Enceladus is the sixth-largest moon of Saturn. It was discovered in 1789 by William Herschel. Enceladus is only 310 miles in diameter, about a tenth of that of Saturn’s largest moon, Titan, and reflects almost 100% of the sunlight that strikes it. It orbits in the densest part of Saturn’s diffuse E ring, indicating a possible association between the two. Despite the moon’s small size, it has a wide range of terrains ranging from old, heavily cratered surfaces to young, tectonically deformed terrain, with some regions with surface ages as young as 100 million years old.
A water-rich plume vents from the moon’s south polar region. Enceladus is one of only three outer solar system bodies (along with Jupiter’s moon Io and Neptune’s moon Triton) with active volcanoes. Analysis of outgassing suggests that it originates from a body of sub-surface liquid water, which along with the unique chemistry found in the plume, has fueled speculations that Enceladus may be important in the study of astrobiology.
Enceladus is one of the major inner satellites of Saturn. It is the fourteenth satellite when ordered by distance from Saturn, and orbits within the densest part of the E Ring, the outermost of Saturn’s rings, an extremely wide but very diffuse disk of microscopic icy or dusty material, beginning at the orbit of Mimas and ending somewhere around the orbit of Rhea.
Enceladus orbits Saturn at a distance of 147886 miles from the planet’s center and 111846 miles from its cloudtops, between the orbits of Mimas and Tethys, requiring 32.9 hours to revolve once (fast enough for its motion to be observed over a single night of observation). Enceladus is currently in a 2:1 mean motion orbital resonance with Dione, completing two orbits of Saturn for every one orbit completed by Dione. This resonance helps maintain Enceladus’s orbital eccentricity (0.0047) and provides a heating source for Enceladus’s geologic activity.
Like most of the larger satellites of Saturn, Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. Unlike the Earth’s moon, Enceladus does not appear to librate about its spin axis (more than 1.5°). However, analysis of the shape of Enceladus suggests that at some point it was in a 1:4 forced secondary spin-orbit libration. This libration, like the resonance with Dione, could have provided Enceladus with an additional heat source.