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Mars photos: The devil is in the details

dust devil animated gif
A montage of images of northern Amazonis Planitia, contrasting the same area before and after the formation of tall dust devils

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July 5, 1999
Web posted at: 3:28 p.m. EDT (1928 GMT)

In this story:

Temporary tornadoes

Batter up


(CNN) -- Pancake batter, whirling dervishes and sports utility vehicles are the apparent inhabitants of Mars these days, if first impressions of NASA's latest red planet images are to be trusted.

Actually, the "batter" is a mysterious crater-filling material and the "SUV track" is a typical feature created by dust devils -- frenzied towers of wind dancing on the planet's highly weathered surface like miniature tornadoes.

One of the newly released images from the Mars Global Surveyor spacecraft shows several thin, dark lines that look like pathways blazed by off-road sport utility vehicles.

The image, taken on April 26, 1998, during the NASA orbiter's 264th orbit, also showed a number of natural features -- small craters formed by meteor impact, several buttes and mesas left by erosion of the surrounding terrain, small dunes and drifts, and a mantle of dust.

But the new picture also showed two dark lines -- each varying in width up to about 15 meters (49 feet) -- that extended several kilometers/miles across the image.

Temporary tornadoes

Lines like these have been seen before on Mars. NASA scientists say they are most likely the result of dust devils, some of which rise as high as 8
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kilometers (5 miles). Dust devils are a common occurrence in dry and desert landscapes on Earth as well as Mars. They form in the mid-afternoon when the ground heats up, warming the air immediately above the surface.

As pockets of warm air rise and interfere with one another, they create horizontal pressure variations that combine with winds to cause the upward moving air to spin.

As the spinning column of air moves across the surface, it occasionally encounters dust on the surface, which it can suck upward. This dust rises into the spinning air, giving the appearance of a tornado-like column that moves across the landscape.

As the column of air moves, its ability to pick up dust varies -- sometimes they hold a lot of dust and are nearly opaque; sometimes you cannot even see them. Dust devils rarely last long, since their motion changes the conditions that allowed them to form in the first place.

The deeply gullied impact crater in Promethei Terra. Click on the image to see a closeup of material that appears to have flowed uphill   

Mars Pathfinder detected several dust devils during its 83 days of operation on Mars in 1997. Mariner 9 and the Viking landers and orbiters of the 1970s also found evidence that dust devils occur on Mars.

Two new pictures from the Mars orbiter mission also show dust devils in northern Amazonis Planitia. Each image covers an area 88 kilometers (55 miles) across, and each shows similar features on the ground, such as the two partially-buried craters at the center left.

Each scene is illuminated by sunlight from the lower left -- thus each towering dust devil casts a long, dark shadow that points toward the right/upper right.

The pictures were taken two days apart--the first on May 13, 1999, the second on May 15, 1999.

Batter up

Finally, the "pancake batter" image illustrates that some geological features of Mars defy simple explanations.

A recent example is on the wall of a 72 kilometer-wide (45 mile-wide) impact crater in Promethei Terra, with inner walls that appear in low-resolution images to be deeply gullied.

A high-resolution image shows that each gully on the crater's inner wall contains a tongue of material that appears to have flowed. Ridges and grooves that converge toward the center of each gully and show a pronounced curvature are oriented in a manner that seems to suggest that material has flowed from the top toward the bottom of the picture.

This pattern is not unlike pouring pancake batter into a pan, NASA scientists said. The viscous fluid will form a steep, lobate margin and spread outward across the pan. Mud- and lava-flows, and even some glaciers, for the most part behave in this manner. From these observations, and based solely on the appearance, one might conclude that the features formed by moving from the top of the image towards the bottom.

But this is not the case. The material cannot have flowed from the top towards the bottom of the area seen in the high-resolution image because the crater floor (which is the lowest area in the image) is at the top of the picture.

Since gravity pulls the material in the gullies downhill -- not uphill -- the pattern of ridges and grooves found on these gully-filling materials is puzzling. An explanation may lie in the nature of the material -- its viscosity, for example -- and how rapidly it moved, but for now this remains an unexplained Martian phenomenon.

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