Which layer of the atmosphere has the lowest density?

The exosphere is the layer of the atmosphere that has the lowest density. This outermost layer extends from about 600 kilometers (372 miles) above the Earth's surface to outer space. In the exosphere, the air is extremely thin, composed mostly of hydrogen and helium, and the molecules are so sparse that they can travel vast distances without colliding with one another.

As we move down through the atmosphere, density increases significantly in the lower layers. The troposphere, for example, is where most weather occurs and has a much higher density due to the accumulation of air molecules. The stratosphere sits above the troposphere, where the density is still relatively high but begins to decrease with altitude as well. The thermosphere, while very hot due to solar radiation, still contains a significant number of air molecules compared to the exosphere, resulting in a higher density.

Thus, it is the exosphere that stands out as having the lowest density, primarily due to its location at the edge of space and its composition of highly dispersed particles.