Other
Scientific paper
Jan 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996icar..119...90y&link_type=abstract
Icarus, Volume 119, Issue 1, pp. 90-111.
Other
18
Scientific paper
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 CS_2 and NH_3. The spectra obtained on July 18 also show the spectral signature of H_2S. 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 CS_2 resides at lower pressures than H_2S, NH_3, and the bulk of the aerosols. The CS_2 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. NH_3 is confined to pressures greater than 5 mbar with a mole fraction of 1 x 10^-7 on July 18 and August 9, decreasing to 3 x 10^-8 on August 23. H_2S is also confined to pressures greater than 5 mbar with a mole fraction of 5 x 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 x 10^9 cm^-2. Using different aerosol models, it is possible to obtain adequate fits to the spectra with mole fractions of H_2S and NH_3 that are 2.5 and 7.5 times smaller. The spectra show no evidence for SO_2 absorption and we derive an upper limit of 10^-7 g-cm^-2 for the July 18 spectrum, assuming that SO_2 has the same altitude distribution as CS_2. Using the same assumptions we derive upper limits of 10^-6 and 3 x 10^-8 for OCS and SO. There is no compelling evidence for either H_2O or C_2H_2 but both can be tolerated with mole fractions of 1 x 10^-7 and 3 x 10^-7, respectively. The altitude distributions of CS_2, H_2S, and NH_3 suggest that CS_2 was created by chemistry in the plume but that H_2S and NH_3 were injected into the stratosphere from below by upwelling over spatial scales of thousands of kilometers associated with the impact. The presence of H_2S on July 18 suggests that the G fragment penetrated at least as deep as the NH_4-SH clouds.
McGrath Melissa A.
Yelle Roger V.
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