Titan Crater Topography Through Stereo

To give a little background, I updated the crater population on Titan by identifying all the craters at the end of Cassini. Then I characterized their morphometry using limited topography in SARTopo, which give thin lines of low resolution topography where overlapping images were taken within a single swath. With this, I define certain parameters to study how the crater was changing. Crater size (diameter) is crucial because the shape of the crater is often presented as a function of diameter. The depth is also important because it gives quantitative constrains on how degraded a crater is from its original (deeper) depths. I took it a step further and measured rim heights as well.

However, the reason I have had to continue studying this is because I have been comparing SARTopo to standard stereo topography. Stereo is similar in that it is acquired where we have entire RADAR swaths overlapping. This provides a 3D look at the crater rather than a single (may 2 or 3) profile through the crater. Furthermore, SARTopo is at ~10km per pixel resolution. Stereo is more like ~2km per pixel. This is relevant because we measured “freshness” of a Titan compared to Ganymede craters. Ganymede is ~same size and density as Titan, so it is expected tat its craters will form in similar manners. The problem is that the topography on Ganymede is ~1.5km per pixel. This is significantly less than Titan, and it becomes a problem when measuring sharp peaks (i.e. low wavelength topography) because lower resolution averages these peaks with surrounding lower topography giving artificially lower rim heights. However, you see that Titan stereo resolution is more on par with Ganymede topography, so we can use it as a test case to see 1) how it compares to SARTopo and 2) if degradation still remains.

Unfortunately, we have come across yet another problem. The Titan stereo results (presented in various Neish et al. papers) was measured using a method that takes statistics of the entire rim and floor values rather than a single profile. I would argue that this is probably the most representative measurement of the crater parameters. However, our comparison is with Ganymede using previously published results from Bray et al. (2012). Brey et al. (2012) averages up to 8 profiles through Ganymede craters; we need Titan stereo measurements that mirror the Bray et al. (2012) methods for it to be comparable.

So, without getting to into the details, I found the 6 craters with stereo (Ksa, Soi, Santorini Facula, Shikoku, Forseti, and Hano). I uploaded the DEM png’s into matlab and manually assigned lat and long values (that was annoying). That may not have been necessary, but I wanted to be able to go back and see where the data was taken. Then I decided to take 8 precise profiles from the crater. I did this by creating a vector (list) of angles from the y-axis then found the right x and y values for each profile. This took a lot of troubleshooting, and I am fairly impressed with my ability to get this done one step at a time. It was particularly difficult where we had partial coverage. I almost just went with the two best craters we had and let it be, but I really wanted to include all the data we had.

The first thing to notice is that not all craters have full coverage, so results are limited to where we do have coverage. I tried to extend up to 5r out (as with Bray et al) where possible. However, I never ended up needing the extra topography because I didn’t go so far as to back out topography. Each black x represents the rim point I found. Then a show 3 of the best examples we have data for with the 8 profiles for Ksa, Soi, and Shikoku where you can see how these values where found. The rims and floor positions are marked with blue marks; the red marks are from previous literature for reference.

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The Final Result

What I have done is re-purposed my previous plot of rim to floor depths (the traditional depth measurement) to include H18 stereo measurements. Ganymede craters are in black. Titan craters are color coded for the 6 stereo craters to compare Neish stereo, H18 stereo, and SARTopo. SARTopo is shown in red if there is no stereo to compare to. What we notice is that there is a 3:2 ratio of craters that have SARTopo depths larger than stereo. Forseti (blue) is lower than SARTopo, well outside the margin out error, but ~same as Neish measurements. Hano (light blue) is lower than SARTopo but more on par than the ~0km depth from Neish et al. Soi (orange) and Ksa (cyan), the ones with the best coverage, show SARTopo slightly below stereo but well within the margin of error for each. While Shikoku (yellow) is below SARTopo, well outside the margin out error. H18 stereo is about the same as Neish et al. for Santorini (green) but slightly above.

 

depth-and-stereo21.png
Rim to floor depths of Titan (circle, triangle, square) and Ganymede Craters (diamond, line). SARTopo is in circles. Neish stereo in squares. H18 stereo in triangles. The 6 craters in stereo are color coded in both stereo and SARTopo (if available). SARTopo without stereo is shown in red.

 

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Starting Off Right: My Trip to Dragon Con and Work Ahead

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So, I spent the weekend visiting friends at Dragon Con in Atlanta. It was a blast, and this year I made sure to get at least 5 hours of sleep each night. Unfortunately, that meant I kept missing early panels I wanted to go see. I ended up spending most of my time playing games with my friends that I don’t see very often. Although, I did manage to attend two panels. One was hosted by Cecil Baldwin, of the Welcome to Nightvale Podcast. It was on queer representation in Horror films. There are a few new horror films I want to watch now, and even more that I want to rewatch with a new point of view. The other panel was on the mature themes in a Handmaids Tale. I still haven’t finished season 2, but I still enjoyed it. The panel asked some interesting questions like what constitutes rape in this world. Even consensual sex between June and Nick is arguable a form of rape because that wouldn’t be happening if she wasn’t in this position. It essentially her best attempt to assert some control over her own life. Then there was a question over the justification for violence, and if it is justified, then when is that threshold where it becomes okay?
All in all, I regret not seeing more. I didn’t make it to a single Space, Science, or Skeptic panel. To be clear, I wanted to, but in the moment, it is easy to get invested in gaming for hours on end. Then its a question of navigating from Hotel A to Hotel B which makes some panels more difficult than others. There is just simply too much to see and do.

The gaming was mostly Ultimate Werewolf. Although as a group, we tend to play all sorts of deception games including secret Hitler, Two Rooms and Boom, etc. It is a great way to have fun and get to know new people.

The trip wasn’t perfect. It’s long and stressful (but I don’t regret it). Still, my car stopped working on my way home and I had to spend the day in Michigan at a mechanic. It was over 90F (32C) and it was terrible. Luckily, they got it going, but I think it may be time for me to begin making arrangements to say goodbye.

Now, lets get to business. My current agenda has me finishing the Titan Craters paper by updating the Stereo results. That is my number one priority at the moment. From there, I need to move on to my new project(s). With that in mind, I have a few ideas to discuss further with Catherine. We’ve already spoken about studying how molecules will freeze in an impact melt pond. I’m also interested in testing the stability of a melt pond in an impact crater. If we have a porous subsurface, water will drain out, but how fast? The slower it is, the more time the pores have to fill with frozen water. My idea is that we may be able to model at what porosities a pond may remain. A shrinking pond also introduces the possibility (if flimsy) of wet-“cold-dry” cycles. This is a stretch, but McMaster University showed us how they are simulating wet-dry cycles with their planetary simulator to encourage polymerization. It likely will not happen at these cold temperatures, but I am curious how these molecules may act when frozen in a rigid ice matrix. Furthermore, if we consider a pond that is draining yet simultaneously freezing, we might imagine a hanging roof of frozen ice that may break and remix partially. Again, its a stretch, but I spent a week hearing about this wet-dry cycle and tried my best to think how if at all it could happen on Titan. Even if we disregard this cycling, the question of melt pond dynamics in a porous environment is still interesting.  I need to talk to Britney and her group, but I think Jacob, who is working on ice-water interactions in Antarctica and Europa because I think this type of project is still similar.

Other courses of action are, investigating Impact craters in RADAR. I assume that would involve RADAR processing (roughness, CPR, etc.) then mapping and inferring and possible ground truth planning. One other project which we spoke about (but one I am not all that excited about) is mapping fractures to river orientations. That is something I still need to discuss with Oz to get feedback about the report I made.

I also remind you all that I will be gone from October 6th to 14th (I think) for the Meteor crater course. I will also be gone for a cruise during Halloween (very sad about that).