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Over the course of a martian year, which consists of 687 Earth days, as much as a third of Mars' tenuous carbon dioxide (CO2) atmosphere "freezes out" during the winter in the northern and southern hemispheres. For years, scientists wondered how the caps changed during the year? How much carbon dioxide 'snow' is deposited each season? Are the frost deposits more like snow or more like ice?

Researchers used measurements of martian neutrons combined with height measurements from the laser altimeter on another NASA spacecraft, Mars Global Surveyor, to monitor the amount of ice during the northern winter and spring seasons. Scientists say that in some regions, the water-ice content (shown in blue) is more than 90% by volume.

During the winter months, the icy soil is covered by a thick layer of carbon dioxide ("dry ice") frost, obscuring the water ice signature. As the carbon dioxide layer dissipates in the spring and summer, the water ice becomes 'visible' to the neutron and gamma ray detectors onboard Odyssey. Top image mosaic shows the northern hemisphere of Mars as seen by the Viking orbiter. The second image shows the concentration of water ice observed by Odyssey during the northern winter, when much of it is buried by carbon dioxide frost.

Third image shows the water ice that is revealed during the martian summer.

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