SIMULATIONS OF THE ATMOSPHERES OF SYNCHRONOUSLY ROTATING TERRESTRIAL PLANETS ORBITING M-DWARF - CONDITIONS FOR ATMOSPHERIC COLLAPSE AND THEIMPLICATIONS FOR HABITABILITY

Planets within the habitable zones of M dwarfs are likely to be synchr onous rotators; in other words, one side is permanently illuminated wh ile the other side is in perpetual darkness. We present results of thr ee-dimensional simulations of the atmospheres of such planets, and com ment on their possible habitability. Near the ground, a thermally dire ct longitudinal cell exists, transporting heat from the dayside to the nightside. The circulation is three-dimensional, with low-level winds returning mass to the dayside across the polar regions. Aloft, the zo nally averaged winds display a pattern of strong superrotation due to these planets' finite (albeit small) rotation rate. With terrestrial v alues of insolation, a CO2/H2O atmosphere collapses, or condenses on t he surface of the darkside, when surface pressure is approximately 30 mb, this value being much lower for a N-2 atmosphere. This temperature contrast is also sensitive to factors such as gravity, planetary radi us, and IR optical depth tau. These results question the suitability o f the concept of a habitable zone around M dwarfs that is independent of planetary parameters. If CO2 partial pressure is controlled by the carbonate-silicate cycle, we find that these planets should have a min imum surface pressure of 1000-1500 mb of CO2, as this is the minimum p ressure needed to support stable liquid water on the darkside at the i nner edge of the habitable zone. We finally conclude that planets orbi ting M stars can support atmospheres over a large range of conditions and, despite constraints such as stellar activity, are very likely to be habitable. (C) 1997 Academic Press.