For a look at what might have been the original material in the solar system, comets are a good bet. Many of them have been under the influence of significant solar radiation for a relatively short time (sometimes only in the millions of years, whereas the planets and asteroids have been under it for billions of years now). The spacecraft missions to Comet Halley, and the future missions in which we will try to land on a comet and penetrate its core, are bound to enhance our ideas about these messengers of times long past.
We can also find clues about the many factors that affect the evolution of a planet – internal structure, tectonics, or atmosphere – on most of the bodies of the solar system. Since they were formed under different circumstances, because of their position in the solar system and the distribution of materials in the sweep of their orbits, and since their interactions with the rest of the solar system are somewhat different from ours, they offer a wide range of instances of those evolutionary factors at work. It is not surprising that under these different circumstances, unusual mechanisms have come into existence. In trying to understand such mechanisms, we modify our ideas about our own planet, as we will see in the next section.
2. Stretching our views of planetary mechanisms
Let us consider anew the standard account of the forces that lead to the geologic evolution of a rocky planet. The denser the planet, the more heat will be available from radioactive elements; and the larger the planet, the more retarded the loss of heat. By this account no planets much less massive than the Earth could still have active volcanoes or relatively young surfaces. Learning to draw the line has not been easy. Mars, for example, shows evidence of recent volcanism (within the last two million years); but even if Mars is not a dead planet, its surface is testimony to a prolonged coma. The Moon, which is definitely much smaller than the Earth, does no longer seem active at all. But matters soon become far more complex than. Harold Urey argued a long time ago for a greater variety of mechanisms that could produce internal heat in a planetary body. Some theorists, following in Urey's footsteps, went as far as speculating about volcanoes on Io, a Jovian moon about the size of ours – an idea that seemed much too fanciful to most researchers until, to their astonishment, they looked through Voyager’s camera and clearly saw the gigantic plume of a volcano rising over Io's horizon.
Another surprise greeted them when Voyager discovered that Enceladus, a small icy moon of Saturn about 1/100,000th the mass of the Earth, might be geologically active. If Enceladus were inert, as a small moon is supposed to be, it could not renew its surface, and thus it would show evidence of the large bombardment that took place during the early stages of the solar system. But the surface of Enceladus looks quite new. A similar argument can be made about Europa, a beautifully smooth satellite of Jupiter that apparently has large water oceans under a frozen surface.
Evidence of early geological activity can be found in many other moons, including Uranus' moons Oberon, Titania, and Ariel. One of the mechanisms that may explain these findings is that each of these moons is caught between two or more masses that exert significant gravitational attraction upon it. As a result of Io's specific position, for example, its mass expands and contracts in tides created by Jupiter on one side and Europa on the other. But we should not suppose that this form of tidal heating is the only additional mechanism able to produce an active geology. The bizarre geological formations in Uranus' moon Miranda (see figure) can perhaps best be explained by supposing that Miranda has broken up one or more times and is now in the process of differentiation, with very large ice formations still side by side with big chunks of dark carbon compounds.
In Ganymede, the largest moon of Jupiter and the solar system, we can see what looks like signs of the beginnings of plate tectonics, now conveniently frozen for our inspection. In other worlds we can see other stages of the generation and dissipation of internal heat.
All these considerations remind me of my own experience concerning the geology of Venus. As the reader may imagine, since Venus is practically a twin of the Earth, comparative planetologists have been itching to take a good look at its atmosphere and geology. Unfortunately exploring Venus has been extremely difficult, for the dense clouds keep the surface hidden from our view. Part of the reason is that in Venus' atmosphere there is 300,000 times more CO2 than in Earth's. The resulting greenhouse effect has helped produce a temperature of almost 900 degrees Fahrenheit, about the melting point of lead. In that oven, volatile substances are kept at a large height from the surface, where they form dense clouds that keep radiating heat downwards (most sunlight is actually reflected by those clouds into space, which explains why Venus is so bright). The density is one hundred times that of the Earth's atmosphere, which turns a wind of 10 miles an hour into a hurricane (albeit without rain). And whereas on Earth rain cleanses the atmosphere and changes the land, on Venus the rain is made of sulfuric acid and the heat evaporates it long before it can touch the ground.
In this inhospitable world, our landers perish in a matter of hours, unable to give us more than the vaguest of glimpses. This was the situation until the arrival of the spacecraft Magellan in the 1990s. Magellan’s radar gave us maps of Venus better in many respects than those we then had of Earth.
Once upon a time Venus might have been very different. When the sun was dimmer oceans, rivers, and perhaps even life may have existed there. According to a plausible scenario, as the sun became more luminous, life could not keep up with the increase in energy and a runaway greenhouse effect began to vaporize Venus' oceans. The increase in water vapor made the atmospheric temperature rise even more, which then vaporized more of the oceans. Eventually the oceans ended up high in the atmosphere, where ultraviolet radiation disassociated the H2O to form atomic hydrogen (H) and the radical OH. Most of the atomic hydrogen was lost to space while the OH entered into a variety of reactions with other substances in the atmosphere.
If anything like this scenario took place one would expect a rather high ratio of deuterium to standard hydrogen. Normally only so many hydrogen atoms should be expected to be in the form of the isotope deuterium (deuterium has a neutron in the nucleus). But since deuterium is heavier, it is not as likely to be blown away from the planet; and thus as time went by, it should have become a larger percentage of the hydrogen still found in the atmosphere of Venus. This is exactly the case.
In conversations with planetary scientists in the early 1980s I floated the suggestion that the absence of water on Venus would change the viscosity of the rocks (viscosity is the resistance to flow) and, thus, plate tectonics was unlikely. The change in viscosity would make subduction and other plate motions very difficult. That is, Venus was unlikely to show much on the way of plate tectonics. Not to worry, I was told: high temperature can make the mantle behave like melting hot butter. By 1984, however, M. Carr and others had shown that lack of water would indeed make Venusian plate tectonics rather unlikely (I am sure they had been thinking along these lines longer than I and, moreover, had the expertise and imagination to come up with convincing explanations). Most observers now agree with Carr. It seems, then, that a runaway greenhouse effect can deprive a planet of plate tectonics. And it occurred to me, using the arguments about the role of life given earlier in this chapter, that we should consider an astonishing corollary: without the climatic regulation by life, plate tectonics might have disappeared from the Earth as well.
This hypothesis, which seemed so fanciful twenty some years ago, is apparently considered quite reasonable nowadays. Indeed the reasoning that takes us to the biological modulation of plate tectonics goes further. As the planetary scientist D.H. Grinspoon puts it,
If you agree to that, you must agree that Earth’s interior thermal evolution has been affected by its changing atmosphere and biosphere, because plate tectonics is the main way that Earth cools its interior. Even such remote quarters as the molten iron outer core, which produces Earth’s singular magnetic field, may not have been immune to the modifying effects of Earth’s quirky air, its unique, biologically touched, gaseous envelope.
This line of thought begins to give us a sense of the extraordinary complexity involved in the global environment of a planet, particularly in the case of a still dynamic planet such as Earth, or Venus. This complexity in turn raises the suspicion that global environments are mathematically complex, and therefore practically unpredictable (“classical” complexity, by contrast, is often a measure of our lack of understanding more than of the possibility of understanding). And even if some features of a planet remain only partially predictable, such as next week’s weather, much can be learned from comparative planetology about the key factors and the trends that can be reasonably expected, just as someone raised on the equator learns that there is a most drastic difference in the mean temperatures of winter and summer in the Northern hemisphere. It is precisely in determining what environmental factors are relevant to what, and how they are relevant, that the study of Venus becomes most useful, as I trust we have seen in the preceding discussion.
. A significant difference, however, may be that Ganymede is about 50% water, and so its crustal movements are closer to ice- tectonics than those on Earth.
. The Geology of the Terrestrial Planets, op. cit., p. 77.
. There are some who still manage to see something resembling the Earth’s mid-ocean ridges, but even if we grant that, it still seems a far shot from the full-blown terrestrial plate tectonics.
. D.H. Grinspoon, Venus Revealed, Addison-Wesley Publishing Co., Inc., 1997, p. 179.