Enceladus: Small but feisty

Saturn’s moon Enceladus, a tiny outpost in the frigid outer solar system, ought to be cold and geologically dead. But observations by the Cassini spacecraft, which flew within 175 kilometers of Enceladus on July 14 (SN: 7/30/05, p. 69: Available to subscribers at Cassini eyes youthful-looking Saturnian moon), reveal that it continues to undergo eruptions “right this minute,” says Cassini researcher John Spencer of the Southwest Research Institute in Boulder, Colo. NASA announced the newest findings on July 29.

A spectrometer on board Cassini has detected a large cloud of water vapor above the moon’s south pole. Large, linear fractures at the pole, some of them dubbed tiger stripes, suggest that ice may continually vaporize from these cracks and replenish the cloud.

In another finding, Cassini detected large amounts of dust emanating from Enceladus. Micrometeoroids blast the particles off the moon, and the particles then form a haze of ice and dust around Enceladus. The dust particles also form the bulk of the E ring, the outermost of Saturn’s large rings, Cassini researchers say.

An infrared spectrometer indicated that, as predicted, temperatures near the equator dip as low as 80 kelvins. Scientists had expected the poles to be even colder. However, the south pole’s average temperature is actually about 85 K. Some areas near the tiger-stripe fractures reach 110 K.

“This is as astonishing as if we’d flown past Earth and found that Antarctica was warmer than the Sahara,” says Spencer. The heat probably comes from inside Enceladus and escapes through the fractures.

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