The Distribution of Carbon Monoxide in the Lower Atmosphere of Venus

TL;DR

This study maps the distribution of carbon monoxide in Venus's lower atmosphere using near-infrared spectroscopy, revealing higher CO concentrations at high latitudes and suggesting atmospheric circulation processes.

Contribution

It provides the first detailed two-dimensional CO distribution map of Venus's nightside and identifies the altitude of CO variation as around 45 km, revising previous assumptions.

Findings

Higher CO at 60° latitudes compared to equator

CO occurs at around 45 km altitude, not 35 km

CO distribution varies over time and latitude

Abstract

We have obtained spatially resolved near-infrared spectroscopy of the Venus nightside on 15 nights over three observing seasons. We use the depth of the CO absorption band at 2.3 microns to map the two-dimensional distribution of CO across both hemispheres. Radiative transfer models are used to relate the measured CO band depth to the volume mixing ratio of CO. The results confirm previous investigations in showing a general trend of increased CO abundances at around 60 degrees latitude north and south as compared with the equatorial regions. Observations taken over a few nights generally show very similar CO distributions, but significant changes are apparent over longer periods. In past studies it has been assumed that the CO latitudinal variation occurs near 35 km altitude, at which K-band sensitivity to CO is greatest. By modeling the detailed spectrum of the excess CO at high latitudes we show that it occurs at altitudes around 45 km, much higher than has previously been assumed, and that there cannot be significant contribution from levels of 36 km or lower. We suggest that this is most likely due to downwelling of CO-rich gas from the upper atmosphere at these latitudes, with the CO being removed by around 40 km through chemical processes such as the reaction with sulfur trioxide.