Doctoral Leaflet

It seems a bit of Vaudeville is still lingering around the Academe…



Zane A. Selvans


Date/Time: 2:30pm, Friday, 20th November, 2009
Bldg./Rm: Benson Earth Sciences (BESC) 380

Examining Committee Members:

  • Karl Mueller
  • John Wahr
  • Robert Pappalardo
  • Bruce Jakosky
  • John Spencer


Major Field: Geological Sciences


A descendant of Dust Bowl migrants, Zane grew up near Fresno in California’s San Joaquin Valley. He left as soon as humanly possible, and got his BS in Computer Science at the Caltech in Pasadena. After a brief stint working in Silicon Valley (which unfortunately did not result in any kind of dot-com stock option fortune), he returned to Caltech via sea kayak to work with Mars Global Surveyor data, mapping Mars’ south polar layered deposits. While he has been a student at CU Boulder since the fall of 2002 you may not have seen much of him lately, because in early 2006 his wife and advisor both moved to Caltech/JPL, and like a long period comet, he slid back down into that place’s deep potential well to be with them. Next year, Zane intends to spend a lot of time on his bicycle.


Time, Tides and Tectonics on Icy Satellites
Faculty Advisor: Karl Mueller


In the outer solar system, we cannot directly use the radiometric dating techniques widely applied in terrestrial geology. We also lack the detailed understanding of the correspondence between crater size-frequency distributions and absolute ages that the radiometric dating of lunar samples has given us in the inner solar system. Additionally, many geologically interesting surfaces on the icy satellites are insufficiently cratered to allow us to infer precise relative ages. Thus it is desirable to find other ways to construct geological chronologies that function well in the outer solar system. In this work I develop two techniques.

The first compares the linear tectonic features covering Jupiter’s moon Europa to modeled tensile fractures resulting from tidal stresses due to the non-synchronous rotation (NSR) of the satellite’s decoupled, icy, lithospheric shell. The amount of shell rotation required to align a feature with the stress field resulting from NSR is used as a proxy for time. This translation is potentially convolved with a phase lag between the tidal potential and the stresses it induces, resulting from the shell’s partially viscous response to the NSR forcing. The geography of individual lineaments is found to be no more consistent with NSR stresses than chance would predict, however, the ensemble of global lineaments displays a non-uniform apparent rate of lineament formation throughout the time period recorded by the surface. This non-uniformity may be explained either by steady state fracture formation, activity, quiescence and erasure, or by a transient episode of tectonics.

The second technique encodes the myriad superposition relationships evident between Europa’s tectonic features as a directed graph enabling algorithmic analysis. The observed superposition relationships are generally insufficient to construct complete stratigraphic stacks, but we can calculate the degree to which they corroborate or contradict another hypothesized order of formation. We find that they tend to corroborate the hypothesis that the lineaments are tensile fractures due to NSR stresses.

Together these results offer cautious support for the idea that Europa’s shell rotates independently of its silicate interior, and demonstrate techniques useful in comparing tectonic features on other icy satellites to hypothesized mechanisms of formation.


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.


Short lineaments aren’t just noise

I’m now able to successfully discriminate plausible NSR fits by:

  • calculating several “good” fits, that is, any local minimum in the fit curve that is within 10% of the overall curve’s amplitude, of the minimum fit.
  • screening these good fits based on whether or not endpoint doppelgangers generated at those amounts of backrotation are with in an MHD of less than 0.1*lin.length() of lin.

This screening process:

  • almost always results in a unique best fit
  • screens out many bad fits (because even at their minima, they can’t create synthetic lineaments)
  • very occasionally permits more than one fit to be included as good enough

I think it’s good enough to be able to avoid doing the monte carlo thing for now.

I looked at several bands of lineament length, especially the short ones, to see if there were perhaps a trend toward noise in the shorter lineaments, which is what I would expect, given how easy it is for them to fit somewhere in backrotational space. But it turned out that they still display approximately the same aggregate fit curve and activity histogram:

Only the shortest lineaments tend toward noise.
Only the shortest lineaments tend toward noise.

It would be good to create a map of the lineaments, color coded by where their good fits occur, and compare that to the map of resolution and illumination angle that I got from Trent, just to see if there’s any kind of correlation.

Now I need to transform the lineaments into the paleo-orientation suggested by Schenk and Nimmo, and re-run the analysis, to see if magically, that shell orientation gives a more convincing story.