[meteorite-list] Rock and roll on Mars

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Wed, 14 Jan 2009 22:37:41 -0500
Message-ID: <qobtm452qlirb22262qqbv6t9h00htq8ep_at_4ax.com>

http://www.foxnews.com/story/0,2933,479873,00.html

Strange Rock Formations on Mars Explained

Wednesday, January 14, 2009

Rocks on Mars are in some areas scattered in a strangely uniform fashion,
puzzling scientists for years. Now they've figured it out.

Researchers had thought the rocks were picked up and carried downwind by extreme
high-speed winds thought to occur on Mars in the past.

Although Mars is a windy planet, its atmosphere is very thin, so it would be
difficult for the wind to carry the small rocks, which range in size from a
quarter to a softball, said Jon Pelletier, a geoscientist at the University of
Arizona in Tucson.

Pelletier and his colleagues now think the rocks are constantly on the move,
rolling into the wind, not away from it, and creating a natural feedback system
that results in their tidy arrangement.

Rock-n-roll

Here's what they think happens: Wind removes loose sand in front of the rocks,
creating pits there and depositing that sand behind the rocks, creating mounds.
The rocks then roll forward into the pits, moving into the wind. As long as the
wind continues to blow, the process is repeated and the rocks move forward.

The rocks protect the tiny sand mounds from wind erosion. Those piles of sand,
in turn, keep the rocks from being pushed downwind and from bunching up with one
another.

"You get this happening five, 10, 20 times then you start to really move these
things around," Pelletier said. "They can move many times their diameter."

The process is nearly the same with a cluster of rocks. However, with a cluster
of rocks, those in the front of the group shield their counterparts in the
middle or on the edges from the wind, Pelletier said.

Because the middle and outer rocks are not directly hit by the wind, the wind
creates pits to the sides of those rocks. And so, instead of rolling forward,
the rocks roll to the side, not directly into the wind, and the cluster begins
to spread out.

The research is published in the January issue of the journal Geology.

Lots of evidence

Several pieces of evidence have come together to support this idea of how rocks
are organized along some areas of the Martian surface.

For instance, when study team member Andrew Leier of the University of Calgary
in Canada was a graduate student at UA, he told Pelletier about an experiment on
the upwind migration of rocks that his thesis advisor James Steidtmann of the
University of Wyoming had conducted.

Steidtmann used a wind tunnel to see how pebbles on sand moved in the wind,
revealing the rocks moved upwind and that over time, a regular pattern emerged.

Some time later, while attending a lecture that showed pictures of uniformly
organized rocks on Mars, Pelletier recalled his conversations with Leier, and it
all came together.

Meanwhile, Leier had noticed a similar phenomenon when observing sand dunes in
Wyoming. Basically, loose pebbles and rocks there seemed to spread away from
each other in an almost organized fashion ? similar to what is seen on the sandy
surface of Mars.

In the recent study, Pelletier tested out the idea with three computer models,
including models of air flow, sand erosion and deposition, and rock movement.

He compared the model results with the distances between each rock and its
nearest neighbor in Mars images taken by the Mars Exploration Rover, Spirit. The
patterns of the Martian rocks matched what the model predicted.

Pelletier plans to apply the same numerical models to larger features on Mars
such as sand dunes and wind-sculpted valleys and ridges called "yardangs."
Received on Wed 14 Jan 2009 10:37:41 PM PST