SUMMARY: It turns out that Mars has been hiding its water underneath a layer of dry ice. By tracking the seasonal changes at Mars northern ice cap, NASA's Mars Odyssey spacecraft has watched how a layer of carbon dioxide permafrost comes and goes. When the permafrost dissipates in the Spring, Mars reveals a soil layer mixed with large amounts of water ice. In some places, the water ice content is more than 90% by volume. During the winter, the dry ice permafrost can reach more than a metre in thickness.
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