New pictures taken by the camera on board the Mars Global Surveyor (above) at last explain some of the mysterious dark streaks and splotches that appear to be spray-painted on the red planet. As it turns out, this graffiti represents nothing more otherworldly than dust devils, polar pits and landslides (right, from top to bottom). Scientists revealed some images in Nature and others at the 31st Lunar and Planetary Science Conference in mid-March.
Dust devils are common in arid locations, such as Earth's deserts. The heat in these regions slowly warms the air just above the surface, eventually forcing the air to rise in spinning columns that kick up loose dust. It is the dust that makes these little, tornado-like vortices actually show up during their short life. On Earth, they dart across the desert surface for a few minutes at most--and the same is most likely true on Mars, explaining in part why they have only now been observed.
"This is the 'smoking gun' that explains the wild, sometimes twisted 'spaghetti' of dark streaks and trails we have been seeing," says Ken Edgett of Malin Space Science Systems, which built and operates the Mars Orbiter Camera. In addition, the dust devils help explain seasonal variations in light on the planet. "The dust devils remove some of the bright dust and cause the surfaces to appear to darken in the the spring and summer seasons," Edgett notes. "This ... seems most common in the midlatitudes of Mars."
New pictures of the Martian poles similarly shed light on the seasonal and climate differences there. Whereas the south pole has large depressions puctuated by tall, flat mesas, the northern cap features only tiny pits. "These landforms may be telling us what the south polar cap is made of," says Andrew Ingersoll of the California Institute of Technology. "The north residual cap--the part that survives the summer--is made of water ice. The south residual cap seems to be made of carbon dioxide or dry ice, but we don't know if this is a veneer a few meters thick or a solid block that extends down two or three kilometers. These images may help us decide."
And other recent images of what appear to be landslides should help scientists decide how long it takes for the Martian landscape to physically change, and so how it has come to look as it now does. These landslide pictures--if they are truly that--are the first to show a geological process other than wind from orbit. "It's not a big surprise," Edgett remarks, "but it is exciting to have captured the results of several new landslides that occurred in less than one Martian year." Exciting indeed.