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Interpretation of Galileo Probe Data and Implications for Jupiter’s Dry Downdrafts

Showman, Adam P. and Ingersoll, Andrew P. (1998) Interpretation of Galileo Probe Data and Implications for Jupiter’s Dry Downdrafts. Icarus, 132 (2). pp. 205-220. ISSN 0019-1035.

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The Galileo probe found the jovian abundance of H_2S to be 30% solar at the 8 bar level, while the abundance of water was less than 3% solar at 12 bars. From 8 to 20 bars, H_2S increased to three times solar, and water apparently increased as well. Since H_2S and water condense at 2 and 5 bars, respectively, the probe probably entered a dry downdraft, wherein dry air above 2 bars is advected to 12 bars or deeper (Owen et al. 1996,Eos (Spring Suppl.) 77, S171). This is consistent with the fact that the probe entered the south edge of a 5-μm hot spot, a local region of Jupiter's atmosphere known from spectral modeling to be unusually low in cloud abundance (Orton et al. 1996, Science 272, 839). We use basic physical constraints to address three problems raised by Galileo probe data. First, it is unclear how the hypothesized downdraft remains dry, since simple models of convection preclude dessication below the 2- and 5-bar condensation levels. We suggest that to suppress moist plumes from below, the downdraft must be of low density below 5 bars and hence thermally indirect, requiring mechanical forcing from other parts of the atmosphere. Second, if geostrophic balance holds, the Galileo probe winds imply that the hot spot (north of the probe site) contains a stable layer from 1 to 5 bars; this is inconsistent with a downwelling, since downwellings should be adiabatic below 2 bars due to the low radiative flux divergence. We show that when the centripetal acceleration of curving parcel trajectories is included in the force balance, however, a variety of density profiles is possible within the hot spot (depending on the radius of curvature of the winds). The most plausible profile implies that the hot spot is nearly dry adiabatic and that the equatorial zone south of the probe site is stable from 2 to 6 bars, suggesting moist adiabatic upwellings with a water abundance of 1–2 times solar. This is consistent with Galileo and Voyager images suggesting upwelling at the equator. The profile further implies that from 1 to 5 bars the hot spot is denser than the equatorial zone south of the probe site. Third, probe data indicate that NH_3 increased with depth below 1 bar and became constant by 8 bars, H_2S began increasing below 8 bars and leveled off by 16 bars, while water only began increasing below 12 bars and was still increasing with depth at 20 bars. We propose that lateral mixing along isopycnals (surfaces of constant potential density) could produce the observed pattern; alternatively, the downwelling might consist of column stretching, so that the NH_3, NH_4SH, and water lifting condensation levels were pushed to 8, 16, and >20 bars, respectively. In either case, the simplest form of this model requires the downdraft to be less dense than the surroundings from 0.5 to 20 bars. In its simplest form, this model is therefore incompatible with our favored interpretation of the winds; more detailed studies will be necessary to resolve the problem.

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Ingersoll, Andrew P.0000-0002-2035-9198
Additional Information:© 1998 by Academic Press. Received July 29, 1997; revised December 4, 1997. We thank J. Cho, D. Hinson, P. Mahaffy, P. J. Gierasch, N. Renno, G. Schubert, A. Vasavada, and R. Young for useful discussions and D. H. Atkinson and A. Seiff for sharing their preliminary data. W. M. Folkner, R. Woo, H. Niemann, and the mass spectrometer team graciously allowed us to present the data shown in Fig. 1. This work was supported by NASA Grant NAGW-1956 and by the Galileo project.
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Subject Keywords:Jupiter (atmosphere); atmospheres (dynamics); atmospheres (structure); spacecraft
Record Number:CaltechAUTHORS:20121212-094023109
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Official Citation:Adam P. Showman, Andrew P. Ingersoll, Interpretation of Galileo Probe Data and Implications for Jupiter's Dry Downdrafts, Icarus, Volume 132, Issue 2, April 1998, Pages 205-220, ISSN 0019-1035, 10.1006/icar.1998.5898. (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35935
Deposited By: Ruth Sustaita
Deposited On:12 Dec 2012 19:41
Last Modified:25 Oct 2017 22:20

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