Cassini: Mission to Enceladus

The Cassini space probe has been in orbit around Saturn since 2004, exploring the planet’s well-known rings and dozens of moons. In 2005, Cassini made three flybys of one of Saturn’s most intriguing satellites, the moon Enceladus. Enceladus resides in the wide blue E-ring encircling the planet, and has long been considered the source of the fine particles that make up the E-ring. As nine papers from international Cassini instrument teams and two “Perspective” articles in the 10 March 2006 issue of Science show, the mystery of the E-ring’s origins may be solved, but the 2005 flybys also revealed new intriguing features on the small moon. Tiny Enceladus is surprisingly dynamic, spewing a vast plume of water vapor, gilding its bright surface with fresh ice and buckling and cracking under tectonic forces.

Tiger Stripes, Hot Spots and Cold Geysers

Images from Cassini's third flyby of Enceladus revealed a distinctive "tiger-stripe" pattern at its southern pole, dark troughs alternating with bright plains, according to surface image analysis by Carolyn Porco and colleagues. The polar region appears to be cracked and littered with house-sized ice boulders. Remote sensing data analyzed by John Spencer and colleagues collected on the flybys revealed a “hot spot” at the moon’s southern polar region in the vicinity of the tiger stripe troughs. The troughs may be surface vents for some kind of underground reservoir of warmer (but still icy) material, according to the analysis by the Porco and Spencer teams. Spencer and colleagues note that Enceladus is only the third known solid planetary body (after the Earth and Jupiter's moon Io) with internal heat that can be detected by remote sensing. While the tiger stripes appear to be a mixture of light organic materials, carbon dioxide and water ice, the rest of the moon's surface is mostly pure water ice, according to Robert Brown and colleagues. The tiger stripes' composition, particularly their abundance of carbon dioxide, suggests these materials are being continually replenished by geological activity, the researchers say.

The landscape’s most distinctive feature, however, is a vaporous plume of fine icy particles and dust spewing like a “cold Yellowstone geyser,” according to Porco and colleagues. Material in the plume rises until its ionized material reaches Saturn’s atmosphere and replenishes the planet’s E-ring. Some of the plume material may fall back to the moon's surface as fresh snow, brightening the plains between the moon's tiger striped troughs.

In a related "Perspective" article, Jeffrey Kargel discusses how such a small moon — no bigger than France — might produce a geologically dynamic hotspot. Kargel believes a model of "shifting, glacier-like tectonic plates" and tidal forces could generate and trap heat to produce Enceladus' current features. It may even be possible that a cold ocean lies beneath the moon's icy crust, perhaps even serving as a severe habitat for some form of life, Kargel speculates.

Anatomy of a Plume

Models of the plume suggest that it may be fed by several smaller jets of material and driven by warmer temperatures below the moon’s icy surface. Candice Hansen and colleagues suggest the plume may have erupted continuously over at least the past 15 years, supplying most of the oxygen gases in Saturn’s magnetosphere. According to data collected by Cassini’s cosmic dust analyzer and ion spectrometer, the plume is mostly made up of water with significant amounts of carbon dioxide and trace amounts of a few other chemicals such as propane and methane. The instruments detected surprisingly small amounts of ammonia, which can act as antifreeze, say Hunter Waite, Jr. and colleagues. The plume also contains a significant amount of dust particles that probably supply material to Saturn’s E-ring, according to Frank Spahn and colleagues.

Plume Meets Plasma

The three flybys of Enceladus also show how the moon's south pole plume interacts with and dynamically changes the plasma of Saturn's magnetosphere. Cassini’s magnometer was the first instrument to detect evidence of dispersing particles at the southern pole. Magnetometer data collected by Cassini clearly show how Saturn's rotating plasma flow is deflected by the outgassing plume, say Michele Dougherty and colleagues. The charge exchange between Enceladus' charge neutral plume products and Saturn's ion plasma produce the cloud of oxygen and hydrogen ions seen in Saturn's magnetosphere, according to Robert Tokar and colleagues. Together the findings point toward the plume as the "missing source" of the large amounts of water and its gaseous byproducts in Saturn's magnetosphere, the researchers say. Enceladus' "imprint" on Saturn's magnetosphere varies over time as the plume's eruption timing and volume changes, say Geraint Jones and colleagues. Margaret Kivelson discusses Enceladus' overall effect on Saturn's magnetosphere in a related "Perspective" article.

Becky Ham

13 March 2006