The exosphere is the outermost layer of the Earth's atmosphere, located above the thermosphere. It extends from about 600 km until it thins out to merge with interplanetary space. This makes the exosphere about 10,000 km or 6,200 miles thick or about as wide as the Earth. The top boundary of Earth's exosphere extends about halfway to the Moon.
For other planets with substantial atmospheres, the exosphere is the layer above the denser atmospheric layers, but for planets or satellites without dense atmospheres, the exosphere is the region between the surface and interplanetary space. This is called the surface boundary exosphere. It has been observed for the Earth's Moon, Mercury, and the Galilean moons of Jupiter.
The word "exosphere" comes from the Ancient Greek words exo, meaning outside or beyond, and sphaira, which means sphere.
The particles in the exosphere are extremely far apart. They don't quite fit the definition of a "gas" because the density is too low for collisions and interactions to occur. Nor are they necessarily plasma, because the atoms and molecules aren't all electrically charged. Particles in the exosphere can travel hundreds of kilometers along a ballistic trajectory before bumping into other particles.
The Earth's Exosphere
The lower boundary of the exosphere, where it meets the thermosphere, is called the thermopause. Its height above sea level ranges from 250-500 km up to 1000 km (310 to 620 miles), depending on solar activity. The thermopause is called the exobase, exopause, or critical altitude. Above this point, barometric conditions do not apply. The temperature of the exosphere is nearly constant and very cold. At the upper boundary of the exosphere, the solar radiation pressure on hydrogen exceeds the gravitational pull back toward Earth. The fluctuation of the exobase due to solar weather is important because it affects atmospheric drag on space stations and satellites. Particles that reach the boundary are lost from the Earth's atmosphere to space.
The composition of the exosphere is different from that of the layers beneath it. Only the lightest gases occur, barely held to the planet by gravity. The Earth's exosphere consists mainly of hydrogen, helium, carbon dioxide, and atomic oxygen. The exosphere is visible from space as a fuzzy region called the geocorona.
The Lunar Atmosphere
On Earth, there are about 1019 molecules per cubic centimeter of air at sea level. In contrast, there are fewer than a million (106) molecules in the same volume in the exosphere. The Moon does not have a true atmosphere because its particles don't circulate, don't absorb much radiation, and have to be replenished. Yet, it's not quite a vacuum, either. The lunar surface boundary layer has a pressure of about 3 x 10-15 atm (0.3 nano Pascals). The pressure varies depending on whether it's day or night, but the entire mass weighs less than 10 metric tonnes. The exosphere is produced by outgassing of radon and helium from radioactive decay. The solar wind, micrometeor bombardment, and the solar wind also contribute particles. Unusual gases found in the Moon's exosphere, but not in the atmosphere's of Earth, Venus, or Mars include sodium and potassium. Other elements and compounds found in the Moon's exosphere include argon-40, neon, helium-4, oxygen, methane, nitrogen, carbon monoxide, and carbon dioxide. A trace amount of hydrogen is present. Very minute quantities of water vapor may also exist.
In addition to its exosphere, the Moon may have an "atmosphere" of dust that hovers above the surface due to electrostatic levitation.
Exosphere Fun Fact
While the exosphere of the Moon is nearly a vacuum it's larger than the exosphere of Mercury. One explanation for this is that Mercury is much closer to the Sun, so the solar wind can sweep away particles more easily.
- Bauer, Siegfried; Lammer, Helmut. Planetary Aeronomy: Atmosphere Environments in Planetary Systems, Springer Publishing, 2004.
- "Is There an Atmosphere on the Moon?". NASA. 30 January 2014. retrieved 02/20/2017