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Planetary geology
Tiny moon, gigantic geyser

A tiny moon of Saturn, no larger than England, is changing researchers’ notions about which celestial bodies can support geologic activity. New, closer images of Enceladus have confirmed that a plume, noticed in previous images, is indeed an enormous geyser emanating from visible cracks in the moon’s surface.

In this color-enhanced image, an enormous plume emanating from fractures on Saturn’s moon Enceladus appears backlit by the sun. Astronomers were surprised to find geologic activity on the small moon, which is seven times smaller than Earth’s moon. Image courtesy of NASA/JPL/Space Science Institute.

NASA’s Cassini spacecraft first revealed the plume-like feature in images taken in January and February last year. But according to Carolyn Porco, Cassini imaging team leader at the Space Science Institute in Boulder, Colo., speculation remained that the feature was an artifact of the camera. Following a special Cassini mission in November to take a closer look, however, the images “confirmed it without question,” Porco says.

The moon and its plume look similar to a comet and its tail, which forms from vapor created when sunlight warms the icy body. Enceladus, however, does not receive much sunlight. Instead, pressurized vapor on Enceladus emanates from below the surface and “shoots out like a jet” through vents the team calls tiger stripes, Porco says. She describes the vapor’s composition of small icy particles to be “like the finest powder you might ski on in Utah.” The cause of the pressurized geyser, Porco says, is due to an internal source of heat on the moon, either from flexing tides or from radioactive material.

Small bodies such as Earth’s moon, which is 3,476 kilometers in diameter, typically lose their internal heat shortly after formation, rendering them geologically dead. Porco says that even though she was not surprised to find geologic activity on Enceladus, she finds it “thrilling” to see geologic activity on another body — especially a moon only 480 kilometers in diameter, with a geyser as tall as the moon is wide. Finding activity on such a small body, Porco says, “has torqued our ideas around about how geologic activity can come about.”

Kathryn Hansen

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