New images taken by NASA's Mars
Reconnaissance Orbiter have revealed rare evidence of an impact crater in Mars'
north polar region.
Around the red planet's north pole is a feature called the north polar layered
deposits, which are a series of ice-rich
layers deposited over time and up to several kilometers thick.
The new images from MRO's High Resolution Imaging
Science Experiment (HiRISE) camera revealed an odd,
solitary hill rising part-way down an eroding slope of the layered terrain. The
exposed section of the deposits is about 1,640 feet (500 meters) thick, and the
conical
mound is about 130 feet (40 meters) high.
"The mound may be the remnant
of a buried impact crater, which is now being exhumed," said HiRISE team member Shane Byrne, of the University of Arizona.
If it sounds backwards for a crater
to be revealed by a mound, there's a fairly simple explanation:
The crater formed as the north polar
ice layers were being deposited. The crater itself would have been filled in by
ice after it formed.
Most of these craters are buried
under the Martian surface and inaccessible to scientists and their instruments.
But this crater and its associated ice mound were exhumed as erosion formed a
trough above and around it.
"For reasons that are poorly
understood right now, the ice beneath the site of the crater is more resistant
to this erosion, so that as the trough is formed, ice beneath the old impact
site remained, forming this isolated hill," Byrne said.
At high resolution, the HiRISE image shows that the mound is made up of polygonal blocks
as big as 33 feet (10 meters) across. The blocks are covered by the reddish
dust that is ubiquitous on Mars, but otherwise resemble ice-rich blocks seen in
other images of the north polar layered deposits.
Another new HiRISE
image showed a small
impact crater on the surface of Planum Boreum, or the north
polar cap. This smaller crater is only about 125 yards (115 meters) in
diameter.
The dearth of craters around Mars' north polar region has led scientists to suggest that either
the north polar cap is only about 100,000 years old, and therefore would have
accumulated fewer impacts, or that the craters disappear as the ice relaxes,
just as imperfections in an old window disappear as the glass relaxes.
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