Many Mars Global Surveyor Mars Orbiter Camera (MOC) images of the middle latitudes of the northern and southern hemispheres of Mars show wild patterns of criss-crossing dark streaks. Many of these streaks are straight and narrow, others exhibit curly arcs, twists, and loops. They often cross over hills, run straight across dunes and ripples, and go through fields of house-sized boulders. The two examples shown above were acquired in the last three months. The first picture, showing dark streaks on the rippled flats of Argyre Planitia, covers an area 1.9 by 3.1 miles at a latitude of 51 degrees South. The second picture shows a similar sized area in Promethei Terra at a latitude of 58 degrees S. Photos: NASA/JPL/Malin Space Science Systems

New images from NASA's Mars Global Surveyor spacecraft have caught dust devils and landslides in the act of changing the surface of Mars, giving scientists more clues about how Mother Nature's vandals are leaving their mark on the changing Martian landscape.

Since Mars Global Surveyor arrived in September 1997, its high-resolution camera has been snapping pictures of puzzling dark streaks and lines that seemed to defy simple explanation -- until now. In December 1999, scientists had their first solid evidence, a picture of a dust devil caught like a graffiti artist in the act of etching the surface of Mars.

"Dust devils are spinning columns of air that move across the landscape and look somewhat like miniature tornadoes," said Dr. Ken Edgett, a staff scientist at Malin Space Science Systems, San Diego, CA. "We've captured them in the midst of sweeping up dust and leaving behind a dark streak. This is the 'smoking gun' that explains the wild, sometimes twisted 'spaghetti' of dark streaks and trails we have been seeing. I get the feeling when I look at these pictures that something is 'moving'. These things send shivers down my spine."

Dust devils are a common occurrence in dry and desert landscapes on Earth as well as Mars. They form when the ground heats up during the day, warming the air immediately above the surface. As the warmed air nearest the surface begins to rise, it spins. The spinning column begins to move across the surface and picks up loose dust. The dust makes the vortex visible and gives it the "dust devil" or tornado-like appearance. On Earth, dust devils typically last for only a few minutes and the same is probably true for Mars.

"What is exciting about this dust devil finding is that we are witness to one of the processes that help explain cause of some of the seasonal variations in the bright and dark surfaces on Mars. The dust devils remove some of the bright dust and cause the surfaces to appear to darken in the spring and summer seasons. Each little dust devil that runs across the landscape makes the surface in that region just a little bit darker," Edgett explained. "This isn't happening everywhere, but it seems to be most common in the mid-latitudes of Mars. In recent weeks, we have seen as many as five to 10 devils at a time running across the floors of the giant impact basins of Hellas and Argyre."

Scientists have known for decades that winds change the surface of Mars, but Global Surveyor has also captured other dark streaks that scientists now believe are the result of recent landslides.

"This is the first time we have been able to detect from orbit a change caused by some other geologic process. Gravity is acting to move loose dust and sand down these crater slopes," said Edgett. "It's not a big surprise, but it is exciting to have captured the results of several new landslides that occurred in less than one Martian year." Mars Global Surveyor's camera is observing how often these streaks form, which will provide scientists with some idea of the rate at which Martian slopes are modified. "Knowing how long it takes for any process that changes the landscape to occur can tell us more about the how the planet came to look the way it does today," said Edgett.

A comparison of the southeastern crater wall as it appeared on Februrary 1, 1998, and again on November 18, 1999. During the time between the two images, three new dark slope streaks formed (arrows). The older streaks are lighter and fainter than these new, dark ones, suggesting that streaks fade with time. This means that, at least for the crater walls shown here, any streak that is dark is younger than any streak that is pale. Photo: NASA/JPL/Malin Space Science Systems

Edgett and imaging team colleagues Dr. Michael Malin, also at Malin Space Science Systems, and Drs. Robert Sullivan, Peter Thomas and Joe Veverka of Cornell University, Ithaca, NY, are presenting these finding this week at the 31st Lunar and Planetary Science Conference, in Houston, TX.

Malin Space Science Systems built and operates the camera on board Mars Global Surveyor, which is managed by the Jet Propulsion Laboratory for NASA's Office of Space Science, Washington, DC. JPL's industrial partner is Lockheed Martin Astronautics, Denver, CO, which developed and operates the spacecraft. JPL is a division of the California Institute of Technology in Pasadena.