This comparison of visible light (blue) and far-ultraviolet (FUV) images of Jupiter taken with the Wide Field Planetary Camera-2 (WFPC-2) on NASA's Hubble Space Telescope show how the appearance of the planet and of comet Shoemaker-Levy 9 impact sites differ at these two wavelengths (1400-2100 and 3100-3600 Angstroms). The images taken 20 minutes apart on July 17,1994 (around 19:00 UT), show the impact sites on the southern hemisphere, from left to right, of comet fragments C, A, and E, about 12, 23, and 4 hours after each collision. Jupiter's satellite 10 is seen crossing above the center of the disk, and the famous Great Red Spot is near the eastern limb.
While visible light reflects off the top of Jupiter's cloud decks, ultraviolet light doesn't penetrate any deeper than Jupiter's stratosphere and higher altitude levels (hundreds of kilometers above the cloud tops). The grainy appearance of Jupiter in the FUV is due to the darkness of the planet at this wavelength. Jupiter's aurora can be seen around the north and south poles where the atmosphere appears dark due the presence of hazes. These emissions are produced when energetic charged particles from Jupiter's magnetic field collide with molecular hydrogen in the upper atmosphere.
In the visible image, the impact sites appear as localized dark spots with diffuse halos. In the ultraviolet image the impact regions appear darker and more extended, because the FUV is more sensitive to smaller amounts of particles and/or that the horizontal winds in the upper atmospheric levels may be faster. The dark appearance is due to the presence of enhanced amounts of UV-absorbing molecules, scattering hazes and dust. This material should be a combination of gases from Jupiter's lower atmosphere as well as comet volatiles and impact by-products that were carried up from deeper in Jupiter's atmosphere and deposited into the stratosphere and thermosphere. Material should also have been deposited from ablation of the fragments and dust during entry.
Tracking the motions with WFPC-2 FUV images of the dark comet fragment "clouds" throughout the impact period should reveal for the first time the magnitude and direction of the high altitude winds on Jupiter. The Jovian auroral emissions will also be monitored with both WFPC-2 and the Faint Object Camera (FOC) to determine if the associated processes are affected by the comet's passage through the magnetosphere or changes in the upper atmosphere.
Credit: John Clarke, University of Michigan and NASA