Mars Odyssey’s Measurements Reveal a Wet, Red Planet

Dry valleys, channels, and networks of gullies scar the arid Martian landscape. Along with other evidence, these physical vestiges of conditions on ancient Mars suggest a planet once saturated with liquid water. Where is this water now? Scientists have posited that a portion of it evaporated into the atmosphere, but that the rest lies beneath the surface. Findings announced today offer the strongest support yet to that hypothesis: according to new data, large deposits of water ice may in fact exist under just tens of centimeters of soil on the Red Planet.

Researchers used a gamma-ray spectrometer on board the Mars Odyssey spacecraft to map the emissions of gamma rays and neutrons from the Martian surface. Interactions between elements and cosmic rays, which constantly bombard all planets, produce these gamma rays and neutrons. Specifically, when a cosmic ray strikes an element, neutrons are released. These neutrons may either escape the planet’s surface or excite the nuclei of surrounding elements, which respond by emitting gamma rays. Each element emits a unique combination of gamma rays and neutrons, and thus has a distinctive fingerprint. In three papers released yesterday by the journal Science, investigators reported having found evidence for a high concentration of the element hydrogen, an indicator of water. The results suggest that an immense quantity of water exists within the nooks and crannies of a rocky, porous layer of soil some 30 to 60 centimeters beneath Mars’s surface. Stretching from the edges of the polar ice caps to the middle latitudes, the thickness of the ice layer is difficult to determine–it may be anywhere from a few hundred centimeters to a kilometer deep.

If confirmed, the locations of such water ice deposits could determine future landing sites for rovers, locations of sample returns, and perhaps even placements of human settlements. “We have suspected for some time that Mars once had large amounts of water near the surface. The big questions we are trying to answer are, ’where did all that water go?’ and ’what are the implications for life?’” remarks Jim Garvin, Mars Program Scientist at NASA headquarters in Washington, D.C. “Measuring and mapping the icy soils in the polar regions of Mars as the Odyssey team has done is an important piece of this puzzle, but we need to continue searching, perhaps much deeper underground, for what happened to the rest of the water we think Mars once had.”

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Rachael Moeller Scientific American

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