Chapter 2 - 5

05 Astronomy

Listen to a discussion in an astronomy class. Fill in the diagram with the information that you hear.

M: OK, many of you had questions about Earth’s earliest atmosphere. So to, uh, answer those questions, let’s start at the beginning.

So, uh, during the first phase, um, the very first gases that existed in the Earth’s atmosphere were hydrogen, helium, and hydrogen-rich gases from when the planet was formed from, uh, from the remains of a star.

Now, these gases are really light, and the solar wind, or fast-moving streams of particles from the sun, were able to blow these gases out into space easily.

So the first gases in Earth’s atmosphere escaped, and the Earth had no atmosphere for a while. Then Earth underwent a lot of geologic activity.

Inside the Earth’s hot interior, heavy metals were sinking toward the center, while lighter materials like rock rose toward the surface.

Let me clarify. The events happening inside Earth’s interior caused a lot of volcanic activity on the surface. We had volcanoes exploding constantly.

Now all this volcanic activity released, um, a lot of gases into the air in a process called outgassing. OK, you all get that?

Outgassing. That’s the second step. Now, most of the gases released by these volcanoes were steam or water vapor, carbon dioxide, and nitrogen, so our early atmosphere was composed chiefly of these three gases.

So does anybody know what Earth’s early atmosphere was like?

W: Um, well, the Earth would have been extremely hot. And since there was no water or oxygen, there wouldn’t be any life. And I think I remember hearing that the atmosphere was denser than it is today.

M: Absolutely. Hot, dense, devoid of life. As in no living things at all. Think of the carbon dioxide-rich atmospheres of Venus or Mars, and you’ll get a good idea of what early Earth may have been like.

Well, the next major thing that happened was water formation. So Debbie, how was water able to form?

W: Uh, well you said much of the Earth’s atmosphere was water vapor. So I guess the Earth eventually cooled down enough for water to form.

M: Yes. Once the atmosphere cooled, the water vapor in the atmosphere began to condense and create precipitation. This is how our oceans were formed.

Much of Earth’s carbon dioxide was dissolved in Earth’s oceans, leaving an atmosphere rich in nitrogen and with less carbon dioxide and water vapor.

Water acted as a catalyst for life. Once water was created the final major step was able to occur. About 3.5 billion years ago, life began to develop in the form of unicellular bacteria.

This was followed by more complex organisms, most notably, photosynthetic blue-green algae. And this is where things really got interesting. Photosynthesis, you’ll remember, is how organisms like plants and algae are able to convert carbon dioxide into energy.

As most of you probably know, one product of photosynthesis is oxygen. The algae absorbed much of the carbon-dioxide and released oxygen into our atmosphere.

As plants continued to evolve, more and more oxygen was released into the atmosphere until our atmosphere transitioned from a carbon-dioxide dominant atmosphere to its current state as an oxygen-rich atmosphere.

1) According to the professor, what caused hydrogen and helium to escape from Earth’s atmosphere?

2) After the process of outgassing, which gases made up most of Earth’s atmosphere? Choose 2 answers.

3) What does the professor say caused life to form?