We have confirmed the discovery of N2 and 40Ar by the Entry Science Team, and we have also detected Ne, Kr, Xe, and the primordial isotopes of Ar. The noble gases exhibit an abundance pattern similar to that found in the terrestrial atmosphere and the primordial component of meteoritic gases. Xenon appears to be underabundant in comparison to the meteoritic ratio, as it is on earth. The isotopic ratios 15N/14N, 40Ar/36Ar, and 129Xe/132Xe are distinctly different from the terrestrial values, implying different evolutionary histories for volatiles on the two planets. The noble gas abundances indicate that at least 10 times the present atmospheric amount of N2 and 20 times the CO2 abundance were released by the planet during geologic time; the outgassing of a large amount of water must also have taken place. There is thus an explanation for the high surface pressure and abundance of water required at some early epoch to cut the dendritic channels observed on the Martian surface.