Volume mixing and isotope ratios secured with repeated atmospheric measurements taken with the Sample Analysis at Mars instrument suite on the Curiosity rover are: carbon dioxide (CO2), 0.960(+/-0.007); argon-40 (Ar-40), 0.0193(+/-0.0001); nitrogen (N-2), 0.0189(+/-0.0003); oxygen, 1.45(+/-0.09) x 10(-3); carbon monoxide, < 1.0 x 10(-3); and Ar-40/Ar-36, 1.9(+/-0.3) x 10(3). The Ar-40/N-2 ratio is 1.7 times greater and the Ar-40/Ar-36 ratio 1.6 times lower than values reported by the Viking Lander mass spectrometer in 1976, whereas other values are generally consistent with Viking and remote sensing observations. The Ar-40/Ar-36 ratio is consistent with martian meteoritic values, which provides additional strong support for a martian origin of these rocks. The isotopic signature delta C-13 from CO2 of similar to 45 per mil is independently measured with two instruments. This heavy isotope enrichment in carbon supports the hypothesis of substantial atmospheric loss.