NASA’s Spirit and Opportunity have been exploring Mars
about three times as long as originally scheduled. The more
they look, the more evidence of past liquid water on Mars
these robots discover. Team members reported the new findings
at a news briefing today.
New findings raise the possibility Opportunity’s work area
was soaked long ago, before it dried and eroded into a wide
plain. There are also signs some rocks may have gotten wet
again, after an impact excavated a stadium-size crater in the
plain.
Evidence of this exciting possibility has been identified in
a flat rock dubbed “Escher” and some neighboring rocks near
the bottom of the crater. These plate-like rocks bear
networks of cracks dividing the surface into patterns of
polygons, somewhat similar in appearance to cracked mud after
the water has dried up here on Earth.
Alternative histories, such as fracturing by the force of the
crater-causing impact, or the final desiccation of the
original wet environment that formed the rocks, might also
explain the polygonal cracks. Rover scientists hope a lumpy
boulder nicknamed “Wopmay,” Opportunity’s next target for
inspection, may help narrow the list of possible
explanations.
“When we saw these polygonal crack patterns, right away we
thought of a secondary water event significantly later than
the episode that created the rocks,” said Dr. John
Grotzinger. He is a rover-team geologist from the
Massachusetts Institute of Technology, Cambridge, Mass.
Finding geological evidence about watery periods in Mars’
past is the rover project’s main goal, because such
persistently wet environments may have been hospitable to
life.
“Did these cracks form after the crater was created? We don’t
really know yet,” Grotzinger said.
If they did, one possible source of moisture could be
accumulations of frost partially melting during climate
changes, as Mars wobbled on its axis of rotation, in cycles
of tens of thousands of years. According to Grotzinger,
another possibility could be the melting of underground ice
or release of underground water in large enough quantity to
pool a little lake within the crater.
One type of evidence Wopmay could add to the case for wet
conditions after the crater formed would be a crust of water-
soluble minerals. After examining that rock, the rover team’s
plans for Opportunity are to get a close look at a tall stack
of layers nicknamed “Burns Cliff” from the base of the cliff.
The rover will then climb out of the crater and head south to
the spacecraft’s original heat shield and nearby rugged
terrain, where deeper rock layers may be exposed.
Halfway around Mars, Spirit is climbing higher into the
“Columbia Hills.” Spirit drove more than three kilometers
(approximately two miles) across a plain to reach them. After
finding bedrock that had been extensively altered by water,
scientists used the rover to look for relatively unchanged
rock as a comparison for understanding the area’s full range
of environmental changes. Instead, even the freshest-looking
rocks examined by Spirit in the Columbia Hills have shown
signs of pervasive water alteration.
“We haven’t seen a single unaltered volcanic rock, since we
crossed the boundary from the plains into the hills, and I’m
beginning to suspect we never will,” said Dr. Steve Squyres
of Cornell University, Ithaca, N.Y., principal investigator
for the science payload on both rovers. “All the rocks in the
hills have been altered significantly by water. We’re having
a wonderful time trying to work out exactly what happened
here,” he added.
More clues to deciphering the environmental history of the
hills could lie in layered rock outcrops further upslope,
Spirit’s next targets. “Just as we worked our way deeper into
the Endurance crater with Opportunity, we’ll work our way
higher and higher into the hills with Spirit, looking at
layered rocks and constructing a plausible geologic history,”
Squyres said.
Jim Erickson, rover project manager at JPL, said, “Both
Spirit and Opportunity have only minor problems, and there is
really no way of knowing how much longer they will keep
operating. However we are optimistic about their conditions,
and we have just been given a new lease on life for them, a
six-month extended mission that began Oct. 1. The solar power
situation is better than expected, but these machines are
already well past their design life. While they’re healthy,
we’ll keep them working as hard as possible.”
JPL, a division of the California Institute of Technology in
Pasadena, manages the Mars Exploration Rover project for
NASA’s Science Mission Directorate, Washington. Images and
additional information about the project are available from
JPL and Cornell at: http://marsrovers.jpl.nasa.gov and http://athena.cornell.edu
