A FROST on Pluto鈥檚 moon Charon is an unexpected sign that the satellite is not completely dead. Mike Brown of the California Institute of Technology in Pasadena has obtained the first detailed infrared spectrum of Charon and found the crystalline form of ice. The spectrum also showed that ammonia was mixed with the ice (Science, vol 287, p 107).
Crystalline water is not stable at Charon鈥檚 frigid temperatures of around -223掳C, and ammonia ices decompose when exposed to cosmic ultraviolet radiation. The two types of ice indicate that the surface of the moon is being continually reworked, says Brown, 鈥減robably by micrometeorite bombardment鈥�.
He thinks the impact vaporises surface ice, which then forms a frost faster than the extreme cold can convert it to the more stable amorphous form of ice. Ammonia helps to lower the melting point of ice, he adds.
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