Ultraviolet spectroscopy of the SL9 impact sites: I. The 175-230 nm region

Roger V. Yelle, Melissa A. Mcgrath

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13 Scopus citations


We present a comprehensive analysis of spectra in the 175-230 nm wavelength region obtained by the Faint Object Spectrograph of the Hubble Space Telescope (HST) to determine the abundance of molecular species in the vicinity of the G and L impact sites. Data were obtained on July 18, roughly 3 hr after the G impact, on August 9, and on August 23. All spectra clearly show signatures of aerosols and gaseous CS2 and NH3. The spectra obtained on July 18 also show the spectral signature of H2S. To determine the abundance of gases and aerosols we compare the observations with calculations based on the scattering properties of three-layer models for the atmosphere. We are able to fit the aerosol-dominated portions of the spectra with aerosol distributions similar to those derived from HST imaging observations by West et al. (Science, 267, 1296-1301, 1995). On all three dates we find that CS2 resides at lower pressures than H2S, NH3, and the bulk of the aerosols. The CS2 column abundance is approximately 10-7 g-cm-2 on July 18, a factor of 2-3 less on August 9, and another factor of 2 less on August 23. NH3 is confined to pressures greater than 5 mbar with a mole fraction of 1 × 10-7 on July 18 and August 9, decreasing to 3 × 10-8 on August 23. H2S is also confined to pressures greater than 5 mbar with a mole fraction of 5 × 10-8 on July 18. These mole fractions depend upon assumptions about the aerosol distribution and are derived from models with an aerosol column density of 2 × 109 cm-2 Using different aerosol models, it is possible to obtain adequate fits to the spectra with mole fractions of H2S and NH3 that are 2.5 and 7.5 times smaller. The spectra show no evidence for SO2 absorption and we derive an upper limit of 10-7 g-cm-2 for the July 18 spectrum, assuming that SO2 has the same altitude distribution as CS2. Using the same assumptions we derive upper limits of 10-6 and 3 × 10-8 for OCS and SO. There is no compelling evidence for either H2O or C2H2 but both can be tolerated with mole fractions of 1 × 10-7 and 3 × 10-7, respectively. The altitude distributions of CS2, H2S, and NH3 suggest that CS2 was created by chemistry in the plume but that H2S and NH3 were injected into the stratosphere from below by upwelling over spatial scales of thousands of kilometers associated with the impact. The presence of H2S on July 18 suggests that the G fragment penetrated at least as deep as the NH4-SH clouds.

Original languageEnglish (US)
Pages (from-to)90-111
Number of pages22
Issue number1
StatePublished - Jan 1996

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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