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Sep 21 / Peter Polito

Titan Channels: What we know four and half years later. [Accretionary Wedge]

This post is for the monthly geoblogosphere carnival called the Accretionary Wedge. This month’s Accretionary Wedge is being hosted by Chris Town at Good Schist.

One of the most fascinating things about the surface of Titan is that five years ago we knew nothing about it.  But with the arrival of Cassini and Huygens that has all changed.

image courtesy of University of Arizona and JPL.

My research into Titan channels is driven by this simple premise:  We kinda think we know how bedrock channels work on earth, what does that tell us about Titan?  While Titan does not have any “bedrock” as we know it, it does have its own variety of bedrock.  Titan is a balmy -200 °C, the surface is predominately water-ice with some hydrocarbons thrown in for good measure, what exactly those hydrocarbons are is hotly debated.  There are several questions that can be asked about these channels.  What process drives incision?  At what rate does the incision occur?  Is the incision active?  Can we “Drill now!” to get those hydrocarbons?  My research is trying to address the second question, how quickly does ice erode?  Another way to ask this question is this: If we were to transport a river from Earth to Titan, keeping the physical characteristics the same (slope, discharge, alluvial cover, etc.) but changing the planetary conditions (gravity, fluid viscosity, density, bedrock type, etc.) how different would the incision rate be?

image courtesy of University of Arizona and JPL.

(And if before going on I can shamelessly advertise: If any of this interests you I will be presenting it at GSA in Houston on Monday, October 6th.)

Let me start out by saying that ice is a strange bird, it is arguably one of the most bizarre materials in our solar system, maybe even weirder than aerogel. We are addressing this problem in phases.  The first phase is making the ice.  This may sound like a simple thing to do, but we quickly realized that freezing water in your freezer makes monocrystalline ice, and once a fracture reaches a critical length it propagates through and shatters it.  Instead we make polycrystalline ice using ice ground up with a Sno-Kone machine and near-freezing water, making the Titan equivalent of sandstone.  The second phase is trying to measure what happens to a single clast as it impacts the stream bed.  To do this we perform drop tests, where we mimic one collision between one clast and one section of “bedrock” over and over again.  The result of this is measuring how much energy is required to erode a unit volume of bedrock.  And presto, we have a Titan-Earth incisional ratio!  See the images of our impact chamber and “eroded” ice disk below.

So what are we finding?  Preliminary results show that ice doesn’t erode that much faster than rock, but there is much still to do.  We still need to take these results, corroborate them with further experiments, and plug them into some fancy-dancy equations to tell us what a river on earth would look like if it were on Titan.  But the moral of the story is that if we think we understand a process on earth, whether it be channel incision or mountain building, then we should be able to test our theory on other planetary bodies.  The results won’t be exactly the same, but they should be similar and so far results are promising.

Cryogenic nitrogen vapor emissions from impact chamber. ©Peter Polito.

Ice disk after ~700 low-velocity impacts. © Peter Polito.

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