[meteorite-list] HiRISE Camera Reveals Rare Polar Martian Impact Craters

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 15 Oct 2008 16:17:12 -0700 (PDT)
Message-ID: <200810152317.QAA25032_at_zagami.jpl.nasa.gov>

FROM: Lori Stiles (520-626-4402; lstiles at u.arizona.edu)

HiRISE Camera Reveals Rare Polar Martian Impact Craters
University of Arizona
October 15, 2008

An odd, solitary hill rising part-way down an eroding slope in Mars' north polar
layered terrain may be the remnant of a buried impact crater, suggests a
University of Arizona planetary scientist who studied the feature in a new,
detailed image from the HiRISE camera onboard NASA's Mars Reconnaissance
Orbiter.

HiRISE, or the High Resolution Imaging Science Experiment, headed by Alfred
McEwen of the Lunar and Planetary Laboratory, is based at the UA. New HiRISE
images are posted weekly on the team's Web site, http://hirise.lpl.arizona.edu

The north polar layered deposits are stacked up to several kilometers thick and
represent one of the largest surface reservoirs of Martian water that interacts
with the planet's atmosphere, said LPL's Shane Byrne. Scientists believe the
deposits record orbitally driven climate changes and study them to learn how
Mars climate evolved.

The new HiRISE image shows an exposed 500-meter thick section (1,640 feet) of
this layering, and also a 40-meter high (130-foot) conical mound sticking out
of the slope.

"The mound may be the remnant of a buried impact crater, which is now being
exhumed," Byrne said.

Impact craters would have been buried by ice as the layered deposits
accumulated, with layers wrapping around the crater, Byrne said. Almost none
exist on the surface of this terrain.

"But in this rare case, erosion formed a trough that uncovered one of these
structures. 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 this
trough formed, ice beneath the old impact site remained, forming this isolated
hill."

Viewing the HiRISE image at full resolution shows that the mound is made up of
polygonal blocks as big as 10 meters, or 33 feet, across, he added. The blocks
are covered with reddish dust, but otherwise resemble ice-rich blocks seen in
other images of the north polar layered deposits.

The seven new HiRISE images released today include another image of an impact
crater where such features are rarely seen ? on the north polar cap. HiRISE
turned up a small crater, only about 115 meters, or 125 yards, in diameter on
the surface of Planum Boreum, popularly known as the north polar cap.

The dearth of craters has led scientists to suggest that either the north polar
cap is only about 100,000 years old or that crater impacts into the ice
disappear as the ice relaxes, just as imperfections disappear as old window
glass relaxes.

Color in the enhanced-color version of the Planum Boreum impact crater comes
from dust and from ice of various grain sizes. Blueish ice has a larger grain
size than the ice that has collected in the crater. Dust is reddish. A smooth
area stretching away from the crater to the upper right of the image may be
caused by winds around the crater or by fine-grained ice and frost blowing out
of the crater, HiRISE scientists say.

HiRISE has returned more than 8,200 gigapixel-size images of Mars' surface since
the start of its science mission in November 2006. The HiRISE team so far has
released a total of about 27 terabytes of data, more than all previous deep
space missions combined.

The Mars Reconnaissance Orbiter is managed by the Jet Propulsion Laboratory,
Pasadena, Calif., for NASA's Science Mission Directorate, Washington. Lockheed
Martin Space Systems of Denver built the spacecraft. The UA operates the HiRISE
camera, built by Ball Aerospace and Technology Corp. of Boulder, Colo.

SCIENCE CONTACTS:
Shane Byrne (520-626-0407; shane at lpl.arizona.edu)
Alfred McEwen (520-621-4573; mcewen at lpl.arizona.edu)

LATEST HiRISE IMAGES: http://hirise.lpl.arizona.edu/nea.php
Received on Wed 15 Oct 2008 07:17:12 PM PDT