UNDERSTANDING THE EARTH AS A PLANET

Chapter 4G

UNDERSTANDING THE EARTH AS A PLANET

A sketch of the view

Our global understanding of the Earth depends on what we think a planet like Earth is like. The mechanisms that regulate Earth's environment interact with each other in many loops and cycles. These loops and cycles are run by energy, and that energy comes either from the Earth itself or from extraterrestrial sources. The energy that comes from the Earth depends very much on the sort of planet the Earth is. And the useful energy that comes from the sun and the rest of the solar system depends on how the planet Earth relates to the rest of the system.

The Earth produces heat that rejuvenates its surface by creating, moving, and breaking up continents; by forming mountain ranges when the tectonic plates that carry the continents collide; by bringing new materials from the mantle into the crust, oceans, and atmosphere through volcanoes and mid-ocean ridges; and by recycling lands and gases through the spreading and subduction of the crust (the sinking of one plate under another when they collide). The rejuvenation of the Earth's surface creates a great variety in the environment, one of the crucial factors in the natural selection of living things. Moreover, the energy injected into the oceans and the atmosphere drives those systems and greatly influences how they interact with one another.

The Earth's gravitational energy keeps the atmosphere from dissipating into space, and thus it determines to a high degree the density of that atmosphere. That density in turn influences the chemistry of the environment and the climate of the planet. To see the point clearly it pays to compare our planet with others. For example, the density of Mars' atmosphere is so low (about one hundredth that of Earth's) that water cannot exist in liquid form: It goes directly from ice to vapor. One of the crucial differences between Mars and the Earth is precisely that the Earth's mass, and therefore its gravitational attraction, is much larger.

Internal heat and gravitation are both functions of the mass and structure of the Earth. Let us discuss heat. The main sources of internal heat are the release of energy from the decay of radioactive elements such as thorium, uranium, and potassium in the interior of the planet, and the energy left over from the gravitational collapse of the matter that formed the planet initially. Associated with this second source is the heat from differentiation, which occurs when denser materials move downwards and displace less dense materials towards the surface. And initially there was also, of course, the extraordinary heat that the bombardment of the Earth by asteroids generated, enough, in the bigger collisions, not only to vaporize oceans and atmosphere but to melt the entire surface.

Our ideas about these sources of heat are based partly on what we think is the structure of the Earth and partly on how we think the Earth was formed. We think that the Earth was formed by the accretion of planetesimals (chunks of the original materials of the solar system) over four and a half billion years ago. As the Earth grew, its gravitational attraction also grew and the Earth captured even more planetesimals. In a short time the Earth was colliding with many objects of diverse sizes. Many of these collisions would have generated a good deal of heat,^[1] as also did the compression of the accreting materials by the increasing gravitation. In the hot terrestrial interior, the heavier elements separated towards the center, eventually creating a metallic, radioactive core. Less heavy materials concentrated first in the mantle, and then in the lithosphere (the crust and the uppermost portion of the mantle upon which the crust rests). The heat from the core stirs up the mantle and leads to the convection currents that push tectonic plates apart at the ridges. Through those long trenches, a new surface is forged from the mantle that spills forth.^[2]

In trying to understand the mantle, the core, and so on, we do not observe them directly. We infer some of their properties from the measurements we make of the Earth, for example with seismic waves, using a technique called "seismic tomography,” which is based on the notion that seismic waves travel at different speeds through different materials (such as molten metal or solid rock). And then we use those measurements in the light of certain theories in order to discern other geophysical properties. Among those theories are our ideas of how a planetary body forms and how it distributes its energy. More specifically, they are theories of the evolution of a planetary body, adjusted to Earth's mass and position within the solar system (not just its distance from the sun but also its having a very large moon as well). To understand the Earth we must understand what kind of object it is. We know that Earth is a planet; thus we need to understand what a planet is, and more specifically what a planet like Earth is.

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[1]. Several theories would posit a cold accretion of the Earth. If the Earth had condensed from a gas nebula, a cold accretion would make sense. But on the prevailing view that the Earth accreted from planetesimals, I do not think it makes as much sense. Recent calculations do suppose a cold accretion -- the reasoning is that when planetesimals come together at low relative velocities they can easily stick together; this result allows for the expected quick planetary accretion. At low relative velocities, however, not much heat is produced. At high relative velocities, on the other hand, the colliding planetesimals would vaporize; and thus accretion would take a long time (see Alan P. Boss, "The Origin of the Moon," Science, January 24, 1986, vol. 231, pp.341-345). Nevertheless, it seems to me, that the quick initial accretion of bodies with low relative velocities leading up to the proto-Earth would be eventually followed by collisions at high relative velocities with bodies of varying sizes, long before the completion of the Earth's accretion. As long as the proto-Earth's mass was significantly higher than those of the other objects, some of the mass of those objects would be quickly accreted into the Earth by gravitational attraction, even if those objects vaporized upon impact. This "hot" phase in the formation of the Earth is quite reasonable in view of the presently favored hypothesis of the origin of the Moon, which would have the Earth colliding with a body the size of Mars (see below). Surely if this is plausible we should favor a scenario in which the proto-Earth is constantly bombarded by bodies too small to break it up but large enough to provide for a "hot" phase during its accretion.

[2]. 70% of the heat from the interior is dissipated in the motions of the plates. M. Carr, R.S. Saunders, R.G. Strom, D. E. Wihelms, The Geology of the Terrestrial Planets, NASA SP-469, 1984, p. 76.