Why is the Earth able to retain and maintain its own atmosphere around it over long periods of time?

Introduction / Context:
The Earth is surrounded by an atmosphere made of gases such as nitrogen, oxygen, and trace components. Not all planetary bodies have significant atmospheres, and understanding why the Earth can hold on to its atmosphere is an important concept in basic physics and planetary science. This question tests whether you know that gravity, rather than winds or clouds, is the fundamental reason the atmosphere stays bound to the planet.

Given Data / Assumptions:

• The Earth has a certain mass and therefore exerts a gravitational attraction on all objects, including gas molecules.
• Gas molecules in the atmosphere have thermal motion and can move randomly.
• Winds and clouds are secondary phenomena occurring within the atmosphere.
• Solar radiation can add energy to the atmosphere but can also cause atmospheric escape for some planets.
• We assume normal planetary conditions, not catastrophic events.

Concept / Approach:
Gravity is the fundamental force that keeps the atmosphere bound to the Earth. Each gas molecule experiences a gravitational pull toward the centre of the planet. As long as the average molecular speeds due to thermal motion are below the escape velocity at the given height, most molecules remain gravitationally bound. Winds and clouds are effects of atmospheric dynamics, but they do not create the binding effect themselves. The rotation of the Earth does influence atmospheric circulation but does not provide a mechanism for retaining gases; in fact, very high rotation would make retention more difficult at the equator.

Step-by-Step Solution:
Step 1: Recognise that any mass, including gas molecules, experiences a gravitational force toward the centre of the Earth.Step 2: Understand that the Earth gravitational field extends far above the surface and acts on the entire atmosphere.Step 3: Note that thermal motion of gas molecules can cause some to escape, but for most molecules the speeds are below escape velocity, so they remain bound.Step 4: Conclude that it is the gravitational attraction of the Earth that allows it to maintain a stable atmosphere over long time scales.

Verification / Alternative check:
Comparing different planets supports this explanation. Massive planets like Jupiter have strong gravity and hold very thick atmospheres. Small bodies like the Moon have weak gravity and cannot retain a significant atmosphere, even though they may have winds or be illuminated by the Sun. Wind patterns and clouds are features of an existing atmosphere, not the cause of its presence. Therefore, gravity is clearly the controlling factor in atmospheric retention.

Why Other Options Are Wrong:
Option a is wrong because winds only move air around within the atmosphere and do not hold it in place. Option b is incorrect since clouds are condensed water droplets or ice crystals suspended within the atmosphere; they depend on the atmosphere but do not cause it to exist. Option d is misleading because the rotation of the Earth can create centrifugal effects and influence weather systems, but it does not bind gas molecules to the planet; if anything, increased rotation would slightly reduce effective weight at the equator. Option e is not correct because solar radiation can heat the atmosphere and may contribute to atmospheric loss under some conditions rather than ensuring retention.

Common Pitfalls:
Students sometimes mix up causes and effects, assuming that visible phenomena like clouds or wind are responsible for the deeper physical reasons. It is also easy to overestimate the role of rotation, imagining that spinning somehow glues the atmosphere in place. Remember that gravitational attraction is the main reason planets can hold atmospheres, and the ability to do so depends strongly on planet mass, radius, and temperature conditions that affect molecular speeds.

Final Answer:
Because of the gravitational attraction of the Earth holding gas molecules.