Both Spirit and Opportunity are equipped with a sophisticated suite of scientific instruments, but you wouldn't think the wheels were among them - or would you? As the wheels move across the martian surface as they are designed to do, they churn up clues that help scientists.

"I would compare the rover tracks to the boot prints of geologists walking around on Earth," said Dr. Lutz Richter, of the German Space Agency and Mars Exploration Rover science team member. "They immediately give us information about the nature of the material on which we are roving."

How far have we sunk?
Scientists have been busy analyzing Spirit's new territory. Since they cannot don their geologic tool belts and go themselves, they are taking advantage of tools on the rover that can virtually put them there.

"The material we are on has given way to the weight of the rover in some places," Richter noted. "We can measure the amount of sinkage and that tells us the strength of the material that we are on. It is a 'cheap' measure of information for us that we can use throughout the mission. So far we have seen a lot of variation."

Lacking any kind of interplanetary ruler, scientists rely on advanced software, developed at NASA's Jet Propulsion Laboratory, called the Science Activity Planner, to make measurements. Using stereo images from the rover cameras and the known weight of the rover and its wheels, scientists can get very accurate information about the surface material.

What are we looking at?
The amount of sinkage into the surface material is leading the science team to believe that there is a thin crust covering the soil. This uppermost material, which measures between a half-centimeter and one centimeter, is relatively young in geologic years - probably not older than several tens of thousands of years old. A mere infant when compared to the underlying material that dates back billions of years to when Gusev Crater might have cradled a lake.

From previous missions to the surface of Mars we've seen similar materials but not such a large area of it, Richter said. Preliminary chemical analyses indicate high amounts of chlorine and sulfur.

Water's role
The debate continues about Mars' mysterious past - was it always as desolate as it is today or was it once warmer and wetter? Richter and his colleagues wonder if water, in some form, played a role in the formation of the thin crust near the lander, the Columbia Memorial Station.

"For climate studies this understanding is very important because there must have been some moisture at work - even if only in low quantities," he said. "There's a water cycle on Mars. There are certain times of year that trace amounts of water are present in certain locations. There is also water vapor in the atmosphere and ice below the surface. Perhaps a few hundred thousand years ago the atmosphere might have been saturated and could have been responsible for this recent crust at the Gusev site."

Seeing Eons Below the Surface
Without the benefit of any major excavating tools, Spirit and Opportunity can still analyze material that formed billions of years ago. Rocks that were violently displaced from craters expose part of Mars' history.

"Below the crust would be any evidence of the lake deposits - perhaps a few meters - but we don't know because there may have been volcanic activity there," Richter explained. "That's why it's so important to go to the nearby crater because there are ejecta rocks there that would give us a clue about what lies far beneath the crust. Ejecta rocks are those that were sent flying when an impact created the crater hole."

Rover wheels aren't just for driving anymore! Proof that driving on Mars is anything but "routine," they reveal a part of Mars that time has covered up.

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