Definition of "exosphere" [exosphere]

• The outermost region of a planet's atmosphere. (noun)
• The outermost layer 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)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "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."
• "What is the nature of that atmosphere, more properly known as an exosphere, and what are its constituents?"
• "The observations are expected to yield new insights into how the magnetosphere "force field" works, as well as how thin the uppermost layer of Earth's atmosphere, called the exosphere, actually is, researchers said."