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By Emily Lakdawalla

The colors of Saturn's moons

Jul. 26, 2006 | 08:28 PDT | 15:28 UTC

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As more and more of Cassini's data is being released to the Planetary Data System, the amateur image processors out there have been able to do more sophisticated stuff with it. The image below is something I've been wanting to see for a long time, and I want to thank Gordan Ugarkovic for producing it.

Click to enlarge >

The colors of Saturn's moons
This montage of images of Saturn's moons (left to right: Prometheus, Pandora, Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, Hyperion and Titan) was produced by Gordan Ugarkovic from Cassini data in NASA's Planetary Data System (PDS). The moons are shown to scale with each other. The images have all been processed in the same way in order to show the actual differences in color and brightness among them. Credit: NASA / JPL / SSI / Gordan Ugarkovic

Unlike the raw images available immediately on the Internet, the data released a year or so later to the PDS includes information on the proper calibration of the images. Calibration allows images taken at different times and of different targets to be compared in this fashion. Other information about the images allows folks who are interested to query the whole image database to find images of specific targets, through specific filters, with specific lighting geometry, and so forth.

Brightness is a function of "phase angle" -- they will look brighter if the Sun is more "overhead" -- so Gordan attempted to choose images of the moons lit at phase angles of approximately 50 degrees. In the picture, Prometheus and Pandora are much more fully lit at 10 and 19 degrees, so they appear brighter than they should; Dione and Rhea are also a little on the fuller side at 34 and 39 degrees phase angle. There aren't very many image sets available with images in the red, green, and blue filters necessary to make approximate-true-color views; this is the most comparable set of pictures he could come up with.

What can we learn from this montage? Enceladus is clearly the brightest of the moons. Mimas, which is closer to the ring system, and has a much older surface, is rather darker.

I was a little surprised at how colorful the next three moons appear. I'd thought they were basically gray, but shown next to each other like this you can see some differences among them. Tethys and Rhea are similar in color and brightness, with a slightly reddish or orangish cast to their gray surfaces, while Dione is much grayer and is stained dark near the "wispy terrain" at the bottom of the image.

Iapetus' Cassini Regio -- its dark-stained leading hemisphere -- is practically invisible. It reflects only 4% of the light that strikes it. Hyperion is nearly as dark, and even Titan appears dim (though it makes up for dimness by its large size). I knew Iapetus was dark, but I didn't realize just how dark Titan's absorbing atmosphere made it.

These pictures make you realize how much work it is to figure out exposure settings for images of all the targets in Saturn's system. And also make me amazed yet again at how much detail Cassini can pick out in Iapetus' dark terrain from so far away, sometimes using only reflected light from Saturn to see it by. Incredible.

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