Cryogeysers erupting on Enceladus

The moons of the outer planets have turned out to be surprisingly interesting, and Saturn's moon Enceladus is one of the most fascinating.

1. William Herschel discovered Enceladus in 1789.
When Herschel discovered Enceladus, it was a featureless dot in a telescope eyepiece, and it stayed that way until 1981 when Voyager 2 visited Saturn. Before that, astronomers had only been able to calculate its orbit, and estimate its mass, density and brightness. The Voyager photographs were intriguing, but what really showed us this little world was the Cassini mission. Cassini was in the Saturnian system from 2004-2017.

2. John Herschel named Enceladus.
For almost two centuries after the 1655 discovery of Saturn's biggest moon, none of Saturn's moons had names. They had numbers. John Herschel, William Herschel's son, found it confusing to remember which was which, so he devised a naming scheme based on the mythology of the god Cronos (Roman Saturn). Enceladus was one of them.

3. Enceladus has the highest albedo of any known Solar System body.
The albedo [al-BEE-doh] is a measure of how much light a surface reflects. Bright Venus reflects about 65% of the Sun's light, but Enceladus reflects 99% of it. This makes it not only brighter than the other Saturnian moons, but also colder, since it absorbs very little sunlight. A warm day on Enceladus would be about −198 °C (−324 °F).

4. Enceladus was geologically active in the past.
Despite the moon's being only 500 km (300 mi) across, it has a variety of terrains. The surface features of some bodies resulted from impacts. On Enceladus most of the features are typical of tectonic activity, i.e., geological activity within the moon. These include fractures, grooves, steep cliffs, and long, deep canyons called rifts. Since impacts occur all the time, heavily cratered surfaces are older than those with few craters. Although there are regions of Enceladus with extensive cratering, there are also areas of smooth plains. These show that geological activity has resurfaced terrains such as Sarandib Planitia.

5. Enceladus is still geologically active.
Planetary scientists say some surfaces formed “recently”, perhaps “only” half a million years ago. That may not sound recent to you, but what if we could see some activity now? Earthly volcanoes are convincing evidence that our planet is geologically active. And so is cryovolcanism — cold volcanism — on Enceladus. Besides taking images of geysers, Cassini flew through a volcanic plume to find out what it's made of. There was water and dissolved substances, as well as ice and salt crystals. Some of the material, including most of the salt crystals, falls back onto the moon as a kind of snow. The rest shoots out into space.

6. South polar region.
The south polar region is a notable contrast to the north pole's cratered terrain. It has few impact craters, but lots of fractures and ridges. It's also where the cryovolcanoes – over a hundred of them – erupt. A puzzling feature of the region is what's been nicknamed the tiger stripes. This is a set of four fractures edged by ridges and surrounded by mint green water ice. The green material is unique to this area, and differs chemically to material elsewhere on Enceladus.

7. Enceladus is layered, and there's a salty ocean covering the moon between the rocky core and the icy crust.
Planets have layers. Although smaller bodies often aren't layered, Cassini's gravity measurements of Enceladus show that it has a rocky core, icy mantle and crust, and surprisingly, a salty ocean. The saltiness of the cryovolcanic plumes supports the idea of a salty ocean, and gravity measurements confirm it. The icy layer is 30-40 km (19-25 mi) thick and the ocean itself about 26-31 km (16-19 mi) deep.

8. Enceladus maintains one of Saturn's rings.
Saturn's rings are made up of chunks of ice – except for the E ring. The E ring is wide and diffuse, and composed of microscopic particles of mostly frozen water. The E ring would have disappeared long ago but for the cryovolcanic eruptions in Enceladus's southern polar region. The particles that escape from Enceladus replenish the ring.

9. There must be a heat source to maintain the geological activity and keep the ocean liquid.
Tidal heating is the main contributor. Tidal heating is the heat generated by gravitational interaction with another body. However, although tidal heating can't account for all of the heat, there's no consensus about what provides the remainder.

10. Many astrobiologists think that Enceladus is a good place to look for life.
Deep within Earth's oceans are hydrothermal vents. They're places warmed by heat from Earth's mantle, and chemicals are released. They form environments whose energy isn't based on plants photosynthesizing sunlight. Food chains start with microbes whose metabolism is based on chemical energy. Perhaps such organisms have evolved in the ocean of Enceladus? It has an energy source which keeps it liquid, it's in contact with the rock layer, and the plumes show that there are traces of organic molecules.