How Martian Winds Make Rocks Walk
(sent by Mari N. Jensen, The University of Arizona, 520-626-9635,
mnjensen{at}email.arizona.edu)

-- Thursday, January 8, 2009

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Researcher contact information is at the end of this release.
Images: available to logged-in reporters on Eurekalert or from the
researcher
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Rocks on Mars are on the move, rolling into the wind and forming
organized
patterns, according to new research.

The new finding counters the previous explanation of the evenly spaced
arrangement of small rocks on Mars. That explanation suggested the
rocks
were picked up and carried downwind by extreme high-speed winds
thought to
occur on Mars in the past.

Images taken by the Mars Exploration Rover Spirit show small rocks
regularly
spaced about 5 to 7 centimeters apart on the intercrater plains
between
Lahontan Crater and the Columbia Hills.

Although Mars is a windy planet, it would be difficult for the wind to
carry
the small rocks, which range in size from a quarter to a softball,
said Jon
D. Pelletier, associate professor of geosciences at The University of
Arizona in Tucson.

Pelletier and his colleagues suggest that wind blows sand away from
the
front of the rock, creating a pit, and then deposits that sand behind
the
rock, creating a hill.

The rock then rolls forward into the pit, moving into the wind, he
said.

As long as the wind continues to blow, the process is repeated and the
rocks
move forward.

This explanation does not require extreme winds, Pelletier said.

"You get this happening five, 10, 20 times then you start to really
move
these things around," he 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
those 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. Therefore, they roll to
the
side, not directly into the wind, and the cluster begins to spread
out.

Pelletier, Andrew L. Leier of the University of Calgary in Alberta,
Canada,
and James R. Steidtmann of the University of Wyoming in Laramie report
their
findings in the paper, "Wind-Driven Reorganization of Coarse Clasts on
the
Surface of Mars." The paper is in the January issue of the journal
Geology.

When Leier was a graduate student at the UA, he told Pelletier about
an
experiment on the upwind migration of rocks that Steidtmann, Leier's
thesis
advisor, had conducted.

Steidtmann had studied upwind migration about 30 years ago. He used a
wind
tunnel to see how pebbles on sand moved in the wind. Steidtmann's
research
showed that the rocks moved upwind and that over time, a regular
pattern
emerged.

Pelletier wasn't sure how he could use the idea.

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

To investigate the regular patterns of the rocks on Mars, Pelletier
combined
three standard numerical computer models. The first modeled air flow,
the
second modeled erosion and deposition of sand and the third modeled
the
rocks' movement, he said.

"We can model it on the computer to try to get a better sense of
what's
actually happening and to provide another sort of documentation or
justification for the idea," he said.

Pelletier was the first to combine the three standard models and apply
them
to this new problem.

He also conducted what is known as a Monte Carlo simulation, which
applies
his combination numerical model over and over to a random pattern of
rocks
to see how the rocks ultimately end up.

Pelletier ran the simulation 1,000 times. The rocks ended up into a
regular
pattern 90 percent of the time, he said.

As an independent verification, he also compared the pattern predicted
by
the numerical model to the distances between each rock and its nearest
neighbor in the Mars images. The patterns of the Martian rocks matched
what
the model predicted.

Pelletier said upwind migration of rocks also occurs on Earth.

Co-author Leier wrote in an e-mail, "Something as seemingly mundane as
the
distribution of rocks on a sandy, wind-blown surface can actually be
used to
tell us a lot about how wind-related processes operate on a place as
familiar as the Earth and as alien as Mars."

However, because plants and animals can alter wind patterns and
rearrange
rocks, it is much more difficult to study this process on Earth,
Pelletier
said.

Of Mars' mysterious walking rocks, he said, "This is a neat problem,
but
there are bigger fish to fry."

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."

He said understanding the climate history of other planets and where
those
climates went awry can help in understanding our own climate system.

 This release was written by University of Arizona NASA Space Grant
Intern
Megan Levardo.

Researcher Contact Information:
Jon D. Pelletier
(520) 626-2126
jdpellet{at}email.arizona.edu


Related Web sites:
Jon D. Pelletier
http://geomorphology.geo.arizona.edu/

NASA Mars Exploration Rover Mission
http://marsrovers.nasa.gov/home/index.html

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