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Post-SL9 Sulfur Photochemistry on Jupiter

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Post-SL9 Sulfur Photochemistry on Jupiter NASA-CR-203023 GEOPHYSICAL RESEARCH LETTERS, VOL. 22, NO. 12, PAGES 1597-1600, JUNE 15, 1995 Post-SL9 sulfur photochemistry on Jupiter Julianne I. Moses Lunar and Planetary Institute, Houston, Texas Mark Allen Jet Propulsion Laboratory, Pasadena, California G. Randall Gladstone Southwest Research Institute, San Antonio, Texas Abstract. We have modeled the photochemical evolu- identified; notably absent are signatures for SO2 and tion of the sulfur-containing species that were observed SO. Although SO2 has been detected in mid-infrared in Jupiter's stratosphere after the SL9 impacts. We find spectra [A. Sprague, personal communication, 1994], its that most of the sulfur is converted to $8 in the first few inferred abundance is several orders of magnitude be- days. Other important sulfur reservoirs are CS, whose low that of the other observed sulfllr species. These abundance increases markedly with time, and possibly observations suggest, that oxygen compounds may not H2CS, HNCS, and NS, whose abundances depend on have played a large role in the impact chemistry [cf. kinetic reaction rates that are unknown at the present. Zahnle et al. 1995]. On the other hand, several Earth- We discuss the temporal variation of the major sulfur based observations imply that oxygen species such as compounds, make abundance and compositional pre- H20 and CO were abundant at the same G impact site dictions useful for comparisons with observations, and [Lellouch ct al., 1995; Bjoraker et al., 1994; Meadows discuss the possible condensation of sulfur-containing et al., 1994]. Because the reason for the discrepancy species. between the ttST and other observations is not known, we concentrate on the dark core regions observed by Introduction the FOS and assume that the oxygen compounds re- main within the HST upper limits. Observational reports indicate that many new molec- The complex nature of the impacts makes it impossi- ular species were deposited in Jupiter's stratosphere ble for 1-D photochemical models to reproduce all the following the Comet. P/Shoemaker-Levy 9 (SL9) im- phenomena that affect, the impact debris; however, such pacts. These new species derive from highly processed models are nseful for several reasons. First of all, the cometary material and tropospheric Jovian air that models can be used to predict the t.emporal variation were modified during the impact explosion, fireball, and of the initially observed species; the degree to which re-entry shock ("splashdown") of the plmne back into the predictions match the observations tells us about the upper atmosphere. To examine the photochemi- the relative effectiveness of dynamical vs. photochenl- cal evolution of the species that were observed or pre- ical processes in the post-SL9 Jovian stratosphere and dicted to have formed following the plume splashdown can greatly enhance our understanding of complex pho- (--,10-15 minutes after the actual impact), we use a one- tochemical mechanisms that are unstudied in labora- dimensional (l-D) chemical kinetics and diffusion model tories on Earth. Secondly, the models can be used to similar to the one described in Yung et al. [1984]. Our direct, the search for undetected molecular species and results regarding the sulfur-containing species are pre- to identify the short- and long-term reservoirs of various sented at. this time -- results regarding nitrogen and elements. Finally, such models can aid in the analysis oxygen species are presented ill a companion paper of the haze observations by predicting the composition, [Moses et al., 1995]. location, formation rate, and lifetime of possible con- The Hubble Space Telescope (HST) Faint Object. densates in Jupiter's stratosphere. Spectrograph (FOS) obtained ultraviolet spectra within the central dark core of the G (and L) site over a period Photochemical Model of several days and weeks following the impacts [Noll et al., 1995; Yelle and McGrath, 1995]. These high- quality spectra indicate the presence of several sulfur The hydrocarbon reaction list, absorption cross sec- compounds at the site: $2, CS9, CS, and H2S are all tions, quantum yields, diffusion coefficients, boundary conditions, and background atmospheric structure of Gladstone et al. [1995] form the basis for our photo- chemical model, but we update the hydrocarbon pho- Copyright 1995 by the American Geophysical Union. tochemistry and include oxygen, nitrogen, and sulfur re- actions. Our photochemical model contains 144 species Paper number 95GL01200 and over 900 reactions. We have tried to identify and 0094-8534/95/95GL-O 1200503.00 include the important reaction schemes that are most 1597 1598 MOSESET AL.: SULFUR PHOTOCHEMISTRY F()I,I,OWING SL9 likely to affect the abundances of the "parent" molecules On the other hand, Ss formation is only rampant in observed after the SL9 impacts. However, the rates for the middle and lower stratosphere at pressures greater several important reactions are not available in tile lit- than ,-_0.1 mbar. At higher altitudes, $2 is effectively erature, and we often had to make estimates. There- photolyzed before larger sulfur molecules can form. The fore, this work represents a preliminary, qualitative at- $2 (B-X) band system lies at 240 360 nm and is clearly tempt to follow the evolution of the important chemical observed by the HST FOS [Noll et al., 1995]. At wave- species; the results should not be regarded as definitive. lengths shorter than _278 nm, $2 dissociates into two Our reaction list is available upon request. ground-state (3p) sulfur atoms. The lifetime against Since some of the observed species form most easily photolysis for the Su molecule at 10 -5 mbar and 44 ° S in low pressure re-entry shocks [Zahnle et al., 1995], we latitude on Jupiter in our diurnally averaged model is assumed that most of the new material was deposited 5.3 hours. At. high altitudes, photolysis seems to provide above 1 mbar in the stratosphere. Based on the results an effective permanent loss mechanism for the $2; how- presented by Noll et al. [1994], we start, with initial ever, in the middle stratosphere near 0.01 - 0.1 mbar, column abundances of reduced sulfilr species as shown the three-body reaction 2S + M --+ $2 + M balances the in Table 1. The initial abundances of the nitrogen- and photolysis, and S., is efficiently recycled. Thus, we find oxygen-containing compounds are described in Moses et that some $2 remains several months after the impacts al. [1995]. Due to a lack of coherent information at this (see Fig. 1 and Table 1). Because this result, depends on time, we do not enhance the hydrocarbon abundances the rates of several reactions that have not been mea- after the plume splashdown, but begin with the back- sured in the laboratory, this conclusion should be re- ground hydrocarbon abundances derived by Gladstone garded as speculative. The HST spectra show no evi- et al. [1995] for the same latitude region. The new post- dence for $2 at the G site 22 days after the impact, but impact photochemistry does not measurably affect the no upper limits are given in Noll et al. [1995], and it. profiles of the major hydrocarbon compounds. is unclear whether models such as are shown in Fig. 1, in which the $2 is confined to a narrow altitude region, are consistent with these observations. Results Carbon disulfide (CS2), which is also observed un- ambiguously in the lIST FOS spectra, has strong ab- The abundances of our initial sulfur species change sorption bands in the 185 230 nm region. At these with time as photochemical reactions are initiated. Ta- wavelengths, CS2 dissociates into CS + S, where _80_ ble 1 illustrates the temporal variation of several of the of the sulfur at.ores emerge in the 1D excited state and more interesting species. Due to the large uncertainties the rest in the 3p ground state. The S(1D) reacts ef- in initial compositions at the impact, sites, one should ficiently with [t2 to form SH + H. CS2, like $2, is not not put great reliance on the actual numbers in this ta- lost from the middle stratosphere as fast as its photol- ble; two significant digits are included solely as an aid ysis lifetime (8.2 hours at 10 -5 mbar) would indicate. in comparing relative changes. Instead, CS2 is partly recycled and is also formed dur- Sulfur Photochemistry ing the photolysis of $2 in reaction schemes that involve S atoms and hydrocarbon radicals and have H2CS and The most abundant sulflar compound observed im- HCS as intermediates. mediately following the impacts is diatomic sulfur ($2). Thioformaldehyde (H2CS) forms in potentially ob- $2 is quickly lost by photolysis in the upper atmosphere servable quantities if we assume (as we did) that the and by the formation of larger sulfur molecules in the lower stratosphere. In fact, the production of molecu- lar sulfur (Ss) by various three-body reactions is very o S_ rapid. We adopt the following scheme for the produc- ? tion of various sulfur molecules: o I h_ - j Sx ----+ S×-a + S for x = 2,3,4 9 - - - - - I "C'_ 2S __0_+ S2 E r M S+$2 ---+ $3 o g "2 4 o 2Sa ---+ Ss J _ %_._ _- _- 5- 5_ 2- 2 _ M [2" $2 + S× ---+ Sx+2 forx= 2, 4,6 S+S× ---+ $2 +S×-1 forx = 3,4,5,6 in,_ 01 M 2S4 _ Ss J ...... ,,LJ where M represents any third molecule or atom. As Ta- 0 10 10 !0 9 113 8 10-7 10 6 10 5 10 4 ble 1 indicates, over 70% of the initial cohnnn budget of $2 molecules has been converted to Ss in the first hour SULFUR ($2) MIXING RATIO after we begin our calculations, and another --,10% is Figure 1.
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