The exosphere has very thin air.
The exosphere is the uppermost layer of Earth's atmosphere, gradually transitioning into the vacuum of space. The air within the exosphere is incredibly thin, almost mirroring the airless void of outer space. This is in stark contrast to lower atmospheric layers, which contain significantly denser air. As you move higher in altitude through the atmosphere's layers (troposphere, stratosphere, mesosphere, thermosphere, and finally exosphere), the density of air consistently decreases.
Several sources confirm this:
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The exosphere includes the uppermost region of Earth's atmosphere as it gradually fades into the vacuum of space. The air in the exosphere is extremely thin - in many ways it is almost the same as the airless void of outer space.
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As you might imagine, the "air" in the exosphere is very, very, very thin, making this layer even more space-like than the thermosphere. (Source: Center for Science Education)
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Like Earth, Mars has an atmosphere and weather, but both differ greatly from what we experience on Earth. · Relative to Earth, the air on Mars is extremely thin. (Source: Mars Education) While not directly referring to a specific Earth atmospheric layer, this highlights the comparative thinness of atmospheres on other planets. The exosphere is the closest Earth analogue to this thin Martian atmosphere.
The Moon also has a very thin atmosphere, but this is significantly different than the Earth's atmospheric layers and should not be considered part of the Earth's atmosphere.
