On this day in 1789 William Herschel discovered a sixth moon orbiting the planet Saturn. It was eventually named Enceladus. It is named after a giant from Greek mythology who is supposed to be buried underneath Mount Etna. I mentioned buried giants when I wrote about Pliny a few days ago. It isn’t a giant though, it’s tiny, only 310 miles (500 km) across and it sits on the edge one of Saturn’s outer rings, the E ring.
We didn’t really know much about Enceladus until in was photographed by the two Voyager spacecraft in the early 1980s. They showed us that it was a very unusual place. It’s northern hemisphere in pitted with craters, as one would expect of an object that has been subject to impacts for millions of years. It’s southern hemisphere though shows far less damage, suggesting that it is a much younger surface. It is also has deep ridges in its icy surface, similar to the fault lines seen between tectonic plates on earth. This led scientists to realise that the moon must be geologically active and was more than likely the source of much of the material in the E ring.
In 2005 another space probe, Cassini, was able to send us much clearer images of the surface of Enceladus. What was discovered were four deep fractures over the south pole. Thermal imaging revealed that the moon’s temperature beneath the ridges was much warmer that elsewhere. Further images showed that huge plumes of vapour were continually spewing out through vents much like a geyser. The vents are throwing out as much as 440 lb (200 kg) of material a second. When tested the vapour was found to be largely water with some salt and a few other minerals. It is this water vapour that produces the lumps of ice found in Saturn’s E ring. Some of the ice though, falls back to the surface and as it builds up, pushes and folds the icy crust much in the same way that mountain ranges are pushed and folded here on earth. This means that somewhere beneath the surface of the moon there is an ocean of liquid water. The surface temperature of Enceladus is -198 °C, so what is it that heats the ice enough to melt, expand and be pushed out through the vents? And why is it happening at one of the poles, we would expect the warmest temperatures to be found at the equator. Although Enceladus is composed of rock, it is much too small to have a molten core. It is possible that the gravitational pull of Saturn is causing friction inside the moon, that could raise the temperature. But the answer is no one knows yet.
Cassini is still in orbit around Saturn, and we may still find out more about Enceladus. With the presence of liquid water and minerals it is, at the moment, the most likely place in our solar system where we might find evidence of extra-terrestrial life, which would be amazing.