The Moon doesn't have any significant atmosphere (surface pressure is $3\times10^{-15}$ bar). Can the absence of atmosphere on the Moon be determined with ground-based observations? When was it first determined that the Moon doesn't have an atmosphere, and with what data?
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asked Jan 12, 2022 at 13:48
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3$\begingroup$ Seems like the kind of thing someone would have noticed during a solar eclipse. $\endgroup$– RBarryYoungJan 13, 2022 at 12:17
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$\begingroup$ When you say "ground-based", do you mean the ground on Earth or on the Moon? $\endgroup$– ParrotmasterJan 14, 2022 at 11:30
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2 Answers
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The Moon is so close that establishment of an atmosphere would be easy:
you take a photo. An atmosphere always is thicker at the surface and gets exponentially thinner outward. This would be visible at the edges of the moon. Just think of the photos of Earth from space with the blue-ish layer the atmosphere shows as.
It would also mean that the edges of the Moon would show in (slightly) different colour due to the presence of an atmosphere. Any atmosphere absorbs and scatters light. Especially the bluer light is scattered more efficiently (that's why the mountains in the distance show blue-ish). This would be visible in colour photography but - depending on the atmosphere's thickness - also with the visible eye.even if you don't resolve the atmosphere: the Moon often passes in front of stars. The light curve of the stars can be captured when the Moon starts to obscure it. The drop is sharp and indicative of an edge. You would not see such diffraction pattern but a smooth transition from visible star to obscured star if the Moon had any significant atmosphere which causes scattering.
Even easier this would be visible during a solar eclips. The edge of the shadow of the Moon would be less clear than it is.Spectroscopy. Any atmosphere consists of atoms. Each atom has its own characteristic properties in absorption but also emission. The spectrum of daylight on Earth is characterized both, by the emission lines on the Sun, but also by the Fraunhofer absorption line in the Earth's atmosphere. No absorption of any kind is observed in the reflected light from the Moon.
Neither of these observations needs very advanced equipment and can be made ground-based.
answered Jan 12, 2022 at 14:05
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13$\begingroup$ I wonder if the observation of Baily's Beads during total solar eclipses would also be evidence of an atmosphere-less moon. $\endgroup$– notovnyJan 12, 2022 at 14:19
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3$\begingroup$ @notovny doh! Sure enough, that's easier than stellar ocultations. Thanks, I added that. $\endgroup$ Jan 12, 2022 at 14:45
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$\begingroup$ The solar spectrum is full of absorption lines superimposed on the underlying quasi-black body spectrum), including Fraunhofer lines. The reflected light from the moon includes all of that. It’s true that wouldn’t see O2 absorption (except that you’d still see O2 absorption from the Earth’s atmosphere, so it would be harder to rule out some additional contribution from a hypothetical thin lunar atmosphere). $\endgroup$ Jan 12, 2022 at 23:30
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$\begingroup$ @PeterErwin it's no problem to measure the direct solar spectrum. Any difference from that must be attributed to the Moon. Sure enough, the thinner the atmosphere, and the more similar to Earth, the harder it is to detect with spectroscopy - but I recon still not a terribly hard task unless you get to atmospheric thicknesses where one can start calling it absence of an atmosphere. $\endgroup$ Jan 12, 2022 at 23:42
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4$\begingroup$ “Especially the bluer light is scattered more efficiently (that's why the mountains in the distance show blue-ish).” – actually it's the opposite: the mountains show more yellowish because the blue components are scattered before reaching you. However, except for snow patches, mountains tend to be too dark-coloured for this to be much notable; what is notable instead is the sunlight that also gets scattered from the air between mountain and observer. That has nothing to do with the mountains, but our eyes don't know that and interpret it as the mountains being blue. $\endgroup$ Jan 13, 2022 at 13:49
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19th century observers determined that the Moon is (almost totally) airless using 19th century instruments and observing techniques.
Roger Joseph Boscovich (1711-1787) is credited with having discovered that the Moon has no atmosphere in 1753.
Wilhelm Beer and Johann Heinrich Mädler established the same conclusion in the 1830s:
They also produced the first exact map of the Moon, Mappa Selenographica, published in four volumes in 1834–1836. In 1837 a description of the Moon (Der Mond) was published. Both were the best descriptions of the Moon for many decades, not superseded until the map of Johann Friedrich Julius Schmidt in the 1870s. Beer and Mädler drew the firm conclusion that the features on the Moon do not change, and there is no atmosphere or water.
The Guinness Book of Astronomy Facts & Feats by Patrick Moore (2nd edition, 1983) discusses solar prominences on page 17:
Prominences were first described in detail by the Swedish observerVassenius at the total eclipse of 1733, though he believed that they belongd to the Moon rather than the Sun. (They may have been seen earlier - by Stannyan in 1706, from Berne.) It was only after the eclipse of 1842 that astronomers became certain that they were solar rather than lunar.
Page 20 discusses the solar corona, visible during eclipses, which was long suspected to be the atmosphere of the Moon.
After observing the eclipse of 16 june 1806 from Kinderhook, New York, the Spanish astronomer Don Jose Joaquin de Ferrer pointed out that if the corona were due to a lunar atmopshere, then the height of this atmosphere wuld have to be 50 times greater than that of the Earth, which was clearly unreasonable. However, it was only after careful studies of the eclipses of 1842 and 1851 that the corana and the prominences were were show unmistakably to belong to the Sun rather than the Moon.
Exploration of the Universe Brief Edition, George Abell, 1964, 1969, discusses the lack of lunar atmosphere on page 184:
Telescopic observations of the moon, as ell as observations from lunar probes, confirm its expected lack of an appreciable atmopshere. On the earth the air scatters the sunlight around into a certain portion of its night side, producing a twilight zone. On the moon there is no evidence such a twilight zone. Furthermore, when the moon occults (passes in front of) a star, the star's light is observed to blink out suddenly, rather than to do dim gradually as it would if had to shine thorugh an atmosphere around the moon.
Triton, the large moon of Neptune, has a very thin atmosphere:
Nitrogen is the main gas in Triton's atmosphere. The two other known components are methane and carbon monoxide, whose abundances are a few hundredths of a percent of that of the nitrogen.
Triton's atmosphere is well structured and global. The atmosphere extends up to 800 kilometers above the surface, where the exobase is located, and had a surface pressure of about 14 microbars as of 1989. This is only 1/70,000th of the surface pressure on Earth.
Voyager 2 photographed atmospheric haze over the horizon of Triton.
It also photographed streaks of dark atmospheric material being blown by winds in the atmosphere of Triton.
Presumably if the lunar atmosphere was as dense as even the ultra thin atmosphere of Triton it would have visible effects.
It is easy and simple to calculate and predict the Moon's orbit roughly. But the precise orbital motions of the Moon is much more complicated to calculate and predict. Those complex calculations are known as lunar theory.
Peter Andreas Hansen (1795-1874) was a famous astronomer. Hansen worked out a new version of the lunar theory in the late 1850s which predicted all the lunar movements perfectly - for a few years. And one of the features of Hansen's theory was that the Moon was sort of egg shaped (it is, but not that much) and the near side of the moon is sort of like a giant mountain projecting toward the Earth, and thus father from the center of the moon than the far side is.
That would mean that the near side would be sticking out above the lunar atmosphere and all the lunar atmosphere would be on the far side of the Moon, and hidden from Earth by the bulk of the Moon.
Simon Newcomb (1835-1909) soon refuted that theory of the shape of the Moon. This was mentioned by The Cornhill Magazine in 1877 on page 724]8.
H.P. Lovecraft (1890-1937) once wrote that when he was 12 (c. 1902) he wrote a story where there was air on the far side of the Moon, even though he knew that Hansen's theory was false. And Lovecraft was probably far from the last person to write a story where there was air and water and life on the far side of the Moon. For example, the Lunar Trilogy by Jerzy Zulawsky (1901-1911) features a far side of the Moon with atmosphere.
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$\begingroup$ There are numerous typos in several of your book quotes. Did you type up the quoted text manually (and thus made errors in transcription), or are those errors in the original books you're quoting? $\endgroup$– V2Blast ♦Jan 14, 2022 at 19:54
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$\begingroup$ I don't remember. I must have typed out the quotes from books that I own, and copied the quotes from online sources. For some reason I keep writing atmopshere instead of atmosphere. $\endgroup$ Jan 15, 2022 at 19:13