Carbón dioxide gas
This is a little like asking 'Why is iron mostly metal?' Atmospheres are made up of gas.
helium
Which gas is air mostly made up from
The atmosphere is the mixture of gas molecules and other materials surrounding the earth. It is made mostly of the gases nitrogen (78%), and oxygen (21%).
Venus is made up of an iron core and rocky mantle. While it's atmosphere mostly consists of carbon dioxide and nitrogen.
It's rocky, but it has an atmosphere made up mostly of carbon dioxide and has clouds made of sulfuric acid.
Gas giants are planets whose atmosphere is predominantly composed of gases, such as hydrogen and helium. These planets have a small solid core surrounded by thick layers of gas. Examples of gas giants in our solar system include Jupiter and Saturn.
The largest planet is Jupiter, with an atmosphere made mostly of hydrogen.
Dust and gas
Carbon dioxide makes up the majority of the Martian atmosphere, accounting for about 95% of the gases present.
The atmosphere is made up of 71% nitrogen and 28% oxygen. The other 1% is mostly made up of the noble gas argon.
it is made up of mostly gas