[meteorite-list] Boulders Dash Hopes for Mars Landing Site for Phoenix

[Ron Baalke]

http://space.newscientist.com/article/dn10654-boulders-dash-hopes-for-mars-landing-site.html

Boulders dash hopes for Mars landing site
Kelly Young
New Scientist
24 November 2006

NASA's Mars Reconnaissance Orbiter (MRO) has started scanning the arctic
plains of Mars for possible landing sites for the next spacecraft, the
Phoenix Mars Lander.

Some of the first pictures returned to Earth have already dashed the
hopes of scientists who wanted to land Phoenix at a place they call
Region B. It turns out Region B is littered with boulders, which could
make a landing very dangerous.

"That sort of took the wind out of our sails," says Peter Smith,
principal investigator for Phoenix at the University of Arizona in
Tucson, US. The lander is set to launch in August 2007. They will pick a
landing site from five candidates.

"Currently we are targeting the HiRISE telescope on MRO to study other
regions we'd rejected in the past, because we didn't know about these
boulders," Smith told New Scientist. "We think we found some in what we
call Region A."
          
High resolution

A camera on an older craft orbiting the Red Planet, the Mars Global
Surveyor, had spotted some boulders at Region B. But MRO's high-powered
HiRISE camera (High Resolution Imaging Science Experiment) revealed the
boulders in much greater detail. In addition to having a
higher-resolution camera, MRO is also flying 25% lower than Mars Global
Surveyor, enabling closer shots.

MRO started its primary science phase on 7 November and has sent over 50
pictures of the proposed landing sites to the Phoenix team. The boulders
in Region B appear to be concentrated around circular features that used
to be craters. They may have been thrown into the air during the initial
impact that created the crater.

HiRISE can resolve boulders just under 1 metre in diameter. But smaller
rocks can pose a risk. "Our lander needs to stay away from rocks that
are even a third of a metre across," Smith told New Scientist. The
boulders cannot be avoided simply by steering away from them during the
descent due to the time it takes commands from Earth to reach Mars.

The landing ellipse that Phoenix will aim for during its descent is
larger, measuring 120 km by 20 km. There could be many of these boulder
clusters within that ellipse. "There are ways to avoid boulders but we
don't have them," Smith says. "I wish we did now."

Even after landing, boulders still pose a problem. The lander's two
solar arrays unfold like Chinese fans. "If boulders are around, we can
slap it right into that," Smith says. And the arrays are crucial for
providing power to the lander.
          
Flat terrain

The five potential landing regions are quite similar to one another. The
general terrain is very flat, so there is little danger of the lander
touching down on a steep hill and toppling over, unless one of its legs
gets caught on a boulder.

All of the potential landing sites have polygonal features on their
surface, probably caused by the expansion and contraction of ice beneath
the surface.

The five regions were chosen because they all fall within the 65° to 72°
north latitude band specified for this mission and because there is a
good chance of water ice being near the surface. Unlike the two NASA
rovers currently exploring the surface, Phoenix will stay put once it 
lands, so mission planners want to make sure they pick a good spot.

Phoenix will scoop up some of the surface material with a robotic arm
and analyse it on the lander. The arm is also capable of digging a
0.5-metre deep trench to hunt for subsurface ice.

Scooping up soil

Though NASA is hunting for water on Mars, it does not want Phoenix to
land on pure water ice. Soil is easier to scoop up than ice. Also,
surface soil would protect ice from ultraviolet radiation, which could
have destroyed the organic molecules that may indicate life on Mars.

At the end of January 2007, scientists will meet to see if there is an
obvious choice for a landing site. If not, they will keep analysing the
data until summer 2007.

They are comparing HiRISE's data with that taken by the Thermal Emission
Imaging System (THEMIS) camera on the Mars Odyssey orbiter. Since
boulders hold heat better than soil or sand, they show up in THEMIS
images taken early in the morning.