Pale Blue Dot II
May 19-21, 1999
One of the fundamental requirements for life as we know it is the presence of liquid water on (or below) a planets 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 planets 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 planets 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 planets 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 planets 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 Earths history. Recent calculations by Pavlov et al. (1999) (Fig. 1) show that CH4 could have provided a
Fig. 1 Surface temperature of Earth at 2.8 Ga as a function of pCO2 and CH4 mixing ratio. Solar luminosity is assumed to be 80% of the present value. The dotted curve is an upper limit on atmospheric CO2 levels derived from paleosol data. (From Pavlov et al., 1999).
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 Saturns 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 planets 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 Earths early climate, and of planetary climates in general, may be more complicated than has generally been assumed.
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