The most troubling aspect of Earth’s origin is our inability to probe the geological record for the first ~0.5-billion years of our planet’s history. Studies of Earth itself are surprisingly useless. Evidence from this critical time domain, which would ordinarily provide clues to the youthful environment in which our planet was born, is missing, having been literally melted, eroded, and chipped away long ago. What we do know is that, in nearly every respect, primordial Earth and its global environment of several billion years ago must have differed substantially from the world we now inhabit.

Drawing a mental picture, we can surmise that shortly after Earth formed, it was hot, oceanless, free of oxygen, and pelted with all sorts of energy from within and without. Solar ultraviolet radiation, fierce thunder and lightning, radioactive rocks, and violent volcanoes all energized our young planet. Intense meteorite barrages, known as the period of late heavy bombardment, must have caused our early planet to resemble a hell on Earth for its first several hundred million years. We need look no further than the heavily scarred and anciently cratered Moon for ample proof that Earth was, in fact, belted frequently by comets and asteroids.

The whole globe of the Earth must have melted right down to its center since, when our planet rings like a bell during earthquakes today, geologists infer a complete differentiation of it’s interior, from core to surface. Not long after Earth’s formative stage, the dense, iron-nickel metals must have sunk to the center while the lightweight, granite-silicate rock rose toward the surface. Gradually, the restless Earth cooled, cracked, exhaled steam, and secreted an ocean and atmosphere.

Atmospheric Change Earth’s original atmosphere almost certainly contained all of the most abundant elements—hydrogen, helium, nitrogen, oxygen, neon, carbon—as well as a long list of trace elements. These gases mimicked those of the interstellar cloud from which our Solar System formed (see Figure 3.46). This primary atmosphere didn’t likely stick around very long, however. Earth’s surface was much hotter during its first billion years than it is today and many of the atmospheric gases present then must have escaped to outer space. Gravity just couldn’t hold back the early hot gases.

The relative scarcity of several noble gases—those that are inert and unable to react with other chemicals—such as neon, argon, krypton, and xenon, is the best evidence that Earth failed to retain its original atmosphere. If our primordial atmosphere were still here, even if modified by later evolutionary events, those inert gases should be present in quantities comparable to those in the Sun where they do in fact exist. Apparently, the heavy bombardment, high surface heat, and fierce solar winds were too much for the small young planets to bear. None of the Terrestrial Planets likely retained their original gaseous atmospheres left over from the primitive solar nebula.

Despite the depletion of Earth’s initial atmosphere, one obviously surrounds our planet today. We wouldn’t be here if it didn’t. Hence, the air we breathe must be a secondary atmosphere acquired by our planet at a later date. What’s more, these secondary gases in turn evolved, owing to the presence of plants as explained in the BIOLOGICAL EPOCH, to become the air we do now breathe—so perhaps we should correctly call the current gases in which we are bathed a “tertiary atmosphere.”

For the same reason that ice cubes congeal from the outside in, the surface of the gradually cooling primordial Earth would have been the first part of the molten planet to solidify into rock. Intense heat trapped below the crust had to get out somehow. The result was surely volcanoes, geysers, quakes, and a variety of other geological events that literally blew off steam and pent-up heat through cracks in the surface. “Outgassing” of this sort happens even today, such as shown in Figure 4.11, though at only a few locations on Earth and rather infrequently at that.

Several billion years ago, this type of geological activity was surely more widespread and frequent. Scrutiny of modern volcanoes shows that lots of steamy water vapor (H2O), carbon dioxide (CO2), and nitrogen (N2) would have then undoubtedly emerged, along with vast quantities of ash and dust. Smaller amounts of hydrogen (H2), oxygen (O2), carbon (C), and other gases doubtless accompanied these early planetary eruptions. Calculations imply that over the course of Earth’s history, enough gas was exhaled through fissures from Earth’s interior to create much of our current atmosphere. The rest presumably came from comets and meteorites that could have salted the young Earth with large quantities of matter, including prebiotic molecules. Even today, ~40,000 tons of extraterrestrial matter fall to Earth each year, almost all of it burning up in the air or splashing into the ocean.

FIGURE 4.11 FIGURE 4.11 – Volcanic activity atop Mt. Etna in Italy releases more energy than the detonation of 1000 nuclear bombs. It also releases much gas and dust into the atmosphere. (World Wide)

The origin of our present atmosphere is therefore a combination of terrestrial outgassing and interplanetary assault (further changed by later biological events). In truth, Earth’s atmosphere is perhaps still adjusting as present-day volcanoes occasionally sputter gas and heat amid incoming debris arriving from space. Today’s atmosphere was not, however, derived directly from a mixture of interstellar gases. The composition of Earth’s secondary atmosphere thus differs considerably from the average cosmic abundance of the elements. By contrast, Jupiter and the other Jovian Planets have atmospheres rich in hydrogen, helium, and many light gases. These planets are large enough to have retained their primitive atmospheres, which were formed directly from interstellar matter.

Primordial Oceans Since the atmosphere and ocean of planet Earth are so closely linked, they almost certainly originated, at least partly, from the same sources—our planet’s interior and interplanetary bolides. As regards the ocean, geologists argue that as the surface cooled sufficiently, the first stock of liquid water pooled as water vapor condensed. After all, steam (which is hot, gaseous H2O) is the main component of volcanically vented matter, and hydrated rocks (those mostly silicates, with water trapped inside) comprising Earth’s mantle today store several times more water than in all the seas combined. But outgassing from within may not be the whole story since our planet’s water has chemical (isotopic) subtleties implying that some of it may have come from beyond.

Debate swirls among geologists concerning the rate and timing of ocean formation. We are unsure whether Earth’s mantle outgassed the global seas all at once early in our planet’s history—known among geologists as the “big-burp” theory. Or perhaps the seas took some time to form, having secreted from Earth’s interior in a series of volcanic events that occurred more gradually. A minority of researchers argue that some (perhaps even a majority) of the waters of Earth could have resulted from a rain of water-rich comets and meteorites that collided with our planet in great numbers during its first billion years. But here, too, there is a chemical anomaly: The makeup of water on Earth doesn’t well match that trapped in interplanetary bodies. Three comets that recently bypassed Earth—Halley in 1986, Hyakutake in 1996, and Hale-Bopp in 1997 (see Figure 4.12)—all emitted radiation that revealed a heavy-water (deuterium) content twice that in Earth’s ocean.

FIGURE 4.12 FIGURE 4.12– Comet Hale-Bopp is seen here hovering above the Alps in its most recent apparition in 1997. (SAO)

As with many other aspects of the cosmic-evolutionary story that are neither black nor white, neither solely this nor cleanly that, Earth’s large bodies of sea water probably emerged from both within and without. Then, as the rate of outgassing and bombardment declined, a global recycling system began to operate. Water locked in rocks was expelled back into the ocean whenever the rocks were heated, such as those near volcanoes or suboceanic faults and ridges. To be sure, most of today’s seawater is thought to have been recycled many times through the world-wide system of oceanic ridges, perhaps as frequently as every 10 million years. Recently, water has been directly observed emanating from certain underwater vents. Whether this is “juvenile” water still originating directly from Earth’s mantle and incrementally adding to the world’s supply, or merely existing seawater cycling through the vents, is not yet known.

Later Gaseous Reactions Earth’s ocean and atmosphere gradually stabilized. As activity on our early planet subsided, the atmosphere cooled, enabling gravity to retard its further escape into space. Nitrogen partly reacted with other gases and partly remained free in the atmosphere where it now comprises the largest fraction of our air. Gaseous water vapor changed into liquid water, which further rained down on Earth’s ocean. And discharged carbon dioxide reacted with silicate rocks in the presence of water to form limestone. Whatever pure oxygen gas existed on primitive Earth would have quickly vanished by reacting either with hydrogen to make more water, or with surface minerals to form oxides such as rust and sand now found throughout the crust of our planet. Breathable oxygen and the protective ozone layer arose only much later, after plants had blossomed across the face of our planet.

Shaded by Earth’s secondary atmosphere, some of its chemicals would have further interacted with one another. No coercion by outside influences was needed for the airy gases to collide, stick, and react, thus forming slightly more complex gases of ammonia and methane. While the actual chemistry is more complicated, we can summarize these basic changes with the following idealized equations:

three hydrogen molecules (3H2) + one nitrogen molecule (N2) --> two ammonia molecules (2 NH3),

two hydrogen molecules (2H2) + one carbon atom (C) --> one methane molecule (CH4),

two hydrogen molecules (2H2) + one oxygen molecule (O2) --> two water molecules (2 H2O),

four hydrogen molecules (4H2) + one carbon dioxide molecule (CO2) -->one methane molecule (CH4) + two water molecules (2 H2O).

Chemists verify these reactions almost daily in industrial and academic laboratories. And theorists well understand the electromagnetic forces among electrons that persuade these simple atmospheric atoms to combine spontaneously, thereby concocting stable gas molecules.

With time, the molecular products of these spontaneous reactions became the interactants of additional chemical reactions. These additional reactions, however, were not spontaneous. Laboratory experiments prove that the simple molecules of ammonia, methane, and water vapor require some energy in order to combine further. This energy is, in some sense, a catalyst that helps produce even bigger molecules. Actually, it’s more than just a catalyst. The application of energy fashions a near miracle: It synthesizes molecules a good deal more complex than those likely to form by chance in a collection of free atoms and simple molecules. As we shall see in the next CHEMICAL EPOCH, the molecules produced are among the very building blocks of life.

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