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Planetary Society Advent Calendar for December 7: Jupiter

Posted by Emily Lakdawalla

07-12-2009 15:11 CST

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Jupiter has been high overhead at sunset for several months, a brilliant light that's easy to spot even when the sky is still bright at dusk; but it's now moving quickly to the west as Earth speeds ahead of Jupiter's more stately march around the Sun. Although Jupiter's moons were discovered by Galileo, the banded and spotted appearance of the planet wasn't observed until Cassini pointed his more powerful telescope at the planet in the 1660s. Since then, it's been a fascinating target for professional and amateur observers; every night it appears different, since each of its dark belts and bright zones, and the bright and dark spots that twist on the boundaries between belts and zones, move with winds of slightly different speeds.

Voyager 2 view of Jupiter

NASA / JPL / color mosaic by Björn Jónsson

Voyager 2 view of Jupiter
Here is a Voyager 2 view of Jupiter that hasn't been seen before. The original data consists of eighteen images, nine each captured through orange and violet filters by Voyager 2 as it approached the greatest planet on June 30, 1979. The white oval below and to the left of the Red Spot is Oval DE, one of three white spots at that latitude that later merged to become Oval BA, also known as "Red Junior."

Jupiter mosaics are notoriously difficult to assemble because of the complicated motions of cloud features and the fast rotation of the planet. To produce this view, Jonsson reprojected each image in 3D space to align them, and had to do additional processing near the terminator to account for the way features rotated into daylight through the nearly half an hour that it took for Voyager 2 to acquire all 18 frames. To create a three-color composite from only two filters' worth of images, he calculated weighted averages of orange and violet images to create synthetic red, green, and blue channels. (Specifically: G=(0.71*O + 0.29*V) * 1.11285; color balance of O, G, V channels adjusted to make bright zones white; then R=1.1*O-0.1*V and B=0.25*G+0.75*V.)

The complex motions of the clouds, coupled with Jupiter's fast rotation (its "day" is 10 Earth hours long) make it beastly difficult to assemble multi-image mosaics or even single-frame color composites on the planet. The view above is both a mosaic and a two-channel color composite, and hasn't been posted on the Web before; Björn Jónsson polished it for me after I hassled him to send me some of his work. Unlike most of the other amateurs whose images I've posted, Bjorn doesn't usually set out to make "snapshot" style images of the planets; he processes many individual views of one body to create a global photographic data set that he can then use as a surface texture for 3D simulated views of the planets and moons in our solar system. Check out his web page for some truly stunning synthetic planetscapes. I should also mention that Bjorn is responsible for probably the most important piece of software I have on my computer after Photoshop: his "IMG2PNG" tool (sorry, Windows only) batch-converts image data downloaded from the Planetary Data System to the more familiar PNG format, which I can open and process in Photoshop or GIMP.

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