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The Role of Atmospheric Dynamics in Planetary Habitability

Wednesday, April 18, 2007
Cooper (NASA Postdoctoral Fellow, University of Arizona)

Traditional views of planetary habitability consider only the average surface temperature, ignoring atmospheric dynamics. Here, we show that rotation rate and radius may also be critical for determining whether liquid water oceans can reside on the surface of terrestrial planets. In the case of the Earth, which has lost a significant amount of its angular momentum to the Moon, the modest rotation rate allows efficient equator-to-pole heat transport. Hence, the poles never reach the condensation point of CO2, which would have a destabilizing effect on planetary climate. On faster rotators, on the other hand, equator-to-pole transport may be suppressed, resulting in lower polar temperatures.

Radius is also important for determining the sizes of Hadley cells, which are more global in scale for small planets (for a given rotation rate). Equator-to-pole heat transport is more efficient on small planets than large planets, leading to smaller temperature gradients overall. From a dynamical standpoint, we therefore expect greater climatic stability on Mars than on Earth.

In this study, we adapt the NCAR Community Atmosphere Model to planetary habitability studies. We first validate the model by comparing with results from the ARIES/GEOS Dynamical Core, which has been successfully used for Mars and giant planet studies. We then provide the same heating rate to models of variable rotation rate and radius, with no other differences. We analyze the temperature, velocity, and heating rates predicted in these models to explore whether high rotation rate and large planetary size inhibit latitudinal energy transfer, which increases the likelihood of condensation of greenhouse gases at the poles and the onset of a fully glaciated climate state. The magnitude of these effects on atmospheric dynamics will be discussed in the context of other considerations that affect a planet's long-term habitability.

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