New views from Galileo
Posted by Emily Lakdawalla
17-12-2007 13:52 CST
Amateur image magician Ted Stryk has just provided me with a glorious set of global views of Jupiter's satellites, which he has produced from the Galileo data set. Galileo gave us our best views of these worlds but the data set has a variety of problems, most of which are rooted in the failure of the high-gain antenna. There are lots of data dropouts, for one thing -- places where there should be image data, but where there are only black pixels. Also, the Galileo imaging team was forced to compress their images onboard the spacecraft before transmitting them to Earth much more than they would have liked; compression introduces ugly artifacts in images, resulting in loss of detail. And they had to be very choosy about which filters they used for imaging -- so it is very rare to have the data necessary to make a view that looks like what human eyes would see. Ted has put a lot of effort into this data set, developing tricks to work around some of these problems, and produced images that look like they came from a different spacecraft. I'm slowly adding them to our image database, so I'll post a few here at a time.
Before I post these images I want to mention that in my enthusiasm for Ted's work I wish in no way to impugn the work done by the original Galileo imaging team. Ted's work is as much art as science, a labor of love that the actual science teams not only don't have time to do but also wouldn't want to do, because to make the images look this good Ted had to fill in gaps here and there, reducing the value of these images for science purposes but adding to their illustrative power.Callisto is an often-neglected satellite of Jupiter, usually regarded as the least interesting of the Galilean satellites because its surface is so very ancient. Here are two very pretty, subtly colored views that Ted managed to produce from the Galileo data. The first is the canonical view of Callisto:For comparison, here's the standard version that you'll see all over the Internet, which is much more garishly colored: And here's Ted's other Callisto view, one that I hadn't seen before. This one is centered on the equator at 56 degrees longitude; north of the equator is the strange, 3,000-kilometer-diameter multiringed structure named "Valhalla." The two big white splotches on the lower right of the limb are named "Lofn" and "Heimdall." Let me just show you what Ted had to work with in order to make this global view. There were six images returned by Galileo, two through each of three filters, all of them only covering part of the disk, all of them cut in resolution by a factor of two before being returned to Earth. Ted overlaid them all and used "superresolution" image processing techniques (pioneered by a Mars geologist named Tim Parker to get more detail out of Pathfinder's images of distant rocks) to reproduce the missing detail, and he borrowed data from more complete images to fill in gaps in less complete images, shifting the colors to match Callisto's brownish spectrum. When he was done with this there was still about 10% of the right side of the disk that was missing, which he had to reconstruct using data from other flybys. Like I said, it's as much art as science, but it's pretty. I just noticed that the distant New Horizons view of Callisto covers similar territory to this one. That's it for Galileo's global views of Callisto. Here's one other little gem, one of only two color views that Galileo ever got on tiny little Amalthea, one of the inner moons of Jupiter. It's not much but it's the best we have, and it reveals Amalthea's strange red hue. Ted sent me much more images of Europa, Io, and Ganymede, which I'll post later.
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