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OCR for page 234
Runaway Greenhouse Atmospheres:
Applications to Earth and Venus
JAMES F. KASTING
The Pennsylvania State University
ABSTRACT
Runaway greenhouse atmospheres are discussed from a theoretical
standpoint and with respect to various practical situations in which they
might occur. The critical solar flux required to trigger a runaway greenhouse
is at least 1.4 times the solar flux at Earth's orbit (SO). Rapid water loss
may occur, however, at as little as 1.1 SO, from a type of atmosphere
termed a "moist greenhouse." The moist greenhouse model provides the
best explanation for loss of water from Venus, if Venus did indeed start
out with a large amount of water. The present enrichment in the D/H
ratio on Venus provides no unambiguous answer as to whether or not
it did. A runaway greenhouse (or "steam") atmosphere may have been
present on the Earth during much of the accretion process. Evidence from
neon isotopes supports this hypothesis and provides some indication for
how long a steam atmosphere may have lasted. 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.
INTRODUCTION
The topic of runaway greenhouse atmospheres has received renewed
attention over the past several years for three different reasons. The first
concerns the history of water on Venus. Although there is still no concensus
as to whether Venus had much water to begin with (Grinspoon 1987;
234
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AMERICAN AND SOVIET RESEA12CH
235
Grinspoon and Lewis 1988), some recent theories of accretion (Wetherill
1985) predict extensive radial mixing of planetesimals within the inner solar
system. If this idea is correct, then Venus must initially have received a
substantial fraction of Earth's water endowment. This water is obviously
not present on Venus today. The mung ratio of water vapor in the lower
atmosphere of Venus is approximately 10-4; thus, the total amount of
water present is only about 10-5 times the amount in Earth's oceans. The
runaway greenhouse theory provides a convenient explanation for how the
rest of Venus' water might have been lost.
A runaway greenhouse atmosphere may also have been present on
Earth during at least part of the accretionary period. Matsui and Abe
(1986a,b) and Zahnle et al. (1988) have shown that an impact-induced
steam atmosphere could have raised the Earth's surface temperature to
1500 K, near the solidus temperature for typical silicate rocks. This implies
the existence of a global magma ocean of unspecified depth. Although
the continuous existence of such a steam atmosphere has been questioned
(Stevenson 1987), an analysis of terrestrial neon isotope data (Kasting 1990)
strongly supports the notion that such an atmosphere existed during some
portion of the accretionary period.
A third reason for interest in runaway greenhouse atmospheres con-
cerns their implications for the existence of habitable planets around other
stars. Any planet that loses its water as a consequence of a runaway green-
house effect is not likely to be able to support life as we know it. Thus,
the idea that runaway greenhouses are possible sets limits on the width of
the continuously habitable zone (CHZ) around our Sun and around other
main sequence stars (Hart 1978, 1979; Kasting and lbon 1989). One of
the most important reasons for studying runaway greenhouses is to tIy to
estimate the chances of finding another Earth-like planet elsewhere in our
galaxy.
Here, the theory of runaway greenhouse atmospheres is briefly re-
viewed, and the consequences for the three problems mentioned above are
discussed.
RUNAWAY GREENHOUSE ATMOSPHERES
The concept of the runaway greenhouse atmosphere was introduced
by Hoyle (1955) and has been further developed by Sagan (1960), Gold
(1964), Dayhoff et al. (1967), Ingersoll (1969), Rasool and DeBergh (1970),
Pollack (1971), Goody and Walker (1972), WaLker (lg7$), Watson et al.
(1984), Matsui and Abe (1986a,b), Kasting (1988), and Abe and Matsui
(1988). The basic idea, as explained by Ingersoll (1969), is that there exists
a critical value of the solar flux incident at the top of a planet's atmosphere
above which liquid water cannot exist at the planet's surface. Intuitively,
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PIANETARY SCIENCES
one expects this to be the case. If Earth were by some means to be pushed
closer and closer to the Sun, one would anticipate that at some point the
oceans would be vaporized and the planet would be enveloped in a dense,
steam atmosphere. The amount of water in Earth's oceans, 1.4 x 1024 g, is
such that the surface pressure of this atmosphere would be about 270 bar.
For comparison, this is ~50 bar greater than the pressure at the critical
point of water (647.1 K, 220.6 bar).
It should be noted that the term "runaway greenhouse" has also been
used to describe the positive feedback between the surface temperature of
a planet and the amount of water vapor in its atmosphere. An increase in
surface temperature causes an increase in the vapor pressure of water which,
in turn, leads to an enhanced greenhouse effect and a further increase in
surface temperature. Although this type of positive feedback certainly
exists, there is no reason to believe that Earth's present climate system is
unstable. 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.
The single most important characteristic of a runaway greenhouse at-
mosphere is the critical solar flux required to trigger it. Only recently have
detailed estimates of this energy threshold been made (Kasting 1988; Abe
and Matsui 1988~. Even these estimates, which were obtained using elab-
orate radiative-convective climate models, cannot be considered reliable.
The greatest uncertainty in performing such a calculation is the effect of
clouds on the planetary radiation budget. Kasting (1988) has derived results
for a fully saturated, cloud-free atmosphere. (Actually, clouds were crudely
parameterized in this model by assuming an enhanced surface albedo.)
The critical solar flux in his model is 1.4 SO, where SO is the present solar
flux at Earth's orbit (1360 W mob. An Earth-like planet with Earth-like
oceans was assumed. Abe and Matsui (1988) have performed a similar
calculation for a case in which part of the energy required to trigger the
runaway greenhouse was derived from infalling planetesimals. (Their study
was specifically directed at the problem of atmospheric evolution during the
accretion period.) The amount of accretiona~y heating required to trigger
runaway conditions in their model, 150 W m~2, is the same as the value
derived by Kasting (1988) for an analogous simulation. (See his Figure 13.)
Thus, the two existing detailed calculations of the energy threshold of the
runaway greenhouse are in good agreement.
Although clouds cannot reliably be parameterized in such an atmo-
sphere, their qualitative effect on the planetary radiation balance is not
difficult to determine (Kasting 1988~. An atmosphere rich in water vapor
would probably exhibit at least as much fractional cloud cover as the current
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AMERICAN AND SOVIET RESEARCH
2237
Earth and possibly much more. Although clouds affect both the incom-
ing solar and outgoing infrared radiation, the solar effect should dominate
because a water vapor atmosphere would already be optically thick through-
out the infrared. Thus, the main effect of increased cloud cover should
be to reflect a greater proportion of the incident solar radiation, thereby
diminishing the amount of energy available to sustain a steam atmosphere.
It follows that the energy threshold for a runaway greenhouse is almost
certainly higher than 1.4 SO. How much higher is uncertain, but values as
high as 5 SO are within the realm of possibility (Kasting 1988, Figure Sc).
These rather speculative theoretical models should be weighed against
an observational fact: our neighboring planet Venus has very little water in
its atmosphere, less than 200 ppm by volume (Moroz 1983; von Zahn et al.
1983). As discussed further below, it is not clear whether this lack of water
is innate to the planet or whether it is the result of an evolutionary process.
One possibility, however, is that Venus was initially water-rich, and that
it lost its water by photodissociation in the upper atmosphere followed by
escape of hydrogen. (See references in opening paragraph.) If this theory
is correct, then Venus must have experienced either a runaway greenhouse
or a phenomenon akin to a runaway greenhouse at some time in the past.
The solar flux at Venus' orbit is currently 1.91 SO. Based on stellar evolution
models, the Sun's output was some 25-30% lower (1.34-1.43 SO) early in
solar system history (Gough 1981~. This implies that the critical threshold
for losing water from a planet is no higher than 1.9 SO and may well be
considerably lower.
If this was all there was to the problem, one could reliably conclude
that the energy threshold for the runaway greenhouse was between 1.4 SO
and 1.9 SO. However, it has recently been demonstrated that there are other
ways for a planet to lose water rapidly besides the runaway greenhouse. An
alternative possibility, proposed by Kasting et al. (1984) and Kasting (1988)
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. This alternative theory is described briefly below.
MOIST GREENHOUSE ATMOSPHERES
The concept of the moist greenhouse atmosphere stems from the
analysis of moist adiabats by Ingersoll (1969). Ingersoll showed that the
vertical distribution of water vapor in an atmosphere should be strongly
correlated with its mass-mixing ratio co(H2O) near the surface. When water
vapor is a minor constituent of the lower atmosphere [co(H2O) < 0.1],
its concentration declines rapidly with altitude throughout the convective
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PLANETARY SCIENCES
region as a consequence of condensation and rainout. This is the situation
in Earth's atmosphere today, where Co(H2O) declines from roughly 0.01
near the surface to about 3 x 10-6 in the lower stratosphere. When water
vapor Is a major constituent [co(H2O) > 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. This allows water vapor
to remain a major constituent even at high altitudes which, in turn, allows
it to be effectively photodissociated and the hydrogen lost to space. (The
important constraint here is that water vapor remain abundant above the
cold trap, lie. the maximum height at which it can condense. When this
criterion is satisfied, hydrogen should escape at close to the diffusion-limited
rate (Hunter 1973), as long as sufficient solar extreme ultraviolet (EUV)
energy is available to power the escape.)
Kasting et al. (1984) and Kasting (1988) have applied the moist green-
house model to the problem of water loss from an Earth-like planet. The
most recent results (Kasting 1988) indicate that hydrogen escape becomes
very rapid (ie. cO(H2O) becomes greater than 0.1) for incident solar fluxes
exceeding 1.1 SO. As before, this calculation was performed for a fully
saturated, cloud-free atmosphere, so the actual value of the solar flux at
which water loss becomes efficient is probably greater than this value. The
calculation does demonstrate, however, that Venus could have lost most of
its water without ever experiencing a true runaway greenhouse. Indeed,
the solar flux at Venus's orbit early in solar system history (1.34 - 1.43
SO) is so close to the minimum value required for runaway (1.4 SO) that
it seems likely that clouds would have tipped the balance in favor of the
moist greenhouse scenario. Thus, if Venus were originally endowed with as
much water as Earth, it may at one time have had oceans at its surface.
With the concepts of runaway and moist greenhouses in mind, let us
now return to the three topics mentioned in the introduction.
LOSS OF WATER FROM VENUS
The real issue concerning Venus is not so much whether it could have
lost its water but, rather, whether it had any appreciable amount of water
initially. It is now well accepted that the D/H ratio on Venus is very high:
approximately 100 times the terrestrial value. The original interpretation
of this observation (Donahue e! al. 1982) was that Venus was once wet.
If Venus and Earth started out with similar D/H ratios, which seems even
more likely now in view of the terrestrial D/H ratio observed in the tail
of comet Halley (Eberhardt et al. 1987), this measurement implies that
Venus once had at least 100 times as much water as it does now. The
current water abundance on Venus, assuming a lower atmosphere mixing
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AMERICAN AND SOVIET RESEARCH
239
ratio of 100 ppmv, is only 0.0014% of a terrestrial ocean. Thus, this
minimal interpretation requires only that Venus start out with about 0.1%
of Earth's water endowment. Even advocates of a dry early Venus would
probably not dispute such a claim, given the potential for radial mixing of
planetesimals during the accretion process (Wetherill 1985~. If, however,
some deuterium was lost along with the escaping hydrogen (which seems,
indeed, to be unavoidable), the amount of water that was lost could be
orders of magnitude greater. Consequently, proponents of a wet origin for
Venus (Donahue et al. 1982; Kasting and Pollack 1983) have suggested that
Venus may well have started out with an Earth-like water endowment.
The wet-Venus model has been challenged by Grinspoon (1987) and
Grinspoon and Lewis (1988), who point out that the present D/H en-
richment on Venus could be explained if the water abundance in Venus'
atmosphere were in steady state. Loss of water by photodissociation and
hydrogen escape could be balanced by a continued influx of water from
comets. Grinspoon and Lewis's steady-state model has, in turn, been crit-
icized (Donahue, private communication, 1988) on the grounds that they
underestimated the amount of water vapor in Venus' lower atmosphere.
The time constant for evolution of the D/H ratio in Venus' atmosphere can
be expressed as (Grinspoon 1987)
T ~ R/(f¢)
(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.
the relative efficiency of D escape compared to H escape). Best estimates
for the values of ~ and f, based on a weighted average of the hydrogen
loss rates from charge exchange with H+ and from momentum transfer
with hot O atoms, are 2 x 107 H atoms cm~2 s~t and 0.013, respectively
(Hunter et al. 1989~. This estimate draws upon a reanalysis of the charge
exchange process by Krasnopolsky (1985~. Grinspoon (1987) assumed that
the Venus lower atmosphere contained only 20 ppmv of water vapor; this
gives R ~ 6 x 1022 H atoms cm~2 and T ~ 7 x 109 years. Even this
value is somewhat longer than the age of the solar system, indicating that
the steady-state model is marginally capable of explaining the observations.
Small increases in the value of either ~ or f could eliminate the time scale
problem. If the H2O mixing ratio is actually closer to 200 ppm, however,
then ~ ~ 7 x 10~° years, and the steady-state model is in serious trouble.
Resolution of this question requires, at a bare minimum, that the
present controversy regarding the H2O content of the Venus lower atmo-
sphere be resolved. Our present understanding of Venus' water inventory
is further muddled by the fact that the H2O mixing ratio apparently varies
with altitude from about 20 ppmv near the surface to 200 ppmv just below
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PLANETARY SCIENCES
the clouds (von Zahn et al. 1983~. Until this variation is explained theoret-
ically, little confidence can be given to any of the measurements, and the
H2O abundance on Venus will remain an enigma.
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. One additional reason for favoring the moist greenhouse
model is that it might have facilitated removal of the last few bars of
Venus' water (Kasting et al., 1984~. If an ocean had been present on
Venus for any significant length of time, it should have drawn down the
atmospheric CO2 partial pressure by providing a medium for the formation
of carbonate minerals. A thinner atmosphere would, in turn, have provided
less of a barrier to loss of water by photodissociation followed by hydrogen
escape. For example, suppose that an initial 100-bar CO2-N2 atmosphere
was reduced to 10 bar of total pressure by carbonate formation. The critical
water abundance at which the cold trap became effective would then be
reduced from 10 bar to 1 bar, based on the criterion co(H2O) < 0.1. Only
1 bar of water would then need to be lost by the relatively inefficient
hydrogen loss processes that would have operated after the cold trap had
formed.
Once surface water was depleted, carbonate formation would have
slowed dramatically, and CO2 released from volcanos should have begun
accumulating in the atmosphere. SO2 concentrations would have likewise
increased, and the modern sulfuric acid clouds would have started to
form. Thus, regardless of its initial condition, Venus' atmosphere should
eventually have approached its modern state.
STEAM ATMOSPHERES DURING ACCRETION
The possibility that Earth was enveloped in a dense steam atmosphere
during the accretionary period was raised by Matsui and Abe (1986a,b,
and earlier references therein). Their model elaborated on earlier studies
(Benlow and Meadows 1977; Lange and Ahrens 1982) that predicted
that infalling planetesimals would be devolatil~zed on impact once the
growing Earth had reached about 30% of its present radius. The water
contained in these impactors would thus have been emplaced directly
into the protoatmosphere, instead of following the more traditional route
of being first incorporated into the solid planet and being subsequently
outgassed from volcanos.
A critical aspect of Matsui and Abe's model was that it considered the
effect of the impact-induced steam atmosphere on the planetary radiation
budget. Based on a relatively crude, gray-atmosphere, radiative-equilibrium
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AMERICAN AND SOVIET RESEARCH
241
calculation, they argued that the surface temperature of such an atmosphere
would rise to the approximate solidus temperature of crustal rocks, about
1500 K The surface pressure would continue to rise until it was of the order
of 100 bar. At this point dissolution of water in the partially molten plane-
tary surface would balance the continued input of water from planetesimals
and thereby stabilize the atmospheric pressure and temperature.
Matsui and Abe's fundamental predictions have been largely borne out
by studies carried out using more detailed models (Kasting 1988; Zahnle
et al. 1988; Abe and Matsui 1988~. Given an accretionary time scale of
107 to 108 years (Safronov 1969), the rate of energy release from infalling
material should indeed have been sufficient to maintain the atmosphere in
a runaway greenhouse state (Kasting 1988, Figure 13~. One objection that
has been raised, however, is that none of these models have taken into
account the stochastic nature of the accretion process (Stevenson 1987~.
If the latter stages of accretion were dominated by large impacts spaced
at relatively long time intervals (Wetherill 1985), 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. Craig and Lupton (1976) pointed out some
time ago that the 20Ne/22Ne ratio in Earth's atmosphere (9.8) is lower
than that in gases thought to originate in the mantle. Their database
has now been expanded to include volcanic gases, along with trapped
gases in diamonds and MORBs (mid-ocean ridge basalts) (Ozima and
Igarashi 1989~. The neon isotope ratio in gases derived from the mantle
is generally between 11 and 14, with the lower values attributed to mixing
with atmospheric neon (Ozima and Igarashi 1989~. Thus, the 20Ne/22Ne
ratio of mantle neon is similar to the solar ratio, which ranges from 13.7 in
the solar wind to 11-12 in solar flares (Ozima and Igarashi 1989~.
The neon isotopic data are most easily explained if Earth formed
from material with an initially solar 20Ne/22Ne ratio, and if 20Ne was
preferentially lost from Earth's atmosphere during rapid, hydrodynamic
escape of hydrogen (Kasting 1990~. The requirements for losing neon
are quite specific and can be used to set rather tight constraints on the
composition of Earth's atmosphere at the time when the escape occurred.
The minimum hydrogen escape flux required to carry off 20Ne is 2 x
10~3 H2 mol cm~2 s~i (Kasting 1989~. If the background atmosphere at
high altitudes was predominantly CO2, the diffusion-limited escape rate of
hydrogen is given by
Slim ~ 3 x 10~3 f(H2~/~1 ~ f(H2~]cm~2s-i
(2)
where f(H2) is the atmospheric H2 mixing ratio (Hunter 1973~. By com-
paring this expression with the escape rate needed to carry off neon, one
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PLANETARY SCIENCES
can see that this is only possible if f(H2) exceeds unity, i.e. the atmosphere
must be composed primarily of hydrogen. Such an atmospheric composi-
tion would have been very difficult to sustain during most of Earth's history.
However, it is entirely reasonable in an impact-induced steam atmosphere,
where copious amounts of H2 could have been generated from the reaction
of H2O with metallic iron.
A second reason that the escape of neon must have occurred early is
that this is the most favorable period from an energetic standpoint. The
solar EUV energy flux required to power an escape rate of 2 x 10~3 H2
mol cm~2 s~i is about 40 ergs cm-2 s-i, or roughly 130 times greater
than the present solar minimum EUV flux (Kasting 1989~. EUV fluxes of
this magnitude are expected only within the first 10 million years of solar
system history (Zahnle and Walker 1982~. Thus, the isotopic fractionation
of neon must have taken place during the accretionary period, most likely
in a steam atmosphere of impact-induced origin.
If this explanation for the origin of the atmospheric 20Ne/22Ne ratio
is correct, it is possible to use this information to estimate the length of
time that a steam atmosphere must have been present on the growing
Earth. According to theory (Zahnle et al. 1988), the surface pressure of the
steam atmosphere should have been buffered at a more or less constant
value of ~ 30 bar. Application of the "constant inventory" model for
hydrodynamic mass fractionation (Hunter et al. 1987) then predicts that
the escape episode must have lasted at least five million years (Kasting
1989~. 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) is that the fractionation occurred during the
loss of a primordial H2 atmosphere captured from the solar nebula (Sasaki
and Nakazawa 1988; Pepin, manuscript in preparation). This theory appears
equally viable in terms of its ability to explain the isotopic data. It differs
from the steam atmosphere model in that it requires that the accretion
process proceed in the presence of nebular gas. If the lifetime of the solar
nebula was much less than the accretionary time scale, as predicted by
Safronov (1969), then the steam atmosphere model is preferred.
THE CONTINUOUSLY HABITABLE ZONE
A third reason that runaway (and moist) greenhouse atmospheres are
of interest is that they set constraints on the inner edge of the continuously
habitable zone (CHZ) around the Sun (Kasting e! al. 1988; Kasting and
lbon 1989~. The concept of the CHZ was introduced by Hart (1978, 1979~.
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AMERICAN AND SOVIET RESEARCH
243
He defined it as that region of space in which a planet could remain
habitable (i.e. maintain liquid water at its surface) over time scales long
enough for life to originate and evolve. Hart concluded, based on what
seems in retrospect to have been an overly simplified model, that the CHZ
extended from only about 0.95 AU to 1.01 AU. In Hart's model, the inner
boundary of the CHZ was determined to be the distance at which an Earth-
like planet would experience a runaway greenhouse at some time during
the last 4.6 billion years. The outer edge of the CHZ was the position at
which runaway glaciation would occur.
The climate models described earlier (Kasting 1988; Abe and Matsui
1988) show that the runaway greenhouse threshold is considerably higher
than Hart had estimated. If the minimum solar flux needed for runaway is
1.4 SO (see above), then the distance at which this would occur should be
< 0.85 AU. On the other hand, the minimum solar flux required to lose
water in the moist greenhouse model is only 1.1 SO. The radial distance at
which this might occur is thus 0.95 AU, in agreement with Hart's original
estimate. Thus, it appears that Hart located the inner edge of the CHZ
correctly, even though his reasoning was slightly flawed.
The outer edge of the CHZ, on the other hand, probably lies well
beyond Hart's estimate of 1.01 AU. Hart erred because he ignored the
important feedback between atmospheric CO2 levels and climate (Walker et
al. 1981~. This story is told in detail elsewhere (Kasting et al. 1988; Kasting
and Won 1989) and will not be repeated here. A modern conclusion,
however, is that the CHZ Is relatively wide, and that the chances of finding
another Earth-like planet elsewhere in our galaxy are reasonably good.
CONCLUSION
Runaway greenhouse atmospheres are much better understood than
they were several years ago. Recent theoretical work has provided better es-
timates 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.
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Representative terms from entire chapter:
steam atmosphere
