Exosphere

The exosphere is the very top layer of the atmosphere that surrounds a planet or moon. It is the thinnest part and is almost like the beginning of space. In this layer, the air is so thin that particles rarely bump into each other.

Earth has an exosphere made mostly of hydrogen and helium. Because it is so thin and hard to study, scientists know very little about this distant layer. Other places like the Moon, Mercury, Ceres, Europa, and Ganymede also have their own versions of exospheres.

The exosphere stretches very far out. On Earth, it ends about halfway to the Moon, or around 200,000 kilometres from our planet. From space, the exosphere can be seen as a faint glow called the geocorona.

Some planets and moons, like Mercury and the Moon, have a special outer layer called a surface boundary exosphere. In these places, molecules move in stretched circle paths before hitting the surface again. This makes their exospheres unique and interesting to learn about.