Summary: Adv. Space Res. Vol. 7, No. 12, pp. (12)79--(12)85, 1987 0273--1177/87 $0.00 + .50
Printed in Great Britain. All rights reserved. Copyright ©1987 COSPAR
NEUTRAL UPPER ATMOSPHERES OF THE
S. K. Atreya
Departmentof Atmospheric and Oceanic Science, The University ofMichigan,
Ann Arbor, MI 48109--2143, U.S.A.
Several recent papers have reviewed the upper atmospheres and ionospheres of Jupiter and Saturn in the post
Voyager era (see, e.g., /1/ and references therein). Therefore, this paper will review only the most salient
characteristics, as far as Jupiter and Saturn are concerned. The emphasis here, however, is placed on the Uranus
upper atmosphere that was probed in January, 1986, by Voyager 2 spacecraft. In particular comparative aspects
of atmospheric composition, thermal structure, photochemistry and the vertical mixing are discussed.
For all practical purposes, the bulk compositions of the atmospheres of Jupiter and Saturn are reflective of the
solar ratios of the elements. The mixing ratios of atmospheric constituents on Jupiter are shown in Figure 1.
Due to the relatively low tropospheric temperatures on Jupiter, H
20, H2S and NH3 would condense at the 2-5,
1.5-2.5 and 0.7-0.4 bar pressure levels respectively. Similar condensation is expected for Saturn. On Uranus,
condensation of CH4 at --~1300mb, and formation of an aqueous-ammonia solution cloud and an H2S-ice cloud
are also expected. Despite the recent Voyager flyby of Uranus, the inventory of detected species on this planet