Scientists and engineers celebrated when they saw the first pictures
NASA’s Opportunity sent from the rim of a stadium-sized crater that
the rover reached after a six-week trek across martian flatlands.

Multiple layers of exposed bedrock line much of the inner slope of the
impact crater informally called “Endurance.” Such layers and their
thicknesses may reveal what the environment on Mars was like before
the salty standing body of water evaporated to produce the telltale
rocks that were explored in the tiny “Eagle” Crater. That’s where
Opportunity spent its first eight weeks on Mars.

click on image to enlarge

“It’s the most spectacular view we’ve seen of the martian surface, for
the scientific value of it but also for the sheer beauty of it,” Dr.
Steve Squyres of Cornell University, Ithaca, N.Y., said about a color
panorama of Endurance Crater released at a news conference today at
NASA’s Jet Propulsion Laboratory, Pasadena, Calif. He is the principal
investigator for the science instruments on both Opportunity and its
twin Mars Exploration Rover, Spirit.

In coming days, Opportunity will circle the rim of Endurance,
observing the crater’s interior from various angles. Scientists and
engineers have begun to identify interesting science targets and
assess how difficult it would be for the rover to descend partway into
the crater and climb back out. “We will need to decide whether the
science is compelling enough to send the rover into a crater it might
never leave, or whether to explore other sites first before entering
Endurance,” said Orlando Figueroa, director of the Mars Exploration
Program, NASA Headquarters, Washington.

At Eagle Crater, an outcrop of bedrock only about the height of a
street curb yielded evidence that the site was once covered by a body
of salty water deep enough to splash in. “That was the last dying gasp
of a body of water,” Squyres said. “The question that has intrigued us
since we left Eagle Crater is what preceded that. Was there a deep
body of water for a long time? Was there a shallow, short-lived playa?
We don’t know.”

The strategy for seeking answers is to examine older rocks from deeper
layers, so Opportunity was sent on drives totaling about 800 meters
(half a mile) to reach the deepest crater nearby, Endurance. This
crater excavated by the impact of a tiny asteroid or a piece of a
comet is about 130 meters (430 feet) wide and, from the highest point
on the rim, more than 20 meters (66 feet) deep, 10 times as deep as
Eagle. An exposure of outcrop in a cliff high on the inner wall across
from the rover’s current position reveals a stack of layers 5 to 10
meters (16 to 33 feet) tall. Other exposures around the inner slope of
the crater may be more accessible than the cliff, and chunks from the
same layers may have been thrown out onto surrounding ground by the
crater-forming impact.

“There is a rock unit below what we saw at Eagle Crater,” Squyres
said. “It looks fundamentally different from anything we’ve seen
before. It’s big. It’s massive. It has a story to tell us.”

Brian Cooper, leader of JPL’s squad of rover drivers for Spirit and
Opportunity, said the initial view of the crater doesn’t settle
accessibility questions yet. “The slope right in front of us averages
18 to 20 degrees. Getting into the crater is no problem, but we have
a lot more work to do to assess whether we could get back out. That
depends on soil properties and slippage, as well as slope.” The
planned circuit around the rim will also require careful navigation.
“If you don’t go close enough to the lip, you can’t look in, but if
you go too far, you could fall in,” he said. “We’re going to have a
very interesting few weeks.”

When NASA sent astronauts to the lunar surface more than 30 years ago,
it was decided not to allow them to enter craters as fresh and steep
as Endurance, but Opportunity may be able to do what no human has done
before on another planet.

Scientists and engineers working with the other rover, Spirit, are
also examining images of a destination area to identify possible
targets of study and to assess how well the rover can get to them.
However, that destination area, informally named “Columbia Hills,”
still lies several weeks of travel ahead of Spirit. Images and
surface-temperature information from the NASA orbiters Mars Global
Surveyor and Mars Odyssey are supplementing Spirit’s own increasingly
detailed pictures of the hills. Nighttime surface temperatures
indicate that some areas within the hills are rockier than others,
said Amy Knudson, a rover science team collaborator from Arizona State
University, Tempe.

“The hills represent a different rock unit, likely older than the
plains we’re on,” Knudson said. “There are intriguing features in the
hills and we want to investigate the processes that formed them. We’re
especially interested to see if water played any role.”