|  Listen to this story via streaming audio, a downloadable file, or get help.
February 19, 2003: When NASA's Mars Global Surveyor spacecraft photographed what looked like fresh rain gullies on Mars three years ago, researchers were baffled. The surface of Mars is extraordinarily dry. What could have carved the curious features? Now, thanks to data from the 2001 Mars Odyssey spacecraft, there's a new answer: melting snow.  Above: Inside Newton Basin on Mars, narrow channels run from the top down to the floor. These and other "gullies" were first discovered in high-resolution pictures taken by NASA's Mars Global Surveyor spacecraft. [more] The gullies were created by trickling water from melting snow packs, not underground springs or pressurized flows, as had been previously suggested, argues Philip Christensen, a professor at Arizona State University and the principal investigator for Odyssey's camera system. The water melts and could flow beneath snow packs where it is sheltered from rapid evaporation in the planet's thin atmosphere, Christensen explains in the Feb. 19th electronic issue of Nature. Christensen's idea took shape while he was looking at a Mars Odyssey image of a Martian impact crater. He noted eroded gullies on the crater's cold northern wall and immediately next to them a section of what he calls "pasted-on terrain." Such unique terrain represents a smooth deposit of material that Mars researchers have concluded is "volatile" (composed of materials that evaporate in the thin Mars atmosphere), because it characteristically occurs only in the coldest, most sheltered areas. The most likely composition of this slowly evaporating material is snow. Christensen suspected a special relationship between the gullies and the snow. "I saw it and said 'Ah-ha!'" recalls Christensen. "It looks for all the world like these gullies are being exposed as this [pasted-on] terrain is being removed through melting and evaporation."  Left: The arrow indicates a region of "pasted-on" terrain, perhaps snow pack, near the lip of a Martian crater. Image credit: NASA/JPL/Malin Space Science Systems/Philip Christensen. [more]
Eroded gullies on crater walls and cliff sides were first observed in images taken by Mars Global Surveyor in 2000. There have been other scientific theories put forward to explain the gullies, including seeps of ground water, pressurized flows of ground water (or carbon dioxide), and mudflows caused by collapsing permafrost deposits, but no explanation to date has been universally accepted. "The gullies are very young," Christensen said. "That's always bothered me. How could Mars have groundwater close enough to the surface to form these gullies, and yet the water has stuck around for billions of years? Second, you have craters with rims that are raised, and the gullies go almost to the crest of the rim. If it's a leaking subsurface aquifer, there's not much subsurface up there. And, finally, why do they occur preferentially on the cold face of slopes at mid-latitudes?" That the coldest place and the least likely place for melting groundwater. Christensen points out that finding water erosion under melting snow deposits answers many of these problems. "Snow on Mars is most likely to accumulate on slopes that face the north or south poles--that is, the coldest areas. It accumulates and drapes the landscape in these areas during one climate period, and then it melts during a warmer one. Melting begins first in the most exposed area right at the crest of the ridge. This explains why gullies start so high up."  Right: The gullies in the top right-center appear to emerge from beneath and within a gradually disappearing blanket of snow. Image Credit: NASA/JPL/ASU [more]
Once he started to think about snow, Christensen began finding a large number of other images showing a similar relationship between "pasted on" snow deposits and gullies in the high resolution images taken by the camera on Mars Global Surveyor. Yet it was the wide field of view of the visible light camera in Mars Odyssey's thermal emission imaging system that was critical for the insight. "The basic idea [of melting snow] comes from having a regional view, which Odyssey's camera system gives," he explains. "It's a kind of you-can't-see-the forest-for-the-trees problem. An Odyssey image made it all suddenly click, because the resolution was high enough to identify these features and yet low enough to show their relationship to each other in the landscape." The Jet Propulsion Laboratory manages the Mars Exploration Program for NASA's Office of Space Science in Washington, D.C. | more information | | Mars Exploration -- (JPL) explore the Red Planet yourself at NASA's Mars Exploration Program home page. See also Mars Global Surveyor and 2001 Mars Odyssey. Mars Surprise -- (Science@NASA) High resolution images from NASA's Mars Global Surveyor spacecraft reveal features that suggest current sources of liquid water at or near the surface of the red planet. Once Upon a Water Planet -- (Science@NASA) Today the Red Planet is dry and barren, but what about tomorrow? New data suggest that the long story of water on Mars isn't over yet. The Case of the Missing Mars Water -- (Science@NASA) Plenty of clues suggest that liquid water once flowed on Mars, but the evidence remains inconclusive and sometimes contradictory. Making a Splash on Mars -- (Science@NASA) On a planet that's colder than Antarctica and where water boils at ten degrees above freezing, how could liquid water ever exist? Scientists say a dash of salt might help. Layers of Mars -- (Science@NASA) Mars Global Surveyor has spotted terrains on Mars that resemble sedimentary rock deposits. Did they form under water? | | 
