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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 17 July 1994 (around 19:00 UT), show the impact sites on the south hemisphere, from left to right, of comet fragments C, A and E about 12, 23, and 4 hours after each collision. Jupiter's satellite Io is seen crossing above the center of the disk.
In both colors the planet is seen in sunlight reflected by the atmosphere. The visible light penetrates to the top of the cloud decks, but the FUV light only reaches the stratosphere and higher altitude levels (100's of kilometers above the cloud tops). In the FUV, the signatures of trace amounts of ultraviolet absorbing gases and haze in the Jovian stratosphere and higher levels are observed. Around the poles the atmosphere appears dark due the presence of hazes, and in addition, ultraviolet auroral emissions are observed. These emissions are produced when energetic charged particles from the magnetosphere collide with molecular hydrogen in the upper atmosphere. Low-contrast banded structure is seen across the disk in the FUV. The salt-pepper appearance of Jupiter in the FUV is due to the darkness of the planet at this wavelength.
In the visible image, the impact sites appear as localized dark spots with diffuse halos. In the ultraviolet 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 presence of enhanced amounts UV absorbing molecules, scattering hazes and dust. This material should be combination of gases from Jupiter's lower atmosphere as well as comet volatiles and impact by-products upwelled 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: Hubble Space Telescope Jupiter Imaging Team
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