Until well into the 1960's, science fiction stories about Venus generally portrayed it as a steamy jungle planet, but one where intrepid explorers could perhaps survive unprotected on the surface. The idea of a jungle and breathable air was of course unfounded speculation, but the general picture of the climate was not wholly without merit. After all, the dense reflective cloud deck of Venus was readily observable - it is what makes Venus so bright as the "evening star" - and the reflection of sunlight could easily make up for the fact that Venus is closer to the Sun than is Earth. In fact in Chapter 3 we'll see that the reflectivity more than makes up for the proximity. In the late 1950's the picture of Venus as a habitable world began to unravel. Recall that the temperature of Earth's Moon was determined by examining infrared radiation from that body. Viewed in the infrared spectrum Venus appeared quite cool, but in the microwave ("radar") spectrum it was far too bright. In fact, seen in the microwave, Venus radiated like a body with a temperature well in excess of 600K (327C). A popular hypothesis at the time was that the anomalous microwave emission arose in the upper portions of the Venusian atmosphere. Another view held that the microwave emission came from the surface of the planet, and that the atmosphere was transparent to microwaves but relatively opaque to infrared. The latter idea suffered from the lack of a plausible mechanism to make the surface of Venus so hot. Then, in 1960 the young Carl Sagan proposed that Venus has a very thick atmosphere rich in greenhouse gases, which would heat up the surface to the required temperature. Little was known about the mass of the atmosphere or its composition at the time, but Sagan developed simple models of the greenhouse effect of a thick atmosphere, which showed that the trick could be accomplished with an atmosphere consisting of mostly carbon dioxide with some water vapor mixed in, having a total mass three or four times that of the Earth's atmosphere. Sagan even recognized that since the planet was too hot for water vapor at the hypothesized concentration to condense and reach the surface as liquid, the Urey reaction (which removes carbon dioxide from the atmosphere and turns it into limestone) could not take place. This would make it easy for carbon dioxide to accumulate in the atmosphere, though even Sagan did not envision just how far this would go.
A series of interplanetary probe missions over the next two decades - four US Mariner missions, two US Pioneer missions and sixteen Soviet Venera missions including eight Venera missions that returned data successfully from the surface - refined the estimates of surface temperature and substantially revised the conception of the atmospheric mass and composition. By the late 1970's, it was known that the surface temperature was nearly uniform at 737K. The atmosphere was found to be much more massive than originally thought, in fact sufficiently massive to raise the surface pressure to 92 times that of Earth's atmosphere. And, it was found that the atmosphere consisted almost entirely of carbon dioxide, with only traces of water vapor remaining. The thick clouds that give Venus its high reflectivity were found to be made not of water, but of droplets of sulfur dioxide and concentrated sulfuric acid. It took the better part of another decade before the challenges of dealing with the effect of such an exotic atmosphere on climate were fully mastered and a fully satisfactory account of the high surface temperature could be given. Still, the initial exploration of the problem was carried out with simple models very like the ones we'll introduce in the first half of Chapter 4. The discovery of the true nature of Venus climate is another illustration of the tenet that big ideas grow from little models.
Mars yielded up its climatic secrets somewhat earlier, because it was not hidden behind the thick atmospheric veil that complicated observation of Venus. Mars was observed in the infrared using the Mt. Wilson telescope during the opposition (time of closest approach) of 1926 and 1927. Like all infrared observations, the interpretation was complicated by interference from the Earth's atmosphere. By 1947, the understanding of the effect of atmospheres on the emission of infrared light had progressed to the point that the mass and composition of the Martian atmosphere could be estimated. Based on these measurements, Kuiper estimated that Mars had an atmosphere that was almost entirely CO2, with a sufficient mass that the surface pressure on Mars would be only .03% of the surface pressure on Earth. This turns out to be an underestimate of the true mass by about a factor of twenty, but even so, the picture of Mars as a nearly airless planet was not far wrong. By way of comparison, infrared observations of Venus interpreted in the 1940's using similar techniques suggested that the surface pressure of Venus was a fifth that of Earth -an underestimate by a factor of nearly 500. The Mt. Wilson infrared observations of Mars also indicated that the atmosphere was almost entirely CO2, with almost no water - so little, in fact, that water vapor wouldn't condense until temperatures fell below —60C, and at those temperatures it would condense into frost, not liquid. Infrared observations showed further that the visible polar ice caps of Mars are most probably made of water ice rather than frozen CO2. Temperature estimates based on the Mt. Wilson infrared observations were less informative. Nicholson and Pettit, in the same paper in which they discuss Lunar temperatures, noted a very large day/night cycle of Martian infrared, indicating extreme diurnal constrasts unlike those found on Earth. They concluded that this was due to the lack of water vapor in the Martian atmosphere, but we shall encounter the true reason in Chapter 7. Writing in 1947, Adel reported quantitative estimates of Martian surface temperature ranging from as low as 236K to as high as 318K (or even higher if the surface was assumed to emit infrared inefficiently, as does granite). Some of the variation in reported temperatures may have been due to the fact that these were not whole-disk observations, insofar as Mars exhibits extreme temperature contrasts. The higher end of the estimates based on Mt. Wilson turned out to be far greater than the actual maximum ground temperature encountered anywhere on the planet.
Given the thin atmosphere, what does theory lead us to expect about the Martian surface temperature? By 1947, it was a simple exercise to compute the expected temperature of airless bodies like the Moon, using arguments like those we'll discuss near the beginning of Chapter 3. Planets with atmosphere present more of a problem. The atmosphere of Mars is thin compared to that of Earth, but how thin does an atmosphere have to be (particularly if it's pure CO2) in order to have a minimal effect on planetary temperature? We'lll learn how to answer that question in the latter portions of Chapter 3. Based on similar reasoning, Kuiper, writing in 1947, inferred correctly that the atmosphere would have only minor effect on T, in particular allowing severe night-time temperature drops. By the 1940's Mars was already looking inhospitable - a mostly cold, dry nearly airless body where (it was still hoped) conceivably lichens might eke out a living but certainly not Thuvia, Maid of Mars.
Ground-based and theoretical estimates of the Martian climate improved gradually over the next decade, but the real breakthrough came with the Mariner flyby of 1965 and the two Viking orbiters and landers of 1976. Spaceborne infrared observations gave the first detailed picture of geographic variations of the ground and air temperature, and Viking provided in situ air temperature measurements of the ground. These observations consolidated the picture of Mars as a planet which (as we find it now) has more in common with the airless moon than with Earth. Even hopes of lichens were dashed, though it is too soon to give up on bacterial life, especially in view of the innovative chemical entrepreneurship shown by non-photosynthetic bacteria on Earth. In discussing Martian temperatures, one must take care to distinguish the air temperature from the temperature of the ground itself, as the two differ considerably. At high noon in the tropics, the ground can indeed briefly get as warm 300K, though temperature drop by 100K or more at night. The air temperature also shows an extreme day/night cycle, but the peak daytime air temperatures are far less than the peak ground temperature; at the Viking Lander 1 site, in the tropics at 22N latitude, the daytime air temperature never exceeds 260K, and plummets to under 200K at night. The reason the air temperatures are so much lower than the ground temperatures will become clear in Chapters 4 and 6.
There are considerable seasonal and latitudinal variations in daytime temperature, and the Southern Hemisphere polar summer is notably warmer than the Northern Hemisphere polar summer. Night-time temperatures are comparatively uniform both geographically and seasonally; the Mars surface cools so fast that that once it is dark, it evidently doesn't matter much whether it has been dark for a few hours or a hundred days. The Southern Hemisphere winter pole does get notably colder than the rest of the planet, dropping to as low as 160K. The Viking landers also provided the first clear picture of surface pressure variations on Mars. They showed that the Martian surface pressure varies from a high of about 1% of Earth's surface pressure to a low of about 0.6%, with the lowest values occurring in Southern Hemisphere winter. Since surface pressure gives a measure of the mass of air in the atmosphere (2), the large variation of pressure indicates that a considerable portion of Mars' CO2 atmosphere snows out over the North pole in Northern winter and the South Pole in Southern winter, only to sublimate back into the atmosphere as spring approaches. A theory for the seasonal cycle of Martian temperature and pressure will be developed in Chapter 7.
So, Venus, like the porridge tasted by Goldilocks is too hot. Mars is too cold, and Earth is just right. One could quite reasonably object that this is a view prejudiced by our own status as a form of terrestrial life, and that conditions "too hot" by our standards could well be "just right" for somebody else. However, it appears that there's nobody home on either Venus or Mars (not even a microbial somebody), so if conditions there are indeed "just right" for somebody, it must not be very easy for such a somebody to evolve, given that it didn't happen in the past four billion years.
Presumably, Venus started out with a composition rather similar to Earth. What went wrong? Why did it keep most of it's CO2 in its atmosphere, whereas most of Earth's CO2 got bound up in carbonate rocks? Where did its water go? The answer came in 1967 with the theory of the runaway greenhouse, formulated first by M. Kombayashi and independently rediscovered shortly thereafter by Andrew Ingersoll of Caltech. This theory puts together two simple bits of physics, the first being that water vapor content of a saturated atmosphere increases exponentially with temperature (Chapter 2), and the second being that water vapor is a greenhouse gas (Chapter 4). When the two are put together, it is found that a planet which receives sufficient solar radiation can get into a runaway cycle where the planet warms in response to absorbed sunlight, which causes more water vapor to enter the atmosphere, which causes more greenhouse effect, which leads to further warming in an unstable feedback loop that doesn't end until the entire ocean is evaporated into the atmosphere. At that point, the water vapor in the upper atmosphere breaks down into hydrogen and oxygen under the influence of high energy solar radiation, and the hydrogen escapes to space while the oxygen reacts with rocks. Without liquid water, the Urey reaction which turns CO2 into limestone can't take place, so all the outgassed CO2 stays in the atmosphere. The runaway greenhouse theory (explored in Chapter 4) gives rather precise predictions of the circumstances under which a runaway can occur, and explains why Earth did not undergo a runaway despite the fact that it has a water ocean. The work of Kombayashi and Ingersoll is another example of the general idea that big ideas come from simple models. Their reasoning was based on simple radiation models of the sort developed in the first half of Chapter 4. This work also illustrates another general principle of planetary climate: profound results can be obtained by combining a few bits of very basic physics in a novel way.
Radar mapping from the Magellan orbiter of 1990 revealed another remarkable fact about Venus: Unlike the Earth, with long-lived continents and a gradually subducted sea floor, Venus has a young-looking uncratered surface, suggesting that the crust may have been engulfed and resurfaced as recently as 500 million years ago. This has important implications for planetary habitability. Evidently, the formation of a planetary crust, as at end of Hadean, is not end of the peril from fires in the deep. For habitability, the crust has to be relatively stable, engulfed slowly in subduction zones (as is the Earth's sea floor) rather than being subject to episodic catastrophic volcanism as seems to have been the case on Venus. However, if the crust is engulfed too slowly, then limestone that forms by the Urey reaction is only sluggishly recycled, leading to a drawdown or atmospheric CO2 and (under some circumstances) a very cold planet; since photosynthesis uses CO2 as a feedstock, low CO2 impedes habitability even if a planet is in an orbit where it doesn't need the greenhouse effect of atmospheric CO2 in order to stay warm. The question of when a planet has plate tectonics like Earth or when it has episodic catastrophic resurfacing like Venus, is another one of the great questions of planetary science, though one we will not take up at any great length in this book.
Mars may be impoverished in atmosphere compared to Venus, but it has something Venus lacks: a geological record of the distant past preserved in its crust. In fact, the ancient features on the surface of Mars are far better preserved than is the case on Earth. Mars appears to have lost most of its atmosphere quite early on, leading to a near-halt in the rate of erosion of surface features. The first high resolution data of Martian surface features, returned by the Mariner mission, revealed a startling fact. Evidently, Mars was not always the dry, frigid planet cloaked in a tenuous atmosphere that we see today. The Mariner photographs revealed dry river-like channels for which the only likely explanation is flowing surface water, which would be impossible under the conditions of the present Martian climate. A more recent image of this type of feature is shown in Figure 1.1. The rate of cratering of a planet goes down with time, so the features can be dated by counting superposed craters; many of the major river-like features date to very early in Mars history, perhaps 4 billion years ago. This led to the concept of a "warm,wet Early Mars." But how could Mars have been so much warmer than it is at present, at a time when the Sun was so much fainter. Mars presents an even more extreme version of the Faint Young Sun paradox than does the Earth. It will probably come as no surprise that it was Carl Sagan who first pointed out the implications of Martian dry river networks. It is still somewhat disputed whether the surface features really demand that Early Mars be warm and wet, but adopting the warm-wet view, the resolution of the Faint Young Sun problem, as was the case for Earth, lies in
Figure 1.1: Nanedi Vallis on Mars, observed by the Mars Orbiter Camera on the Mars Global Surveyor Mission. The image covers an area 9.8km wide and 18.5km tall.
the supposition that the Early Mars atmosphere had a substantially stronger greenhouse effect. What kind of atmosphere could warm Mars to the point that liquid water could flow long distances at the surface? That question is taken up in Chapters 4 and 5.
If Mars started out with such a dense atmosphere, where did it go? Some possible answers are suggested in Chapter 8. Modern high-resolution images suggest other forms of massive climate change on Mars. In particular, there are tropical landform features suggesting that at some time in the past, glaciers formed in the Martian tropics, whereas virtually all the ice is today sequestered at the cold polar regions. The tropical glacier landforms suggest that at some time in the past, the equatorial regions of Mars were colder than the poles? How could that be? The answer is provided in Chapter 7.
The fact that Earth maintained habitable conditions while Venus succumbed to a runaway greenhouse and got too hot while Mars lost its atmosphere and got too cold raises the question of just how narrowly Earth escaped the fate of Mars and Venus. How much could Earth's orbital distance be changed before it turned into Mars or Venus, and how would the answer to this question change if Earth were more massive (making it easier to hold onto atmosphere) or less massive (making it easier to lose atmosphere? If Mars were as large as Earth, would it still be habitable today? What if Venus were as small as Mars? Perhaps if the orbits of Mars and Venus were exchanged, our solar system would have three habitable planets, instead of just one.
The range of orbital distances for which a planet retains Earthlike habitability over billions of years is known as the habitable zone. Determing the habitable zone, and how it is affected by planetary size and composition as well as the properties of the parent star, is one of the central problems of planetary climate.
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