An atmosphere is a layer of gases surrounding a celestial body, such as a planet or moon. It is held in place by the body's gravity and can vary in composition, density, and thickness. Earth's atmosphere, for example, is made up of nitrogen, oxygen, and trace gases, which are essential for life. Different planets have unique atmospheres. For instance, the atmosphere of Mars is thin and mostly carbon dioxide, while Venus has a thick atmosphere rich in carbon dioxide and sulfuric acid clouds. These differences affect temperature, weather, and the potential for supporting life.