Spinning Europa 1: Introduction

Aaron suggested that my paper would would be much better if it read more like one of my blog posts, and less like a litany of torture lab notebook.  So here it is in parts, written as if I intended for you, dear reader, to read it.  (But don’t worry Bob, I’m actually working on the real paper).  It’ll probably be cathartic, as one of the things I hate about writing papers is the formalistic language.  It makes the content less readable, less enjoyable, less human.  I just don’t see the point.  If the content is up here, then anyone who feels the same way can get an idea of what’s going on without wading through all the passive voice crap.  It’ll also help me enjoy writing it, and let me feel like I got it out of my system.  Plus, on the internet, color figures are free (not $350 for the first page, $175 for each additional page… I mean jeez, that’s like a year’s worth of hosting fees just for one paper), you can insert links, and nobody has to pay $3975 per year for a subscription.  Oh, and sweet, I also get to retain the copyright.  Honestly, paper journals are so sad.  Of course there’s that pesky peer review, but you’ll find a comment form at the bottom of the page, and if you actually make it that far, by all means let me know what you think.  In a production environment, the publication would be hosted on a neutral third party site, precluding me from editing or deleting comments, verifying everybody’s identities, and ensuring that the content was archived effectively.  Alas, we’re not there yet.  Maybe this will seem ridiculous at this point in the grad school experience, but I actually maybe for the first time understand why someone would want to give a talk.  I have results, they’re interesting (if you’re into this kind of thing), but I don’t really know what they mean.

A Little Background:

Europa is one of Jupiter’s four big moons.  It’s about the same size as Earth’s moon.  While it’s often called an “icy” moon, compared to other satellites in the outer solar system it’s actually pretty rocky, with just a couple hundred km of H₂O (as water or ice) covering a silicate interior.

The thick ice shell is very likely floating on top of a deep salty ocean.  We know this because the Galileo spacecraft detected an induced magnetic field around Europa, which varied depending on where within Jupiter’s magnetic field the satellite (that is, Europa) was.  Okay, really we don’t know for sure that there’s an ocean (it isn’t as if anyone has ever been there, or drilled a hole through the ice… and actually, it’s fairly unlikely that we ever will, as it’s something like 20 km thick).  But we know the surface is water ice (from spectrometry) and we know there’s something under the surface that’s conductive, and the only plausible idea anyone has had is that it’s a salty ocean.  We also know that the first 100 km or so of the moon is made of low density stuff (based on measurements of Europa’s gravitational field).  So, low density, conductive, and we know the surface is water ice: ocean.  And incidentally, this ocean contains more water than all the Earth’s oceans combined, by a long shot.

So the ice shell floats on top of this ocean, like the ice shelves off Antarctica, or the sea ice in our own arctic ocean.  But on Europa, it very well might never touch bottom, anywhere.  That means it’s free to slide around; it can move independently of the moon’s rocky core.  There’s a catch though, which is that Europa isn’t actually quite spherical.  Because of Jupiter’s intense gravitational field, the satellite is stretched out toward its parent planet, and shaped a little more like a symmetric egg (a prolate spheroid) than a perfect sphere.  Because of this tidal bulge, for the shell to move relative to the interior (and relative to Jupiter) actually quite a torque needs to be applied, and ultimately, if the shell moves, it will experience internal stress, some of which may relax away viscously (solid state flow), and some of which may either be stored elastically (as in a spring), or released through material failure (breaking).

Looking at the surface of Europa, we see a huge collection of linear features.  We don’t really know how they formed, and so they’re often just called lineaments.

The most plausible suggestion so far is that they are faults of some kind, cracks in the ice.  After the ice cracks, apparently some other kind of icy geologic process takes over, building up ridges on either side of the crack, and spewing out sulfurous material, maybe similar to whatever has Saturn’s moon Enceladus erupting with geysers.  We don’t really know what it is that’s caused these cracks, but one idea, the one that I’ve been working on, is that it’s stresses from the shell rotating independently of the interior, in a particular way.  The idea is that as Europa goes around Jupiter, because it’s in an eccentric orbit (closer to Jupiter sometimes, and further away at others) the tidal bulge isn’t always pointing right toward the planet, and so there’s a little net torque in one direction over time, that causes the shell to rotate (there’s a lot more math and physics in there, don’t worry…).  Most satellites this small, and this close to their parent planet, end up always putting the same face toward the planet, which is called being tidally locked or synchronously rotating.  Our moon does this too: the “dark” side of the moon is really just the far side; it isn’t any darker than any other part of the moon on average.

If the shell is getting a little tug each time it goes around, and so rotating ever so slightly faster than the interior (which has to rotate once per orbit, or Europan “day”, in order to keep the same face toward Jupiter) then it would be experiencing non-synchronous rotation, or NSR for short.  A more general kind of re-orientation of the shell is true polar wander (TPW), in which the shell can wobble around to any new position, without maintaining the same rotational axis.  Either of these kinds of shell re-orientation could create stresses, and both move and change the orientation of pre-existing features on the surface.  My research has been to look at whether the lineaments can really support the hypothesis that either of these things has happened, and if so, whether they can tell us anything about how…