noun. The outermost region of a planet's atmosphere.

noun. The outermost region of the earth's atmosphere, lying above the thermosphere and extending thousands of kilometers into space, from which molecules having sufficient velocity can escape the earth's gravitation.

noun. the uppermost layer of a planet's atmosphere

noun. an extremely thin atmosphere, as on Saturn's moon Dione

noun. the outermost atmospheric layer

"One surprise is that in the initial data, sodium was seen in the spectroscopic data, and Colaprete said sodium exists in the Moon's tenuous atmosphere called the exosphere, and perhaps something got thermalized during the impact excite the sodium atoms to where strong visible emission lines showed up in the data."

layer