Snowball Moon

Enceladus is, without question, one of the most interesting locations in our solar system, and for a lot of reasons. Quite simply, this tiny Cronian moon is a mystery. We know what we see, but we have utterly no idea what makes it tick!

Oh, and that yellow glow in this picture? That's saturnshine. Sunlight reflected off the yellowy cloudtops of saturn.

What we see, as it happens, is geysers of pure water spraying out of four fissures in Enceladus’ surface. They spray so high and with such force that much of the water escapes the moon’s feeble gravitational pull and actually create one of Saturn’s rings! (The rest eventually falls back down as snow, giving Enceladus a ratheryouthful appearance). The thing is, a diminutive object like Enceladus shouldn’t, by rights, be active at all. It’s so small that it’s likely only just large enough to even be spherical. Smaller objects cool faster, so while a reasonably sized planet like Earth still has a molten core, our planet’s own moon is mostly solid by now.

Small but mighty

Enceladus, however, contains a lot of internal heat and it’s still not entirely clear where it all comes from. Because when I say it’s tiny, I’m really not exaggerating. The image above shows a handful of tiny worlds recently visited by humanity, compared with Earth’s moon. As you can see, Enceladus is so small that it could easily become a moon for an object the size of our moon (which would make it a moon moon, I suppose).

There are hotspots near it’s south side, resulting in a series of fissures nicknamed “tiger stripes”, and it certainly receives tidal heating due to Saturn’s considerable gravity pulling and squeezing it as it orbits. But it’s still difficult to explain why this little moon is so warm. Or why it seems to contain liquid water. Or why it sprays that water so high into Saturn’s orbit the way it does. Studies have suggested that tidal heating alone can account for about 1.1 gigawatts of power being supplied to Enceladus. But the Cassini probe’s observations suggest that there are about 4.7 gigawatts powering Enceladus’ hotspot. There’s a lot of heat which is still going unaccounted for…

While it’s been accepted for some time that there must be a subsurface ocean under Enceladus’ south pole, the latest suggestions are that this ocean may actually be global (just like Europa!). Actually, the evidence is fairly compelling. Essentially, as it orbits, Enceladus shows a slight wobble. It’s not very pronounced, but it appears to be more apparent than it would be if Enceladus was mostly rigid. This suggests that its surface is actually detached from its core, as it would be if its icy crust was floating on a global ocean of liquid. Given that an ocean of water would be a good place to look for signs of extraterrestrial life, many consider Enceladus to be a priority target in space exploration.

So excitingly enough, NASA planetary scientists have schemed something quite fun. Cassini’s taken a few close passes of Enceladus lately, giving us some more detailed views. Following up on this, on October 28th (that’s this Wednesday), they’re going to plough Cassini straight through the plume of icy water being sprayed out of Enceladus. Cassini is, to date, one of the most successful robotic probes ever launched. It’s been orbiting Saturn for over a decade now. As a result, I guess its pilots aren’t afraid to take a few risks (it’s always fun when spacecraft pilots aren’t afraid to be a little gung-ho). After all, there surely can’t be that much more data it can usefully collect. I’m quite excited to see what they might find out…

What secrets are you hiding, little moon...?

And as we all know, 1.1 gigawatts isn’t even enough power to travel into the future

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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