Saturday 27 November 2010

Cassini Set For Enceladus Flyby

Our Solar system, the Sun and all those who sail with her, is pretty ordinary as far as we can tell.  There's a very average star surrounded by a few gas giants, some small rocky types and a fair bit of rubble.  Examinations of other stars have so far shown much the same setup and there's no reason to suspect our system is anything other than fairly mundane, with the possible exception of some interesting biology on the third rocky type.

And yet we still find fascinating things everywhere we look.  One of the funkiest ongoing projects is the Cassini Equinox Mission, a space probe orbiting Saturn and paying some flying visits to the attendant moons.  Cassini has been producing some stunning data over the last six or so years, some of it highly technical, and some of it simply very, very pretty.
Based on an original from NASA/JPL (cropped by author)

This, for example, is Enceladus, an ice moon with a diameter of about 500km.  There are two particularly interesting things in this image:

The stripes and flat areas are the first.  Look at almost any object in the Solar system that doesn't have a thick atmosphere (which Enceladus doesn't) and you'll find one main feature: craters.  Big craters, small craters, overlapping craters, craters inside craters...there's usually lots and lots of craters.  Everything in the Solar system is being constantly bombarded by lumps of rock, ice and metal left over from the original formation, and that leaves a few scars.  Even the Earth has a few big ones from objects that made it through the atmosphere.  Enceladus has craters too, but not as many as you'd expect.  There are big areas which are fairly crater-free, meaning that the surface is being wiped clean somehow.  The stripes are a clue as to how this is happening.

The second interesting thing in the picture is the fuzzy plume at the bottom of the picture.  Enceladus is too small to hold an atmosphere down, so what is the plume, and where did it come from?

The most likely solution to both curiosities, the plumes and the unusual surface, is that Enceladus isn't entirely frozen.  The suggestion is that under the surface shell of ice there is liquid water, possibly huge amounts of it.  The current theory is that because Enceladus' orbit isn't perfectly circular it's kneaded by Saturn's gravitational pull.  If you take a snowball, stick it in a plastic bag and crunch it about in your hand it'll melt fairly quickly, and that appears to be what's happening with Enceladus.  And a liquid ocean under the ice would produce tectonic activity very similar to what we see on Earth.  The icy crust will slide about on the water, crack open, melt and refreeze, potentially explaining the lack of craters.

This is where this article becomes a bit speculative.  There's no suggestion that there's life anywhere in the known universe other than on Earth, but we do have a few likely candidates:  hardy microbial life on Mars perhaps, or possibly even some funky methane eating organisms on Saturn's gigantic moon Titan.  One of the best bets however, is anywhere you find liquid water, which puts Enceladus firmly in the "just maybe" category.  We've got life on Earth which exists in a very similar environment, under the Antarctic ice cap for example, or deep in the ocean.  There might be nothing - Enceladus might just be a dead snowball hovering too far from the Sun.  The potential is certainly interesting though.

Cassini will be making another close pass on Enceladus this Tuesday (30th November, 2010), skimming past at a mere 48km.  Even if there's no extraordinary new game-changing data from the plucky little space probe, we should at least get some very pretty pictures back.

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