A Cassini spacecraft image of Enceladus, Saturn’s small geologically-active moon
Plumes of icy particles, visible on the upper limb of back-lit Enceladus, erupt from the south polar region of Enceladus. This is a rare type of volcanic activity, termed cryovolcanoes.
Credit: NASA/JPL/SSI

NASA's Saturn-orbiting Cassini spacecraft has relayed wonderful images of two of Saturn’s bizarre icy moons. On 21 December 2010, at 1:08 GMT, Cassini approached Enceladus, Saturn’s small, geologically-active moon to within only about 48 km, and passed approximately 100,000 km from Dione, a larger Saturnian moon, whose surface features intriguing terrains, about eight hours earlier.

Several pictures show Enceladus (504 km across) back-illuminated, with plumes of icy material, emanating from fissures, in the south polar region of the moon. These plumes are unique in the Solar System, and represent a phenomenon of a rare type of volcanism, known as cryovolcanoes. Cryovolcanoes spew very cold material, not lava. The fissures are known as "tiger stripes," as they resemble the stripes that characterize the tiger’s fur. Scientists will use the images to study the shape and activity of the plumes, and identify their origin.

This was the 13th flyby of Enceladus, in the highly-successful Cassini mission. Instruments onboard Cassini searched for a possible tenuous atmosphere around Enceladus. Recently, a thin atmosphere was found around Rhea (1,528 km across), Saturn’s second largest moon. Scientists also hope Cassini’s data will help them investigate the impacts of tiny meteorites, termed micrometeoroids, on Saturn’s moons.

A Cassini spacecraft image of Dione, obtained on 20 December 2010
Credit: NASA/JPL/SSI

Cassini’s images of Dione (1,123 km across) show wispy terrains, consisting of ridges and faults, believed to have been formed by past geologic activity on Dione. Dione orbits Saturn every 2.7 days, at a distance of approximately 377,000 km. Enceladus orbits Saturn every 1.4 days, at a distance of approximately 295,000 km. For comparison, the Moon orbits the Earth every 27.3 days, at an average distance of 384,000 km. Most of the planetary moons have been named after mythological characters. The names Enceladus and Dione come from Greek mythology. 

References

NASA’s JPL Press Release

http://www.jpl.nasa.gov/news/news.cfm?release=2010-426

The Official Websites of the Cassini Mission

http://saturn.jpl.nasa.gov

http://www.nasa.gov/cassini .

Aymen Mohamed Ibrahem

Senior Astronomy Specialist