A striking image from the High Resolution Imaging Science Experiment
(HiRISE) camera on NASA's Mars Reconnaissance Orbiter shows a mound within
the area of a trough cutting into Mars' north polar layered deposits. The
camera took this image on Sept. 2, 2008.
The north polar layered deposits are a stack of discernable layers that
are rich in water-ice. The stack is up to several kilometers or miles
thick. Each layer is thought to contain information about the climate that
existed when it was deposited, so the stack may represent a record of how
climate has varied on Mars in the recent past. We can see these internal
layers exposed in troughs and scarps where erosion has cut into the stack.
One of these troughs is shown in this image and contains a 500-meter
(1,640-foot) thick section of the layering.
A conical mound partway down the slope stands approximately 40 meters (130
feet) high. One possible explanation for this unusual mound is that it may
be the remnant of a buried impact crater now being exhumed. As the north
polar layered deposits accumulated, impacts occurred throughout their
surface area, then the impact craters were buried by additional ice. These
buried craters are generally inaccessible to us, but, in a few locations,
erosion that forms a trough (like this one) can uncover these buried
structures. For reasons poorly understood, the ice beneath the site of the
crater is more resistant to this erosion, so when material is removed by
erosion the ice beneath the old impact site remains, forming this isolated
hill.
An inspection of the full-resolution data shows that polygonal blocks, up
to 10 meters (33 feet) across, make up this mound. Although covered with
reddish dust, the blocks resemble ice-rich blocks seen in other exposures
of the north polar layered deposits.
The image, catalogued as http://hirise.lpl.arizona.edu/PSP_009855_2625, is
centered at 82.6 degrees north latitude, 265.8 degrees east longitude. The
range to the target site was 323.2 kilometers (202.0 miles). At this
distance the image scale of the full-resolution version is 32.3
centimeters (12.7 inches) per pixel (with 1 x 1 binning), so objects about
97 centimeters (38.2 inches) across are resolved. The image was taken at a
local Mars time of 1:37 p.m. The scene is illuminated from the west with a
solar incidence angle of 62 degrees, thus the sun was about 28 degrees
above the horizon. At a solar longitude of 121.4 degrees, the season on
Mars was northern summer.
NASA's Jet Propulsion Laboratory, a division of the California Institute
of Technology in Pasadena, manages the Mars Reconnaissance Orbiter for
NASA's Science Mission Directorate, Washington. Lockheed Martin Space
Systems, Denver, is the prime contractor for the project and built the
spacecraft. The High Resolution Imaging Science Experiment is operated by
the University of Arizona, Tucson, and the instrument was built by Ball
Aerospace and Technology Corp., Boulder, Colo.