Table 1. Chemical composition of the Martian atmosphere. Spacecraft data are in black, ground-based spectroscopy is in red, spectroscopy from Earthorbiting observatories in blue. Species Mixing ratio Instrument CO2 0.955 GS N2 0.027 Viking/MS Ar 0.016 Viking/MS Ne 2.5 ppm Viking/MS Kr 0.3 ppm Viking/MS Xe 80 ppb Viking/MS H 2.7×10 cm Mariner/UVS O 0.01-0.02 Mariner/UVS O2 (1.2−2)×10 FPI CO 8×10...