NASA Mars Phoenix Lander: How Do They Know It's Water Ice?

The most common questions I've heard regarding the Mars Phoenix Lander's discovery of ice near Mars' North Pole are, "How do they know it's ice?" and "How do they know it's water?" They know it's ice because it evaporated in sunlight, much like ice on Earth will do on a cold and windy day. If the white material shown in the photo was salt, it might be disturbed by wind, but it wouldn't vanish altogether. At Martian temperatures, ice could be from carbon dioxide or water or both. The reason NASA believes the ice is water is because the temperature was too high and air pressure was too low for carbon dioxide ice.

Why is water ice important? Water is necessary for life. Now... just because ice is present doesn't mean there must have been liquid water on Mars. The next step is to analyze the soil to see if it contains carbonates and sulfates, which could have been formed from the reaction between liquid water and minerals. Finding ice on Mars isn't a surprise... it's part of what the Lander went to find, after all. It's still exciting to see frozen water, rather than predict it or see indirect evidence for its existence.

Photo: Image of sublimated ice, taken by NASA's Mars Phoenix Lander (NASA)