February 19, 2004
Rovers Digging
Up Discoveries
By inspecting the sides and floor of a
hole it dug on Mars, NASA's Opportunity rover is finding some
things it did not see beforehand, including round pebbles that
are shiny and soil so fine-grained that the rover's microscope
can't make out individual particles.
"What's underneath is different than
what's at the immediate surface," said Dr. Albert Yen, rover
science team member at NASA's Jet Propulsion Laboratory, Pasadena,
Calif.
Meanwhile, NASA's other Mars Exploration
Rover, Spirit, has reached a site with such interesting soil
that scientists have decided to robotically dig a hole there,
too. Spirit's trenching at a shallow depression dubbed "Laguna
Hollow" could answer questions about whether traits on the
soil surface resulted from repeated swelling and shrinking of
an upper layer bearing concentrated brine, among other possibilities.
Opportunity has manipulated its robotic
arm to use its microscope on five different locations within
the trench the rover dug on Monday. It has also taken spectrometer
readings of two sites. "We've given the arm a very strenuous
workout," said JPL's Dr. Eric Baumgartner, lead engineer
for the arm. The accuracy of the tool placements -- within 5
millimeters, or less than a quarter inch -- is remarkable for
mobile robotics on Earth, much less on Mars.
Once data are analyzed from the alpha particle
X-ray spectrometer and the Mössbauer spectrometer about
what elements and what iron-bearing minerals are present, the
differences between the subsurface and the surface will be easier
to interpret, Yen said.
While Opportunity has been digging and
examining its trench this week, it has also been catching up
on transmission of pictures and information from its survey last
week of a rock outcrop along the inner wall of the small crater
in which the rover is working.
Both rovers can communicate directly with
Earth, but JPL's Andrea Barbieri, telecommunication system engineer,
reported that 66 percent of the 10 gigabits of data they have
returned so far has come via relays by NASA's Mars Odyssey orbiter
and another 16 percent via relays by NASA's Mars Global Surveyor.
Based on the outcrop survey, scientists
have chosen a feature they have dubbed "El Capitan"
as the next target for intensive investigation by Opportunity.
"We've planned our assault on the
outcrop," said Dr. Steve Squyres of Cornell University,
Ithaca, N.Y., principal investigator for the rovers' science
instruments. "The whole stack of rocks seems to be well
exposed here," he said of the chosen target. Upper and lower
portions appear to differ in layering and weathering characteristics.
Planners anticipate that Opportunity's arm will be able to reach
both the upper and lower parts from a single parking spot in
front of "El Capitan."
Halfway around the planet, Spirit will
be told to use a front wheel to dig a trench during the martian
day, or "sol," that will end at 12:36 p.m. Friday,
PST.
Some soil in "Laguna Hollow"
appeared to stick to Spirit's wheels. Possible explanations include
very fine-grained dust or concentrated salt making the soil sticky,
said Dr. Dave Des Marais, a rover science team member from NASA
Ames Research Center, Moffett Field, Calif. Pictures of the surface
there also show pebbles arranged in clusters or lines around
lighter patches Des Marais described as "miniature hollows."
This resembles patterned ground on Earth that can result from
alternating expansion and shrinkage of the soil. Possible explanations
for repeated expanding and contracting include cycles of freezing
and thawing or temperature swings in salty soil.
After trenching to seek clues about those
possibilities, Spirit will continue on its trek toward the rim
of a crater nicknamed "Bonneville," now estimated to
be about 135 meters (443 feet) away from the rover. Spirit has
already driven 128 meters (420 feet).
The rovers' main task is to explore their
landing sites for evidence in the rocks and soil about whether
the sites' past environments were ever watery and possibly suitable
for sustaining life.
Previous article: Opportunity
Digs, Spirit Advances
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