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 310 miles in diameter, it had been considered too small 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 in La Canada Flintridge. That makes Enceladus 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.
Because the moon's gravity is too low to keep an atmosphere, scientists believe some internal process, 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 Jupiter's Io and Neptune's Triton.
Ice volcanoes continually coating the surface with ice particles would also explain why the moon is the most reflective object in the solar system, reflecting about 90% of the sun's light.
The Cassini-Huygens mission launched on Oct. 15, 1997, and has discovered two new moons. The Huygens probe, built by the European Space Agency, has revealed the bizarre nature of Titan's smog-like atmosphere and its gummy surface, where liquid methane flows like rivers.