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Venus
& Mars
In the 1960s and 1970s, observations of Mars and Venus showed that
planets that seemed much like the Earth could have frightfully different
atmospheres. The greenhouse effect had made Venus a furnace, while lack
of atmosphere had locked Mars in a deep freeze. This was visible evidence
that climate can be delicately balanced, so that a planet's atmosphere
could flip from a livable state to a deadly one.
A planet is not a lump
in the laboratory that scientists can subject to different pressures
and radiations, comparing how it reacts to this or that. We have only
one Earth, and that makes climate science difficult. To be sure, we
can learn a lot by studying how past climates were different from
the present one. And observing how the climate changes in reaction
to humanity's "large scale geophysical experiment" of emitting greenhouse
gases may teach us a great deal. But these are limited comparisons
different breeds of cat, but still cats. Fortunately our solar
system contains wholly other species, planets with radically different
atmospheres. |
- LINKS -
<=Climate cycles
<=Simple
models
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Until the 1950s, science fiction writers gave about as accurate
a picture of the nearest planets as scientists themselves held (the
writers, after all, read up on the science). Mars seemed much like
the Earth, although its atmosphere was certainly far colder and dryer.
Strange life might crawl over the Red Planet's sands under an indigo
sky brushed with wispy clouds. Visitors to Venus might find something
still more Earth-like, a steam bath under perpetual clouds, although
scientists knew that the atmosphere was largely carbon dioxide gas
( CO2). Could study of these strange atmospheres
provide, by comparison, insights into the Earth's weather and climate?
With this ambitious hope Harry Wexler, head of the U.S. Weather Bureau,
instigated a "Project on Planetary Atmospheres" in 1948. Several leading
scientists joined the interdisciplinary effort. But the other planets
were so unlike the Earth, and information about their atmospheres
was so minimal, that the scientists could reach no general conclusions
about climate. The project was mostly canceled in 1952.(1)
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Telescopic observations continued, benefitting
from improved photographic techniques. By the mid 1950s, scientists,
if not science fiction writers, knew that the atmosphere of Mars was
unbreathable mainly CO2, very tenuous
and very cold, occasionally stirred up with yellowish dust storms.
If Mars had features resembling "canals" they were not full of water,
for water could not exist as a liquid on the planet's surface.(2) The Mariner 4 spacecraft of 1965, sending back pictures that
showed a surface scarred with craters like the Moon, confirmed that
the planet was an unlikely abode of life. As for Venus, radio observations
published in 1958 showed that the surface temperature was amazingly
hot, upwards of 600°K, around the melting point of lead. "It
was very disappointing to many people," one of the discoverers recalled,
"who were reluctant to give up the idea of a sister planet and perhaps
even the possibility of life." Some astronomers worked up arguments
that the radio measurements were misleading and life might still exist
on Venus.(3) |
= Milestone
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Already back in 1940 Rupert Wildt had made a rough calculation
of the greenhouse effect from the large amount of CO2
that others had found in telescope studies of Venus, predicting the
effect could raise the surface temperature above the boiling point
of water. But it to raising it as high as 600°K seemed impossible.(4*) Nobody mounted a serious attack on the problem (after all,
very few people were doing any kind of planetary astronomy in those
decades). Finally in 1960 a young doctoral student, Carl Sagan, took
up the problem and got a solution that made his name known among astronomers.
Using what he later recalled as "embarrassingly crude" methods, taking
data from tables designed for steam boiler engineering, he confirmed
that Venus could indeed be a greenhouse effect furnace.(5)
The atmosphere would have to be almost totally opaque, and this "very
efficient greenhouse effect" couldn't all be due to CO2.
He pointed to absorption by water vapor as the likely culprit. |
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Sagan, a science fiction fan from his early years, was among those
who had dreamed of a living sister planet of swamp and ocean, but
now he had to admit that "Venus is a hot, dry, sandy... and probably
lifeless planet." Most significantly, the situation was self-perpetuating.
The surface was so hot that whatever water the planet possessed remained
in the atmosphere as vapor, helping maintain the extreme greenhouse
effect condition. It was later found that Sagan was mistaken, for
Venus's atmosphere has little water. If the greenhouse effect is strong
anyway, that is because Venus has a much denser CO2
atmosphere than astronomers of the time thought (in 1978, the Pioneer
space probe found the atmosphere was almost entirely CO2).
But mistakes in science can be as useful as valid results, when they
stimulate further work and point in the right direction.(6) |
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A few researchers tried putting a feedback
between temperature and water vapor into a simple system of equations;
the results were strange. In 1969, Andrew Ingersoll reported "singularities,"
mathematical points where the numbers went out of bounds. That signaled
"a profound change in the physical system which the model represents."(7) (Link from below) Ingersoll pointed at CO2
as the key ingredient. Sagan had estimated that Venus had started
out with roughly the same amount of CO2 as the
Earth. On our planet, most of the carbon has always been locked up
in minerals and buried in sediments. The surface of Venus, by contrast,
was so hot and dry that carbon-bearing compounds evaporated rather
than remaining in the rocks. Thus its atmosphere was filled with a
huge quantity of the greenhouse gas. Perhaps Venus had once enjoyed
a climate of the sort hospitable to life, but as water had gradually
evaporated into the warming atmosphere, followed by CO2,
the planet had fallen into its present hellish state? It seemed that
such a "runaway greenhouse" could have turned the Earth too into a
furnace, if the starting conditions had been only a little different.(8*) |
<=>Simple models
|
In the late 1970s Michael Hart pursued the
idea with a more complex computer model, and concluded that the balance
was exceedingly delicate. Hardly any planets in the universe, he said,
orbited in the narrow "habitable zone" around a star where life could
flourish. For our solar system, the orbits in which a planet would
be too close to the Sun so that at some point the planet would
suffer a runaway greenhouse effect from which it could never recover
were separated by only a 5% gap from orbits in which the planet
would be so far away that runaway glaciation would freeze any ocean
solid. The Earth was a lucky place, then. Hart's calculations were
riddled with untested assumptions, and many scientists denied (rightly,
as later calculations showed) that our situation was so extremely
precarious. Hart defended his ideas energetically among his colleagues,
and also in public, including an appearance on television in "Walter
Cronkite's Universe."(9*) |
=>Public opinion
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The atmosphere of Venus was filled not only with CO2
but also with an opaque haze. Its nature was unknown, and in the 1960s
scientists could only say that the haze was probably caused by some
kind of tiny particles.(10)
"The clouds on Venus had long been a mystery," as one expert recalled,
"in which stratospheric aerosols now appeared to play a key role.
The unraveling of the precise role of aerosols in the Venus atmosphere
would certainly benefit studies of chemical contamination of Earth's
atmosphere." (11*) In the
early 1970s, ground-based telescope observations produced extraordinarily
precise data on the optical properties of these aerosols, and at last
they were identified. The haze was from sulfur compounds.(12*) |
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The greenhouse effect of the sulfates could
be calculated, and by the late 1970s climate modeler James Hansen
could state confidently that the sulfates together with CO2
"are responsible for the basic climatic state on Venus." Hansen had
originally become interested in the greenhouse effect when, in response
to Sagan's primitive calculations, he tried to derive a better explanation
of why the planet's atmosphere was so hot. Now Hansen's findings about
sulfate aerosols strengthened his belief that these particles could
make a serious difference for the Earth's climate as well. Sulfates
were emitted by volcanoes and, increasingly, by human industry, so
Venus had things to tell us about climate change at home.(13*) |
=>Aerosols
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And then there was Mars. That planet too
inspired important new thinking about the Earth's atmosphere, even
before spacecraft paid a visit. In the 1960s, NASA asked some scientists
to think about ways to detect life on Mars. A few of them noticed
that life on Earth makes its presence blatantly evident by driving
the atmosphere far from chemical equilibrium. In particular, the abundant
oxygen in the air would swiftly drain away, by combining with surface
minerals, except the oxygen is renewed by a daily emission from plants.
Telescopic studies found practically no oxygen in the Red Planet's
atmosphere. Overall the Martian atmosphere showed no signs of any
chemical disequilibrium. Biochemist James Lovelock dismayed his peers
by arguing that this showed any search for life there would be fruitless.
The sterile atmosphere of Mars, so strikingly different from the Earth's,
helped Lovelock, and eventually others, to recognize that life plays
a central role in determining the nature of our own planet's atmosphere.
(14*) |
<=>Biosphere
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In 1971 the spacecraft Mariner 9, a marvelous
jewel of engineering, settled into orbit around Mars and saw... nothing.
A great dust storm was shrouding the entire planet. Such storms are
rare for Mars, and this one was no misfortune for the observers, but
great good luck. They immediately saw that the dust had profoundly
altered the Martian climate, warming the planet by tens of degrees.
The dust settled after a few months, but its lesson was clear. Haze
could warm an atmosphere. More generally, anyone studying the climate
of any planet would have to take dust very seriously. In
particular, it seemed that on Mars the temporary warming had reinforced
a pattern of winds that had kept the dust stirred up. It was a striking
demonstration that feedbacks in a planet's atmospheric system could
flip weather patterns into a drastically different state. That was
no longer speculation but an actual event in full view of scientists
"the only global climatic change whose cause is known that
man has ever scientifically observed."(15)
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= Milestone
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Crude speculations about just such radical
instabilities in the Earth’s own climate had been published
independently by two scientists a few years before, about the same
time as Ingersoll’s calculation of the Venus greenhouse runaway.(16)
(See above) Pursuing such thoughts
before Mariner arrived at Mars, Carl Sagan had made a bold prediction.
He suggested that the Red Planet's atmosphere could settle in either
of two stable climate states. Besides the current "ice age" there
was another possible state, more clement, which might even support
life. The prediction seemed to be validated by crisp images of the
surface that Mariner beamed home after the dust cleared. The canals
some astronomers had imagined were nowhere to be seen, but geologists
did see strong signs that vast water floods had ripped the planet
in the far past. Calculations by Sagan and his collaborators now suggested
that the planet's climate system was balanced so that it could have
been flipped from one state to the other and back by relatively minor
changes in its orbit, or in the strength of the Sun's radiation, or
in the reflection of sunlight off the polar icecap. These were also,
as the authors remarked, "fashionable variables in theories of climatic
change on Earth." |
<=Simple
models
![](images/mariner9-sm.jpg)
Scarred Mars
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The speculations recently
published about the Earth’s climate had suggested possibilities
as spectacular as anything proposed for Mars. A small change could
start a warming in which the Earth's polar ice caps would shrink,
lowering the planet's reflectivity and pushing the warming further
into a self-sustaining climate shift. Much the same thing could perhaps
happen on Mars, releasing the CO2 frozen at its
poles, starting a greenhouse effect process that would melt the water
ice buried in the soil. In fact some kind of drastic climate shift
had happened on Mars, if the evidence of ancient floods was
correct. In these arguments, dust stood at the fore, since storms
that deposited dust on the polar caps and darkened them seemed the
most likely mechanism for pushing the planet into its warm phase.(17) |
=>Aerosols
=>Simple
models
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In later years, spacecraft probed the exotic
weather of Venus and Mars in great detail. Meanwhile as computer models
of atmospheres improved, the Earth's two neighbors, plus more exotic
planets such as Jupiter, occasionally served as testbeds to probe
the limits of the modelers' methods. If a set of equations gave plausible
results for such utterly different atmospheres, that gave more confidence
in their applications on Earth.(18*) But the main lesson was a larger one.
The idea that feedbacks involving the greenhouse effect could have
huge consequences for a planet's climate was not a mere speculative
theory. It was an observation of real events. |
=>Radiation math
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RELATED:
Home
Simple Models of Climate
1. Doel (1996), pp. 57-67.
BACK
2. Kuiper and Middlehurst
(1961), pp. 386-88, 438.
BACK
3. The first observation was in 1956 by Mayer, McCullough, and
Sloanaker. Mayer (1983), p. 271.
BACK
4. The actual temperature is around 750°K. Wildt predicted
roughly 400, "higher than the terrestrial boiling-point." Wildt
(1940); something other than CO2 would be necessary
to get to 600 according to Kuiper in Kuiper (1952), ch. 12. BACK
5. Sagan, interview by Ron Doel, Aug. 27, 1991, AIP, tape 4 side
1.
BACK
6. Sagan (1960); Sagan (1960), "efficient" p. vii, "lifeless" p. 20; see also Sagan (1961); arguments against a waterless Venus were
developed by Gold (1964); see Davidson (1999), pp. 101-106.
BACK
7. Ingersoll (1969), p.1191.
BACK
8. Rasool and de Bergh (1970);
they found that water would always have boiled on Venus, but others speculated its climate had
once been "clement," e.g., Wang et al. (1976); the ever prescient
Tommy Gold had already speculated in a 1963 symposium about a "runaway process" when
water boiled away, Gold (1964), p. 250. The question of whether
Venus once had a more Earth-like climate remains unresolved today.
BACK
9. Hart (1979). More accurate
calculations in the 1990 found that for our Sun, a considerably larger zone should be habitable
despite the gradually increasing brightness of the Sun itself.
BACK
10. Mayer (1961), p. 458.
BACK
11. Newell (1980), ch. 20.
Newell also says that "study of the role of halogens in the atmosphere of Venus... led to the
suspicion that chlorine produced in Earth's stratosphere from the exhausts of Space Shuttle
launches or from Freon used at the ground in aerosol sprays might dangerously deplete the ozone
layer.".
BACK
12. Sulfuric acid was "the most probable constituent of the
Venus clouds," Young (1973), p. 564; Young relied especially
on measurements by Hansen, who had identified the acid but was dissuaded from publishing the
idea. Hansen, interview by Weart, Oct. 2000, AIP. Hansen's contribution to the identification was
noted by Prather (2002).
BACK
13. He thought "a great deal stands to be gained" by studying
other planets' climates alongside the Earth's. Hansen et al.
(1978), p. 1067.
BACK
14. Hitchcock and Lovelock
(1967); see Lovelock (2000), pp. 228ff.; the absence of
detectable oxygen on Mars was long considered no definitive argument against the presence of
vegetation, and in the early 1960s, NASA's ideas on detecting life through atmospheric analysis
centered on a search for complex organic molecules. Dick
(1998), pp. 48, 175.
BACK
15. Observations: Hanel et al.
(1972); Feedbacks: Leovy et al. (1973); "observed": Toon et al. (1975), p. 495.
BACK
16. Budyko (1968); Sellers (1969).
BACK
17. Prediction (hoping that Martian life was only hibernating
through the winter of a 50,000-year cycle): Sagan (1971); Sagan et al. (1973), quotes pp. 1045, 1048.
BACK
18. According to O.B. Toon, a radiative transfer model
developed by Sagan and J. Pollack was influential for atmospheric studies in general. Davidson (1999), p. 244; also, studies of Mars helped stimulate
discussion of the "faint early sun problem" billions of years back when the sun was
dimmer, the greenhouse effect had saved the Earth from a perpetual ice age. Sagan and Mullen (1972).
BACK
copyright
© 2003-2004 Spencer Weart & American Institute of Physics
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