Below is Part II of my “The Origin of the Earth” paper. Part III will follow tomorrow. Again, figures are from the books and papers referenced (I’ll provide my full references at the end of the last section of the paper) or are stolen from the internet, if unreferenced. Enjoy!
The Origin of the Earth
Part II: Crustal Chemistry, the Solar Nebula, and the Solar System
Terrestrial Abundances of the Elements:
After one has a basic understanding of the cosmic abundances of the elements, one may notice that the terrestrial planets, Earth included, have a chemical composition that differs significantly from cosmic composition. The differences between the cosmic and terrestrial abundances of the elements can provide important clues about the processes of Earthâ€™s formation and evolution. Earthâ€™s crust is mostly oxygen (about 47% by weight) and also contains significant amounts of silicon, aluminum, iron, calcium, sodium, potassium, and magnesium as shown in Table 1 (Lutgens and Tarbuck, 2003).
The reasons why certain elements are depleted or enriched in Earthâ€™s crust will be explained further in subsequent sections, but for now it is worth bearing in mind that elements can be either volatile or refractory. Volatile elements are those elements with low boiling points and are easily vaporized. Refractory elements, on the other hand, have high boiling points and are fairly robust when heated. Earthâ€™s elements can also be thought of as generally being atmophile, lithophile, siderophile, or chalcophile (see Figure 5). Atmophile elements (e.g. H, He) are highly volatile and are concentrated in Earthâ€™s atmosphere and easily lost to space. Because of their volatility, atmophile elements are depleted in the Earth relative to their cosmic abundances. Lithophile elements have a strong affinity for oxygen and are concentrated in the silicate portion of the Earth (i.e. the crust and mantle). Siderophile elements, which have an affinity for iron, and chalcophile elements, which have an affinity for sulfur, are concentrated in Earthâ€™s iron core.
The Solar Nebula:
The solar nebula was a dense, rotating cloud of interstellar gas and dust that collapsed to form our solar system. Scientists gain information about what our solar nebula may have looked like from the composition of the sun, our star, and also by studying other nebulae. In the Milky Way Galaxy there are many nebulae scientists can observe. In general, nebulae are dozens of light years wide and are composed of gases, mostly hydrogen but also some nitrogen and oxygen, as well as tiny dust particles, which are mostly carbon, silicates, and iron that were produced in stars (Norton, 1994). These dust particles may also be encased in layers of methane, ammonia, and water ice (Norton, 1994). Significantly for the origin of life, some of the densest nebulae are observed to contain organic compounds of varying complexities (Norton, 1994). Our own nebula likely consisted of approximately 98% hydrogen with 2% heavier elements (Dickin, 2006). This composition indicates that the solar nebula formed from the ashes, so to speak, of dead stars as these heavier elements had to be produced through stellar processes. As the well-known astronomer Carl Sagan was fond of pointing out, we are all made of stardust as is everything on our planet and in our solar system. In general, nebulae with higher abundances of heavier elements produced in stars are more likely to contain terrestrial planets such as Earth (Dickin, 2006).
The Development of the Solar System:
The solar system began forming when the solar nebula started collapsing gravitationally. What triggered the collapse of the solar nebula is debatable, but one likely cause is a shockwave produced by a supernovae explosion during the death of a nearby star (Norton, 1994). As the solar nebula collapsed gravitationally, three important developments occurred: the solar nebula became hotter, it began to spin quickly, and it flattened into a disk (Norton, 1994) The solar nebula became heated as gravitational potential energy was converted into kinetic energy. Also, the solar nebula became heated because as particles fell into the gravitational well, they moved more quickly as the gravitational forces became stronger because the particles were closer together. The solar nebula began spinning quickly because of the conservation of angular moment. When the solar nebula was a spherical gas cloud before collapse, the nebula was rotating very slowly and had a direction of net angular momentum. Angular momentum is defined as L = Iw where w is the angular velocity of the object, and I = mr^2 where m is the mass of the object and r is the perpendicular distance from the object to the axis of rotation. As objects move closer to the axis of rotation about which they are rotating, they move faster. Think of a spinning ice skater for a simple example: as the skater pulls his arms closer to his body, he spins more quickly. Similarly, as the solar nebula collapsed, the particles had to move faster in order to conserve angular momentum. Finally, the solar nebula flattened into a disk because the direction of net angular momentum was favored and so the particles began preferentially moving in the same direction.
Eventually, the central, dense center of the collapsing solar nebula reached the pressure and temperature conditions under which nuclear fusion could begin and our star the sun was born. Currently, our sun is a middle-aged star main sequence star steadily burning hydrogen and releasing energy through nuclear fusion. When the sun was first born, however, it was a T-Tauri star (see Figure 6), a young star which is larger, less hot, and had a much more powerful solar wind than a main sequence star (Norton, 1994). One important point to note is that the powerful solar wind associated with this T Tauri star would have blown away other gases in the developing solar system. Thus, any planets or planetesimals forming from these gases would end their growth once the powerful solar wind of the T Tauri protosun developed.
As the sun was forming, the protoplanetary disk around the sun was cooling and condensing. Dust (metals, silicates) and ice (water, methane, and ammonia) condensed out of the solar nebula and began sticking together to form larger particles (Norton, 1994). Some of the particles gradually began growing larger by collisions which led to the capture of other particles. Eventually, the largest particles began growing very large in a process known as oligarchic or runaway growth (Wetherill and Stewart, 1993). The larger the objects became, the faster they would grow because these large objects were moving more slowly and had larger volumes, making collisions with other particles more likely. Additionally, very large growing objects could also gravitationally capture smaller, nearby particles. From condensation and oligarchic growth, fairly large (up to Mars-sized, perhaps) planetesimals could form (Canup and Agnor, 2000). However, as will be described below the production of a large terrestrial planet such as Earth seems to require giant collisions between large planetesimals.
There are three main types of planets in our solar system: terrestrial, gas giant, and icy (see Figure 7). The terrestrial planets are Mercury, Venus, Earth and Mars. The gas giant planets are Jupiter, Saturn, Uranus, and Neptune. Further out, there are some icy planetesimals such as Pluto– which I still consider a planet! Because most of the solar system is hydrogen and helium, terrestrial planets with concentrations of heavier elements can only form close to the sun where the temperatures are too warm for hydrogen compounds to condense out of the solar nebula, so only heavier, less volatile elements condense. Further from the sun, where temperatures are cooler then about 150 K, hydrogen compounds are able to condense and gas giants are able to form. Because there is so much more material available for these gas planets to form since hydrogen is so abundant, the gas planets become giants, capturing gases from the solar nebula because they are so large. One the T Tauri solar wind sweeps away the solar nebula gases, however, the growth of the gas giant planets is stopped. Finally, at the very furthest, coldest reaches of the solar system icy planets, such as Pluto, form.