Why dont most moons have an atmosphere?
There are two reasons. First they don’t have enough mass. They just don’t have enough gravity to hold on to an atmosphere. Secondly, and more importantly, they lack a magnetic field. The Sun heats up a planet’s (or moon’s) atmosphere. Whatever molecules that might be in this atmosphere move faster, and faster and eventually ‘evaporate’ into space. The larger a planet’s mass, the greater it’s gravity and the harder it is for these molecules to escape the planet’s gravity well. If a planet (or moon) lacks a magnetic field this problem is compounded. Not only is the atmosphere heated thermally, the solar wind actually erodes it over time. The solar wind is actually full of highly charged particles. When these particles interact with a planet’s atmosphere, they can actually knock atoms and molecules out of a planet’s atmosphere. This is exactly what happened to Mars. It the past (a couple billion years ago) Mars had a much thicker atmosphere than it does today. It is small enough that it c
Some moons don’t have enough mass to have the gravity required to hold one down. Another important factor is geological activity. The Moon is a “dead”, inactive piece of rock, where there has been no volcanic activity for billions of years. Volcanic activity produces gases, and is vital to life. The greenhouse gases produced by volcanoes help to keep the Earth warm.
