Uranus photochemistry and prospects for Voyager 2 at Neptune

Abstract

Methane is the only photochemically active constituent in the atmospheres of Uranus and Neptune. NH3, H2O and H2S are all removed by condensation at pressures greater than 1.5 bars. Although the bulk mole fraction (~2%) of CH4 is 20-30 times its solar value on both planets, it drops to its saturation limit (~ 10-4) 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 those planned at Neptune will be discussed, along with their implications for the upper atmospheric physics and thermochemistry.