Enceladus' Old Faithful
We begin our tour of the universe with a trip to Saturn, and one of its tiny moons, Enceladus. When the Cassini spacecraft flew past in 2005, researchers noticed some strange hot spots around Enceladus' South Pole. Scientists have now realised that these hot spots give rise to a gigantic geyser, which erupts regularly, a bit like Old Faithful here on Earth.
The geyser on Enceladus is on a completely different scale. When it erupts, it does so with enough force to send material out into space and some of this settles out to become part of one of Saturn's famous rings.
We spoke to NASA's Terry Hurford to find out more.
Chris - What powers this beast that you've seen on Enceladus?
Terry - There's some question about what the heat source is, as Enceladus is so small that it should have cooled off a long time ago. Something has kept this moon warm, and it's most likely tidal friction: Because of its orbit [around Saturn] it's constantly re-shaping itself, and this re-shaping leads to friction, which ultimately powers the geysers.
Chris - So how does that heating process work? Why should just orbiting a planet make this moon warm?
Terry - Well, if it were in a circular orbit, it probably wouldn't be warm, but it's in an orbit which is not completely circular. Because it's not circular, the distance to Saturn is constantly changing, as is the position of Saturn in its sky. This constantly causes the body to re-shape itself, and in the process of re-shaping itself it dissipates heat.
Chris - So it's a bit like rubbing your hands together, and getting some warmth between them?
Terry - Right.
Chris - So when that happens, how does it give rise to the phenomenal 500 mile high jet of water on the south pole of this moon?
Terry - In the process of warming itself, it can create some liquid water around the south pole. When that liquid water is exposed to the vacuum of space it can then erupt at speeds that then allow it to escape the surface. Because the body [Enceladus] is so small, it doesn't take much velocity to get things off the body and into orbit around Saturn.
Chris - Why is it only the south pole that gives rise to this phenomenon, why aren't we seeing these things spurting out all over this moon? It must be getting tugged and squashed all over the place.
Terry - That's a good question too. In the process of heating the body some of the material becomes warmer and that material has a lower density so it causes the whole body to reorient itself so that the warmer area is located at one of the poles, either north or south. In this case, it just happened to go to the south pole.
Chris - You're now studying this using Cassini, what does this tell you about what the structure of that moon must be? There is, presumably, a big source of liquid water underneath somewhere, could there be life in there even?
Terry - The work that I've done is looking at stresses in the south pole, and specifically when the features in that open and close because of the tides. The tidal stresses pull open some of the fractures and when they pull open it then allows the liquid water to be exposed and this allows the eruptions to occur. In order for the stresses to be high enough to do this, I speculate that there's got to be a global ocean of water underneath an icy crust, similar to Europa, a moon of Jupiter.
Chris - So if you've got liquid water and it's warm enough to give rise to a geyser phenomenon, that suggests that you've got similar situations to those found here on Earth around hydrothermal vents, so you could see life there.
Terry - Right, that makes it a very interesting place to look for microbial life or some sort of simple life forms.
Chris - And how are you going to get to the bottom of that? Its obviously not convenient to take a 3 million mile trip to drop by Saturn and take samples. Is there a way to answer that question simply?
Terry - The fact that we have the geysers erupting means that the liquid water is somehow getting out to the surface and ultimately into space. So you may not have to actually land something on Enceladus to look and see what kinds of things are inside the water, since it's being transported to you. It's much more convenient on Enceladus to look for this than on Europa, as Europa has a very thick ice shell and it would be very hard to get to the liquid water underneath it. Enceladus may not be as thick, and the water is getting right up to the surface.