A surface feature thought to be ice
beneath NASA's Phoenix Mars Lander visibly changed sometime between mid-June
and mid-July, close-up images show.
Phoenix's robotic arm camera took the first
close-up
image of the hard feature, dubbed Snow Queen, on May 31, six Martian days
(called sols) after the craft landed. The $420 million dollar mission is
digging up and testing samples of Martian dirt and ice to see if the red planet
might have been habitable at some point in the past.
Thruster exhaust, intended to slow
the spacecraft's descent, blew away the surface layer of dirt covering Snow
Queen as the craft landed, exposing the hard layer, which had featured several
smooth, rounded cavities.
More recent images show that some of
the smooth texture of Snow Queen has slightly roughened since then. Cracks as long as 4 inches (10 centimeters) have
appeared in the hard layer. A pebble or clod about one-third of an
inch long (seven millimeters) not seen before also popped up to the surface.
"Images taken since landing
showed these fractures didn't form in the first 20 sols of the mission,"
said Phoenix co-investigator Mike Mellon of the University of Colorado
in Boulder.
"We might expect to see additional changes in the next 20 sols."
Unprecedented observations
Mellon said that the long-term
monitoring of Snow Queen and other patches of ice exposed by Phoenix
is unprecedented for science. This mission has offered the first chance for
scientists to see visible changes in Martian ice at a place where temperatures
are cold enough to keep the ice from immediately sublimating, or vaporizing,
after it is exposed to the air.
The Phoenix team has also been watching ice in
the Dodo-Goldilocks and Snow White trenches with its stereo camera and the
robotic arm camera (only the robotic arm camera can reach to see Snow Queen
under the lander).
Previous images showed that
centimeter-sized chunks of
ice scraped up in Dodo-Goldilocks lasted several days before disappearing,
presumably having sublimated away.
Scientists have been working to
scrap up a sample of the ice-dirt mixture in the Snow White trench to test in
the lander's Thermal and Evolved-Gas Analyzer, which
heats up samples in its tiny ovens and then analyzes the vapors to determine
the sample's composition. The mixture has proven stickier
than expected and mission controllers are moving ahead to analyze a second
sample of the drier surface dirt while they work on methods for delivering an
icy sample.
Several ideas
Mellon said he has several
hypotheses for what caused the cracks to form in Snow Queen, but he added,
"There are difficulties with all of them."
One possibility is that the
temperature changes throughout the Martian day have expanded and contracted the
surface enough to create stress cracks. It would take a fairly rapid change to
form the fractures seen in the robotic arm camera images, Mellon said.
Another hypothesis is that the
exposed layer has undergone a phase change that caused it to shrink. For
example, salts in the layer could have lost water molecules attached to them
after a few days of exposure, causing the layer to shrink and crack.
"I don't think that's the best
explanation because dehydration of salt would first form a thin rind and finer
cracks," Mellon said.
"Another possibility is that
these fractures were already there, and they appeared because ice sublimed off
the surface and revealed them," he said.
The small pebble that popped up
after 21 sols could be a piece of the original surface that broke free or it
might have fallen down from somewhere else.
"We have to study the shadows a
little more to understand what's happening," Mellon said.
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