Ly alpha-induced fluorescence of H-2 and CO in Hubble Space Telescope spectra of a comet Shoemaker-Levy 9 impact site on Jupiter

Far-ultraviolet spectra of Jupiter obtained on 1994 July 20 in the region of the Shoemaker-Levy 9 G-fragment impact site contain features that differ markedly from the typical Jovian dayglow spectrum. These features are identified as fluorescence of H-2 due to pumping by solar Ly alpha photons, which requires the presence of a substantial population of H, in the second excited vibrational state; normal atmospheric temperatures preclude such an elevated H-2 (v'' = 2) population in thermal equilibrium. The observed fluorescence includes approximately 600 rayleighs (R) of Ly alpha-induced fluorescence in addition to the 500 R of solar fluorescence estimated for typical Jovian dayglow at the time of the observations. The weaker (14, v'') transitions of the CO fourth positive band system are also excited primarily by resonant and fluorescent scattering of solar Ly alpha photons. As a result of their low-oscillator strengths and the large Ly alpha flux, these bands remain unsaturated up to column densities greater than or equal to 10(17) cm(-2), therefore providing reliable upper limits for the CO band emission produced by the (14, 3), (14, 4), and (14, 5) transitions at 1317, 1354, and 1392 Angstrom, respectively. Comparing the derived upper limits to estimates inferred from millimeter-wave data, we demonstrate that there is no contradiction between observations in the two spectral regions.