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Mark A. Bullock, David H. Grinspoon (Southwest Research Institute)
The powerful greenhouse effect on Venus exists because pressure-broadened CO2 absorption bands, interlaced with water absorption features, effectively block most of the upwelling thermal radiation coming from the surface. The sulfuric acid clouds and small amounts of SO2, OCS, CO, HCl and HF are responsible for some absorption of radiation at wavelengths greater than 2 \mum. In particular, these constituents of Venus' atmosphere absorb thermal radiation in a crucial part of the spectrum -- the 2.1 to 2.6 \mum range where the CO2-H2O thermal absorption conspiracy is weak. Much of the radiation on the short-wavelength side of Venus' surface blackbody curve (which has a peak at 4 \mum), leaves the planet through this window. Variations in the abundance of the trace atmospheric species have a large effect on the efficacy of this window, both directly through their infrared absorption, and indirectly through their effect on clouds. Increased atmospheric absorption, say through an increase in atmospheric water abundance, can heat the surface. However, as the surface of Venus heats up, the peak of its Planck function moves towards the 2.1 to 2.6 \mum window, allowing more direct thermal radiation loss to space. This high temperature shift of radiative loss into the window will act as a thermostat. For moderate perturbations in atmospheric trace species therefore, such as those expected from volcanism or large impacts, there is a limit to how hot the surface of Venus can get.
Using a one-dimensional, non-gray coupled cloud/radiative transfer model, we will show what the theoretical limits on the surface temperature of Venus are. We will discuss the fairly broad constraints on these conclusions, and make some general predictions for terrestrial planets with CO2-H2O atmospheres in other solar systems. These results may be relevant for models of tectonic and convective history of Venus and other planets.
The author(s) of this abstract have provided an email address for comments about the abstract: bullock@boulder.swri.edu