Note: This blog post is taken from a series of posts that I originally posted on the Skepchick blog back in 2007. These posts came from a paper that I wrote for a Marine Geology & Geophysics class back in 2007, so the information here may be slightly out-of-date since the early Earth and Cosmos is a hot research topic. If you spot any errors (this is just a classroom paper, and I am not an early Earth expert) or want to add anything, please do so in the comments. Finally, I apologize that some of the figures do not have proper references. In my early blogging days, I stole figures and images from the interwebs with wild abandon. These days, I am more careful and try to reference sources. I eventually hope to go through this post and reference or replace the ambiguous figures. Most of the figures are hosted on image shack (I'll move them once I have fast, non-African internet again), so you may need an image shack account to view them properly. For now, I'll have to leave the post as is. This post is long, but I've broken it up into 5 parts if you wish to digest it in pieces. Enjoy!
For the past couple of weeks [note: back in Spring 2007], 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 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.
For the past couple of weeks [note: back in Spring 2007], 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.Introduction:
-Carl Sagan, Cosmos
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.
