Posted by Emily Lakdawalla
06-11-2008 15:44 CST
Nearly a week after yet another super-close flyby of Enceladus by Cassini, I am finally taking the time to try and work through the pictures and make sense of them. Unfortunately, I am not sure I can say anything much more intelligent about these images than, "wow, these are cool."
To set the stage: the Cassini Saturn orbiter flew by Saturn's active moon Enceladus again on October 31, 2008, passing at a closest approach distance of 197 kilometers at 17:14:51 UTC. Cassini's path was fairly similar to the one it took for the previous two flybys, coming in from over the north pole, approaching closest in the high southern latitudes, and getting excellent views of the south pole as it departed. This flyby was a bit higher-altitude than the two previous ones, though, so the spacecraft passed through a less-dense part of the south polar plume. That's not so good for the fields and particles instruments, but it's great for imaging, so Cassini's activities during this flyby were controlled by the various optical remote sensing instruments, the cameras and spectrometers.
The imaging team, by which I really mean Paul Helfenstein, planned a second amazing "skeet shoot" imaging sequence for this encounter, similar to the one he produced for the August 11 flyby. Just after closest approach, they wanted to use Cassini to take very high resolution photos of the south polar vents, but the relative speed between Cassini and Enceladus was so high that their usual "point and shoot" style of targeting images wouldn't work. Instead, they planned a series of spacecraft motions that spun the spacecraft at its top angular speed to partially compensate for the fast motion of Enceladus through Cassini's camera field of view, snapping the shutter at specifically timed moments to capture images of interesting spots near the south pole. It worked as well this time as it did the first time. Many, many kudos to Paul.
Before I show you the photos, here is a map of the south pole of Enceladus for some context. The green squares are the locations of images taken in the first skeet shoot back in August. The yellow squares are the locations of images taken during the October 31 skeet shoot. And the two squashed blue shapes are the locations of single wide-angle and narrow-angle images taken during a much earlier close flyby, on July 14, 2005. Those appear squashed because Cassini wasn't looking directly downward as it snapped the shutter; it was gazing off to one side, so it got an oblique view of the landscape.The first thing I noticed when I looked at this map is that there is one area where there was considerable overlap, with one set of images taken in August and one in October. The overlap happens over two active plumes, the plume sources labeled "II" and "III." We know Enceladus is a dynamic place, and we know those plumes are erupting every time Cassini checks, so it's natural to ask: was there any visible change to the surface between August and October? You can check for yourself with the two images below. I looked myself, and I didn't see any obvious differences that I couldn't attribute to a slight difference in lighting angle. But then I didn't study the images for very long. Here's the August set (two overlapping images mosaicked together): And here's the October image: The best stuff from the October 31 flyby came in the form of four closely clustered high-resolution shots of an area that was not seen at high resolution during the August 11 flyby. The four images don't completely overlap, so the imaging team pieced them together atop a lower-resolution background to make this composite: There are a lot of things that capture my attention in this image, though I cannot claim to be able to explain any of them. First, the shape of Baghdad sulcus, the "tiger stripe" that crosses the image from the left to right, isn't simply a trough with symmetrical ridges on each side. Some of the sulci are that simple in some places, but it most definitely isn't here. I do see two parallel ridges bounding Baghdad, but in between them is a complex and weird pile of ridgy stuff -- I suppose this must be what Paul Helfenstein was calling "shark fins" in his talk at the Division of Planetary Sciences meeting. That also happens to be the area from which one of the plumes is supposed to emanate. Hmmm.
Zooming in on the upper left of that mosaic, to the highest-resolution part of it, I'm struck by how busted-up the surface is. What looks like a neat if slightly rumpled set of nearly parallel ridges at lower resolution is breaking up into an intensely fractured and rubbly area -- I imagine that an explorer on foot would have a really tough time scrambling over the ground here, though of course they'd be helped a great deal by the very low gravity. All those parallel, horizontal ridges are chopped up, vertically, in a million places into skinny triangular wedges; on top of all that, it looks like there are huge boulders just lying on the ground everywhere, on top of ridges and in between them. Where did those come from? How can there be forces acting simultaneously in so many different directions to munge up the terrain so completely? Here's that image at its original resolution:Finally, it's fun to look back at the context map I posted at the top of this entry and realize that the first high-resolution image Cassini got of the south polar terrain, back in July of 2005, actually plots very close on the map to all these skeet-shoot images. Here's both the wide-angle camera view and the narrow-angle camera view, which were taken simultaneously. Cassini's wide-angle and narrow-angle cameras have the same kind of detectors -- both take images 1024 pixels square -- and they have the same boresight, so the centers of two images taken at the same time are coincident. But their optics are different, so that the wide-angle camera has a field of view precisely 10 times broader than the narrow-angle camera. It sees 10 times as much width, but at a resolution 10 times poorer than the narrow-angle camera. Paul Helfenstein had not yet developed the "skeet shoot" technique at the time of the July 2005 flyby. Instead of taking many high resolution images, Cassini just stared in one direction and quickly snapped a bunch of simultaneous wide- and narrow-angle camera shots, with this pair being the only one that actually scored a "hit" on the moon. Even though it was a hit, there was a lot of motion blur. This just goes to show how much better a mission can get over time, as its scientists and engineers get more comfortable, and more creative, with how they operate a spacecraft. It's why extended missions can be even more productive than primary missions, even though they're working with an aging spacecraft.
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