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12/12/1996 08:00:00

Question of the Week: Why Didn't the Ice Found at the South Pole of the Moon Sublimate Away, Like Ice Cubes Do in a Refridgerator?

Submitted by Dean Bessette, Huntington Beach, Calif., and answered by Dave Stevenson, George Van Osdol Professor of Planetary Science, California Institute of Technology

As everyone with a refrigerator knows, ice cubes tend to shrink over time. And if you have an old-style refrigerator, you may have observed that the ice molecules go directly from the ice cubes to the walls of the freezer compartment without ever becoming liquid water. This is the process of sublimation, and your question about its relevance to the water found on the moon is a good one.

The answer is that it takes more energy than is available in a crater at the south pole of the moon to make the molecules fly off the ice in significant numbers. The refrigerator analogy is appropriate to a point, but there is far more energy in your typical freezing compartment than in a shadowy area on the moon where the sun never shines. And since there is not much internal heat coming from within the moon, there simply isn't enough energy available to sublimate this very cold ice.

It has been estimated that if you have water ice sitting in a vacuum, you need to keep the temperature at about 150 degrees Kelvin—or minus 123 degrees Celsius—for sublimation to be significant over geological time. And since the temperature in a shadowy crater of the moon could be much lower than this, there simply hasn't been enough time for the ice to sublimate, assuming there was a reasonable amount of ice to begin with. So the ice the space probe Clementine detected could have been there for billions of years. How's that for a refrigerator?