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Emily LakdawallaDecember 18, 2008

Jupiter and Ganymede from Hubble

I'm practically the last space blogger in the world to get to this (and it was only released this morning! How can I compete with that?), but I think it's worth giving my take on it anyway: a beautiful photo captured by the Hubble Space Telescope in natural colors, showing Jupiter and its largest moon Ganymede. Ganymede was just setting behind Jupiter's limb when the photo was taken; the photo was actually part of a series taken through many filters, watching as Ganymede passed behind Jupiter. Most people are showing a version of the image that has been rotated to make Jupiter's cloud bands horizontal, so, just to be different, I'm posting the image in its original orientation, which reveals much more of Jupiter.

Jupiter and Ganymede from Hubble

NASA, ESA, and E. Karkoschka (University of Arizona)

Jupiter and Ganymede from Hubble
On April 9, 2007, Hubble caught Ganymede in the act of setting behind Jupiter. Ganymede is the largest moon in the solar system, bigger than the planet Mercury.
One thing that immediately strikes me about this photo is that next to Ganymede, which looks sharp and crisp, Jupiter looks kind of fuzzy. There could be a couple of reasons for this -- I am not sure what the correct reason is. Firstly, Jupiter's features are actually clouds, while Ganymede is a cloudless world -- perhaps, when viewed adjacent to each other like this, Jupiter just looks fuzzier. The other possibility is that in the time between exposures, Jupiter rotated just a little bit, so the boundaries of the cloud features got fuzzed out. Also, since time did elapse between the capturing of the three different-filter images necessary to put this color view together, Ganymede should have been moving significantly from filter to filter; I'm guessing they had to do a little cut and pasting within the original files in order to make Ganymede not look smeared out.

The image is neat, but why was it taken? Time on the world's most expensive optical telescope isn't just given to anybody to take pretty pictures with. Thankfully, the Hubble website does an unusually good job of documenting their press release photos. There's always a "Fast Facts" page giving lots of technical details on the photo, and they usually include the proposal number submitted by the scientist when he/she was seeking to be granted the time on the telescope in the first place. In this case, the scientist was Erich Karkoschka of the University of Arizona, and the proposal number was 10468. Plugging those details into Google quickly gets me to the page at the Space Telescope Science Institute website with all the details on the proposal, and from there I can follow some links to find a detailed explanation, written by Karkoschka, of the justification for the sequence of photos.

I propose to observe a disappearance of Ganymede behind the dark limb of Jupiter with five filters of the ACS/HRC camera. Two exposures in each filter can be taken during such an event. The images will provide the spectral variation of the altitude of the apparent limb of Jupiter. The altitude of the apparent limb is dependent on the presence of hazes in Jupiter's stratosphere. Hazes of vertical optical depths below 0.001 could be detected with these observations, providing an extremely sensitive probe of high hazes. The observations probe altitudes levels near the 1-mb pressure level, for which we have very limited data.

The creation of aerosols, their growth, and their transport by winds is currently a mostly theoretical study. It would significantly benefit from constraints derived from the proposed observations. ACS/HRC is the only instrument capable of the required spatial resolution in the ultraviolet. Furthermore, a favorable geometry of Ganymede's orbit occurs only once every six years. This proposal achieves unique results with a minimum of HST time.

In other words, he's using the brightly reflective surface of icy Ganymede as a light source, shining from behind as it sets behind Jupiter, to illuminate hazes high in Jupiter's atmosphere, above those visible clouds. The fact that the images captured for this study were also very pretty was just a happy by-product. But I suspect that for most planetary astronomers, the fact that the pictures that they take are very pretty is no accident -- it's what attracted them to studying the sky in the first place!
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Emily Lakdawalla

Solar System Specialist for The Planetary Society
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