Investigating the Martian Atmosphere 1931

Spectroscopic studies have long been a means to determine the atmospheric composition of planets. From the start, there was a problem, however: Distinguishing the gases of another planet from those of Earth's atmosphere.

An old c 1940 work, The Story of the Starry Universe mentioned this problem in regards to studies of the Martian atmosphere, and the solution. Two Mt Wilson astronomers, Walter S. Adam and Theodore Dunham jr, used "very high dispersion." The book did not go into detail on the method employed, which involved the Doppler effect. The astronomers timed their observations to make the best use of it.

In January 1931 there was an aphelic opposition of Mars. The Red Planet was virtually at its greatest distance from the sun, which slowed its orbital speed. At the same time, Earth had just passed the perihelion point of its orbit, which meant it was orbiting the sun at virtually maximum speed. As Mars was moving at its slowest while Earth was at its fastest, the former was receding from Earth at its most rapid rate. Mars always moves more slowly than Earth, but in January 1931 it was falling behind, i.e. moving away, more quickly than usual. Under those circumstances, the spectrum of Mars was redshifted. The Doppler effect was sufficient to separate the Martian absorption lines from those of Earth (or its atmosphere).

No doubt, Adam and Dunham noted the presence of CO2 in the Martian atmosphere, as CO2 is its primary constituent. They may well have also detected H2O.

On Mars as on Earth, aphelion coincides more or less with the onset of northern summer. The north polar cap of Mars wanes, releasing large quantities of water vapor. Owing to the limited heat received while Mars is farthest from the sun, polar water does not melt but it does sublimate. It forms wispy clouds drifting southward. The research of Adams and Dunham may have been well-timed to detect water.

However, based on the limited evidence available, the researchers also concluded the Martian atmosphere was quite thin. This realization doused cold water on the popular notion of an inhabited Red Planet.

Mars during its latest aphelic opposition (January 2025). Mars presumably looked like that when at opposition on January 27, 1931.