Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability

✍ Scribed by M.M. Joshi; R.M. Haberle; R.T. Reynolds

• Publisher: Elsevier Science
• Year: 1997
• Tongue: English
• Weight: 887 KB
• Volume: 129
• Category: Article
• ISSN: 0019-1035
• DOI: 10.1006/icar.1997.5793

✦ Synopsis

nous of main sequence stars, having masses 0.1-0.5 times the mass of the Sun. The habitable zones around such stars Planets within the habitable zones of M dwarfs are likely to are therefore very close, being typically between 0.03 and be synchronous rotators; in other words, one side is permanently illuminated while the other side is in perpetual darkness. We 0.4 AU [see Fig. 16 of Kasting et al. (1993)]. A planet lying present results of three-dimensional simulations of the atmothis close to its star will tend to become tidally locked, or spheres of such planets, and comment on their possible habitin other words be permanently illuminated on one side, ability. Near the ground, a thermally direct longitudinal cell as the threshold for tidal locking after 4.5 byr is r lock Ȃ exists, transporting heat from the dayside to the nightside. The 0.5(M star /M sun ) 1/3 [see Fig. 16 of Kasting et al. (1993) and circulation is three-dimensional, with low-level winds returning Dole (1964)]. Such planets are termed synchronous rotamass to the dayside across the polar regions. Aloft, the zonally tors, and their rotation rates are therefore governed by averaged winds display a pattern of strong superrotation due to the size of their orbits (this is dealt with below).

these planets' finite (albeit small) rotation rate. With terrestrial If the atmosphere of such a planet is in radiativevalues of insolation, a CO 2 /H 2 O atmosphere collapses, or conconvective equilibrium, the surface temperature T 0 on the denses on the surface of the darkside, when surface pressure dayside will be very high, while the nightside will be so is approximately 30 mb, this value being much lower for a N 2 cold that the major atmospheric constituent will condense atmosphere. This temperature contrast is also sensitive to facout on the surface. When this happens, the surface tempertors such as gravity, planetary radius, and IR optical depth .

ature of the darkside, as well as the mean surface pressure

These results question the suitability of the concept of a habitp 0 , is set by a balance between upwelling thermal radiation able zone around M dwarfs that is independent of planetary and release of latent heat by condensing constituents, simiparameters. If CO 2 partial pressure is controlled by the carbonlar to the scenario that has been postulated for the martian ate-silicate cycle, we find that these planets should have a atmosphere at those times when a permanent polar cap minimum surface pressure of 1000-1500 mb of CO 2 , as this is forms (see, e.g., Toon et al. 1980). In this latter case, surface the minimum pressure needed to support stable liquid water pressures have been modeled as being as low as 1-3 mb on the darkside at the inner edge of the habitable zone. We finally conclude that planets orbiting M stars can support atmo- (Toon et al. 1980). This phenomenon is termed atmospheric spheres over a large range of conditions and, despite constraints collapse, and has been put forward as the primary reason such as stellar activity, are very likely to be habitable. © 1997 against searching for habitable planets around M stars.

Academic Press

In reality, atmospheric motions reduce the day/night temperature gradient ⌬T DN by transporting heat. Simplistically, it can be seen that the higher the atmospheric mass,