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Physics LibreTexts

7.28 Saturn's Enceladus

Enceladus is one of several modest-sized inner satellites of Saturn. It was discovered in 1789 by William Herschel. Its icy surface is the whitest and brightest in the Solar System. It reflects over 90% of the sunlight striking it, which is a higher albedo than any other body in the Solar System. Enceladus is nearly as shiny as a mirror! The Cassini satellite has imaged geysers ejecting ice that are in part responsible for the resurfacing and also responsible for replenishing Saturn's E ring. Near the south polar region, these geysers eject 250 kilograms of icy vapor into space each second, at speed of up to 1400 mph.


Saturn's moon Enceladus, as seen by the Cassini spacecraft. Click here for original source URL

 


Dramatic plumes, both large and small, spray water ice out from many locations along the famed "tiger stripes" near the south pole of Saturn's moon Enceladus. Click here for original source URL

The significance of this satellite lies in its unusual combination of old, cratered terrain and smooth, icy plains. In this sense, it is a "missing link" between ancient cratered moons like Jupiter’s Ganymede and the smooth, young surface of Jupiter’s Europa. Some kind of heating apparently caused enough watery eruptions to resurface about half of the satellite, leaving the other half in a more primitive, heavily cratered state.

Scientists can tell by counting craters on the smooth areas that they are less than 100 million years old. Compared to the much more heavily cratered surfaces, that’s relatively young – indicating whatever process resurfaced the smooth areas was active in the recent past, and may still be active today.

The problem is, Enceladus is much too small to have any radioactive elements remaining that could heat the interior. Tidal heating may play a part, but it probably wouldn’t be strong enough to melt water. Another possibility is that Enceladus isn’t actually made of pure water ice, but some other type of ice with a lower melting temperature. This idea isn't supported by data though: When Cassini flew thru material from the geyser in 2005 it detected water and some nitrogen, methane, and carbon dioxide. The ice is simply water ice. Rather than different ices being present, something else must be going on. In 2008 ammonia was detected, and this element may act as a form of anti-freeze. It may be that this ammonia allows liquid water to be created with less tidal heating than might otherwise be needed. Models suggest a liquid water ocean below an ice shelf 30 to 40 kilometers thick, and the ocean is salty with an alkaline pH of 11-12.


Saturn's moon Mimas. Click here for original source URL.

The details of Enceladus inner working are made somewhat more complex to understand when this moon is viewed in contrast to it's neighboring moon Mimas. This small icy moon has an orbit closer to Saturn than Enceladus' and its elliptical orbit causes it to experience more tidal forces than Enceladus. Despite these factors it is simply an average, cratered, lumpy moon. Why only Enceladus and not Mimas has these geysers is a tangled problem for geophysicists