Enceladus, the Moon

Enceladus’ Place in the Solar System

Enceladus, the 13th moon in the Saturnian system,[1] orbits two hundred and thirty seven thousand kilometres [2] away from Saturn and is twenty six thousand kilometre away from it’s nearest neighbour, Pallene.[3] It lies within the densest part of Saturn’s ‘E-Ring’ [4] and the volcano on it’s south pole is thought to have provided some of the material for it.[5] Enceladus takes 32.8 hours to complete a full orbit of Saturn.

Saturn itself orbits 9.5 AU (1,429,400,000 km) away from the sun meaning that, when aligned, it is 8.5 AU (1,279,548,866 km) from Earth. At these distances, it takes light from Saturn 71 minutes to reach Earth [1] and it took NASA, ESA and ASI’s Cassini probe 6 years, 7 months and 15 days to finally reach Saturn, having taken an indirect path.[6]

Radius 252 km
Distance from Saturn 237,378 km
Orbital Period 32.8 hours
Distance from Nearest Neighbour 26,378 km
Albedo 0.99

Comparison to Other Bodies in the Solar System

A size comparison of Enceladus and Dione

Enceladus is very small,[7] being just 1/25th [8] the size of Earth and 1/6.8th the size of our Moon;[9] to put that into perspective: if our Moon was the size of a football, Enceladus would be smaller than a golf ball.

It’s icy surface is very reflective, making it the brightest body in our solar system in terms of light reflected, having an albedo of 0.99, meaning that it reflects 99% of the light that falls on it back into space. In contrast, our own moon has an albedo of only 0.12 despite how bright it may appear from Earth.[10]

Of Saturn’s “major” moons—that is: Dione, Enceladus, Rhea, Tethys, Titan and Mimas—Enceladus has the shortest year; just 32.8 hours! Dione’s year (65.6 hours) is exactly twice that of Enceladus, while Iapetus has the longest year: 79.3 days.[11]

Relationship with Saturn’s Rings

Saturn’s rings have long been a source of mystery for scientists, especially the question of how they formed.[5] Some of the rings were likely formed from micro-meteorites striking the surface of a small moon and blasting material off it. Because the moon is small it’s gravity is unable to hold on to it and so it drifts off to become part of one of Saturn’s rings.[12]

Enceladus lies in the densest part of the E-Ring,[4] a diffuse ring lying one hundred and eighty thousand kilometres away from Saturn and measuring three hundred thousand kilometres across.[14] This ring is now thought to have been “fed” by particles ejected from the volcano on Enceladus’ south pole.[5]

Enceladus

E-Ring

Enceladus’ Internal Structure

Enceladus is covered in ice—hence it’s high albedo. Whatever lies beneath the surface however, cannot be directly observed—but this doesn’t mean it is totally unknown; we can use a variety of clever methods to infer it’s internal structure.

Ke Zhang and Francis Nimmo looked at the orbits of Enceladus and Dione and calculated the depth of this ice/water layer to be 91km—although they concede that their calculations are an oversimplification. They also argue that the layer of ice does not convect and so the moon must loose it’s heat through conduction.[15]

For this model to work, there must also be an ocean of liquid water between the ice and the silicate core[15]—but Enceladus is some 1,400 million kilometres from the Sun [1], Mars is much closer yet it’s water is frozen—how has it managed to maintain an ocean of liquid water?

Robert Tyler, Ph.D. (2010) proposes that it is rather difficult for an ocean like Enceladus’ to freeze completely. Not only will it’s icy shell act as an insulator—impeding the loss of heat to space—but as the ocean froze, the reduction in water depth drove the ocean into a resonance that actually generates heat through dissipation of tidal energy and opposes further melting. Couple these factors together and you have an ocean that will generate heat that is prevented from escaping, thus maintaining liquid water so far away from the sun.[15]

Enceladus’ internal structure taking Ke Zhang and Francis Nimmo’s value for the depth of the ice and water layer as 91 kilometres and arbitarily selecting a 4:1 ratio of ice to water. The radius of Enceladus is 252 kilometres.

Ice Layer

Liquid Ocean

Silicate Core