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Emily LakdawallaDecember 17, 2007

New views from Galileo

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:

Callisto in color

NASA / JPL / Ted Stryk

Callisto in color
Galileo captured this global view of Callisto on its 30th orbit of Jupiter, on May 26, 2001. The filters used for this image cover a broader range of the spectrum than human eyes can see. Galileo ranged from 740,000 to 744,000 kilometers from Callisto when it took this image, at a phase angle of just 3°. Image scale is 7.5 kilometers per pixel.
For comparison, here's the standard version that you'll see all over the Internet, which is much more garishly colored:
Jupiter's Moon Callisto


Jupiter's Moon Callisto
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."
Callisto in color

NASA / JPL / Ted Stryk

Callisto in color
Galileo captured this global view of Callisto on its 11th orbit of Jupiter, on November 5, 1997. The filters used for this image cover a broader range of the spectrum than human eyes can see. Galileo was 687,000 kilometers from Callisto when it took this image, at a phase angle of just 1°. Image scale is 6.9 kilometers per pixel.
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.
Data from Galileo's E11 global observation of Callisto


Data from Galileo's E11 global observation of Callisto
These six images represent the data set obtained by Galileo during its November 5, 1997 flyby of Callisto. The images were reduced in size by 50% (by binning 4 pixels into 1) before being returned to Earth, and none of them covers the entire disk of Callisto. Ted Stryk managed to produce this global view through careful reassembly of the images and filling-in of gaps using data from other flybys.
I just noticed that the distant New Horizons view of Callisto covers similar territory to this one.


New Horizons captured this image of Callisto as it zipped by the Jupiter system on February 27, 2007. With a diameter of about 4,800 kilometers, Callisto is the second largest moon of Jupiter and the 12th largest object in the solar system.
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.
Amalthea in color

NASA / JPL / Ted Stryk

Amalthea in color
Galileo took this three-color image of Jupiter's little moon Amalthea on November 5, 1996, during its third orbit of Jupiter. Galileo was 876,000 kilometers away, viewing Amalthea at a phase angle of 31°. Amalthea is highly elongated, 270 by 166 by 150 kilometers in size. It is unusually red in color; the red probably results from sulfur compounds that came from Io.
Ted sent me much more images of Europa, Io, and Ganymede, which I'll post later.

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Emily Lakdawalla

Solar System Specialist for The Planetary Society
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