We Earthlings think of our Moon as being bright, but that’s only because it’s close to us: the Moon reflects only about 12% of the light that falls on it during its full phase, with some areas being slightly more reflective than others. The Moon, to put it succinctly, is as reflective as a chalkboard. Now imagine a moon with a surface of nearly pure, smooth ice, a landscape of such frozen austere beauty that reflects 99% of the light it receives. Even a tiny moon like that would be visible a long way away.
That moon is Enceladus, Saturn’s sixth-largest satellite. It’s large enough to be (mostly) spherical, but still quite tiny: about 500 kilometers (311 miles) in diameter, or roughly the distance between Richmond, Virginia (where I live) and New York City. William Herschel reported the first observation of Enceladus in 1789, which again reflects how bright it appears: most things only 500 kilometers across as far away as Saturn are very difficult to spot, even by a careful astronomer like Herschel.
However, small means neither uninteresting nor unimportant: Enceladus is considered by many scientists to be the most likely candidate for harboring life in the Solar System (other than Earth, of course). As with Europa, another potentially habitable world, Enceladus lacks a substantial atmosphere, but has a global shell of ice that protects liquid water beneath. The moon’s proximity to Saturn helps keep the interior warm through tidal forces, which also keep one face of Enceladus pointing toward its host planet (as with our Moon and indeed many other satellites in the Solar System). There is probably internal heating as well from radioactive decay of materials in the core, similar to the internal heat source on Earth.
How do we know about this water and warm interior? As with Earth and Io, the effect of the hidden turmoil beneath the surface is volcanism, though the volcanoes themselves are very different. Plumes of water and ice jet up from fissures in the surface of Enceladus known as tiger stripes: you can see these formations as bluish lines on the image to the left. Since the volcanoes are ice- rather than magma-based, they are called cryovolcanoes (where the prefix cryo– refers to cold, as in “cryogenic”; extra credit will be given to someone who can pun on the word “cryovolcano”).
The Cassini probe (my favorite robotic explorer) has flown through the plumes from the cryovolcanoes and analyzed the chemical composition. Though the jets are mostly water, they also contain ammonia, methane, carbon dioxide, nitrogen, and trace amounts of hydrocarbons—molecules containing hydrogen and carbon. While hydrocarbons are pollutants on Earth (which create that lovely yellow smog over our cities), they also are naturally-occurring compounds that may have played a role in the early biochemistry of life on Earth. To scientists, the word organic refers to the presence of carbon: all life as we know it is organic—carbon-based—in character. The jets of water are also the likely source of Saturn’s E ring, a tenuous watery ring encompassing the orbit of Enceladus and several other moons.
Though Enceladus be but little, it is
fierce important. The combination of liquid water and organic compounds make it highly interesting to those studying the possibility of life beyond our planet. This distant icy moon has taught us much already; might there also be some form of alien life beneath the ice?