Emily Lakdawalla • Mar 12, 2018
Moon Monday: Tethys from Voyager
Here's a little moon for your Monday. I recently wrote an article for Sky and Telescope about NASA's Planetary Data System (on newsstands now!) and it inspired me to dig into the data to find some old gems. Here's one of Saturn's moon Tethys, taken by Voyager 2 on August 26, 1981.
There were a lot of targets for the Voyagers to point at in the Saturn system -- planet, amazing rings, and seven large moons -- but even so, it always astonishes me how few good pictures there were to fire my imagination about outer planet moons in the 1980s. The image above is really the only reasonably high-resolution one of Tethys, and it's barely 400 pixels across. There were a few other, smaller ones, but that's it. (There's a tragic reason Tethys is poorly imaged: the scan platform for Voyager 2's cameras borked only 100 minutes after closest approach, as it was just beginning to shoot a high-resolution mosaic of Tethys. They did fix the problem, but it was too late for the back half of the close-encounter Saturn observations they'd planned, and you only get one chance at a flyby.)
It's just a monochrome photo, a picture of a moon with some craters and a funky wrinkle thing on its upper left side. The wrinkle is grandly named Ithaca Chasma, ostensibly for Odysseus' home island, but I strongly suspect that certain Cornell University-based imaging team members were thinking of a different Ithaca. When Cassini arrived at Saturn and I worked on learning to recognize all the moons including Tethys, I imagined that the craters at lower right marked out the letter "T" for Tethys.
Tethys is the archetypal medium-sized moon. It's almost exactly 1000 kilometers in diameter (1062 at the equator by 1055 pole-to-pole, to be exact). Other worlds in this size class are Dione, Ariel, Umbriel, Ceres, and Charon. Tethys has craters but also some interesting tectonic features. Worlds of this size mostly seem like they haven't changed a lot during most of the age of the solar system, but they had one exciting episode of tectonism in their youths. I wonder what similar-size worlds in the Kuiper belt look like? 1000-kilometer-class objects include 2007 OR10, Orcus, and Quaoar.
Here are a few image processing notes on this photo, C4400357.
Moon Monday: Tethys from Voyager https://t.co/dXyGxWzJrG— Emily Lakdawalla (@elakdawalla) March 12, 2018
I decided my regular blogging schedule wasn't affording me enough time to play with data, so I'm going to try out posting about a moon once a week with a newly processed image, and sometimes explain my process. 1/n pic.twitter.com/BqltKuypjM
Original data came from @pdsopus, the Rings Node of the Planetary Data System: https://t.co/FBFBh29r7g— Emily Lakdawalla (@elakdawalla) March 12, 2018
Once I figured out which image I wanted, I used this process to open the raw data in Photoshop: https://t.co/kFIYg0YgsT 2/n
Here's what the image looks like when I open it. This is a raw EDR (Experiment Data Record), pretty much as it was received from the spacecraft. 3/n pic.twitter.com/VOuEbyXEe3— Emily Lakdawalla (@elakdawalla) March 12, 2018
First two steps are simple, crop out the garbage at top and left (it's really not garbage, it's data relevant to calibration and some metadata, but I don't need it for this) and then adjust levels to make space (mostly) black and the moon a little brighter 4/n pic.twitter.com/sd8FXGMrMy— Emily Lakdawalla (@elakdawalla) March 12, 2018
Next comes the subjective/art part. Voyager cameras had black dots painted on them that helped in removing geometric distortion, but they also interrupt prettiness of image. Using some Photoshop healing brush and some more careful painting, I remove them. 5/n pic.twitter.com/hGhMp3f1ET— Emily Lakdawalla (@elakdawalla) March 12, 2018
Another crop to center it and a bit of a fiddle with levels to make the subtle topography a little more clear, and I have my final image. It's an illustration product, not a science product, because of the inpainting that I did. 6/6 pic.twitter.com/dMtnhceOEe— Emily Lakdawalla (@elakdawalla) March 12, 2018
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