Mars dunes: On the move?

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 last 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 upper portion of this map is from an observation by the Context Camera on the Mars Reconnaissance Orbiter of a field of dark sand dunes in the Nili Patera region of Mars. The white inscribed rectangles show overlapping footprints of observations made by the same orbiter's High Resolution Imaging Science Experiment camera 15 months apart. The black inscribed boxes show locations of two portions of the image seen in magnified view below, "b" on the left and "c" on the right. The site is at 9 degrees north latitude, 67 degrees east longitude. The white scale bar in the top image is 1 kilometer (0.62 mile) long. The scale bars in the two bottom images are 150 meters (492 feet) long. North is toward the top in all three images. Credit: NASA/JPL-Caltech/MSSS




Before and after images from the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter illustrate occurrence of new streaks on the slip face of a dark sand dune in the Nili Patera region of Mars. The slip face is on a dune's downwind side. The image on the left is from a June 30, 2007, observation (late autumn at the site). The one on the right is of the same ground observed 15 weeks later, on Oct. 13, 2007 (winter at the site). In each image, an inset box in the lower left holds a diagram of streaks discernable on the portion of the slip face within the inscribed box near the center of the image. The new streaks in the later image are interpreted as grainflow events, suggesting dune activity between late autumn and winter. White scale bars in each image are 20 meters (66 feet) long. North is toward the top. Credit: NASA/JPL-Caltech/University of Arizona/International Research School of Planetary Sciences




Two pairs of side-by-side, before and after images from the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter illustrate changes in the shape of edges of dark sand dunes in the Nili Patera region of Mars. The two images on the left are excerpts from a June 30, 2007, observation (late autumn at the site). The two on the right are of the same ground observed 15 weeks later, on Oct. 13, 2007 (winter at the site). Dark dunes overlaid with smaller ripples fill the bottom of the upper pair and cross the middle of the lower pair. In each image, an inset box in the upper left holds a diagram of the ripple crests (yellow lines) and dune edges (green lines) within the inscribed box to the right of the diagram. Changes both in the dune edges and in the ripple crests occurred during the interval between the June observation and the October observation. White scale bars in each image are 10 meters (33 feet) long. North is toward the top. Credit: NASA/JPL-Caltech/University of Arizona/International Research School of Planetary Sciences




Three pairs of before and after images from the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter illustrate movement of ripples on dark sand dunes in the Nili Patera region of Mars. The three images on the left are excerpts from a June 30, 2007, observation (late autumn at the site). The three on the right are of the same ground observed 15 weeks later, on Oct. 13, 2007 (winter at the site). Ripple crests discernable in the central portion of each image are diagrammed in the lower right portion of each image, with blue lines highlighting the largest changes. White scale bars in the bottom right of each of the six images are 20 meters (66 feet) long. North is toward the top. Credit: NASA/JPL-Caltech/University of Arizona/International Research School of Planetary Sciences

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."

Source: Jet Propulsion Laboratory
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