Oxygen chemistry in the Venus middle atmosphere
Decades of research have sought to understand the similarities and differences between Venus and Earth. Yet, it is still not clear what chemical processes maintain the long-term stability of Venus' primarily CO_2 atmosphere because, until recently, the observed limit on O_2 was an order of magnitude smaller than predicted by photochemical model calculations. CO_2 dissociates into CO and O after absorbing photons at wavelengths <205 nm. These O atoms should combine to form O_2, and observations of intense airglow, produced as oxygen molecules in the O_2 (a^1 Δ) and O_2 (c^1 Σ) states decay radiatively to the ground state, confirm rapid production of O_2 on both day and night sides. Achieving an appropriate balance in numerical models between this rapid production of O_2 and the rapid destruction implied by the observational upper limits on O_2 has been a challenge for the past 25 years. Numerical modeling shows that recent proposals may resolve this gap between theory and observations, depending on the rates of poorly constrained reactions. The laboratory and observational studies needed to help resolve remaining questions regarding oxygen chemistry in the Venus middle atmosphere are outlined.