiments that will be done if Spirit is permanently stuck.
  • Seismometer: The data systems on the rover can also be used to detect seismic activity on Mars if any occurs in its Gusev crater area.
  • Spectrometer runs: The half life of the cobalt 57 radiation source in the Mossbauer spectrometer used to determine chemical compositions has been largely depleted during 5 years of use, compared with an expected 90 day of operations when designed.

This means that data acquisition on surface targets that earlier took only three hours now requires several days of contact by the sensor head. If the rover is stuck forever, these long data analysis periods will be done to acquire more information on the chemistry of the sulfate rich area.

Topographic map illustrates how Spirit is straddling the edge of a hidden crater with its left wheels imbedded in water related sulfate sand. The right wheels outside the crater are on totally different material. Credit: NASA/JPL/Ohio State
As depicted in a topographic map (above) prepared by rover science researchers at Ohio State University, Spirit is straddling a hidden geologic boundary separating an extremely old shallow crater on its left or west side and the original volcanic surface level on the right or east side.

The 26 ft. diameter crater now only 10 in. deep was punched into sulfate rich material billions of years ago. The presence of so much sulfate means that potentially life sustaining water was abundant during at least two different periods at that location in Gusev crater.

Those periods would have been the time before a meteorite punched into the spot and churned up the sulfate soil, and again when the material that made up the volcanic deposits were reprocessed later by water . The latter water events apparently led to formation of the unusual crust that covered the top of the crater, tricking rover drivers into thinking this was solid soil to drive over.

In imagery taken over the last three decades from Mars orbit, the entire Troy area is near the mouth of what looks like a giant river channel that during early Martian history carried water into the crater. It was these images from orbit that led to selection of Gusev crater as Spirit's landing site in the first place.

Whether water that carved the channel was related to the evidence for wet periods that Spirit has found is not yet determined.

The evidence for that water episode, when Gusev could have been a deep lake, has been buried by volcanic debris that initially led NASA managers to believe the site would be "scientifically unproductive," says Doug McCuistion director of the Mars Exploration Program at NASA Headquarters. But Spirit found many signs of past water not on the crater floor as expected, but rather in the hills where engineers thought initially the rover could never reach.

No matter how sturdy the rover has proven to be over the last 5 years, NASA and contractor engineers are now genuinely pessimistic about Spirit's chances of freeing itself. They plan to attempt such maneuvers only until about February when a decision could be made to declare Spirit a fixed station rather than a roving vehicle, McCuistion says.

Rover engineers and soil experts from across the country have worked since May to simulate traction and steering options that could be used by Spirit to drive out of the slick powdery soil that also cakes to the rover wheels.

Nothing the engineers tried was able to free the two engineering rovers during weeks of simulations in an Earthly soil mixture made to duplicate the Martian volcanic sand where the rover is stuck. Only its three left wheels are mired, however, resulting in a 12 deg. tilt to the northwest.

The data from all of the simulations and computer analysis led to a decision to simply try and drive out directly over the tracks created when the rover drove in, says John Callas, JPL rover project manager.

The first drive command will be for enough wheel rotations that if the rover on solid ground it would travel about 15 ft. That will not happen however. Engineers will be happy if the wheel rotations only move the rover a fraction of an inch. They will be vigilant that the wheel motions do not drop the rover belly closer to the surface.

This is especially important because of a large rock under the rover belly. Spirit's microscopic imager on its instrument arm managed to photograph the rock. The rocks under the rover belly are part of a group designated the "Rock Garden" that Spirit imaged from several feet away just before it became entrapped when passing over them.

One of the pointed rocks could be touching the rover's belly. This could potentially prevent Spirit's wheels from gaining traction stranding it like a turtle left atop a rock. An equally dangerous risk is if the pointed rock hooked into one of the small cavities on the belly originally used to latch the rover onto its air-bag lander. Stroupe said all steering commands will be made in directions that maneuver away from risking the rock from catching on a belly cavity.

The plan is to pause for a day of data analysis and planning after each day's drive command. That way precise measurement can be made about whether any motion toward the south can be determined using images from the rover cameras.

Spirit hazard camera images Ulysses sulfate rich white sand dug up by stuck wheel as well as Cyclops Eye drill hole where more sulfates were found. The nearby Polyphemus Eye drill hold had no sulfates illustrating how Spirit is straddling separate geologic zones. Credit: NASA/JPL
Both Spirit and Opportunity have been making continuous discoveries since landing in January 2004. Spirit has continued that by making startling subsurface findings related to water and volcanism that it would not have made had it only driven by the spot dubbed Troy as planned.

Instead Spirit plunged through an unknown type of water related crust and accidentally began excavating a trench with its spinning wheels exposing some of the most important water related material seen by either rover on Mars.

"No place is a nice place to be imbedded but this place turns out to be a geological treasure trove," says Ray Arvidson of St. Louis University, deputy rover principal investigator.

The rover's rock abrasion tool mounted on the instrument arm was critical to verifying that Spirit was indeed straddling the edge of a hidden crater containing materials much different than more common soils a few feet away.

The key tests are evident in the labeled image of the white sulfate soil dug up by the rover wheel. The material thrown out by the wheel is labeled Ulysses.

The Rat was used to bore through the crust nearly in line with the wheel. The circular cut was labeled Cyclops Eye. Spectrometer data from that area shows sulfate rich soil just as in the Ulysses trenched soil.

The arm was then used to drill another hole about 3 ft. to the right. That drilled area was named Polyphemus Eye and sampled several times by the spectrometers, but turned up no sulfate rich soil. This confirmed that Spirit was sitting abeam two totally different geologic zones, says Arvidson.

When it became stuck Spirit was circling the large Home Plate geologic feature it has explored for the last two years. Rover drivers were trying to set up approach angles toward a small but extremely well preserved volcanic cone named Von Braun by the rover team. An adjoining circular feature was dubbed Goddard by the team.

Home Plate is an explosive volcanic deposit. It is surrounded by deposits of basalt which exploded on contact with water. The presence of brine at the location is further supported by the high concentration of chloride ions found by Spirit in the surrounding rocks.

A patch of 90% pure opaline silicon dioxide was also unearthed by Spirit in the vicinity of Home Plate. The patch is believed to have been formed in acidic hydrothermal conditions that further supports the theory that Home Plate is of an explosive volcanic origin. Discoveries at Tory provide even more evidence for water in an area that may have even had warm pools and geysers, says Arvidson.

"This has been an exciting area to be in scientifically," Arvidson says.

"Probably an impact crater punctured into older sulfate rich deposits associated with the Home Plate's volcanic eruptions," he says.

Spirit imaged rock garden pike of rocks directly in the middle of the area where it got stuck after driving a few feet further. One of these rocks is pointed against rover belly now. The Von Braun volcanic cone with capped top is ahead, Spirit's next objective should it break free. Credit: NASA/JPL
"We know based on the sulfate deposits measured at Troy and the three other places that we have encountered it, together with the opal and silica discoveries from last year, that this place was warm and wet.

"There were steam eruptions and there may have even been hydrothermal pools back billions of years ago," Arvidson said.

"We never have been at such a juicy site for such a long time. There has always been pressure to move to go to new site."

"But it would be nice to expand that by heading south to these beautiful volcanic exposures that we call Von Braun and Goddard," Arvidson concluded.

Spirit will have to back out of the sand trap before that is possible, however.