The Origin of the Earth: Part I

For the past couple of weeks, I have been busy with final projects at school. For one of my geology classes I did a research project about the early Earth and Cosmos and wrote a paper on the subject, which I find fascinating. I thought some of the readers here might enjoy reading my early Earth paper, so I’m going to post it in installments along with appropriate images stolen from various websites and research articles. This paper is just a class research paper and is by no means comprehensive or perfectly polished, but I think many of you may enjoy it nonetheless.

The Origin of the Earth
Part I: Introduction, the Scientific Toolbox, and Cosmic Starstuff

The Cosmos is all that is or ever was or ever will be. Our feeblest contemplations of the Cosmos stir us—there is a tingling in the spine, a catch in the voice, a faint sensation, as if a distant memory, of falling from a height. We know we are approaching the greatest of mysteries… in the last few millennia we have made the most astonishing and unexpected discoveries about the Cosmos and our place within it… they remind us that humans have evolved to wonder, that understanding is a joy, that knowledge is prerequisite to survival. I believe our future depends on how well we know this Cosmos in which we float like a mote of dust in the morning sky.

-Carl Sagan, Cosmos

Introduction:

The study of the origin and development of the early Earth is one of the most intriguing and important research topics in science today. Understanding Earth’s history in the context of the larger Cosmos is both awe-inspiring and humbling. Earth is but a tiny speck in the solar system and is dwarfed by the greater Cosmos. Yet, Earth is our home and harbors the only life of which we know. So, to us, study of the Earth—both past and present– is fascinating and also essential for our survival as a species. Psychologically, there is also something especially powerful about studying the early Earth. Just as we value learning about our ancestors and family histories, we value learning about the history of our planet. More practically, a better understanding of early Earth may provide information on how our planet will respond to stresses such as pollution, greenhouse gas emissions, and magnetic field reversal. Furthermore, if humans ever colonize planets in other solar systems, knowing what conditions and processes lead to an Earth-like planet may help scientists locate habitable planets. Study of early Earth is also relevant because today, at least in America, religious creationism has a strong hold in many places. Better understanding Earth’s long history will aid those who confront the creationists and fight to teach valid science in our school systems.

The goal of this paper is to summarize the processes that scientists believe, to the best of their current understanding, led to the formation and development of the Earth. Because Earth formed from stardust, this paper will begin with an overview of nucleosynthesis, which occurs primarily in stars. The processes leading to the formation of various elements in the Cosmos will be described and trends in elemental abundances explained. Much information about Earth’s history can be gleaned from a comparison of the abundances of elements in the Cosmos and solar system with the abundances of elements on Earth. Therefore, differences in the composition of the Cosmos, the sun, and the Earth will be noted and comments made about how elements are fractionated and concentrated in the solar system. Similarly, a general discussion of the solar nebula and the early development of the sun and solar system will be provided in order to put the Earth in a larger context.

Next, a brief overview of meteorites will be provided as Earth is believed to have accreted from planetesimals with compositions similar to meteorites, in particular carbonaceous chondrite meteorites. Understanding why and how geologists use meteorites to estimate the intial composition of the Earth is crucial to understanding the assumptions inherent in any geophysical or geochemical model using chondrite values for the composition of the early Earth. Following this discussion of meteorites, the accretion of the Earth from planetesimals will be discussed. The importance of late-stage collisions between large planetesimals will be emphasized. At the same time, the origin of the Moon from Earth via a Mars-sized impactor will be examined. Theories about a magma ocean that may have formed as a result of this giant impact will be presented as this magma ocean may have played a significant role in the differentiation of Earth’s iron core.

Finally, the formation of Earth’s core and the subsequent differentiation of the upper Earth into an enriched continental crust and a depleted mantle (which now melts and forms oceanic crust) will be described. Possible evidence for an early protocrust, supposedly subducted deep in Earth’s mantle, will also be analyzed. Clearly, discussing all these topics related to the origin and development of the Earth is a large task for a small paper. However, this paper will hopefully at least provide a sense of how scientists are able to decipher the early history of our home planet and a general overview of Earth’s origin and history.

How Scientists Study Early Earth:

In order to appreciate the story scientists have reconstructed concerning the origin and development of Earth, one must first have an understanding of how scientists are able to glean information about the early Cosmos. What are the objects that scientists study in order to learn about the early Earth, solar system, and universe? Scientists start with objects close-to-home: the Earth and her solo moon. The Earth is a terrestrial, differentiated planet with active plate tectonics and an atmosphere, hydrosphere, and—uniquely so far as we know—a biosphere. The moon is smaller and more barren but is closely related to the Earth. Next, scientists venture further and study the sun and other planets in our solar system. Scientists can also study asteroid belts and meteorites, which are small pieces of asteroids and planets that fall to Earth. Scientists can also glean information from objects outside the solar system. Scientists study galactic cosmic rays and also remotely observe other stars, solar systems, and solar nebulae.

In addition to appreciating the objects scientists study to learn about the origin and development of the Cosmos, one must also appreciate the tools which scientists use to study these objects and to develop theories about the origin of the Cosmos, our solar system, and the Earth. There are three important categories of tools scientists use to study the early Cosmos: spectroscopy, cosmochemistry, and computer modeling. Spectroscopy is the study of the interaction of electromagnetic radiation and matter. Study of absorption lines created by the interaction of radiation and matter can be used to determine the chemical composition of stars, solar nebulae, and cosmic rays. The cosmochemical toolbox includes major elements, trace elements, and isotopes. Within isotopic analysis there are short-lived isotope systems, which are particularly useful for constraining the time of various events in the solar system, and long-lived isotope systems. Computer models are also widely used to simulate conditions in the early solar system and Earth. Computer models are very useful for visualizing processes in the early Cosmos and determining their likelihood, but one must carefully understand the assumptions of the models and must work towards constraining the parameters of the models.

Cosmic Abundances of the Elements and Nucleosynthesis:

From the study of stars, galactic cosmic rays, and chondrite meteorites, scientists have determined the cosmic abundances of the elements (Figure 1).

Figure 1: Cosmic Abundances of the Elements

Looking at a plot of abundance verses atomic number (Z), one can make several observations about the chemistry of the Cosmos:

1. Hydrogen (H) is by far the most abundant element, ~75% of the Cosmos by weight (Dickin, 2006).

2. Hydrogen (H) and helium (He) together comprise ~99% of the Cosmos by weight (Dickin, 2006).

3. Generally, heavier elements are less abundant than lighter elements.

4. Certain elements are depleted relative to their neighbors. In particular, the light elements lithium (Li), beryllium (Be), and boron (B) are especially depleted.

5. Certain elements are enriched relative to their neighbors. Most notable among these are iron (Fe), particularly the isotope 56Fe (Dickin, 2006), and lead (Pb).

6. The elements in general have a sawtooth pattern with elements alternately enriched and depleted with atomic number.

As we will see, these observations can be explained by looking at how elements, and stable isotopes in particular, are made. One must understand the various processes and stages of nucleosynthesis as well as understand which elements are stable and store the most potential energy. Perhaps the easiest of the above observations to explain is the characteristic sawtooth pattern of the elements. Generally, isotopes with an even number of protons and an even number of electrons are energetically favored over isotopes with odd numbers of protons and neutrons. By far, most stable isotopes have an even number of protons and neutrons and an overall even atomic number (protons + neutrons). Thus, the alternating even-odd pattern of abundances can be explained by the fact that even atomic numbers are favored and thus more abundant than odd atomic numbers.

The next major observation that can be explained is the abundance of H and He. H has one proton, one neutron, and one electron and is the easiest element to form because of this. He is essentially just two hydrogens put together and has two protons, two neutrons, and two electrons generally. Together, these two elements were created in large quantities in the big bang, an explosive event ~13.7 billion years ago in which the universe was created from an initially extremely hot, dense state (Norton, 1994). During the big bang, there was rapid expansion during which free quarks and gluons condensed into larger particles and the first elements: H, He, and a small amount of Li (Dickin, 2006).

Beyond H, He, and Li– the three lightest elements– larger elements must be produced by synthesis in stars and through a process called spallation. The elements from carbon to calcium are produced by nuclear fusion in a star. Elements heavier than carbon are produced by neutron capture (either the s-process or r-process), proton capture, or e-process interactions between nuclei and free protons and neutrons (Dickin, 2006). S-process or slow-process neutron capture produces about half of the elements heavier than iron and occurs at moderate neutron density and temperature conditions in middle-aged stars. The e-process occurs at high-temperatures in stars just before they explode as supernovae. The e-process produces elements such as iron and other first-series transition elements (Dickin, 2006). R-process or rapid-process neutron capture and proton capture occur in supernovae and produce unstable isotopes which rapidly decay back to more stable isotopes. Certain light elements can also be produced through cosmic ray spallation, a process that produces elements through the fission of heavier elements. Spallation occurs when an element is bombarded with a cosmic ray (Dickin, 2006). Cosmic rays are primarily a high-energy proton stream, and when a heavy nucleus is hit with a high-energy proton it may release nucleons and form lighter elements. Spallation is the primary way in which the light elements Li, Be, and B are produced. Figure 2 relates atomic number with various nucleosynthetic processes. Note that Figure 2 is a simplification as many elements are produced by more than one process. Figure 3 shows the general evolution of a star and the times at which various nucleosynthetic processes occur.

Figure 2: Atomic Number and Nucleosynthetic Processes

Figure 3: The Evolution of a Star and Nucleosynthetic Production (Dickin, 2006)

Returning now to an explanation of the cosmic abundances of the elements, the general abundance of lighter elements relative to heavier elements can be explained by the fact that heavier elements take longer and are harder to make in stars as you must keep adding neutrons to a nucleus. The depletion of Li, B, and Be can be explained by the fact that these elements are bypassed in stellar fusion because the nuclear binding energies of these elements are very low. Fe, on the other hand, is at the peak of the nuclear binding energy curve, which is shown in Figure 4. Nuclear binding energy is the energy required to dissemble a nucleus into individual protons and neutrons and can be thought of as the energy a nucleus can store. Nuclear systems that are bound have less potential energy than unbound systems and are favored. Thus, elements with high binding energies (such as iron) are favored over elements with lower binding energies because such elements can store more potential energy.

Figure 4: Nuclear Binding Energy Curve

One last comment about the cosmic abundance of the elements regards neutron “magic numbers,” which lead to abundances of isotopes with certain numbers of neutrons such as 50, 82, and 126 (Dickin, 2006). The abundance of these isotopes has to do with something called neutron-capture cross-section, which is a measure of how readily a nucleus can absorb a neutron (Dickin, 2006). Elements with low neutron-capture cross-sections will not be converted to species of higher atomic mass as easily and so will be more abundant relative to their neighbors.