From: Andy Soos, ENN
Published December 19, 2011 12:43 PM

Martian Landslide and Meteorite Strike

Cause and effect are not easy to distinguish. Dust avalanches around impact craters on Mars appear to be the result of the shock wave preceding the actual impact, according to a study led by an undergraduate student at the UA. When a meteorite careens toward the dusty surface of the Red Planet, it kicks up dust and can cause avalanching even before the rock from outer space hits the ground, a research team led by an undergraduate student at the University of Arizona has discovered.

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When a meteorite careens toward the dusty surface of the Red Planet, it kicks up dust and can cause avalanching even before the rock from outer space hits the ground, a research team led by an undergraduate student at the University of Arizona has discovered.

Landslides are common features in canyons on both Earth and Mars, and they happen the same way. Erosion undermines a canyon wall until abruptly millions of tons of rock come crashing down.

The event causing a landslide may be quite small: a trickle of water in the right place or even winds from a storm tipping a precarious balance. Or the impulse may come from a more substantial kick: seismic waves from an earthquake or a meteorite that shake the wall-rock, widening cracks until gravity starts the slide moving.

"We expected that some of the streaks of dust that we see on slopes are caused by seismic shaking during impact," said Kaylan Burleigh, who led the research project. "We were surprised to find that it rather looks like shockwaves in the air trigger the avalanches even before the impact."

Because of Mars' thin atmosphere, which is 100 times less dense than Earth's, even small rocks that would burn up or break up before they could hit the ground here on Earth crash into the Martian surface relatively unimpeded.

Each year, about 20 fresh craters between 1 and 50 meters (3 to 165 feet) show up in images taken by the HiRISE camera on board NASA's Mars Reconnaissance Orbiter. The High Resolution Imaging Science Experiment, or HiRISE, is operated by the UA's Lunar and Planetary Laboratory.

For this study, the team zoomed in on a cluster of five large craters, which all formed in one impact event close to Mars' equator about 512 miles south of the boundary scarp of Olympus Mons, the tallest mountain in the solar system. Previous observations by the Mars Global Surveyor orbiter, which imaged Mars for nine years until 2006, showed that this cluster was blasted into the dusty surface between May 2004 and February 2006.

The results of the research, which Burleigh first took on as a freshman under former UA Regents Professor H. Jay Melosh, are published in the planetary science journal Icarus.

The authors interpret the thousands of downhill-trending dark streaks on the flanks of ridges covering the area as dust avalanches caused by the impact. The largest crater in the cluster measures 22 meters, or 72 feet across and occupies roughly the area of a basketball court. Most likely, the cluster of craters formed as the meteorite broke up in the atmosphere, and the fragments hit the ground like a shotgun blast.

Narrow, relatively dark streaks varying from a few meters to about 50 meters in length scour the slopes around the impact site.

When Burleigh looked at the distribution of avalanches around the impact site, he realized their number decreased with distance in every direction, consistent with the idea that they were related to the impact event.

But it wasn't until he noticed a pair of peculiar surface features resembling a curved dagger, described as scimitars, extending from the central impact crater, that the way in which the impact caused the avalanches became evident.

"Those scimitars tipped us off that something other than seismic shaking must be causing the dust avalanches," Burleigh said.

As a meteor screams through the atmosphere at several times the speed of sound, it creates shockwaves in the air. Simulating the shockwaves generated by impacts on Martian soil with computer models, the team observed the exact pattern of scimitars they saw on their impact site.

In the absence of plate tectonic processes and water-caused erosion, the authors conclude that small impacts might be more important in shaping the Martian surface than previously thought.

"This is one part of a larger story about current surface activity on Mars, which we are realizing is very different than previously believed," said Alfred McEwen, principal investigator of the HiRISE project and one of the co-authors of the study.

For further information: http://uanews.org/node/43798

Photo: Image painted by William K. Hartmann, co-founder of the Planetary Science Institute, Tucson, Ariz.

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