Study with the several resources on Docsity
Earn points by helping other students or get them with a premium plan
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Explore the evolution of earth's early atmosphere, from its primordial state to its secondary composition. Understand the roles of hydrogen, helium, water vapor, and carbon dioxide in shaping the atmosphere. Learn about the geological timetable and the conditions that led to the formation of oceans and the eventual development of life-sustaining oxygen levels. Discover how earth's atmospheric composition differs from venus and mars, and the factors that contributed to these differences. This module provides a detailed look at the pre-biotic earth and the physical changes that influenced its atmosphere over billions of years. It also sets the stage for understanding how lifeforms further altered the atmospheric composition. Charts and explanations to enhance comprehension.
Typology: Slides
1 / 12
This page cannot be seen from the preview
Don't miss anything!
In this section, we will study how the atmosphere changed from what it was probably like when the Earth first formed, to its present composition, and see how the appearance and evolution of lifeforms influenced these changes. We will also take a detailed looked at how humans are creating or could create atmospheric composition changes.
In this first module, we will be covering the first topic, “Has the atmosphere always been like this?” This will be a look at the pre-biotic Earth, before the appearance of lifeforms. Scientific research suggests that about 4.5 to 5 billion years ago, the Earth formed from material that had been spread around the universe by the “Big Bang”. The first atmosphere around Earth was merely the gases that already existed in space at the time. Eventually, the atmosphere would change over billions of years from physical changes to Earth and then, as we will see in another module, from the influence of lifeforms.
Here is a chart of the current atmospheric compositions of Venus, Earth, and Mars. <> The atmospheres of Venus and Mars are nearly all carbon dioxide, while the carbon dioxide content of Earth’s atmosphere is very small. Meanwhile, molecular oxygen makes up a significant portion of Earth’s atmosphere— 21%—while there are negligible amounts of oxygen in the atmospheres of Venus and Mars. In addition, the surface temperatures and pressures of Venus and Mars are extremely large or small compared to Earth. <> So the questions we have are, why is there so little carbon dioxide and so much oxygen in Earth’s atmosphere while the opposite is the case for Venus and Mars? <> Does it have anything to do with life on Earth? Well, yes, but that’s in the next module. <> Was the Earth’s atmosphere always like it is today? Let’s see…
In the beginning… Twenty or so billion years ago, give or take five or ten billion, the Big Bang spread material all over the universe, however we define “universe”, or space. Hydrogen and helium gases, some other minor gases, and dust particles formed an expanding cloud in this universe. <> At some point, the gravitational attraction between particles and molecules of material, plus the contraction of a cooling space caused the universe to stop expanding and to start contracting. As material moved together, some parts became more dense and massive than others, which increased the gravitational attraction, and thus pulled even more material toward these density anomalies. Stars were formed from the compressed material, large clusters of stars formed galaxies, and around the stars, some more material formed smaller bodies that would not be large or massive enough to themselves form stars, but could become objects that orbited in the gravitational fields around the stars.
And so here is our solar system. The inner planets—Mercury, Venus, Earth, and Mars— have a more or less solid, rocky composition, while the “outer” planets are much larger but also seem to be mostly gas. It is the inner planets that are interesting to us from an evolutionary standpoint because they all started with the same types of atmospheres and planetary compositions; the main differences being size and distance from the Sun.
This geologic timetable shows that most of the billions of years of the Earth’s lifetime had been spent physically changing to a form that would be hospitable to life and to very simple lifeforms, mainly unicelluar, that existed in the oceans. This is represented by the red zone in the lefthand column. We will now look at the first billion or so years, before life appeared.
Meanwhile, the hot molten Earth was releasing, or “outgassing”, gases upward. These gases were mostly water vapor and carbon dioxide, with a small amount of nitrogen, methane, sulfur dioxide, and others. It is believed that some asteroids and comets may have also collided with the Earth at this time, bringing some more water into the Earth system. <> The secondary atmosphere of Earth was formed at this time. It was mostly water vapor and carbon dioxide. The other inner planets were doing the same thing. All the planets were cooling, since without nuclear fusion reactions, the planets could not remain hot and molten.
As the Earth cooled down, so did the atmosphere. <> The cooler atmosphere could not sustain the high amount of water vapor that was there, so the water vapor condensed out, precipitated out, and helped form the oceans. Carbon dioxide is water-soluble, so most of the atmospheric carbon dioxide eventually dissolved into the oceans… <> …and formed carbonate compounds that settled out as ocean sediments. Having the carbon locked into solid material prevented the outgassing of carbon dioxide back to the atmosphere; thus, a “sink” for carbon had formed. <> This left the small amounts of nitrogen that had already been outgassed, plus much smaller amounts of water vapor and carbon dioxide than were previously in the Earth’s atmosphere. <> Note that there is nearly no oxygen, like the current atmospheres of the other inner planets. So it appears that the lack of oxygen in the current atmospheres of the other inner planets is a holdover from the time when the secondary atmosphere was present and there was no life. But things went on to become very different on Earth…
