Diurnal and Secular Variability of the Venus Mesosphere

Space Science Institute, Boulder CO

Investigators

Abstract

AST 0098614 Clancy The Venus mesosphere is a dynamical transition region between the zonal rotation of the massive lower atmosphere and the subsonic day-to- night circulation of the thermosphere. It is also a region of key photochemical activity, in which SOx, COx, and HOx catalytic cycles control formation of the massive sulfate cloud layer at its lower boundary, maintain stability of the C02 atmosphere to photolytic dissociation, and limit long-term escape of atmospheric water. The extreme differences in atmospheric circulation and chemistry above and below the Venus mesosphere lead to very large diurnal and interannual variations in its circulation and composition, which have confounded our understanding of this remarkably dynamic regime. Dr. R. Todd Clancy, of the Space Science Institute, will lead a data analysis and observing research program to investigate the variable thermal and compositional state of the Venus middle atmosphere. Millimeter spectral lines of 12CO, 13CO, and HDO enable unique vertical profile retrievals for temperature, and CO and water vapor densities at 70-110 km altitudes within the mesosphere. Over 100 separate measurements of existing, unanalyzed 12CO, 13CO, and HDO millimeter and sub-millimeter spectra will be modeled to determine the diurnal and interannual variabilities of CO and temperatures over the 1990-1999 period. Cursory analysis of these observations indicates abrupt, short-term variations in global mesospheric temperatures at 80-95 km altitudes, a dayside mesopause at 95-100 km altitudes, and strong diurnal variation in mesospheric water vapor abundances. These are unique discoveries of diurnal and secular variabilities in the Venus middle atmosphere that require comprehensive analyses of the full data set to quantify and publish results. In addition, new millimeter and sub-millimeter measurements will be taken over the 2000-2003 period to extend the temporal and vertical coverages of the C0/temperature/water retrievals, and to determine mesospheric abundances for SO, S02, H2SO4, and O2. A primary objective of the analysis is to define the diurnal and large interannual variations of the Venus mesosphere with the broadest vertical, parameter {temperature and composition) , and temporal description possible. The 1990-2003 period of these observations will be placed in the context of prior millimeter (1982-1990), Pioneer Venus orbiter (1978-1985) , and Galileo (1991) flyby observations to maximize the scientific return of these existing mesospheric measurements, and to support future spacecraft exploration of the Venus atmosphere. Study of the Venus mesosphere, in particular, draws strong parallels from the chemical and dynamical processes only recently defined to control the fragile terrestrial stratosphere. ***