What is – the Atmosphere?

About the blog series: What is [geology]

What is [geology] is a blog series where we address key vocabularies in the field of geology (i.e., a study of the history of the Earth and other planets). At Rock Archive, we strive to communicate complex jargon in geology to the public by using simple illustrations and language. The study of geology is often overlooked in the K-12 educational system, which is one of the many reasons why we lack diversity in our field. Our hope is to get more public engagement to increase diversity, literacy, and excitement in the field of geology.

Q1: What is the atmosphere?

A: The atmosphere is a layer of gas covering a planet and other material bodies. Gravity prevents gases that make up the atmosphere from escaping to space (aka. the cosmos). Note: Gravity is a natural force of attraction between all things with mass or energy.

Q2: What is the composition of the Earth’s atmosphere today (also known as air)?

Figure 1. Scale of the Earth from the mantle to the atmosphere (figure by Rock Archive)!


A: The Earth’s atmosphere today is composed of 78% nitrogen (N2), 21% oxygen (O2), 0.9% argon (Ar), and trace amounts of water vapor (H2O), carbon dioxide (CO2), methane (CH4), ozone (O3), and sulfur dioxide (SO2).


Earth’s atmosphere has a thickness of ~3000 km and can be divided into five major layers: troposphere (0 to 10 km above ground), stratosphere (10 km to 50 km above ground), mesosphere (50 km to 85 km above ground), thermosphere (85 to 800 km) and exosphere (800 to 3000 km; data from World Atlas). Each of these layers holds different properties: e.g., differences in density, composition of gases, temperature etc.

Q3: Does every planet have the same atmospheric composition?


A: No. Different planets have different atmospheric compositions. For example, Earth is the only known planet with an O2-rich atmosphere. Some planets even lost most of their atmosphere to space!


Figure 2. Comparison between the atmospheric composition of Mars and Earth (modern; image by Rock Archive).

Q4: Earth is 4.6 billion years old. Was the composition of the Earth’s atmosphere the same in the past?


A: No. The Earth’s atmosphere has changed in composition through the course of Earth history. From the formation of the Earth’s atmosphere to ~2.4 billion years ago, scientists believe that the atmosphere was rich in reduced gases (e.g. hydrogen (H2), CH4, ammonia (NH4), hydrogen sulfide (H2S)) and lacked free O2. The emergence of O2-producing organisms (i.e., cyanobacteria, photosynthetic life) on early Earth led to an increase in atmospheric O2 concentrations, ultimately leading to what scientists call the “Great Oxidation Event (GOE)” (~2.4 billion years ago). The GOE marks a period when biological O2 production exceeded its consumption by chemical reactions, allowing for O2 to accumulate in the Earth’s atmosphere. The exact duration, nature, and magnitude of the GOE are still under debate, but O2 levels have remained above 0.001% present atmospheric level (PAL) since then.

Q5: Why is the atmosphere so important?


A: Broadly speaking, the atmosphere is necessary to sustain life on our planet (and possibly other planets too). Organisms use different gases in the atmosphere to breathe, but the atmosphere also protects us from harmful cosmic radiation. For example, ozone (O3) helps shield us from ultraviolet (UV) radiation, which is known to cause health problems. Although, the ozone layer only protects us from certain types of UV radiation, which is why we need to use sunscreen! Other gases, like CO2, H2O vapor, and CH4, help keep our planet warm – we refer to this as the greenhouse effect. While these so-called greenhouse gases prevent the Earth from freezing, excess emissions of CO2 and CH4 by human activity are driving modern-day climate change.


Educational resources


More on the Great Oxidation Event (GOE) click here!

More on the history of atmospheric discovery click here!

More on the composition of the Earth’s modern atmosphere click here!

More on Ozone and human health click here!

More on Mars’ atmospheric composition click here!

About the authors

Kärt Paiste

Kärt is originally from Pärnu, Estonia. She is currently an MSCA postdoctoral fellow at Washington University in St Louis, where she investigates the uncertainties under which biogeochemical sulfur cycling occurred at critical junctions in Earth’s history. In addition to her research work, she engages with K–12 schools via outreach activities (i.e., European Researchers Night, Back to School program, WashU’s SciZoom) and works with high school students on mentored research projects (WashU STARS) to increase diversity and encourage participation in STEM fields. To raise awareness and address racism in geoscience, she is an Unlearning Racism in Geoscience pod member at WashU (URGE).

Ichiko Sugiyama

Ichiko is originally from Tokyo, Japan, and is a Ph.D. student at the Weizmann Institute of Science investigating the role of iron minerals in regulating metal and nutrient budgets in ancient oceans. She is passionate about communicating science to the public in all STEM fields and contributes by volunteering for different organizations (EAG, EGU, GSA, SEG, etc.). As part of her outreach and DEI work, she has mentored K-12 educators, undergraduate and graduate students in North America and in the Middle East. To find out more about Ichiko, please visit her website.