Atmospheric Composition

The first attempted in situ chemical analysis of the Martian atmosphere was by a mass spectrometer carried by the Mars 6 Lander. The mass spectrometer unfortunately failed to operate, but the ion pump current was successfully telemetered back to Earth. The ion current was observed to be large and to decay slowly, suggestive of a large mole fraction of a poorly pumped inert gas, which, by analogy with Earth, was assumed to be radiogenic argon. From that time until the Viking missions, discussions of the origin and evolution of the atmosphere were predicted upon the apparent upon the apparent necessity of explaining the presence of about 30% 40Ar in the Martian atmosphere. The basic conclusion was that the high amount of outgassed 40Ar required the release of proportionately large amounts of other volatiles from the interior. So much 40Ar require essentially complete outgassing of a chondritic Mars. If one assumes that the relative abundance of volatile elements in Mars are similar to those in Earth, masses of H2O and CO2 sufficient to produce kilometer-thick ice layers would have been released. If one allows for the lower formation temperature, higher oxidation state, and higher volatile content of pre-planetary Mars material, then even more H2O and CO2 must have been released relative to 40Ar. But such enormous masses of volatiles are hard to find on Mars today. Without the most extreme exertions, such a large amount of H2O cannot be accommodated in permafrost and hydrous minerals. Hiding a comparable amount of CO2 is an even more severe problem, requiring the presence of several kilometers of CaCO3 over the entire surface of the planet. The existing data on the composition of the surface show clearly that carbonates cannot be more than a very minor surface constituent. The need thus arises for escape of vast amounts of CO2 with simultaneous complete retention of Ar.

Composition of the Atmosphere of Mars


Abundance (Mole Fraction)


Abundance (Mole Fraction)




2.5 ppm




0.5 ppm




0.3 ppm




0.08 ppm




0.01 ppm (varies)


<0.03% (varies)




The abundance of the Martian rare gases displays some interesting similarities and differences relative to those seen on Earth and meteorites. Detection and positive identification of Ne is difficult because of the masking of the 20Ne+ ions by 40Ar2+ at high ionizing electron voltages. However, the neon abundance was successfully measured at the level of 2.5 ppm. The analytical instrument, Gas Chromatograph-Mass Spectrometer (GCMS) that was attached to Viking probe to Mars, was used to analyze gases released by stepwise heating of surface dirt samples. By this means, extremely small concentrations of a very wide variety of organic molecules could have been detected. Detection limits for a large number of species ranged from 0.1 to 1.0 ppb. Surface samples heated to 500OC were observed to release abundant CO2 and H2O as expected, but no evidence was found for the presence of any organic material on Mars.

NASA's Hubble Space Telescope have taken the space-based observatory's sharpest views yet of the Red Planet.

The telescope obtained four images, which, together, show the entire planet. Each view depicts the planet as it completes one quarter of its daily rotation. In these views the north polar cap is tilted toward the Earth and is visible prominently at the top of each picture. The images were taken in the middle of the Martian northern summer, when the polar cap had shrunk to its smallest size. During this season the Sun shines continuously on the polar cap. Previous telescopic and spacecraft observations have shown that this summertime "residual" polar cap is composed of water ice, just like Earth's polar caps.