The Moist Greenhouse Effect

The process by which Venus lost its initial water ocean is slightly more complicated than the straightforward evaporation of H2O and its dissociation in the upper atmosphere. Indeed, the loss of the oceans of Venus is believed to have been a runaway catastrophe called the moist greenhouse effect. The amount of water vapor in the young Venusian atmosphere would have been controlled by the evaporation rate of the oceans, but since water vapor is a very efficient greenhouse gas, a positive feedback cycle is readily established. In this manner, a small increase in the atmospheric temperature results in a greater ocean evaporation rate, which places more water vapor in the atmosphere, which then warms further, and so on—an unstoppable runaway evaporation of the oceans will set in. Ultimately, the oceans will be entirely denuded and in essence they will have become airborne, making for a very hot, steamy Venusian atmosphere. Atmospheric water vapor will be dissociated into its three components, with the two lighter hydrogen atoms being lost to interplanetary space (recall again, Figure 5.11).

Of great importance to the debate concerning the possible existence of life in the young Venusian oceans is, ''How long did it take before the oceans were boiled away?'' Various estimates concerning this time interval have been published. Andrew Ingersoll (California Institute of Technology, Pasadena), interpreting the most recent results from the ESA Venus Express spacecraft, has suggested that it might have taken about 1 billion years after their formation for the Venusian oceans to have evaporated away. This is an uncomfortably short time for life to have evolved, but not an impossibly short one.

David Grinspoon (Denver Museum of Nature and Science, Colorado) has argued, however, that the Venusian oceans could have realistically survived for more than 2 billion years, which allows for life to have possibly evolved on Venus on a similar time-scale to that in which it appeared on the Earth.4 Greenspoon and fellow researcher Mark Bullock (Southwest Research Institute, Boulder) further point out that there is a possible test for determining how long the Venusian oceans might have survived. Key to the proposed test is the mineral tremolite, which is a metamorphic silicate rich in calcium and magnesium that forms in the presence of water. Since the temperature-sensitive weathering rate of tremo-lite is known from laboratory studies, it can be used as a chemical clock to determine when the oceans finally evaporated. A further, particularly interesting, point about the tremolite test is that it could be performed in situ by a robotic Venusian Lander. There are currently no funded missions for a Venusian Lander or sample-return mission, but the case for funding such missions within the next several decades is growing.

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