But Longer Path Has Scientific Advantages
Loose soil piled against the northern edge of a low plateau
called "Home Plate" has blocked NASA's Mars Exploration Rover
Spirit from taking the shortest route toward its southward
destinations for the upcoming Martian summer and following winter.
The rover has instead begun a trek skirting at least partway around
the plateau, instead of directly over it.
In more encouraging news for the five-year-old rover, Spirit has
also gotten a jump start on its summer science plans, examining a
silica-rich outcrop that adds information about a long-gone
environment that had hot water or steam. And even a circuitous
route to the destinations chosen for Spirit would be much shorter
than the overland expedition Spirit's twin, Opportunity, is making
on the opposite side of Mars.
Both rovers landed on Mars in 2004 for what were originally
planned as three-month missions there.
Spirit spent 2008 on the northern edge of Home Plate, a
flat-topped deposit about the size of a baseball field, composed of
hardened ash and rising about 1.5 meters (5 feet) above the ground
around it. There, the north-facing tilt positioned Spirit's solar
arrays to catch enough sunshine for the rover to survive the
six-month-long Martian winter.
The scientists and engineers who operate the rovers chose as
2009 destinations a steep mound called "Von Braun" and an
irregular, 150-foot-wide bowl called "Goddard." These side-by-side
features offer a promising area to examine while energy is adequate
during the Martian summer and also to provide the next north-facing
winter haven beginning in late 2009. Von Braun and Goddard intrigue
scientists as sites where Spirit may find more evidence about an
explosive mix of water and volcanism in the area's distant past.
They are side-by-side, about 200 meters, or yards, south of where
Spirit is now.
It's mid-spring now in the southern hemisphere of Mars. The sun
has climbed higher in the sky over Spirit in recent weeks.
The rover team tried to drive Spirit
onto Home Plate, heading south toward Von Braun and Goddard. They
tried this first from partway up the slope where the rover had
spent the winter. Only five of the six wheels on Spirit have been
able to rotate since the right-front wheel stopped working in 2006.
With five-wheel drive, Spirit couldn't climb the slope. In January
and February, Spirit descended from Home Plate and drove eastward
about 15 meters (about 50 feet) toward a less steep on-ramp.
Spinning wheels in loose soil led the rover team to choose another
of its options.
"Spirit could not make progress in the last two attempts to get
up onto Home Plate," said John Callas of NASA's Jet Propulsion
Laboratory, Pasadena, CA, project manager for both rovers.
"Alternatively, we are driving Spirit around Home Plate to the
east. Spirit will have to go around a couple of small ridges that
extend to the northeast, and then see whether a route east of Home
Plate looks traversable. If that route proves not to be
traversable, a route around the west side of Home Plate is still an
option."
During the drive eastward just north of Home Plate in January,
Spirit stopped to use tools on its robotic arm to examine a
nodular, heavily eroded outcrop dubbed "Stapledon," which had
caught the eye of rover-team scientist Steve Ruff when he looked at
images and infrared spectra Spirit took from its winter
position.
"It looked like the material east of Home Plate that we found to
be rich in silica," said Ruff, of Arizona State University, Tempe.
"The silica story around Home Plate is the most important finding
of the Spirit mission so far with regard to habitability. Silica
this concentrated forms around hot springs or steam vents, and both
of those are favorable environments for life on Earth."
Sure enough, Spirit's alpha particle X-ray spectrometer found
Stapledon to be rich in silica, too. "Now we have found silica on a
second side of Home Plate, expanding the size of the environment we
know was affected by hot springs or steam vents," Ruff said. "The
bigger this system, the more water was involved, the more habitable
this system may have been."
The contact measurement with the X-ray spectrometer also gave
the team confidence in its ability to identify silica-rich outcrops
from a distance with the rover's thermal emission spectrometer,
despite some dust that has accumulated on a periscope mirror of
that instrument. Researchers plan to use Spirit's thermal emission
spectrometer and panoramic camera to check for more silica-rich
outcrops on the route to Von Braun and Goddard. However, the team
has set a priority to make good progress toward those destinations.
Winds cleaned some dust off Spirit's solar panels on February 6 and
February 14, resulting in a combined increase of about 20 percent
in the amount of power available to the rover.
Opportunity, meanwhile, shows signs of increased friction in its
right-front wheel. The team is driving the rover backwards for a
few sols, a technique that has helped in similar situations in the
past, apparently by redistributing lubricant in the wheel.
Opportunity's major destination is Endeavour Crater, about 14 miles
in diameter and still about 7 miles away to the southeast.
Opportunity has been driving south instead of directly toward
Endurance, to swing around an area where loose soil appears deep
enough to potentially entrap the rover.