Summary: Adv. Space Res. Vol. 10. No. 1. (1)113--(1)119, 1990. 0273--1177/90 $0.00 + .50
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URANUS PHOTOCHEMISTRY AND
PROSPECTS FOR VOYAGER 2 AT
S. K. Atreya
Department of Atmospheric, Oceanic and Space Sciences, The University of
Michigan, Ann Arbor, MI 48109--2143, U.S.A.
Methane is the only photochemically active constituent in the atmospheres of Uranus and Neptune. NH
3, H20 and H2S are all removed
by condensation at pressures greater than 1.5 bars. Although the bulk mole fraction (--.2%) of CF!,5 is 20-30 times its solar value on
both planets, it drops to its saturation limit (-- 10--i) at the Uranus tropopause, but remains high (--`2%)at the Neptune tropopause. This
results in much greater mixing ratios of the product hydrocarbons (ethane, acetylene, ethylene and polyacetylenes) in the stratosphere
of Neptune. On both planets, the photolysis products of methane undergo condensation near the tropopause and the upper stratosphere.
Voyager observations of the hydrocarbons at Uranus, and thoseplanned at Neptune will be discussed, along with their implications for
the upper atmospheric physics and thermochemistry.
Although all major planets show a C/H ratio that is enhanced relativeto solar, it increases progressively from Jupiter to Uranus. On
Neptune it is at least comparable to, or somewhat greater than at Uranus. Such a trend in the C/H ratio is a reflection of the variation in