Tales in the Tracks

http://marsrovers.jpl.nasa.gov/spotl..._20040224.html

Tales in the Tracks
Jet Propulsion Laboratory
February 24, 2004

Even before Spirit set "foot" on martian soil, she was
returning stunning images of her new surroundings. And, with
her first triumphant roll off the lander, she set out to
accomplish lofty science goals. What she left in her wake
makes for great bonus science too.

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.