A critic might raise two objections at this point. The first is that to understand the global environment of the Earth we need at most to have some knowledge of the present structure of the Earth. We need to take into account only the present mass and energy distribution of the Earth, not what happened billions of years ago. The second objection is that to understand the present structure of the Earth we do not need to think of Earth as a planet. The structure of Earth does not depend on that of Mars or Neptune. Why then do we need to know how they are structured in order to know how the Earth is structured?
A simple consideration alone disposes of the first objection: the history of the Earth is important to determine its possible range of behavior in the future. Take as basic a matter as the age of the Earth. If the Earth is indeed four and a half billion years old, certain mechanisms are plausible candidates to account for the transformation of the environment. Plate tectonics needs tens of millions of years for some of the feats that we impute to it. Radical changes in the chemistry of the atmosphere (e.g., the rise in oxygen from a trace gas to a large component) might have taken bacteria tens, or perhaps hundreds, of millions of years. Imagine now for the sake of argument that all the evidence for the age of the Earth is wrong, and that the Earth is only ten thousand years old. In that case, if the Earth formed roughly as we believe, it must have dissipated energy at such a high rate that the global environment must have been run by completely different mechanisms. And since many of those mechanisms would be the same ones that operate today, or would have caused them, our understanding of today's Earth would have to be seriously mistaken. Thus to understand the present global environment, and glimpse its future, we need to have some idea of how the Earth started and of how it evolved. And without planetary science, including the evidence collected by the astronauts on the Moon, the only measure we would have of the age of the Earth would be the chain of “begots” in the Bible.
I will answer the second objection in two stages. First, the structure of the Earth may be seemingly independent from those of Mars and Neptune right now, but unless we reject the theory of planetesimals off hand, Mars and Neptune did have a lot to do with how the Earth came to have the structure it has today, to be the planet it is now. And since history is important after all, as we have just seen, it follows that studying Mars and Neptune, as well as the other members of the solar system, may be very instructive to those of us Earthbound. Second, the critic seems to ignore how the rest of the solar system affects today’s planet Earth more directly. For example, energy and materials arrive constantly from outer space. If the atmosphere did not absorb ultraviolet, X-ray, and gamma radiation, life on land would be very unlikely. And life continues to survive because the Earth is the kind of planet that it is and no other, within the context of the solar system. A smaller, less dense Earth, or an Earth far closer to the sun might have defeated life's best efforts to gain a foothold and flourish.
The complex interactions between our planet’s systems presently regulate in a fortunate manner our share of solar energy. But that energy does not remain constant. It appears that the luminosity of the sun was much less during the first stages of the formation of the Earth, before its nuclear fires were ignited. And even afterward, the sun’s luminosity, according to some hypotheses, may have been 30% lower from what it is now. The sun also seems to undergo a variety of cycles in its output of energy. To complicate matters even more, the Earth's tilt with respect to the solar plane may vary slightly (the spin axis of the Earth oscillates between 22 and 24.4 degrees every 41,000 years).
The eccentricity of the Earth's orbit also changes slightly in cycles of 100,000 years (the orbit departs from its nearly circular shape). M. Milankovitch suggested many decades ago that this cycle was the cause of the Earth's ice ages, which also have a cycle of about 100,000 years. Since the two cycles could not initially be shown to coincide, and since no one proposed a generally accepted mechanism by which the expected change in luminosity would lead to an ice age, Milankovitch's hypothesis was met with skepticism. Nevertheless, recent studies of the history of the oceans provide strong evidence that the two cycles do coincide. Of course, if variations in the energy output of the sun, or in received luminosity, influence the Earth's climate, they will also influence that of other planets. We may then look in those worlds for evidence of such influence, and for a determination of the mechanisms by which that influence is exercised. A better understanding of those mechanisms will give us a better grasp of the evolution of our global environment, and consequently a better idea of its future.
. For an account see S. Schneider, op. cit., pp. 225-229. Since presumably life could not have survived under the corresponding lower temperatures, several writers have proposed a variety of mechanisms. C. Sagan and G. Mullen first suggested a large greenhouse effect driven by ammonia and methane. Then T. Owen and others argued that large concentrations of CO2 were more likely than ammonia (up to 1000 times today's CO2 levels). Most hypotheses depend on a large greenhouse effect created by the large out gassing from the interior of a hot young planet.
. Ibid. p. 261.
. For a report see R.A. Kerr, "Milankovitch Climate Cycles Through the Ages," in Science, February 27, 1987, vol. 235, pp. 973-74.
. O.B. Toon, J.B. Pollack, and K. Rages, "A Brief Review of the Evidence for Solar Variability on the Planets," in R.O. Peppin, J.A. Eddy, and R.B. Merrill, (eds.), Proceedings of the Conference on the Ancient Sun, 1980, pp. 523-531.