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Galaxy Evolution Explorer (GALEX) observations of comet 9P/Tempel 1 using the near-ultraviolet (NUV) objective grism were made before, during and after the Deep Impact event that occurred on 2005 July 4 at 05:52:03 UT when a 370 kg NASA spacecraft was maneuvered into the path of the comet. The NUV channel provides usable spectral information in a bandpass covering 2000-3400 A with a point source spectral resolving power of R [?] 100. The primary spectral features in this range include solar continuum scattered from cometary dust and emissions from OH and CS molecular bands centered near 3085 and 2575 A, respectively. In particular, we report the only cometary CS emission detected during this event. The observations allow the evolution of these spectral features to be tracked over the period of the encounter. In general, the NUV emissions observed from Tempel 1 are much fainter than those that have been observed by GALEX from other comets. However, it is possible to derive production rates for the parent molecules of the species detected by GALEX in Tempel 1 and to determine the number of these molecules liberated by the impact. The derived quiescent production rates are Q(H2O) = 6.4 x 1027 molecules s-1 and Q(CS2) = 6.7 x 1024 molecules s-1, while the impact produced an additional 1.6 x 1032 H2O molecules and 1.3 x 1029 CS2 molecules, a similar ratio as in quiescent outgassing.

The rich A1P - X1S+ system of carbon monoxide accounts for most spectral features seen in long-slit HST STIS observations of comets 153P/Ikeya-Zhang, C/2001 Q4 (NEAT), and C/2000 WM1 (LINEAR), as well as in the HST GHRS spectrum of comet C/1996 B2 (Hyakutake). A detailed CO fluorescence model is developed to derive the CO abundances in these comets by simultaneously fitting all of the observed A - X bands. The model includes the latest values for the oscillator strengths and state parameters, and accounts for optical depth effects due to line overlap and self-absorption. The derived CO abundances relative to water in these comets span a wide range, from 0.44% +- 0.03% for C/2000 WM1 (LINEAR), 7.2% +- 0.4% for 153P/Ikeya-Zhang, and 8.8% +- 0.8% for C/2001 Q4 (NEAT) to 20.9% +- 0.3% for C/1996 B2 (Hyakutake). The radial profiles of the CO column density are consistent with a predominantly native source for all the comets observed by STIS. A complete fitting of the CO spectral features using the fluorescence model leads to the first identification of a molecular hydrogen line pumped by solar H I Lyb longward of 1200 A in the spectrum of comet 153P/Ikeya-Zhang. CO fluorescence due to pumping by solar H I Lya and O I l1302 is detected the spectrum of comet C/1996 B2 (Hyakutake).

We present a determination of the molecular gas mass in the AU Microscopii circumstellar disk. Direct detection of a gas component to the AU Mic disk has proven elusive, with upper limits derived from ultraviolet absorption line and submillimeter CO emission studies. Fluorescent emission lines of H2, pumped by the O VI l1032 resonance line through the C-X (1-1) Q(3) l1031.87 A transition, are detected by the Far Ultraviolet Spectroscopic Explorer. These lines are used to derive the H2 column density associated with the AU Mic system. The derived column density is in the range N(H2) = 1.9 x 1017 to 2.8 x 1015 cm-2, roughly 2 orders of magnitude lower than the upper limit inferred from absorption line studies. This range of column densities reflects the range of H2 excitation temperature consistent with the observations, T(H2) = 800-2000 K, derived from the presence of emission lines excited by O VI in the absence of those excited by Lya. Within the observational uncertainties, the data are consistent with the H2 gas residing in the disk. The inferred N(H2) range corresponds to H2-to-dust ratios of [?]img1.gif : 1 and a total M(H2) = 4.0 x 10-4 to 5.8 x 10-6 M[?]. We use these results to predict the intensity of the associated rovibrational emission lines of H2 at infrared wavelengths covered by ground-based instruments, HST NICMOS, and the Spitzer IRS.

We report the detection of atomic chlorine emissions in the atmosphere of Io using Hubble Space Telescope observations with the Goddard High Resolution Spectrograph (GHRS). The Cl I l1349 dipole allowed and Cl I] l1386 forbidden transition multiplets are detected at a signal-to-noise ratio of 6 and 10, respectively, in a combined GHRS spectrum acquired from 1994 through 1996. Oxygen and sulfur emissions are simultaneously detected with the chlorine, which allows for self-consistent abundance ratios of chlorine to these other atmospheric species. The disk-averaged ratios are Cl/O = 0.017 +- 0.008, Cl/S = 0.10 +- 0.05, and S/O = 0.18 +- 0.08. We also derive a geometric albedo of 1.0% +- 0.4% for Io at 1335 A, assuming an SO2 atmospheric column density of 1 x 1016 cm-2.

Simultaneous ultraviolet spectra and images of the north polar aurora of Jupiter were obtained on 1995 March 9 with the Hopkins Ultraviolet Telescope (HUT) and the HubbleSpaceTelescope(HST). This unique data set contains a well-calibrated flux measurement that includes all of the H2Lyman and Werner band emission from 900 to 1650 Å, as well as information about the spatial extent of the aurora. For these observations, we oriented the 10'' × 57'' slit of the HUT spectrograph parallel to the equator of Jupiter so that it was underfilled by the aurora and minimally contaminated by extraneous reflected solar light from low to mid latitudes. The aurora was imaged simultaneously using the HSTWide Field and Planetary Camera 2 (WFPC2). Images were taken through a Wood's filter and also through a Wood's filter/CaF2filter combination. These images have recorded the aurora in the light of atomic and molecular hydrogen with relatively low reflected solar contamination. The molecular hydrogen aurora is found to extend over 4.5 ± 0.5 × 10-10sr, with an average brightness of 106 ± 12 kR integrated over the band of wavelengths from 900 to 1650 Å, excluding H I?1216. We measure a 13% larger emitting area (5.1 ± 0.5 × 10-10sr) and significantly different morphology for the combined atomic and molecular hydrogen aurora (Ly? + H2), although these differences could be a result of time variability. Color ratios derived from the data indicate a total auroral energy deposition rate of 13 ergs s-1cm-2by 3-30 keV electrons. We find little evidence for ions as primary auroral particles in the HUT spectra. Our analysis places upper limits on sulfur ion emissions that are an order of magnitude lower than those from the IUEobservations, and, therefore, the emissions could be entirely from the Io torus in the background of the planet.

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 H2due to pumping by solar Ly? photons, which requires the presence of a substantial population of H2in the second excited vibrational state; normal atmospheric temperatures preclude such an elevated H2(v'' = 2) population in thermal equilibrium. The observed fluorescence includes approximately 600 rayleighs (R) of Ly?-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? photons. As a result of their low-oscillator strengths and the large Ly? flux, these bands remain unsaturated up to column densities ?1017cm-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 Å, 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.

Partial frequency redistribution (PRD), describing the formation of the line profile, has negligible observational effects for optical depths smaller than [?]103, at the resolving power of most current instruments. However, when the spectral resolution is sufficiently high, PRD modeling becomes essential in interpreting the line shapes and determining the total line fluxes. We demonstrate the effects of PRD on the H2 line profiles observed at high spectral resolution by the Far-Ultraviolet Spectroscopic Explorer (FUSE) in the atmospheres of Jupiter and Saturn. In these spectra, the asymmetric shapes of the lines in the Lyman (v''-6) progression pumped by the solar Lyb are explained by coherent scattering of the photons in the line wings. We introduce a simple computational approximation to mitigate the numerical difficulties of radiative transfer with PRD and show that it reproduces the exact radiative transfer solution to better than 10%. The lines predicted by our radiative transfer model with PRD, including the H2 density and temperature distribution as a function of height in the atmosphere, are in agreement with the line profiles observed by FUSE. We discuss the observational consequences of PRD and show that this computational method also allows us to include PRD in modeling the continuum pumped H2 fluorescence treating about 4000 lines simultaneously.

We present a measurement of the lifetime of ground state atomic carbon, C(3P), against ionization processes in interplanetary space and compare it to the lifetime expected from the dominant physical processes likely to occur in this medium. Our measurement is based on analysis of a far ultraviolet (FUV) image of comet C/2004 Q2 (Machholz) recorded by the Galaxy Evolution Explorer (GALEX) on 2005 March 1. The bright C I 1561 A and 1657 A multiplets dominate the GALEX FUV band. We used the image to create high signal-to-noise ratio radial profiles that extended beyond 1 x 106 km from the comet nucleus. Our measurements yielded a total carbon lifetime of (7.1-9.6) x 105 s (ionization rate of (1.0-1.4) x 10-6 s-1) when scaled to 1 AU. This compares favorably to calculations assuming solar photoionization, solar wind proton change exchange, and solar wind electron impact ionization are the dominant processes occurring in this medium and that comet Machholz was embedded in the slow solar wind. The shape of the C I profiles inside 3 x 105 km suggests that either the CO lifetime is shorter than previously thought and/or a shorter-lived carbon-bearing parent molecule, such as CH4, is providing the majority of the carbon in this region of the coma of comet Machholz.