Hey science gurus! What determines whether a planetary mass, planet ,moon etc will have an atmosphere or not?
All the sun’s planets probably had similar atmospheres when they first formed, billions of years ago. The most common gases were the light gases, hydrogen and helium, with smaller amounts of oxygen, nitrogen and other gases. The immense gravity of the giant planets, Jupiter, Saturn, Uranus and Neptune, was able to hold the hydrogen and helium, which remain the predominant gases in their atmospheres, along with methane formed by the combination of hydrogen with free carbon. However, hydrogen and helium gradually escaped from the atmospheres of the smaller planets, where gravity is insufficient to hold the light gases permanently. Even the heavier gases, such as oxygen and nitrogen, as well as carbon dioxide and water vapor, can gradually escape from smaller planets, such as Mars and Mercury. So, these planets now have quite thin atmospheres. The Moon and other small bodies in the solar system have essentially no atmosphere. In our solar system, only the Earth and Venus are the right siz