Mars images reveal elegant polarity of ice caps
Some portions of the martian south polar residual cap have long, somewhat curved troughs instead of circular pits.
By Richard Stenger
CNN Interactive Staff Writer
March 9, 2000
Web posted at: 11:47 a.m. EST (1647 GMT)
PASADENA, California (CNN) -- Besides showcasing elegant and
contrasting patterns around the polar caps of Mars, new
images snapped by Mars Global Surveyor offer evidence that
the northern and southern extremes have experienced markedly
different geologic pasts.
The pictures, released Wednesday, revealed for the first time features usually hidden underneath the top layers of
seasonal ice. A camera onboard Surveyor
obtained the photos as it orbited the poles during their
The northern residual ice cap, or region that remains frosted
in the summer, has a mostly flat and pitted surface that
varies in appearance from a cottage cheese to a sponge
texture. The southern one, pocked with larger pits, troughs
and mesas, often resembles slices of Swiss cheese.
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"It was just so different from anything we have seen on
Mars," said Dr. Peter Thomas, an astronomer at Cornell
University and lead researcher on the polar cap project.
"We knew there were difference before (between the caps). But
it wasn't clear until now if it was a differences in ice or
topography," he said.
Thomas said the contrast in topography on the two polar
regions indicate that they experienced different climates and
geological histories for thousands of years or longer.
Different poles, different ices
The ice on the north polar residual cap is believed to
consist primarily of water, based on data from the Mariner 9
and Viking orbiters in the 1970s.
The portion of each martian polar cap that remains frosted through the summer is known as the Residual Polar Cap. The two pictures above show examples of the north (top) and south polar residual caps as they appeared in summer.
Sponge-like surface patterns in the north have many small
pits, generally 10 to 20 meters (33-66 feet) wide. They
probably form during the summer as water ice sublimes, or
changes directly from solid to vapor.
The temperature there usually is near water's freezing point
in the summer. And the Viking spacecraft observed water vapor
rising from the cap during the hot season.
After the ice sublimes to vapor, wind could push it away from
the polar cap, according to Mars Global Surveyor scientists.
In contrast, erosion in the south likely happens through a
greater range of physical processes. They include collapse,
slumping, slope movement, and probably sublimation, although
a different kind than in the north.
The temperature at the south residual cap remains chilly
enough to freeze carbon dioxide. Little or no water vapor has
been seen coming off the south cap in the summer.
The erosion processes in the south produce layered hills,
buttes and mesas with surrounding triangular "aprons," and
the large pits and circular depressions that resemble Swiss
The exact cause of the large pits remains unclear, but it
seems that they have collapsed because material underneath
them has disappeared, according to Malin Space Science
Systems, which operates the camera onboard the Mars Global
The Mars Global Surveyor has orbited the red planet since
September 1997. A NASA mission, it is managed by the Jet
Propulsion Laboratory in Pasadena, California.
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