Spirit Struggles While Opportunity Rocks
Perched on a rippled Martian plain, a dark rock not much bigger than a basketball was the target of interest for Opportunity during the past two months. Dubbed "Marquette Island," the rock is providing a better understanding of the mineral and chemical makeup of the Martian interior.
"Marquette Island is different in composition and character from any known rock on Mars or meteorite from Mars," said Steve Squyres of Cornell University in Ithaca, N.Y. Squyres is principal investigator for Opportunity and its twin, Spirit. "It is one of the coolest things Opportunity has found in a very long time."
During six years of roving, Opportunity has found only one other rock of comparable size that scientists conclude was ejected from a distant crater. The rover studied the first such rock during its initial three-month mission. Called "Bounce Rock," that rock closely matched the composition of a meteorite from Mars found on Earth.
Marquette Island is a coarse-grained rock with a basalt composition. The coarseness indicates it cooled slowly from molten rock, allowing crystals time to grow. This composition suggests to geologists that it originated deep in the crust, not at the surface where it would cool quicker and have finer-grained texture. Studying the rock could help astrobiologists understand whether or not ancient Mars was capable of supporting life.
"It is from deep in the crust and someplace far away on Mars, though exactly how deep and how far we can't yet estimate," said Squyres.
The composition of Marquette Island, as well as its texture, distinguishes it from other Martian basalt rocks that rovers and landers have examined. Scientists first thought the rock could be another in a series of meteorites that Opportunity has found. However, a much lower nickel content in Marquette Island indicates a Martian origin. The rock's interior contains more magnesium than in typical Martian basalt rocks Spirit has studied. Researchers are determining whether it might represent the precursor rock altered long ago by sulfuric acid to become the sulfate-rich sandstone bedrock that blankets the region of Mars that Opportunity is exploring.
The rover team used Opportunity's rock abrasion tool to grind away some of Marquette Island's weathered surface and expose the interior. This was the 38th rock target Opportunity has ground into, and one of the hardest. The tool was designed to grind into one Martian rock, and this rock may not be its last.
"We took a conservative approach on our target depth for this grind to ensure we will have enough of the bit left to grind the next hard rock that Opportunity comes across," said Joanna Cohen of Honeybee Robotics Spacecraft Mechanisms Corp., in New York, which built and operates the tool.
Opportunity currently is about 30 percent of the way on a 19-kilometer (12-mile) begun in mid-2008 from a crater it studied for two years. It is en route toward a much larger crater, Endeavour. The rover traveled 5.3 kilometers (3.3 miles) in 2009, farther than in any other year on Mars. Opportunity drove away from Marquette Island on Jan. 12.
"We're on the road again," said Mike Seibert, a rover mission manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "The year ahead will include lots more driving, if all goes well. We'll keep pushing for Endeavour crater but watch for interesting targets along the way where we can stop and smell the roses."
Spirit Swims in Sand
Spirit performed the first backwards drive (toward the south) on Sol 2045 (Jan. 14, 2010). Until then, all drives since extrication attempts began two months earlier had been with forward driving, with Spirit facing northward. The rover first entered its present location driving backward in April 2009. The backward driving in recent days includes the additional technique of steering the wheels side-to-side before performing each step. The hypothesis for the wheel steering is two-fold. The process clears out material in front of the wheel and allows material to slough off the face of the wheel trench and provide traction under the wheel. Also, the flat surface of the wheel's side "kicks" against loose material, like a swimmer's frog kick or breast stroke, to provide some push. This Sol 2045 drive included enough wheel rotations to move the rover backward about 30 meters (98 feet) in six steps of 5 meters (16 feet) each, if the rover were in a situation with good traction. However, as Spirit is in a sand trap, the drive moved the rover backward a total of just over 3 centimeters (1.2 inches) and raised it in altitude just over 1 centimeter (0.4 inch). This is the first time the rover has climbed since extrication attempts began.
Northerly tilt also improved by just over a degree. The explanation here is that the rover's rear wheels are climbing, raising the back of the rover. Images from the rear hazard avoidance camera confirm this. A tilt toward the north would be favorable for energy production in the coming Martian winter, as it would gain more sunshine on the solar array.
A second backward drive was commanded on Sol 2047 (Jan. 16, 2010). It was also six steps of 5 meters backward with the steering "frog kicks." The rover moved about 3.5 centimeters (1.4 inches) backward and climbed 0.3 centimeters (0.1 inch). However, this time the northerly tilt deteriorated by over a degree, undoing the prior drive's improvement. The explanation here is that the rover yawed counterclockwise, swinging the angled solar arrays away from north. But the rear wheels continued to climb, suggesting that the middle wheels are gaining traction. The rover is now about 3.5 centimeters (1.4 inches) south of the point where it started extrication two months ago, meaning the backward driving has already covered all of the distance achieved with forward driving and then some. Spirit is still down about 3 centimeters (1.2 inches) in altitude since extrication started. It is important to remember that the right-rear wheel is still non-functional, along with the right-front wheel.
As for other techniques to consider for extrication, the rover team has examined the two options that would use the robotic arm: pushing with it and re-sculpting the terrain by the left-front wheel. The assessment of pushing with the arm reveals that only about 30 newtons of lateral force could be achieved, while a minimum of several hundreds of newtons would be needed to move the rover. Further, such a technique risks damaging the arm and preventing its use for high-priority science from a stationary rover. The other technique of re-sculpting the terrain and perhaps pushing a rock in front of or behind the left-front wheel is also assessed to be of little to no help and, again, risks the arm. There is also a large risk of accidentally pushing the rock into the open wheel and jamming.