The Spatial-Distribution and Abundance of Greenhouse Gases in the Venus Lower Atmosphere

V. S. Meadows, D. Crisp (JPL)

Emission from the Venus night side in spectral windows between 1.0 and 2.5 tex2html_wrap_inline11 m originates from the lower atmosphere. In the shorter wavelength windows (1.0-1.18 tex2html_wrap_inline11 m) the radiation detected is primarily from the surface and lowest scale height ( tex2html_wrap_inline15 16 km), whereas the 1.74 and 2.3 tex2html_wrap_inline11 m windows probe the 15-25 and 25-45km altitude regions respectively. These windows include high-J transition water absorption lines, which are much stronger in the Venus atmosphere than in Earth's, and can be used to probe the water abundance near the Venus surface from ground-based observations. These windows also contain absorption from H tex2html_wrap_inline19 S, OCS, CO, HCl and SO tex2html_wrap_inline19 . Images taken within the short-wavelength windows show thermal emission contrasts associated with surface topography.

Using a sophisticated radiative transfer model, we have analyzed tex2html_wrap_inline23 near-IR (1.0-2.5 tex2html_wrap_inline11 m) spectral image cubes of the Venus night side acquired in 1993 and 1994 with the infrared imaging spectrometer (IRIS) on the Anglo-Australian Telescope. By combining Pioneer Venus altimetry and synthetic spectra for a given water vapor profile, we created synthetic maps within water absorption features in the 1.18 tex2html_wrap_inline11 m window and compared these with the observations until a best fit water vapor profile was found for all surface elevations. This method provides enhanced vertical resolution for water vapor in the lowest 6km of the atmosphere by obtaining differential column abundances between different surface elevations. For the 1993 data, our best fit H tex2html_wrap_inline19 O mixing ratio profile increases from 20 ppmv at the cloud base to tex2html_wrap_inline15 40 ppmv at 30km, and then remains constant between 30km and the surface. This confirms the result derived from 1991 IRIS data. We are using the 1993 data to retrieve near-surface temperature lapse rates and greenhouse gas abundances as a function of latitude and local time. Spatial variations in these properties will have important implications for the dynamics of the deep atmosphere and the efficiency of the greenhouse mechanism.