Question

Although the earth was formed from the same interstellar material as the sun, there is little elemental hydrogen \(\left(\mathrm{H}_{2}\right)\) in the earth's atmosphere. Explain.

Short answer

Although the Earth and the Sun were formed from the same interstellar material, which was primarily composed of hydrogen and helium, there is little elemental hydrogen in the Earth's atmosphere due to two primary factors: differentiation and escape velocity. Differentiation is a geological process that causes lighter elements like hydrogen to move towards the surface, while heavier elements sink towards the core. The escape velocity of a gas, influenced by a planet's weaker gravity, allows light gases like hydrogen to eventually escape into space from the Earth's atmosphere. Thus, geological and gravitational processes explain why the Earth's atmospheric composition is different from the Sun or the original interstellar material.

Step-by-step solution

Understanding the Composition of the Interstellar Material

The first thing to understand is that the interstellar material, from which the Sun and Earth (and other planets) were formed, was primarily made up of hydrogen and helium, with trace amounts of heavier elements.

Understanding the Process of Planetary Formation

The process of solar system formation consists of the aggregation of matter into a central mass (the Sun) and the leftovers form into smaller planetary bodies (like the Earth). As the solar system evolves, the Sun becomes hot enough to ignite the fusion of hydrogen into helium, creating light and heat.

Understanding Factors Affecting Atmospheric Composition

While the Sun consists mostly of hydrogen and helium, the planetary bodies such as Earth have vastly different atmospheric compositions. The reason lies in two primary factors: the process of differentiation and the escape velocity of gases in a planet's atmosphere.

Understanding Differentiation

Differentiation is a geological process through which denser materials move towards the center of a planet, and lighter materials move towards the surface. The Earth's core, for example, is made up of heavy elements such as iron and nickel, while the lighter elements, including hydrogen, moved towards the surface in the past.

Understanding Escape Velocity

The escape velocity of a gas is the minimum velocity necessary for that gas to escape from the gravitational pull of a planet. Smaller planets like the Earth with weaker gravity have lower escape velocities, meaning light gases, such as hydrogen and helium, can reach speeds (due to their thermal energy) that allow them to escape into space.

Conclusion

Based on the factors discussed above, we can conclude that although the Earth was formed from the same interstellar material as the Sun—and hence initially had a large amount of hydrogen—due to the processes of differentiation and the escape of light gases into space, there is little elemental hydrogen in the Earth's atmosphere today. These natural processes explain why the atmospheric composition of the Earth is different from the Sun or the original interstellar material from which they both were formed.