This is a little like asking 'Why is iron mostly metal?' Atmospheres are made up of gas.
Carbón dioxide gas
The atmosphere is made mostly of gas because gases have low molecular densities, allowing them to be easily suspended in the Earth's gravity. The gases in the atmosphere help regulate temperature and provide elements essential for life, such as oxygen.
Nitrogen
carbon dioxide
The four outer gas giants.
No Mars is composed mostly of rock. The atmosphere is very thin.
The atmosphere of Titan is mostly nitrogen with a small percentage of methane and hydrogen.
it is mostly made of rock I don't think there is really any gas but its atmosphere.
helium
Mercury is predominantly composed of metal and rock, with a small iron core. It does not have a significant atmosphere, so it is not mostly gas like the gas giants in our solar system.
The largest planet is Jupiter, with an atmosphere made mostly of hydrogen.
Jupiter is the planet with the most abundant hydrogen gas in its atmosphere. It is composed mostly of hydrogen and helium, with hydrogen making up about 90% of the gas in its atmosphere.