NASA MARS HiRise Image: The Serpent Dust Devil

A towering dust devil casts a serpentine shadow over the Martian surface in this image acquired by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter.

The scene is a late-spring afternoon in the Amazonis Planitia region of northern Mars.

The view covers an area about four-tenths of a mile (644 meters) across. North is toward the top.

The length of the dusty whirlwind's shadow indicates that the dust plume reaches more than half a mile (800 meters) in height. The plume is about 30 yards or meters in diameter.

A westerly breeze partway up the height of the dust devil produced a delicate arc in the plume. The image was taken during the time of Martian year when the planet is farthest from the sun.

Just as on Earth, winds on Mars are powered by solar heating. Exposure to the sun's rays declines during this season, yet even now, dust devils act relentlessly to clean the surface of freshly deposited dust, a little at a time.

This view is one product from an observation made by HiRISE on Feb. 16, 2012, at 35.8 degrees north latitude, 207 degrees east longitude.

Other image products from the same observation are at http://www.uahirise.org/ESP_026051_2160 .

Argentina from the ISS

This view of eastern Argentina and its coastline, taken by an astronaut on board the space station, shows a large plume of dust blowing from the interior to the Atlantic Ocean.

A docked Russian Progress spacecraft is visible at image upper right.

Picture: NASA / AFP/Getty

MARS Recon Orbiter: Airbursts trigger dust avalanches

The surface of Mars may be cold and desolate, but it is not unchanging. 

New images show that avalanches of dust scour dozens of Martian sites each year.

Without the abundant water and plate tectonics that keep Earth's surface in motion, the surface of Mars is much slower to change. But in one way it is more active. 

While Earth's atmosphere shields us from asteroids smaller than 30 metres across, which burn up or shatter too high above the ground to have much effect on us, Mars's atmosphere is just 1 per cent the density of Earth's. Even rocks less than a metre across make it to the ground and gouge out craters.

NASA's Mars Reconnaissance Orbiter spots about 20 new craters between 1 and 50 metres across on Mars each year - scars that were not present in earlier images. Now closer scrutiny of these images has found thousands of small avalanches near 16 of the craters.

The avalanches appear as dark streaks on the hilly terrain that surrounds the craters (a similar but more dramatic avalanche is shown in the image). 

They show up only in areas where there is a lot of light-coloured dust on the ground. To form, it seems the surface's dust coating was shaken loose and slid downhill, revealing the darker rocks beneath, says a team led by Kaylan Burleigh of the University of Arizona in Tucson (Icarus, DOI: 10.1016/j.icarus.2011.10.026).

The team carried out computer simulations that showed that, surprisingly, the avalanches do not seem to be caused by meteorites hitting the ground, but by the shock wave generated by a rock's passage through the atmosphere. 

This spreads across an area about a million times larger than the craters. "It was astonishing that a relatively small impact could affect a large area," says team member Jay Melosh of Purdue University in West Lafayette, Indiana.

In one case, a cluster of 20-metre-wide craters is surrounded by thousands of dust avalanches in an area 4 kilometres square. The many small avalanches give the whole area a darker hue, like a giant black eye around the craters - except for a narrow light zone shaped like a curved dagger.

That light zone is telling. As a rock tears through the atmosphere at supersonic speed it generates a shock wave, before triggering a second blast when it hits the ground. 

The team's simulations show that the second shock interferes with the first, reinforcing it in some places and cancelling it out in others. 

Where it is cancelled out, a narrow curved strip of relatively undisturbed ground is left behind - just like the light zone seen around the crater cluster, says Melosh.

The Martian surface may be the best place in the solar system for recording the effects of these shock waves, since fewer impactors are blocked by its atmosphere than on Earth, says Mark Boslough of Sandia National Laboratory in Albuquerque, New Mexico.

The New Dust Ball

Dust Ball from Dave Hakkens on Vimeo.

MARS Dust Devil image captured by Opportunity

Image Credit: NASA/JPL-Caltech/Cornell University/Texas A and M

This is the first dust devil that NASA's Mars Exploration Rover Opportunity has observed in the rover's six-and-a-half years on Mars.

The whirlwind appeared in a routine drive-direction image taken by Opportunity's panoramic camera right after a drive during the 2,301st Martian day, or sol, of the rover's mission on Mars (July 15, 2010).

Contrast has been stretched, and the image has been carefully calibrated to make the dust devil easier to see against the Martian sky.

Opportunity's twin, Spirit, has observed dozens of dust devils at its location in Gusev Crater halfway around Mars from Opportunity's location in the Meridian Planum region.

Opportunity conducted systematic searches for dust devils in past years without seeing any. A rougher and dustier surface at Gusev makes dust devils form more readily there than at Meridiani.