Cassini Makes New Discovery

March 19, 2005|By JOHN JOHNSON Los Angeles Times

Enceladus, another one of Saturn's moons, has an atmosphere in spite of its 310-mile diameter.

The Cassini spacecraft, which has been unveiling the secrets of Saturn's giant moon Titan, has found an atmosphere on a second moon circling the ringed planet.

The discovery of an atmosphere around the icy moon Enceladus is perplexing scientists because, at only 310 miles in diameter, it had been considered far too small to be able to hold on to an atmosphere.

Scientists discovered something unusual was going on at Enceladus on March 9, when Cassini approached to within about 300 miles of the surface. The spacecraft measured fluctuations in the moon's magnetic field that were consistent with a surface atmosphere, according to researchers at NASA's Jet Propulsion Laboratory.

That makes Enceladus only the second moon in the solar system to have an atmosphere. Titan is the other one.

Scientists are uncertain what the atmosphere is made of. Cassini's instruments detected ionized water vapor, water that carries a positive or negative charge.

Because the moon's gravity is far too low to keep an atmosphere from floating into space, scientists believe some internal process on the moon, such as volcanism, must be continually resupplying the atmosphere. If volcanoes or geysers are maintaining the atmosphere, that would make Enceladus one of three geologically active moons in the solar system, along with Io at Jupiter and Triton at Neptune.

Ice volcanoes continually coating the surface with new ice particles also would explain why the moon is the most reflective object in the solar system, reflecting as much as 90 percent of the sun's light.

The Cassini-Huygens mission was launched Oct. 15, 1997, and so far has discovered two new moons.

The Huygens probe, built by the European Space Agency, has revealed the bizarre nature of Titan's smoglike atmosphere and a gummy surface where liquid methane flows like rivers. *

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