Observations of Night OH in the Mesosphere of Venus

Satellite measurements of the terrestrial night-side mesosphere from the MLS/Aura MLS and SABER instruments show a layer of OH in the mesopause. This layer confirms earlier measurements by ground-based UVFTS. These observations allow the study of the lowest vibrational state (MLS and UVFTS) and the chemically related emission from the high OH states (SABER) that originate the OH Meinel bands in the near infrared. The Caltech 1-D KINETICS model has been extended to include vibrational dependence of OH reactions and shows good agreement with MLS OH data and with observations of the Meinel bands (Pickett et al, 2006). The model shows a chemical lifetime of HOx that increases from less than a day at 80 km to over a month at 87 km. Above this altitude transport processes become an important part of HOx chemistry. The model predicts that ground state OH represents 99% of the total OH up to 84 km. Similarly, Venus airglow emissions detected at wavelengths of 1.40-1.49- and 2.6-3.14 micron in limb observations by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on the Venus Express spacecraft are attributed to the OH (2-0) and (1-0) Meinel band transitions as well (Piccioni et al., 2008). The integrated emission rates for the OH (2-0) and (1-0) bands were measured to be 100±40 and 880±90 kR respectively, both peaking at an altitude of 96±2 km near midnight local time for the considered orbit. We use the same Caltech 1-D KINETICS model to model these observations for Venus as was used for the Earth (Pickett et al., 2006) and discuss the conclusions, highlighting the similarities and differences between Venus and Earth.