In atmospheric chemistry and geophysics, the upper part of the heterosphere is composed almost completely of which gas?

Introduction / Context:
This question examines your understanding of the structure and composition of the Earth atmosphere, particularly the region known as the heterosphere. In school level geography and environmental science, the atmosphere is often divided into layers based on temperature, composition, and mixing characteristics. At higher altitudes, gas molecules become separated according to their molecular mass, forming the heterosphere. Knowing which gas dominates the upper heterosphere helps you understand how light gases behave in the gravitational field of the planet and why certain space based measurements detect specific gases at very high altitudes.

Given Data / Assumptions:
- The question focuses on the upper part of the heterosphere in the atmosphere.
- You must identify which gas is present almost exclusively in that upper region.
- The options are ozone, nitrogen, oxygen, and hydrogen, which are all known atmospheric gases.
- It is assumed that you are familiar with the basic idea that lighter gases can rise higher in the atmosphere.

Concept / Approach:
The key concept is that the atmosphere can be divided into the homosphere, where gases are well mixed, and the heterosphere, where gases are stratified by molecular weight. In the lower layers of the atmosphere, nitrogen and oxygen are dominant and almost uniformly mixed. However, in the heterosphere at very high altitudes, heavier gases remain closer to the lower boundary, while lighter gases gradually dominate as you move upward. Since hydrogen is the lightest of the gases listed, it tends to be found in the uppermost parts of the atmosphere. The task is to link the idea of molecular mass and gravitational separation with the composition of the upper heterosphere.

Step-by-Step Solution:
Step 1: Recall that the lower atmosphere, known as the homosphere, has a fairly uniform composition dominated by nitrogen and oxygen. Step 2: Understand that the heterosphere lies above the homosphere and extends into regions where gases separate based on density and molecular mass. Step 3: Recognize that in the lower heterosphere, heavier gases like nitrogen and oxygen may still be present, but as altitude increases, lighter gases become more common. Step 4: Identify hydrogen as the lightest gas among the options given, significantly lighter than ozone, nitrogen, or oxygen. Step 5: Conclude that at the upper part of the heterosphere, hydrogen becomes the predominant gas, so Hydrogen is the correct answer.

Verification / Alternative check:
You can verify this by consulting diagrams of atmospheric composition in standard geography or environmental science textbooks. These diagrams often show layers such as the troposphere, stratosphere, mesosphere, and thermosphere, alongside the distinction between homosphere and heterosphere. In the very high altitude exosphere, which lies beyond the upper heterosphere, hydrogen and helium form the bulk of the gas content. Since the question specifically asks about the upper heterosphere, which transitions toward this region, the dominance of hydrogen is consistent and widely accepted in atmospheric science literature.

Why Other Options Are Wrong:
Ozone is concentrated mainly in the ozone layer within the stratosphere and does not dominate the extreme upper atmosphere. Nitrogen is the most abundant gas in the lower atmosphere but does not remain the principal gas at the highest altitudes because it is heavier than hydrogen and helium. Oxygen is also abundant in the lower and middle atmosphere, but its proportion falls off in the upper heterosphere. Hence, ozone, nitrogen, and oxygen do not match the description of the gas that almost completely composes the upper heterosphere.

Common Pitfalls:
A common mistake is to mechanically choose nitrogen because it is the most abundant gas in the atmosphere overall, without considering how composition changes with altitude. Another pitfall is to think of ozone whenever high altitude atmospheric chemistry is mentioned, even though ozone is concentrated much lower. Students may also overlook the role of molecular mass in gravitational separation of gases. To avoid these errors, always relate the concept of layers to the physical properties of gases and remember that lighter gases can dominate at extreme heights above the Earth surface.

Final Answer:
The upper part of the heterosphere is composed almost completely of Hydrogen.