The phrase "percent methane" refers to the amount or proportion of methane gas present in a particular substance or mixture. It tells us how much of the substance is made up of methane gas. Full definition
A rocky, ocean - bearing planet with more than 0.1 percent methane in its atmosphere should be considered a potentially inhabited planet, the researchers say. (scientificamerican.com)
Comets are about 1 percent methane by weight, but they strike Mars only once every 60 million years on average. (scientificamerican.com)
The gas produced during anaerobic digestion consists roughly of 60 percent methane and 40 percent CO2. (fdbusiness.com)