Question

How and when did Earth's secondary atmosphere form? Compare the composition of this atmosphere with that of the modern one.

Short answer

The secondary atmosphere formed about 3.8 billion years ago from volcanic gases, with little oxygen. Today, it's 78% nitrogen, 21% oxygen.

Step-by-step solution

Identify the Secondary Atmosphere Formation

Earth's secondary atmosphere formed due to volcanic outgassing and the accumulation of gases that were released from the interior of the Earth. This process took place during the Archean Eon, approximately 3.8 billion years ago, after the primary hydrogen-helium atmosphere was lost to space.

Describe the Composition of the Secondary Atmosphere

The secondary atmosphere was composed primarily of water vapor (H2O), carbon dioxide (CO2), nitrogen (N2), and sulfur gases, with little to no free oxygen (O2). Methane (CH4) and ammonia (NH3) were also present in smaller amounts.

Contrast with the Modern Atmosphere

The present-day atmosphere is composed of roughly 78% nitrogen (N2), 21% oxygen (O2), and trace amounts of argon (Ar), carbon dioxide (CO2), water vapor (H2O), and other gases. The critical difference is the presence of free oxygen, which was absent in the secondary atmosphere and is largely due to the evolution of photosynthetic organisms.

Key concepts

Volcanic Outgassing

Volcanic outgassing played a pivotal role in shaping Earth's secondary atmosphere. This natural process involves the release of gases from the planet's interior through volcanic eruptions.

When volcanoes erupt, they emit a variety of gases. These include water vapor ( H_2O ), carbon dioxide ( CO_2 ), sulfur gases, and nitrogen ( N_2 ), among others. Through continuous volcanic activity, these gases gradually accumulated around the planet, forming what is known as the secondary atmosphere.

Unlike the primary atmosphere, which largely consisted of hydrogen and helium and was lost to space, the secondary atmosphere was denser and more enduring. It protected the Earth and set the stage for life to develop. Volcanic outgassing is not just a historical process; it still occurs today, contributing to the modern atmospheric composition but without significantly altering it.

Archean Eon

The Archean Eon was a crucial period in Earth's history, stretching from about 4 billion to 2.5 billion years ago. During this time, the Earth's secondary atmosphere began to form.

This eon was marked by significant geological and atmospheric changes. The surface of the Earth solidified into the first continents, and the cooling of the planet allowed the atmospheric gases from volcanic outgassing to become stable. This resulted in a thick secondary atmosphere, rich in water vapor, carbon dioxide, and nitrogen.

One of the most important events of the Archean Eon was the gradual development of simpler life forms. Although oxygen was nearly absent in the atmosphere, the conditions were ripe for primitive organisms to evolve. They would later drive the transformation of Earth's atmosphere, leading to today's more oxygen-rich environment.

Atmospheric Composition

Earth's secondary atmosphere had a distinctive composition that was vastly different from what we experience today. It consisted mainly of water vapor, carbon dioxide ( CO_2 ), nitrogen ( N_2 ), and sulfur gases.

There was little to no free oxygen ( O_2 ), which is a stark contrast to the present-day atmosphere where oxygen comprises about 21%. Methane ( CH_4 ) and ammonia ( NH_3 ) were also present, albeit in smaller quantities.

Over billions of years, the composition has evolved significantly. The modern atmosphere contains about 78% nitrogen, 21% oxygen, and traces of argon, carbon dioxide, and other gases. The major catalyst for this transformation was the emergence of photosynthetic life, which began to release oxygen into the atmosphere, slowly altering its makeup to support more complex life forms.

Photosynthetic Organisms

Photosynthetic organisms have been vital in transforming Earth's atmosphere from its early, oxygen-poor state to the oxygen-rich environment we know today.

The earliest of these organisms were cyanobacteria, which appeared around 2.5 billion years ago. They performed a type of photosynthesis that absorbed carbon dioxide ( CO_2 ) and released oxygen ( O_2 ) as a by-product. This process contributed significantly to what is known as the Great Oxidation Event, where Earth's atmosphere saw a rise in oxygen levels to support more complex life.

These organisms laid the groundwork for the development of higher plants and, eventually, animals. Their role in shaping the atmospheric composition cannot be overstated, as they set the stage for the emergence and evolution of diverse life forms on our planet.