James F. Kasting
Department of Geosciences
443 Deike
Penn State University
University Park, PA 16802
Email: kasting@essc.psu.edu

One of the fundamental requirements for life as we know it is the presence of liquid water on (or below) a planet’s surface. If one is interested in detecting life remotely with Terrestrial Planet Finder (TPF), then it is important that the liquid water environment be in contact with the planet’s atmosphere, as it is only by potential biological modifications of atmospheric composition that we can can hope to do this. Possible subsurface liquid water habitats such as those that might exist on Mars or Europa are interesting with respect to our own Solar System but would be difficult or impossible to investigate on planets around other stars.

A theory for estimating the width of the (surface liquid water) habitable zone (HZ) around main sequence stars has been advanced by Kasting et al. (1993). (See also Kasting, 1996, 1997; Williams et al., 1997). The inner edge of the HZ is determined by loss of water by way of photolysis followed by hydrogen escape to space. This, in turn, is governed by the stratospheric H₂O mixing ratio. The modern Earth has a dry stratosphere because it has an effective "cold trap" for water at the tropopause. A planet with a significantly higher surface temperature would have more water vapor in its lower atmosphere and, hence, the tropopause cold trap would be essentially washed out (Ingersoll, 1969). If one ignores cloud feedback, the effective solar flux that would trigger this situation on Earth is ~1.1 times the present value (Kasting, 1988). This puts the inner edge of the HZ at ~0.95 AU. This estimate is conservative because the buildup of water clouds would probably cool a planet’s surface and tend to make the ocean more stable than in a cloud-free calculation.

The outer edge of the HZ is determined by the distance at which CO₂ and other greenhouse gases can no longer compensate for the lower solar flux. A key assumption of the Kasting et al. (1993) model is that atmospheric CO₂ levels should tend to rise as a planet’s surface becomes colder. The reason is that removal of CO₂ by silicate weathering followed by carbonate deposition should slow down as the climate cools, and would cease almost entirely if the planet were to globally glaciate. On planets like Earth that have abundant carbon (in carbonate rocks) and some mechanism, like plate tectonics, for recycling this carbon, volcanism should provide a more-or-less continuous input of CO₂ into the atmosphere. In the Kasting et al. (1993) model, the outer edge of the HZ was taken to be the distance at which this volcanic CO₂ would condense to form dry ice clouds. Recent work by Forget and Pierrehumbert (1997) indicates that this assumption may be too conservative because CO₂ clouds should tend to warm a planet’s surface radiatively. (They primarily scatter outgoing infrared radiation, rather than absorbing and re-radiating it as H₂O clouds do, but their net effect is generally to warm because they scatter more efficiently at IR wavelengths than at solar wavelengths.) Thus, the outer edge of the present-day HZ may lie in the vicinity of 2 AU. Mars, at 1.52 AU, would probably be habitable today if it had been big enough to retain its volatiles and to keep recycling them throughout its history.

The continuously habitable zone, or CHZ, around a star is narrower than the HZ because stars increase in brightness as they age. Thus, a planet that was initially near the inner edge of the HZ may become uninhabitable as the star brightens. Conversely, a planet near the outer edge of the HZ today might not have been habitable in the past when its parent star was dimmer. A conservative estimate for the width of the 4.6-billion-year CHZ around our own Sun is 0.2 AU (Kasting et al., 1993). This estimate is almost certainly too pessimistic because of the aforementioned warming effect of CO₂ clouds. If the present outer edge of the HZ is at 2 AU, then the 4.6-b.y. CHZ should extend from at least 0.95 AU to 1.7 AU, meaning that both Earth and Mars would lie within it. This suggests that habitable planets may be relatively commonplace, provided that the other requirements of forming Earth-like planets are met.

Finally, it is worth mentioning the contribution to surface warming that could have been made by other reduced greenhouse gases early in Earth’s history. Recent calculations by Pavlov et al. (1999) (Fig. 1) show that CH₄ could have provided a

substantial amount of surface warming prior to ~2.2 b.y. ago when atmospheric O₂ levels first rose. These calculations extend previous work on CH₄ warming by Kiehl and Dickinson (1987). Our new model predicts greater warming, however, because the revised HITRAN database includes more CH₄ lines. An additional complication, pointed out by Sagan and Chyba (1997), is that CH₄ may be photolyzed to give hydrocarbon smog similar to that seen on Saturn’s moon, Titan. This smog could shield other greenhouse gases (e.g., NH₃) from photolysis, but it might also produce an "anti-greenhouse effect" (McKay et al., 1991), thereby cooling a planet’s surface. Photochemical model calculations (Zahnle, 1986; Brown, 1999) predict that hydrocarbon smog should start to form when the atmospheric CH₄/CO₂ ratio exceeds unity. This situation may have been reached on Earth during the Late Archean Era, around 2.2-2.8 b.y. ago. Hence, the story of Earth’s early climate, and of planetary climates in general, may be more complicated than has generally been assumed.

References:

Brown, L. L. 1999. "Photochemistry and climate on early Earth and Mars." Penn State University.

Forget, F., and R. T. Pierrehumbert. 1997. Warming early Mars with carbon dioxide clouds that scatter infrared radiation. Science 278: 1273-76.
My Perspective is on p. 1245.

Ingersoll, A. P. 1969. The runaway greenhouse: A history of water on Venus. J. Atmos. Sci. 26: 1191-98.

Kasting, J. F. 1997. Habitable zones around low mass stars and the search for extraterrestrial life. Origins of Life 27: 291-307.

Kasting, J.F. 1996. Planetary atmosphere evolution: Do other habitable planets exist and can we detect them? Astrophys. Space Sci. 241: 3-24.

Kasting, J.F. 1988. Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus. Icarus 74: 472-94.

Kasting, J. F., D. P. Whitmire, and R. T. Reynolds. 1993. Habitable zones around main sequence stars. Icarus 101: 108-28.

Kiehl, J. T., and R. E. Dickinson. 1987. A study of the radiative effects of enhanced atmospheric CO2 and CH4 on early earth surface temperatures. J. Geophys. Res. 92: 2991-98.

McKay, C. P., J. B. Pollack, and R. Courtin. 1991. The greenhouse and antigreenhouse effects on Titan. Science 253: 1118-21.

Pavlov, A. A., J. F. Kasting, L. L. Brown, K. A. Rages, and R. Freedman. submitted. Greenhouse warming by CH4 in the atmospheres of early Earth and Mars. J. Geophys. Res.

Sagan, C., and C. Chyba. 1997. The early faint Sun paradox: Organic shielding of ultraviolet-labile greenhouse gases. Science 276: 1217-21.

Williams, D. M., J. F. Kasting, and R. A. Wade. 1997. Habitable moons around extrasolar giant planets. Nature 385: 234-36.

Zahnle, K. J. 1986. Photochemistry of methane and the formation of hydrocyanic acid (HCN) in the Earth's early atmosphere. J. Geophys. Res. 91: 2819-34.