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Project Progress

Habitable Planets (dm)

Part of our work has examined the possible evolution of meteorites that might have played a role in bringing water and other volatiles to the early Earth. Results support a scenario in which low-temperature aqueous alteration of an anhydrous CM parent body and melt water from H2O and CO2 ices produce the altered assemblage observed in CM carbonaceous meteorites.

Three-dimensional simulations of thermohydrologic behavior on a planet with a frozen surface heated from below were carried out. The results suggest that hydrothermal circulation may occur in the martian regolith and may significantly thin the surface layer on Mars at some locations due to the upwelling of warm convecting fluids driven solely by background geothermal heating.

Laboratory and theoretical studies were accomplished concerning how carbon dioxide ice clouds affect the greenhouse effect, and hence the outer boundary of the habitable zone. Study of the microphysical properties of carbon dioxide clouds show that such clouds are unlikely to play an important role in the early greenhouses on Earth or Mars. Essentially the clouds warm the atmosphere, which tends to dissipate the clouds. The model was based on lab studies of the nucleation and growth of carbon dioxide. In circumstances where carbon dioxide clouds might have existed in the early martian atmosphere, we find that carbon dioxide clouds could be strongly warming, but the clouds can cool the surface if they are low and/or optically thick.

A review by Kasting of Ward and Brownlee’s book “Rare Earth” hypothesis was published in Perspectives in Biology and Medicine. Ward and Brownlee’s book argues that Earth-like planets are rare, based on a number of biological, geological, and astronomical arguments. Several of these are challenged in the review by Kasting.

We have examined the the carbon dioxide cycle on the Earth immediately after it formed, and the question of whether a hot (~100 C) surface could long be maintained is investigated. That Earth formed hot has been more-or-less accepted since at least the time of Kelvin. The issue is how long did it stay hot. No stable feedback mechanism that could maintain a surface temperature in the vicinity of 100 C was identified.

Analysis of data returned from the Clouds and Earth’s Radiant Energy System (CERES) instrument onboard the Earth Observing System (EOS) satellites shows the same behavior as we found in data from the predecessor Earth Radiation Budget Experiments (ERBE) over the warm pool in the Pacific Ocean. Namely, the outgoing infrared (IR) radiative energy flux maximizes and then decreases as a function of increasing sea surface temperature, a signature of the runaway greenhouse effect.