Mars Dunes: On the Move?

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http://www.jpl.nasa.gov/news/news.cfm?release=2010-076

Mars Dunes: On the Move?
Jet Propulsion Laboratory
March 04, 2010

PASADENA, Calif. -- New studies of ripples and dunes shaped by the winds
on Mars testify to variability on that planet, identifying at least one
place where ripples are actively migrating and another where the ripples
have been stationary for 100,000 years or more.

Patterns of dunes and the smaller ripples present some of the more
visually striking landforms photographed by cameras orbiting Mars.
Investigations of whether they are moving go back more than a decade.

Two reports presented at the 41st Lunar and Planetary Sciences
Conference near Houston this week make it clear that the answer depends
on where you look. Both reports used images from the High Resolution
Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance
Orbiter, which allows examination of features as small as about a meter,
or yard, across.

One report is by Simone Silvestro of the International Research School
of Planetary Sciences at Italy's G. d'Annunzio University, and his
collaborators. They investigated migration of ripples and other features
on dark dunes within the Nili Patera area of Mars' northern hemisphere.
They compared an image taken on Oct. 13, 2007, with another of the same
dunes taken on June 30, 2007. Most of the dunes in the study area are
hundreds of meters long. Ripples form patterns on the surfaces of the
dunes, with crests of roughly parallel ripples spaced a few meters apart.

Careful comparison of the images revealed places where ripples on the
surface of the dunes had migrated about 2 meters (7 feet) -- the largest
movement ever measured in a ripple or dune on Mars. The researchers also
saw changes in the shape of dune edges and in streaks on the downwind
faces of dunes.

"The dark dunes in this part of Mars are active in present-day
atmospheric conditions," Silvestro said. "It is exciting to have such
high-resolution images available for comparisons that show Mars as an
active world."

The other report is by Matthew Golombek of NASA's Jet Propulsion
Laboratory, Pasadena, Calif., and collaborators. They checked whether
ripples have been moving in the southern-hemisphere area of Mars'
Meridiani Planum where the Mars Exploration Rover Opportunity has been
working since 2004. They used observations by Opportunity as well as by
HiRISE, surveying an area of about 23 square kilometers (9 square
miles). Examination of ripples at the edges of craters can show whether
the ripples were in place before the crater was excavated or moved after
the crater formed.

"HiRISE images are so good, you can tell if a crater is younger than the
ripple migration," Golombek said. "There's enough of a range of crater
ages that we can bracket the age of the most recent migration of the
ripples in this area to more than 100,000 years and probably less than
300,000 years ago."

Winds are still blowing sand and dust at Meridiani. Opportunity has seen
resulting changes in its own wheel tracks revisited several months after
the tracks were first cut.

Golombek has a hypothesis for why the ripples at Meridiani are static,
despite winds, while those elsewhere on Mars may be actively moving.
Opportunity has seen that the long ripples in the region are covered
with erosion-resistant pebbles, nicknamed "blueberries," which the rover
first observed weathering out of softer matrix rocks beside the landing
site. These spherules -- mostly about 1 to 3 millimeters (0.04 to 0.12
inches) in diameter -- may be too large for the wind to budge.

"The blueberries appear to form a armoring layer that shields the
smaller sand grains beneath them from the wind," he said.

HiRISE Principal Investigator Alfred McEwen, of the University of
Arizona, Tucson, said, "The more we look at Mars at the level of detail
we can now see, the more we appreciate how much the planet differs from
one place to another."

The Mars Reconnaissance Orbiter and the Mars Exploration Rover missions
are managed by JPL for NASA's Science Mission Directorate in Washington.
Lockheed Martin Space Systems in Denver was the prime contractor for the
orbiter and supports its operations. The University of Arizona operates
the HiRISE camera, which was built by Ball Aerospace & Technologies
Corp., Boulder, Colo.

See related images
http://www.jpl.nasa.gov/news/features.cfm?feature=2511

Guy Webster 818-354-6278
Jet Propulsion Laboratory, Pasadena, Calif.
guy.w...@jpl.nasa.gov

2010-076