Rock samples were to be passed by the PAW into a mass spectrometer and gas chromatograph in the body of the lander - the GAP (Gas Analysis Package), to measure the relative proportions of different isotopes of carbon and methane. Since carbon is thought to be the basis of all life, these readings could have revealed whether the samples contained the remnants of living organisms. Atmospheric methane is another signature of existing life, although geological processes can also be a source.