Which layer of the atmosphere is closest to the Earth's surface and contains 75% of the atmosphere's mass?

The correct answer is the troposphere because it is the lowest layer of Earth's atmosphere, extending from the surface up to about 8 to 15 kilometers high, depending on the geographic location. This layer is where we live and where most of the weather phenomena occur. It contains approximately 75% of the atmosphere's total mass, primarily composed of nitrogen and oxygen, which are essential for life.

In contrast, the stratosphere, which lies above the troposphere, contains less atmospheric mass and is where the ozone layer is found. The mesosphere, located above the stratosphere, is even less dense and is where meteoroids typically burn up upon entering the atmosphere. Lastly, the thermosphere is the uppermost layer that extends much higher and contains very sparse air molecules, leading to low overall atmospheric mass in that region. Thus, the distinction of the troposphere as the layer closest to Earth's surface containing the majority of the atmosphere's mass underlines its critical role in supporting life and weather systems.