1.   Fuel cells use stored hydrogen and oxygen from the air to create electricity, and the only emission from engines they power is water vapor.

2.   Fuel cells use stored hydrogen and oxygen to create electricity, emitting only water vapor.

3.   In fuel cells, stored hydrogen and oxygen drawn from the air create electricity, with water vapor as the only emission.

4.   The automated production plant would combine its stored hydrogen with the carbon dioxide that laces the Martian atmosphere.