Planetary Sciences American and Soviet Research/Proceedings from the U.S.-U.S.S.R. Workshop on Planetary Sciences (1991) / Chapter Skim
Currently Skimming:
17. Runaway Greenhouse Atmospheres: Applications to Earth and Venus
17. Runaway Greenhouse Atmospheres: Applications to Earth and Venus
Pages 234-245
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 234... ...
Finally, the theory of runaway and moist greenhouse atmospheres can be used to estimate the position of the inner edge of the continuously habitable zone around the Sun. Current models place this limit at about 0.95 AU, in agreement with earlier predictions.
|
From page 235... ...
A third reason for interest in runaway greenhouse atmospheres concerns their implications for the existence of habitable planets around other stars. Any planet that loses its water as a consequence of a runaway greenhouse effect is not likely to be able to support life as we know it.
|
From page 236... ...
Surface temperature is simply a monotonically increasing function of the incident solar flux. Thus, the phrase "runaway greenhouse" is best reserved to describe a situation in which a planet's surface is entirely devoid of liquid water.
|
From page 237... ...
is that Venus experienced a so-called "moist greenhouse," in which the planet lost its water while at the same time maintaining liquid oceans at its surface. This turns out to be slightly favored from a theoretical standpoint; it also requires a significantly lower energy input than does the runaway greenhouse model.
|
From page 238... ...
> 0.1i, however, its behavior is quite different. The amount of latent heat released by condensation becomes so large that the temperature decreases very slowly with altitude, and the water vapor mixing ratio remains nearly constant.
|
From page 239... ...
(1) where R is the vertical column abundance of water vapor in the atmosphere, ~ is the hydrogen escape rate, and f is the D/H fractionation factor (i.e.
|
From page 240... ...
Setting aside the problem of the initial water endowment, subsequent aspects of the evolution of Venus' atmosphere are now reasonably well understood (Kasting and ~on, 1989~. If Venus had water originally, most of it was lost through either the runaway or moist greenhouse processes described above.
|
From page 241... ...
, a steam atmosphere may have existed only for short time periods following these events. Some light can be shed on this otherwise difficult question by an analysis of neon isotopic data.
|
From page 242... ...
Thus, even if large impacts were important and steam atmospheres were essentially a transient phenomenon, the neon isotope data implies that such conditions may have obtained during an appreciable fraction of the accretionary period. An alternative theory for explaining the isotopic abundance pattern of atmospheric neon (and xenon)
|
From page 243... ...
Recent theoretical work has provided better estimates of the amount of heating required to trigger runaway and new ideas about where such conditions may have applied. Future advances in our understanding of the evolution of Earth and Venus will require continued theoretical work in conjunction with new data on the isotopic composition of noble gases on both planets and on the water vapor distribution in Venus' lower atmosphere.
|
From page 244... ...
1990. Steam atmospheres and accretion: evidence from neon isotopes.
|
From page 245... ...
1987. Steam atmospheres, magma oceans, and other myths.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.