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.
January 1931 witnessed 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. Since Mars was moving at its slowest while Earth was at its fastest, the Red Planet 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. Almost certainly, they 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 ice does not melt. It does however sublimate. H2O forms wispy clouds drifting southward. The research of Adams and Dunham may have been well-timed to detect water.
Yet 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.
posted by starman at 7:26 AM
8 Comments:
Wow Tim, the astronomers in 1931, did have a great understanding of spectral and orbital science then. Salute you for a wonderful astronomy post. Interesting to see how they concluded that Mars with that thin atmosphere would not have supported life. Although most diehard scientists even today clung on to the idea that the Red planet might have long ago harbored life. Anyway it's still so nice that astronomers today are still seeing ice or its sublimate during the aphelic period of Mars I'm sure the space orbiter probe will have wonderful images closeup of that
Hi Martin,
Thanks for the great input. Scientists didn't conclude Mars was completely uninhabitable in 1931. As late as the '50s, the dark areas of Mars were still thought to represent vegetation, which seemed to wax and wane with the seasons. That idea disappeared in the '60s, but microbial life is still considered possible. A thin atmosphere does, however, argue against complex or intelligent life.
January 23, 2026
If Mars has ever had complex life, it has been in the distant past. Mars obviously does not have advanced life forms that are inhabiting it now. Of course, some extraterrestrials from other solar systems might occasionally visit Mars.
Even in the distant past Mars probably had no complex life; the planet became virtually uninhabitable before complex life could evolve.
Sophisticated multicellular life didn't appear on Earth until about a billion years ago. Mars was already nearly uninhabitable 3 billion years before then.
January 24, 2026
I think that life may have existed on Mars, but it was most likely only microorganisms.
Agreed Neal. On the basis of photos taken by rovers, some researchers claim stromatolites, annelids and mushroom-like organisms are present on Mars, but I'm skeptical.
February 4, 2026
I don't think that extraterrestrials from elsewhere colonized Mars. No evidence of architecture has been observed. Of course, subterranean dwellings could have been built. Any beings who did visit Mars would most likely have only stayed temporarily to do research on the planet.
You know, Neal, some people claim ETs, even indigenous Martians, built pyramids and other structures on the red planet. Hoagland used to say that and there's book by Haas THE GREAT ARCHITECTS OF MARS. But he's a charlatan in my view.
February 5, 2026
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