That has big implications for extraterrestrial biology, since life is based on carbon, and methane - which is made up of carbon and hydrogen - is a byproduct of metabolism.

Other Japanese automakers have been working to create fuel cell cars, which produce electricity through a chemical reaction between hydrogen and oxygen, with water the only by-product.

Fuel cells combine hydrogen and air to produce electricity by running them through a chemical catalyst. The only byproducts are heat and water.