exosphere -
An extremely thin atmosphere, as on Saturn's moon Dione.
Wikipedia
The exosphere is the uppermost layer of the atmosphere. In the exosphere, an upward travelling molecule moving fast enough to attain escape velocity can escape to space with a low chance of collisions; if it is moving below escape velocity it will be prevented from escaping from the celestial body by gravity. In either case, such a molecule is unlikely to collide with another molecule due to the exosphere's low density.
OmegaWiki Dictionary
Ω
exosphere The uppermost layer of the atmosphere, its lower boundary is estimated at 500 km to 1000 km above the Earth's surface.
