Broca's Brain

by Carl Sagan

  We might obtain some clues about the nature of Titan by examining the two major worlds in the outer solar system—Jupiter and Saturn. Both have a general reddish or brownish coloration. That is, the upper layer of clouds that we see from the Earth has this hue primarily. Something in the atmosphere and clouds of these planets is strongly absorbing blue and ultraviolet light, so that the light that is reflected back to us is primarily red. The outer solar system, in fact, has a number of objects that are remarkably red. Although we have no color photographs of Titan because it is 800 million miles away and has an angular size smaller than the Galilean satellites of Jupiter, photoelectric studies reveal that it is, in fact, very red. Astronomers who thought about the problem once believed that Titan was red for the same reason that Mars is red: a rusty surface. But then the reason for Titan’s red color would be different from the reason for Jupiter’s and Saturn’s, because we do not see to a solid surface on those planets.

  In 1944 Gerard Kuiper detected spectroscopically an atmosphere of methane around Titan—the first satellite found to have an atmosphere. Since then, the methane observations have been confirmed, and at least moderately suggestive evidence for the presence of molecular hydrogen has been provided by Lawrence Trafton of the University of Texas.

  Since we know the amount of gas necessary to produce the observed spectral absorption features, and we know from its mass and radius the surface gravity of Titan, we can deduce the minimum atmospheric pressure. We find it is something like 10 millibars, about one percent of the Earth’s atmospheric pressure—a pressure that exceeds that of Mars. Titan has the most Earth-like atmospheric pressure in the solar system.

  Not only the best, but the only visual telescopic observations of Titan have been made by Audouin Dollfus at the Meudon Observatory in France. These are hand drawings done at the telescope during moments of atmospheric steadiness. From the variable patches that he observed, Dollfus concluded that things are happening on Titan that do not correlate with the satellite’s rotation period. (Titan is thought always to face Saturn, as our Moon does the Earth.) Dollfus guessed that there might be clouds, at least of a patchy sort, on Titan.

  Our knowledge of Titan has made a number of substantial quantum jumps forward in recent years. Astronomers have successfully obtained the polarization curve of small objects. The idea is that initially unpolarized sunlight falls on Titan, say, and is polarized on reflection. The polarization is detected by a device similar in principle to, but more sophisticated and sensitive than, “polaroid” sunglasses. The amount of polarization is measured as Titan goes through a small range of phases—between “full” Titan and slightly “gibbous” Titan. The resulting polarization curve, when compared to laboratory polarization curves, gives information on the size and composition of the material responsible for the polarization.

  The first polarization observations of Titan, made by Joseph Veverka, indicated that the sunlight reflected back from Titan is most likely reflected off clouds and not off a solid surface. Apparently there is on Titan a surface and a lower atmosphere that we do not see; an opaque cloud deck and an overlying atmosphere, both of which we do see; and an occasional patchy cloud above that. Since Titan appears red, and we view it at the cloud deck, there must, according to this argument, be red clouds on Titan.

  Additional support for this concept comes from the extremely low amount of ultraviolet light reflected from Titan, as measured by the Orbiting Astronomical Observatory. The only way to keep Titan’s ultraviolet brightness small is to have the ultraviolet absorbing stuff high up in the atmosphere. Otherwise Rayleigh scattering by the atmospheric molecules themselves would make Titan bright in the ultraviolet. (Rayleigh scattering is the preferential scattering of blue rather than red light, which is responsible for blue skies on Earth.)

  But material that absorbs in the ultraviolet and violet appears red in reflected light. So there are two separate lines of evidence (or three, if we believe the hand drawings) for an extensive cloud cover on Titan. What do we mean by extensive? More than 90 percent of Titan must be cloaked in clouds to match the polarization data. Titan seems to be covered by dense red clouds.

  A second astonishing development was inaugurated in 1971 when D. A. Allen of Cambridge University and T. L. Murdock of the University of Minnesota found that the observed infrared emission from Titan at a wavelength of 10 to 14 microns is more than twice what is expected from solar heating. Titan is too small to have a significant internal energy source like Jupiter or Saturn. The only explanation seemed to be the greenhouse effect in which the surface temperature rises until the infrared radiation trickling out just balances the absorbed visible radiation coming in. It is the greenhouse effect that keeps the surface temperature of the Earth above freezing and the temperature of Venus at 480°C.

  But what could cause a Titanian greenhouse effect? It is unlikely to be carbon dioxide and water vapor as on Earth and Venus, because these gases should be largely frozen out on Titan. I have calculated that a few hundred millibars of hydrogen (1,000 millibars is the total sea-level atmospheric pressure on Earth) would provide an adequate greenhouse effect. Since this is more than the amount of hydrogen observed, the clouds would have to be opaque at certain short wavelengths and more nearly transparent at certain longer wavelengths. James Pollack, at NASA’s Ames Research Center, has calculated that a few hundred millibars of methane might also be adequate and, moreover, might explain some of the details of the infrared emission spectrum of Titan. This large amount of methane would also have to hide under the clouds. Both greenhouse models have the virtue of invoking only gases thought to exist on Titan; of course, both gases might play a role.

  An alternative model of the Titan atmosphere was proposed by the late Robert Danielson and his colleagues at Princeton University. They suggest that small quantities of simple hydrocarbons—such as ethane, ethylene and acetylene—which have been observed in the upper atmosphere of Titan absorb ultraviolet light from the Sun and heat the upper atmosphere. It is then the hot upper atmosphere and not the surface that we see in the infrared. On this model there need be no enigmatically warm surface, no greenhouse effect, and no atmospheric pressure of hundreds of millibars.

  Which view is correct? At the present time no one knows. The situation is reminiscent of studies of Venus in the early 1960s when the planet’s radio-brightness temperature was known to be high, but whether the emission was from a hot surface or a hot region of the atmosphere was (appropriately) hotly debated. Since radio waves pass through all but the densest atmospheres and clouds, the Titan problem might be resolved if we had a reliable measure of the radio-brightness temperature of the satellite. The first such measurement was performed by Frank Briggs of Cornell with the giant interferometer of the National Radio Astronomy Observatory in Green Bank, West Virginia. Briggs finds a surface temperature of Titan of −140°C with an uncertainty of 45°. The temperature in the absence of a greenhouse effect is expected to be about −185°C. Briggs’s observations therefore seem to suggest a fairly sizable greenhouse effect and a dense atmosphere, but the probable error of the measurements is still so large as to permit the zero greenhouse case.

  Subsequent observations by two other radio astronomical groups give values both higher and lower than Briggs’s results. The higher range of temperatures, astonishingly, even approaches temperatures in cold regions of the Earth. The observational situation, like the atmosphere of Titan, seems very murky. The problem could be resolved if we could measure the size of the solid surface of Titan by radar (optical measurements give us the distance from cloudtop to cloudtop). The problem may have to await studies by the Voyager mission, which is scheduled to send two sophisticated spacecraft by Titan—one very close to it—in 1981.

  Whichever model we select is consistent with the red clouds. But what are they made of? If we take an atmosphere of methane and hydrogen and supply energy to it, we will make a range of organic compounds, both simple hydrocarbons (like the sort that are needed to m
ake Danielson’s inversion layer in the upper atmosphere) and complex ones. In our laboratory at Cornell, Bishun Khare and I have simulated the kinds of atmospheres that exist in the outer solar system. The complex organic molecules we synthesize in them have optical properties similar to those of the Titanian clouds. We think there is strong evidence for abundant organic compounds on Titan, both simple gases in the atmosphere and more complex organics in the clouds and on the surface.

  One problem with an extensive Titanian atmosphere is that the light gas hydrogen should be gushing away because of the low gravity. The only way that I can explain this situation is that the hydrogen is in a “steady state.” That is, it escapes but is replenished from some internal source—volcanoes, most likely. The density of Titan is so low that its interior must be almost entirely composed of ices. We can think of it as a giant comet made of methane, ammonia and water ices. There must also be a small admixture of radioactive elements which, while decaying, will heat their surroundings. The heat conduction problem has been worked out by John Lewis, of MIT, and it is clear that the near-surface interior of Titan will be slushy. Methane, ammonia and water vapor should be outgassed from the interior and broken down by ultraviolet sunlight, producing atmospheric hydrogen and cloud organic compounds at the same time. There may be surface volcanoes made of ice instead of rock, spewing out in occasional eruptions not liquid rock but liquid ice—a lava of running methane, ammonia and perhaps water.

  There is another consequence of the escape of all this hydrogen. An atmospheric molecule that achieves escape velocity from Titan generally does not have escape velocity from Saturn. Thus, as Thomas McDonough and the late Neil Brice of Cornell have pointed out, the hydrogen that is being lost from Titan will form a diffuse toroid, or doughnut, of hydrogen gas around Saturn. This is a very interesting prediction, first made for Titan but possibly relevant for other satellites as well. Pioneer 10 has detected such a hydrogen toroid around Jupiter in the vicinity of Io. As Pioneer 11 and Voyager 1 and 2 fly near Titan, they may be able to detect the Titan toroid.

  Titan will be the easiest object to explore in the outer solar system. Nearly atmosphereless worlds such as Io or the asteroids present a landing problem because we cannot use atmospheric braking. Giant worlds such as Jupiter and Saturn have the opposite problem: the acceleration due to gravity is so large and the increase in atmospheric density is so rapid that it is difficult to devise an atmospheric probe that will not burn up on entry. Titan, however, has a dense enough atmosphere and a low enough gravity. If it were a little closer, we probably would be launching entry probes there today.

  Titan is a lovely, baffling and instructive world which we suddenly realize is accessible for exploration: by fly-bys to determine the gross global parameters and to search for breaks in the clouds; by entry probes to sample the red clouds and unknown atmosphere; and by landers to examine a surface like none we know. Titan provides a remarkable opportunity to study the kinds of organic chemistry that on Earth may have led to the origin of life. Despite the low temperatures, it is by no means impossible that there is a Titanian biology. The geology of the surface may be unique in all the solar system. Titan is waiting …

  CHAPTER 14

  THE CLIMATES

  OF PLANETS

  Is it not the height of silent humour

  To cause an unknown change

  in the earth’s climate?

  ROBERT GRAVES,

  The Meeting

  BETWEEN 30 and 10 million years ago, it is thought, temperatures on Earth slowly declined, by just a few Centigrade degrees. But many plants and animals have their life cycles sensitively attuned to the temperature, and vast forests receded toward more tropical latitudes. The retreat of the forests slowly removed the habitats of small furry binocular creatures, weighing only a few pounds, which had lived out their days brachiating from branch to branch. With the forests gone, only those furry creatures able to survive on the grassy savannas were to be found. Some tens of millions of years later, those creatures left two groups of descendants: one which includes the baboons and the other called humans. We may owe our very existence to climatic changes that on the average amount to only a few degrees. Such changes have brought some species into being and extinguished others. The character of life on our planet has been powerfully influenced by such variations, and it is becoming increasingly clear that the climate is continuing to change today.

  There are many indications of past climatic changes. Some methods reach far into the past, others have only a limited applicability. The reliability of the methods also differs. One approach, which may be valid for a million years back in time, is based on the ratio of the isotopes oxygen 18 to oxygen 16 in the carbonates of shells of fossil foraminifera. These shells, belonging to species very similar to some that can be studied today, vary the oxygen 16/oxygen 18 ratio according to the temperature of the water in which they grew. Somewhat similar to the oxygen-isotope method is one based upon the ratio of the isotopes sulfur 34 to sulfur 32. There are other, more direct fossil indicators; for example, the widespread presence of corals, figs and palms denotes high temperatures, and the abundant remains of large hairy beasts, such as mammoths, indicate cold temperatures. The geological record is replete with extensive evidence of glaciation—great moving sheets of ice that leave characteristic boulders and erosional traces. There is also clear geological evidence for beds of evaporites—regions where briny water has evaporated leaving behind the salts. Such evaporation occurs preferentially in warm climates.

  When this range of climatic information is put together, a complex pattern of temperature variation emerges. At no time, for example, is the average temperature of the Earth below the freezing point of water, and at no time does it even approach the normal boiling point of water. But variations of several degrees are common, and even variations of twenty or thirty degrees may have occurred at least locally. Fluctuations of a few degrees Centigrade happen over characteristic times of tens of thousands of years, and the recent succession of glacial and interglacial periods has this timing and temperature amplitude. But there are climatic fluctuations over much longer periods, the longest being on the order of a few hundred million years. Warm periods appear to have occurred about 650 million years ago and 270 million years ago. By the standards of past climatic fluctuations, we are now in the midst of an ice age. For most of the Earth’s history, there were no “permanent” ice caps, as in the Arctic and Antarctic today. We have, over the past few hundred years, made a partial emergence from our ice age caused by some as yet unexplained minor climatic variation; and there are certain signs that we may plunge back into the global cold temperatures characteristic of our epoch as seen from the perspective of the immense vistas of geological time. It is a sobering fact that 2 million years ago the site of the city of Chicago was buried under a mile of ice.

  What determines the temperature of Earth? As seen from space, it is a rotating blue ball streaked with varying cloud patches, reddish-brown deserts and brilliant white polar caps. The energy for heating the Earth comes almost exclusively from sunlight, the energy conducted up from the hot interior of the Earth amounting to less than one thousandth of one percent of that arriving in the form of visible light from the Sun. But not all the sunlight is absorbed by the Earth. Some is reflected back to space by polar ice, clouds, and the rocks and water on the surface of the Earth. The average reflectivity, or albedo, of the Earth, as measured directly from satellites and indirectly from Earthshine reflected off the dark side of the Moon, is about 35 percent. The 65 percent of sunlight that is absorbed by the Earth heats it to a temperature which can readily be calculated. This temperature is about −18°C, below the freezing point of seawater and some 30°C colder than the measured average temperature of the Earth.

  The discrepancy is due to the fact that this calculation neglects the so-called greenhouse effect. Visible light from the Sun enters the Earth’s clear atmosphere and is transmitted through to the surface. The surface, how
ever, in attempting to radiate back into space, is constrained by the laws of physics to do so in the infrared. The atmosphere is not so transparent in the infrared, and at some wavelengths of infrared radiation—such as 6.2 microns or 15 microns—radiation would travel only a few centimeters before being absorbed by atmospheric gases. Since the Earth’s atmosphere is murky and absorbing at many wavelengths in the infrared, the thermal radiation given off by the surface of the Earth is impeded in escaping to space. In order to have a close equality between the radiation received by the Earth from the Sun and the radiation emitted by the Earth to space, the surface temperature of the Earth must then rise. The greenhouse effect is due not to the major atmospheric constituents of the Earth, such as oxygen and nitrogen, but almost exclusively to the minor constituents, especially carbon dioxide and water vapor.

  As we have seen, the planet Venus is probably a case where the massive injection of carbon dioxide and smaller amounts of water vapor into a planetary atmosphere has led to such a large greenhouse effect that water cannot be maintained on the surface in the liquid state; hence, the planetary temperature runs away to some extremely high value—in the case of Venus, 480°C.

  We have so far been talking about average temperatures. The temperature of the Earth varies from place to place. It is colder at the poles than at the equator because, in general, sunlight falls directly on the equator and obliquely on the poles. The tendency for the temperatures to be very different between equator and poles on Earth is moderated by atmospheric circulation. Hot air rises at the equator and moves at high altitudes to the poles, where it settles and returns to the surface; it then retraces its path, but at low altitudes, from pole back to equator. This general motion—complicated by the rotation of the Earth, its topography and the phase changes of water—is responsible for weather.