Most geysers on a moon

The natural satellite with the most active geysers is Saturn's moon Enceladus, which has 101 known geysers clustered in an area called the South Polar Terrain, located poleward of 55 degrees south.

Enceladus was discovered in 1789, and for many years was known primarily for its curiously bright surface. It has an albedo (reflectance) of 0.81 using the Bond system, in which a perfectly black sphere has an albedo of 0, and a perfectly white sphere has an albedo of 1. This means that 81 percent of the light that hits it is reflected back.

This high albedo was explained when the moon was imaged by Voyager 2 in 1981, revealing an almost pure-white ice world with very few visible craters. Enceladus' smoothness intrigued scientists, as it implied that some active geological process was continuously renewing the surface.

When the Cassini probe arrived in the Saturn system in 2004, this intriguing little moon was high up on the list of observation targets. Cassini made its first flyby in February 2005, and followed it up with additional closer passes in 2008 and 2009. Over the course of the 13-year mission, Cassini made 23 passes over the moon, flying past at altitudes of as little as 200 km (120 mi) above the surface.

From these observations, a fascinating picture began to emerge. Measurements of the moon's gravity showed that, instead of being a solid mass of ice down a rocky core like Saturn's moon Tethys, Enceladus had a substantial rocky core. These readings also suggested that Enceladus might have a liquid water ocean under all that ice. Most excitingly, they revealed that some of the water from that buried ocean was being vented into space by a network of geyser-like ice plumes near the moon's south pole.

The source of the energy that drives these geysers is still not clearly understood, but analysis of the plumes has given researchers a good working model of how they form. The leading theory is that water from the subsurface ocean seeps into the crust of the moon's rocky core, where it is superheated and ejected through hydrothermal vents on the ocean floor.