Exosphere

The exosphere is a very thin layer that surrounds a planet or moon. It is the outermost part of the atmosphere. In this layer, the air is 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.

Surface boundary exosphere

Some planets and moons, like Mercury, Ceres, the Moon, Europa, and Ganymede, have a special outer layer called a surface boundary exosphere. In these places, molecules move in stretched circle paths before hitting the surface again. Smaller objects, like asteroids, don’t have this because their molecules escape into space.

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 planet or moon has a very thin atmosphere, like the atmosphere of the Moon or that of Mercury, the whole atmosphere is called 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. These impacts can vaporize materials and create clouds that carry gaseous materials up to Mercury’s exosphere. Scientists have observed materials like Na, NaOH, and O₂ in this way.

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 where solar wind can reach Mercury’s surface and sputter materials into the exosphere. This process can wear away elements like sodium and move them into the atmosphere.