Editing Exosphere (section)

{{Short description|Outermost layer of an atmosphere}}
{{For|the racehorse|Exosphere (horse)}}
{{Use dmy dates|date=August 2015}}

[[File:EarthAtmosphereBig.jpg|thumb|80px|right|Diagram showing the five primary layers of the Earth's atmosphere: exosphere, [[thermosphere]], [[mesosphere]], [[stratosphere]], and [[troposphere]]. The layers are to scale. From the Earth's surface to the top of the stratosphere (50km) is just under 1% of Earth's radius.]]

The '''exosphere''' is a thin, atmosphere-like volume surrounding a [[planet]] or [[natural satellite]] where [[molecule]]s are gravitationally bound to that body, but where the density is so low that the molecules are essentially collision-less.<ref name="NASA-20150817">{{cite web |last=Steigerwald |first=William |title=NASA's LADEE Spacecraft Finds Neon in Lunar Atmosphere |url=http://www.nasa.gov/content/goddard/ladee-lunar-neon |date=17 August 2015 |publisher=[[NASA]] |access-date=18 August 2015 }}</ref> In the case of bodies with substantial atmospheres, such as [[Atmosphere of Earth|Earth's atmosphere]], the exosphere is the uppermost layer, where the atmosphere thins out and merges with [[outer space]]. It is located directly above the [[thermosphere]]. Very little is known about it due to a lack of [[research]]. [[Mercury (planet)|Mercury]], the [[Moon]], [[Ceres (dwarf planet)|Ceres]], [[Europa (moon)|Europa]], and [[Ganymede (moon)|Ganymede]] have surface boundary exospheres, which are exospheres without a denser atmosphere underneath. The Earth's exosphere is mostly [[hydrogen]] and [[helium]], with some heavier [[atoms]] and [[molecules]] near the base.<ref>{{cite book |doi=10.1016/B978-0-12-804492-6.00001-0 |chapter=Chemicals and the Environment |title=Environmental Organic Chemistry for Engineers |date=2017 |last1=Speight |first1=James G. |pages=1–41 |isbn=978-0-12-804492-6 }}</ref>