Chapter 20, Problem 1RQ

To determine

The stages in the formation and evolution of Earth’s atmosphere. Also describe from where the ozone in the atmosphere come from and its importance.

Answer to Problem 1RQ

The early atmosphere of Earth was primarily made up of hydrogen and helium. The second atmosphere was composed of volcanic gas with minor amounts of carbon dioxide and sulfur dioxide along with the traces of other gases such as nitrogen and ammonia. Later, the third atmosphere that is dominated by the gases such as nitrogen and oxygen are formed. Ozone is a gas that absorbs the ultraviolet radiation. It is supplied by oxygen.

Explanation of Solution

The early atmosphere of Earth was composed primarily of hydrogen and helium with the traces of other gases. However, it survived only for a short period. Due to the Sun’s heat, the rapid movement of lightweight atoms occur and they eventually attain escape velocity and zoom into space. By the effect of intense solar wind that reached the surface of the Earth, the atoms that did not escape on their own were carried away.

New gases were generated in the atmosphere by the activity of volcano with the disappearance of Earth’s primary atmosphere. Later, the second atmosphere was formed by the accumulation of gases following the differentiation of Earth and the development of a magnetic field that can deflect the solar wind. The second atmosphere was composed of volcanic gas, which consists of 70% to 90% water with minor amounts of carbon dioxide and sulfur dioxide along with the traces of other gases such as nitrogen and ammonia. Apart from this, the other gases such as methane and carbon monoxide that were carried into the Earth by comets are also present in the second atmosphere. The evolution of the Earth’s second atmosphere occurs over geologic time with the sufficient cooling of Earth for the condensation of water, and generation of liquid oceans. The conversion of water from a gaseous state in the atmosphere to a liquid state on or near the surface results in the drastic decrease of the proportion of water in the atmosphere.

Later, the concentration of carbon dioxide in the atmosphere is reduced with the accumulation of liquid water on or near the surface. The ultraviolet radiation from the Sun split the molecules of ${\text{NH}}_{\text{3}}$ that were present on the second atmosphere into the atoms of nitrogen and hydrogen. The lightweight hydrogen atoms are escaped into the atmosphere while the nitrogen atoms are combined to form ${\text{N}}_{\text{2}}$ molecules. It is an inert gas that can remain in the air for a long time and thereby it increases the proportion of nitrogen in the atmosphere. Now, it is the dominant gas present in the atmosphere.

Later, oxygen is appeared in the atmosphere as a result of photosynthesis and its concentration slowly increases during the Proterozoic Era. With the appearance of photosynthetic algae, the production of ${\text{O}}_{\text{2}}$ in the atmosphere increases at about 1.2Ga. However, only by about 600 million years ago, the level of oxygen in the air becomes substantial and thereby forms the third atmosphere that is dominated in ${\text{N}}_{\text{2}}\text{and}{\text{O}}_{\text{2}}.$

Ozone is a gas that absorbs harmful ultraviolet radiation from the Sun and it accumulates mainly at an elevation of about 30km. The raw components for the production of ozone is supplied by oxygen.