Playing around with some New Horizons data

The data from New Horizons' encounter with Jupiter last year has just been released to NASA's Planetary Data System, and I have been fiddling around with it all day while slowing down the network at my office by downloading the enormous files. I don't have very much to show yet for all my fiddling, but thought I'd better post something today, so here is a montage of most of the good images of Jupiter's innermost, volcanically active moon Io that were taken by the MVIC instrument. Each image is shown twice; on the left, you can see the sunlit crescent of Io (and sometimes some sunlit volcanic plumes, particularly Tvashtar, erupting off of Io's north pole, to the right in these images); the right-hand version of each image has been stretched to show details on Io's nightside. The nightside doesn't always show up too well -- more on that below.

One nifty thing in these pictures: for the stretched versions on the right, I stretched all the images by the same amount, brightening the sunlit side by the same amount. However, the night side of Io appears to be bright in some of the images, and dark in others. What gives? It has to do with something happening off-screen in these images. Io's night side, like Earth's night side, sometimes receives reflected light from another source. Just like the Moon can light up our nights, Io's nights can be lit up by Jupiter. But in order for Io's night side to receive light from Jupiter, Io's night side has to see Jupiter in its sky, and furthermore Jupiter has to be at least partially lit (that is, it can't be a "new Jupiter" night for Io). The amount of illumination that New Horizons saw on Jupiter's night side in each image depended upon the relative position of New Horizons, Io, and Jupiter.

Another nifty thing to notice is how, as New Horizons recedes from the Jupiter system and Io is seen at a higher and higher phase (a skinnier and skinnier crescent), the Tvashtar plume gets brighter and brighter. It does that for the same reason that Saturn's most tenuous rings, like the F ring, get brighter when Cassini is on the north (shadowed) side of the rings -- because the tiny particles in the rings or in Tvashtar's plume are scattering sunlight forward to the camera, like dust motes in a sunbeam.