We are familiar with geysers, especially Old Faithful with its graceful fountain of water towering into the sky. Its eruptions, which are predictable hence the name, are powered by geothermal energy, the heat of the Earth from deep within. It has been speculated that some planets should still have active volcanoes and geysers, but the moons were thought to be cratered, cold, and dead.
That all changed in March 1979 when Voyager 1 discovered several huge volcanoes violently erupting on Io. The powerful tidal forces of Jupiter and the other large moons, Europa, Ganymede, and Callisto conspire to flex Io like putty heating its interior to nearly 3000degrees Fahrenheit F. There were theories in the wake of this discovery that some of the moons could be flexed just enough by their parent planet to melt the ice beneath the crust and produce geysers of liquid water, methane, nitrogen, and/or ammonia, which would quickly turn to gas upon eruption. The suspicion emerged as a result of Europa's unusually smooth appearance. Some kind of activity must have resurfaced it. Enceladus was also found to be vastly smooth and is the brightest object in the Solar System because it reflects 95% of the light that falls on it. Its surface is a sheet of pure ice crystals, which has to be quite new because ultraviolet radiation from the Sun rapidly damages and reddens ice crystals darkening them.
Theory became reality from an unexpected place that is truly frozen. Voyager 2 flew past Neptune and Triton on August 25, 1989 making a stunning discovery when it found geysers erupting on Triton. These huge plumes erupt up to five-mile high and drift over 100 miles downwind. The geysers are powered by nitrogen, which is ice on the -392 degrees Fahrenheit surface, but liquid beneath the surface where it is warmer. The warmth creates just enough pressure for the liquid to breach a weak spot and erupt into a geyser. This big discovery turned attention once again to Europa when the Galileo orbiter arrived at Jupiter on December 7, 1995. Galileo lasted until September 21, 2003 and flew as close as 124 miles from the surface of Europa but detected nothing. The Hubble Space Telescope "may" have recently detected weak geyser activity near its South Pole, but it'll take another spacecraft, possibly the newly approved Europa Clipper, to resolve the question.
Enceladus was the next target of interest after the Galileo mission ended. The highly sophisticated Cassini orbiter arrived at Saturn on July 1, 2004. The trick was to catch Enceladus while backlit so that the forward scattering of any ice and dust from the suspected geysers would be brighter and easier to see. Scientists struck gold when the first opportunity came in early 2005. The entire South Polar Region was covered in hundreds of fountains along several fissures. These geysers shot ice particles so high that they not only recoated the surface in bright ice, they also created a ghostly ring orbiting Saturn. Tidal flexing is strong enough to heat and melt the ice under Enceladus' crust to create a large underground ocean. Cassini flew as close as 12 miles from the surface through the plumes without harm proving how delicate they are and that they are composed of water. This is exciting because where there is water, there may be potential for life, making Enceladus an exciting target for future exploration.
The mechanism behind an erupting geyser is simple. Heat from beneath the surface warms the subsurface ice causing it to melt, heat up and expand. The increasing pressure eventually finds a weak spot in the crust along a crack or fissure, and then there is only one thing left to do on a frozen world, let it go!