Exosphere

The exosphere is a very thin layer that surrounds a planet or moon. It is the outermost part of the atmosphere, where the air becomes so thin that particles rarely bump into each other. For planets with thicker atmospheres like Earth, the exosphere is the top layer, right before space begins.

Earth's exosphere is made mostly of hydrogen and helium, with a few heavier particles closer to its base. Because it is so thin and hard to study, scientists know very little about this distant layer. Other celestial bodies such as Mercury, the Moon, Ceres, Europa, and Ganymede also have their own versions of exospheres, though these do not sit above a thicker atmosphere like Earth's does.

Surface boundary exosphere

Some planets and moons, like Mercury, Ceres, the Moon, Europa, and Ganymede, have a special kind of outer layer called a surface boundary exosphere. In these places, molecules are thrown out in paths that look like stretched circles until they hit the surface again. Smaller objects, such as asteroids, don’t have this because their molecules escape into space instead.

Earth's exosphere

The exosphere is the thinnest part of Earth's atmosphere. It is made up mostly of hydrogen, with some helium, carbon dioxide, and atomic oxygen. Because it is so thin, the exosphere is often considered part of outer space.

The lower edge of the exosphere is called the thermopause or exobase. This is the height where the air becomes so thin that temperature stays almost the same no matter how high you go. On Earth, this boundary is usually between 500 and 1,000 kilometres above the surface, depending on the Sun’s activity.

The exosphere stretches very far out. It ends where the Sun’s energy starts to push hydrogen atoms away more strongly than Earth’s gravity can hold them. This happens about halfway to the Moon, or around 200,000 kilometres from Earth. From space, the exosphere can be seen as a faint glow called the geocorona, and it extends at least 100,000 kilometres from our planet’s surface.

Main article: Thermopause

Exosphere of other celestial bodies

If a celestial body has a very thin atmosphere, like the atmosphere of the Moon or that of Mercury, the whole atmosphere is considered an exosphere.

The exosphere of Mercury

There are many ideas about how Mercury’s surface boundary exosphere formed. It includes elements such as sodium (Na), potassium (K), and calcium (Ca). These materials may come from processes like impacts, solar wind, and degassing from the planet, which cause atoms or molecules to form the exosphere.

Meteoroids often hit Mercury’s surface at very high speeds, up to 80 km/s. These impacts can vaporize both the meteor and surface materials, creating clouds of mixed materials. These clouds can carry gaseous materials and compounds up to Mercury’s exosphere. During these impacts, the materials from the colliding bodies mostly break down into atoms, which can then reform during cooling. Scientists have observed materials like Na, NaOH, and O₂ in this way. However, impacts are thought to be a small factor in the overall amounts of sodium and potassium in Mercury’s exosphere. Calcium is more likely to come from impacts, but its movement to the exosphere is thought to happen through photolysis of its former oxides or hydroxides, rather than directly from the impact.

Another way Mercury’s exosphere may form is due to its unique relationship with its magnetosphere and the solar wind. Mercury’s magnetosphere might not fully protect the planet from solar wind. If this is true, there may be gaps in the magnetosphere where solar wind can reach Mercury’s surface and sputter materials from the surface into the exosphere. This process can wear away elements like sodium and move them into the atmosphere. However, this does not happen all the time and cannot explain all the atoms or molecules in the exosphere.