Observations of the the Venusian Mesosphere

The massive Venus atmosphere presents extremely inhospitable conditions, ranging from crushing pressures (equivalent to ~ 1km ocean depths) and searing temperatures (hot enough to melt lead) at the surface, to thick sulfuric acid clouds extending over 40-60 km altitudes above the surface. The circulation of the lower Venus atmosphere is characterized by global east to west (zonal) winds, which peak at 70-100 m/sec velocities within the acid clouds. Above these cloud layers, both the chemistry and circulation of the atmosphere change significantly in character. Solar ultraviolet flux drives sulfur, chlorine, and hydrogen catalytic cycles critical to the formation of the lower sulfuric acid clouds and the stability of the Venus atmosphere to CO2 photolysis. Enhanced levels of CO within the Venus mesosphere (at 60-110 km altitudes) result from dayside photolysis of CO2 (the primary gas constituent of the Venus atmosphere) above the sulfuric acid cloud region. In addition, the general circulation of the Venus atmosphere transitions from the zonal rotation of the lower atmosphere below and within the mesosphere to a dominant solar-to-antisolar (SAS) flow (Dickinson and Ridley, 1980) above the Venus mesosphere.

Millimeter spectral line observations have played an important role in investigation of the poorly constrained Venus mesosphere, due to relatively strong transitions for CO in this wavelength region and the pressure-broadened lineshapes of these absorptions, which support vertical profile retrievals of temperature and CO as well as Doppler wind determinations. Millimeter spectral line observations have shown large nightside enhancements (>200% relative to dayside) in CO abundances above 90 km altitudes (Gulkis et al., 1997; Clancy et al., 2002). This CO diurnal variation is driven by SAS-driven transport within and above the mesosphere of Venus, but the complex and apparently unstable transition between the zonal and SAS circulation leads to global-scale variations on uncertain timescales. Combined measurements of optically thick 12CO and optically thin 13CO millimeter lines constrain both the CO mixing ratio and temperature profile, a technique which has been employed to determine surprisingly large secular variations in Venus nightside mesospheric temperatures (40 K at 90-100 km altitudes- Clancy and Muhleman, 1991) and CO distribution.

JCMT measurements of sub-mm CO spectra yield much improved temperature profiling due to the increased line optical depths, and so have provided the first definition of a global diurnal variation in upper mesospheric temperatures, and the presence of a dayside mesospause at 90-100 km altitudes (figure 1 above left, from Clancy et al., 2003). Sub-millimeter observations from JCMT also offer critical improvements to Doppler wind measurements: very narrow, deep absorption lines (figure 2 right) for much improved wind sensitivity (figure 3 below left); and higher spatial resolution (smaller beam size), which greatly reduces the effects of geometrical and spatial smearing across the Venus disk. We have pursued JCMT mapping observations of 330 and 345 ghz CO line absorptions, for four separate weekend periods centered on Venus inferior conjunctions in 2001 and 2002. Venus presents it maximum angular diameter (~60 arcsec) and a full range of nightside local times at inferior conjunction, which occurs roughly every 1.5 years. Consequently, we were able to retrieve winds, CO, and temperature profiles of the nightside mesosphere with extensive local time and latitudinal coverage, and over a range of diagnostic timescales. The improved sensitivity and temporal sampling of these sub-mm CO observations prove critical to defining a chaotic circulation regime for the upper mesosphere.

Previous disk-resolved millimeter observations of CO line Doppler winds around Venus inferior conjunction have implied conflicting results for mesospheric circulation at 90-110km altitudes, corresponding to interpretations for both predominately zonal (Shah et al., 1991) and predominately SAS (Lellouch et al., 1994) circulation in this region. Our 2001 and 2002 JCMT observations of the Venus nightside mesosphere show that both distinct circulations, accompanied by characteristic temperature differences, were present at these separate times (by 1.5 years). In addition, there are surprisingly large variations in the global wind, temperature, and CO mixing distributions between consecutive days and weeks within each observing period, although the basis circulation character (zonal vs. SAS) remained intact. We do not understand the full implications of this time variability, but it appears that the Venus mesosphere is one of the most dynamically unstable atmospheric regions observed in the solar system. For such a temporally dynamic system, a long- term record of diagnostic, self-consistent observations is critical for analysis. Spacecraft have not regularly observed the Venus atmosphere since the 1980s, and orbiting observatories such and HST preclude Venus observing due to solar direction pointing restrictions In this regard, earth-based observing of Venus over many years with an especially sensitive and diagnostic platform such as the JCMT is the primary method of advancement for study of the remarkably dynamic Venus atmosphere.