Atomic Hydrogen on Mars: Measurements at Solar Minimum

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Science  02 Jun 1978:
Vol. 200, Issue 4345, pp. 1048-1051
DOI: 10.1126/science.200.4345.1048


The Copernicus Orbiting Astronomical Observatory was used to obtain measurements of Mars Lyman-α (1215.671-angstrom) emission at the solar minimum, which has resulted in the first information on atomic hydrogen concentrations in the upper atmosphere of Mars at the solar minimum. The Copernicus measurements, coupled with the Viking in situ measurements of the temperature (170° ± 30°K) of the upper atmosphere of Mars, indicate that the atomic hydrogen number density at the exobase of Mars (250 kilometers) is about 60 times greater than that deduced from Mariner 6 and 7 Lyman-α measurements obtained during a period of high solar activity. The Copernicus results are consistent with Hunten's hypothesis of the diffusion-limited escape of atomic hydrogen from Mars.