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 Moessbauer 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.

This image, taken by the microscopic imager, an instrument located on the Mars Exploration Rover Opportunity ‘s instrument deployment device, or “arm,” reveals shiny, spherical objects embedded within the trench wall at Meridiani Planum, Mars. Scientists are highly intrigued by these objects and may further investigate them. The area in this image measures approximately 3 centimeters (1.2 inches) across.

Image Credit: NASA/JPL/USGS