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

The exosphere — the skin of the venue — will show real-time driver information during the race, along with live content and the post-race podium celebration.

From Los Angeles Times

We were somewhere above the Earth, on the edge of the exosphere, when the gummies began to take hold.

From Los Angeles Times

The mission of the Luna 25 spacecraft was to spend one year studying the composition of the moon's soil, as well as its exosphere.

From Salon

New research in the journal Nature Communications details the weak, atmosphere-like exosphere surrounding the planet Mercury.

From Salon