Methane

Methane was a type of gas. The atmosphere of the planet Merj was made up of methane, meaning that the native Morseerian species evolved to breath methane instead of oxygen, and thus had to wear breath masks when on planets with oxygen-based atmospheres. The Skakoans also breathed methane. The clouds of the planet Bastatha were made of methane and sulfur. Vix, a moon of Endor, had methane rain.