Enceladus: Saturn's Mystery World

Cryovolcanic vents spew ice and water out from beneath the frozen crust of this little moon at Saturn. NASA/Cassini mission

There's a bright, shiny moon circling at Saturn that has intrigued scientists for many years. It's called Enceladus (pronounced "en-SELL-uh-dus") and thanks to the Cassini mission orbiter, the mystery of its glittering brightness may be solved. It turns out, there's a deep ocean hidden under the icy crust of this little world. The crust is about 40 kilometers thick, but it is split by deep cracks over the south pole, which allows ice particles and water vapor to vent out to space.

The term for this activity is "cryovolcanism", which is volcanism but with ice and water instead of hot lava. The material from Enceladus gets swept up into Saturn's E-ring, and scientists suspected that was happening even before they had visual evidence. That's a lot of fascinating activity for a world that's only 500 kilometers wide. It's not the only cryovolcanic world out there; Triton at Neptune is another, along with Europa at Jupiter

Finding The Reason for the Enceladus Jets

Seeing the cracks that split the surface of Enceladus is the easy part of exploring this moon. Explaining why they are there required a close fly-by, so the scientists managing the Cassini mission programmed a detailed look with cameras and instruments. In 2008, the spacecraft sampled the material from the plumes and found water vapor, carbon dioxide, carbon monoxide and organic chemicals. The fact that the plumes exist is probably due to tidal forces acting on Enceladus from Saturn's strong gravitational pull.

That stretches and compresses it, and causes the cracks to pull apart and then pinch together. In the process, material spews out to space from deep inside the moon.

So, those geysers provided the first hint that an Enceladean sea existed, but how deep was it? Cassini made gravity measurements and found that Enceladus wobbles ever so slightly as it orbits Saturn.

That wobble is good evidence of an ocean under the ice, one that is about 10 kilometers deep beneath the south pole (where all the venting action is taking place).

It Could be Hot Down There

The existence of a liquid ocean inside Enceladus is one of the great surprises of the Cassini mission to Saturn. It's so cold out in that part of the solar system, and any liquid water freezes solid as it hits the surface and spews into space. Scientists have speculated about a heat source inside this moon creating hydrothermal vents similar to what we have on Earth's ocean floor. There is a warm region near the south pole as a result of core heating. The best ideas about the core heating are that it could be from decay of radioactive elements (called "radiogenic decay"), or from tidal heating — which would come from the stretching and pulling delivered by Saturn's gravitational pull and perhaps some tug from the moon Dione.

Whatever the heat source, it's enough to send those jets out at a rate of 400 meters per second. And, it also helps explain why the surface is so bright — it keeps getting "resurfaced" by the icy particles that shower back down from the geysers. That surface is very cold — hovering around -324° F/-198° C —, which explains the thick icy crust pretty well.

Of course, the deep ocean and the presence of warmth, water, and organic materials raises the question of whether or not Enceladus could support life. It's certainly possible, although there's no direct evidence of it in Cassini's data. That discovery will have to wait for a future mission to this little world.

Discovery and Exploration

Enceladus was discovered more than two centuries ago by William Herschel (who also discovered the planet Uranus). Since it appears so small (even through a good ground-based telescope), not much was learned about it until the Voyager 1 and Voyager 2 spacecraft flew past in the 1980s. They returned the first close-up images of Enceladus, revealing the "tiger stripes" (cracks) at the south pole, and other images of the icy surface. The plumes from the south polar region weren't found until the Cassini spacecraft arrived and began a systematic study of this icy little world.

The discovery of the plumes came in 2005 and on subsequent passes, the spacecraft's instruments did a more nuanced chemical analysis.

The Future of Enceladus Studies

There are, at present, no spacecraft being built to go back to Saturn after Cassini. That will likely change in the not-too-distant future. The possibility of finding life beneath the icy crust of this little moon is a tantalizing driver for exploration.