exosphere

[ ek-soh-sfeer ]

noun

1. the highest region of the atmosphere, where the air density is so low that a fast-moving air molecule is more than 50 percent likely to escape from the atmosphere instead of hitting other molecules.

exosphere

/ ˈɛksəʊˌsfɪə /

noun

1. the outermost layer of the earth's atmosphere. It extends from about 400 km above the earth's surface

exosphere

/ ĕk′sō-sfîr′ /

1. The outermost region of the Earth's atmosphere, beginning at an altitude of approximately 550 km to 700 km (341 to 434 mi) and merging with the interplanetary medium at around 10,000 km (6,200 mi). The exosphere consists chiefly of ionized hydrogen, which creates the geocorona by reflecting far-ultraviolet light from the Sun. On the remote edges of the exosphere, hydrogen atoms are so sparse that each cubic centimeter might contain only one atom; furthermore, the pressure and gravity are weak enough that atoms in the exosphere can escape entirely and drift into space. Artificial satellites generally orbit in this region.
2. See also mesosphereSee illustration at atmosphere

Other Words From

• ex·o·spher·i·cal [ek-s, uh, -, sfer, -i-k, uh, l, -, sfeer, -], exo·spheric adjective

Word History and Origins

Origin of exosphere¹

First recorded in 1950–55; exo- + -sphere

Example Sentences

For 48 hours, a French instrument mounted on the lander will study the origin and dynamics of the lunar exosphere, a wispy layer of gases that hugs the surface.

From Science Magazine

One goal is to explain sharp variations seen in the exosphere’s density by time and location.

From Science Magazine

There are 1.2 million LED “pucks” that make up the 580,000-square feet exosphere, each of which can display more than 1 billion colors.

From Seattle Times

Additionally, as the authors write, "we know that while the Moon does not have an atmosphere or magnetosphere, it does have a delicate exosphere composed of dust and gas, as well as ice inside permanently shadowed areas, and both are susceptible to exhaust gas propagation."

From Salon

"We know that while the Moon does not have an atmosphere or magnetosphere, it does have a delicate exosphere composed of dust and gas, as well as ice inside permanently shadowed areas, and both are susceptible to exhaust gas propagation," the authors wrote.

From Science Daily