A Rhea flyby and a cloudy Titan with Tethys in color
Posted by Emily Lakdawalla
20-10-2010 14:02 CDT
Over the last few days, the Cassini Saturn orbiter's raw images website has been bursting with awesome pictures. In the past week, Cassini passed reasonably close to, and imaged, Titan, Mimas, Pallene, Dione, and Rhea. (Pallene is a tiny moon located between the orbits of Mimas and Enceladus). Here, just for fun, is an animation of five wide-angle photos of Rhea.I don't post wide-angle photos of moons very often because most of Cassini's images of the icy moons are taken from far enough away that the moon doesn't fill Cassini's narrow-angle camera field of view. The wide-angle camera has a field of view ten times wider than the narrow-angle camera; I use wide-angle views of Saturn a lot, and sometimes Titan, but usually the other moons are just a speck within its frame. But Cassini got close enough to Rhea, and Rhea is big enough (Saturn's second largest moon at 1,538 kilometers across) that it filled the wide-angle field of view nicely as Cassini swept slowly past.
To make the animation, I selected five of the wide-angle camera images (1, 2, 3, 4, 5), adjusted their levels to make space black, painted over stray bright marks in what should be black space from cosmic ray hits on the detector, sharpened four of the five frames using the Photoshop Unsharp Mask tool (the middle frame appeared sharper than the other four for some reason), lined them up to give the "feel" of flying past them, and then flipped the whole thing 180 degrees to put north up. (EDIT: Gordan Ugarkovic pointed out in the comments that the middle image was sharper because it was taken through the green, not clear, filter, and the Cassini's focus is optimized at that wavelength.)
There's an even cooler set of images that appeared on the website earlier today. They show that Cassini doesn't need to be close to the moons to take spectacular photos. We're seeing Tethys -- and its iconic Ithaca Chasma snaking across its surface -- poised in front of Titan, with the entire ring system foreshortened almost into nonexistence in the background.The perspective is a bit of an optical illusion. Even though Tethys and Titan look very close to each other, they're not. Tethys orbits closer to Saturn than Titan does. So for Tethys to appear in front of Saturn to Cassini, Titan needs to be on the opposite side of Saturn from Cassini in its orbit. I fiddled with the Solar System Simulator a bit to figure out the time of this observation, and you can see that at 15:00 on October 18 Titan was about 2.5 million kilometers away from Cassini (confirming the numbers quoted on the raw images website) while Tethys was 40% closer at 1.5 million kilometers away. Being 40% closer also exaggerates Tethys' size relative to Titan, making it appear almost half again larger than its true relative size. Titan is a big moon!
This view was composed by Gordan Ugarkovic from the raw JPEG images. The two moons are in the relative position they occupied in this raw image, which was taken through a blue filter. Titan is darker in blue wavelengths than it is in red wavelengths, and on top of that there is a very bright icy moon in the same field of view, which pushes Titan's levels very dark. So to compose the color information for Titan, Gordan employed red-, green-, and blue-filter images taken before Tethys wandered into the field of view. This bit of extra work is only necessary for the versions of the images that are posted to Cassini's raw images website. The archived science data is 12-bit, with 4096 different gray levels, so there is plenty of room to see Titan's muted colors and Tethys' bright surface in the same image. But when these 12-bit images are converted to JPEG format for posting on the Web, their contrast is automatically stretched and they are squished down to 8 bits (only 256 gray levels), which tends to destroy the ability to see any interesting details in Titan's smoggy atmosphere. (Would a joke about this process probably destroying all evidence of the mathematically perfect alien pyramids on Tethys be in bad taste here? I know, I know, "don't feed the trolls," but it's hard to resist. If you don't understand why I'm saying this at all, read, for example, this.)
The squishing of the data down to 8 bits is a bit of a shame because another image from the same sequence reveals that there is something very exciting happening right now! (Although I call this a "shame," I recognize that the destruction of detail that is performed on the images before they get posted to the raw images website is a necessary compromise which allows the Cassini images to get out to the public in near real time while still permitting the science team to have a proprietary period to look at the actual high-quality data, and I'm glad that the compromise allows me to see any version of the data, no matter how squished, so quickly.)
Anyway, getting back to the Titan images: at the same time that those other images were taken, Cassini also took one with a special filter, one that only permits certain infrared wavelengths (near 938 nanometers) to pass through to its detector. At this particular wavelength, methane gas -- an important component of Titan's atmosphere that is opaque in visible wavelengths of light -- is unusually transparent. When viewed through this filter, here's what Titan looked like.What's so exciting about this photo? Here's an enhanced version, created by dividing the image above by the blue-filter photo. This cancels out some of the blurring due to the atmosphere, making certain features clearer, namely an enormous band of clouds crossing the equatorial regions. There's also some clouds in the mid-southern latitudes, and some at the north pole. I wonder if that north polar cloud is the lakes up there evaporating as the springtime Sun hits them? The stuff evaporating from the northern lakes should, in one theory, wind up precipitating in the relatively dry south. But I am not sure if I am totally up-to-date on my Titan climate theory....
For a very different take on this image sequence, visit Lights in the Dark.
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