NASA's plucky Mars Exploration Rover Spirit, which is still stuck in a patch of soil, has been taking advantage of the situation to learn more about the planet's environmental history.

The sand trap, dubbed Troy, has been holding the rover hostage since April, with Spirit's rotating wheels digging more than hub deep into the sand. Engineers have prepared test escape maneuvers at NASA's Jet Propulsion Laboratory in order to help free the stranded rover. Not to be beaten by the situation, Spirit is studying the composition of the soil while it waits further instruction on how to leave the sandpit.

"By serendipity, Troy is one of the most interesting places Spirit has been," says Ray Arvidson of Washington University and deputy principal investigator for the science payloads on Spirit and its twin rover, Opportunity. "We are able here to study each layer, each different colour of the interesting soils exposed by the wheels."

As Spirit's wheels churned the soil, it exposed coloured sandy materials and a miniature cliff of cemented sands in tan, yellow, white and red. Scientists analysing the data suggest that differences in colours may come from differences in the hydration states of iron sulfates.

"The layers have basaltic sand, sulfate-rich sand and areas with the addition of silica-rich materials, possibly sorted by wind and cemented by the action of thin films of water," says Arvidson. "We're still at a stage of multiple working hypotheses. This may be evidence of much more recent processes than the formation of Home Plate. Or is Home Plate being slowly stripped back by wind, and we happened to stir up a deposit from billions of years ago before the wind got to it?"

The analysis was further enhanced by the fact a dust storm recently gave Spirit's solar panels a spring clean. "The exceptional amount of power available from cleaning of Spirit's solar arrays by the wind enables full use of all of the rover's science instruments," says Richard Moddis of the Johnson team. "If your rover is going to get bogged down, it's nice to have it be at a location so scientifically interesting."

The mock-up Martian soil combines diatomaceous earth, powdered clay and play sand. Engineers are preparing several tons of the mixture into contours to match Troy. A test rover will be put through its paces to find the safest way to proceed on Mars, a problem magnified by Spirit's lame right-front wheel.

"With the improved power situation, we have the time to explore all the possibilities to get Spirit out," says JPL's John Callas, project manager for Spirit and Opportunity. "We are optimistic. The last time Spirit spun its wheels, it was still making progress. The ground testing will help us avoid doing things that could make Spirit's situation worse."

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