Solar and Lunar Atmospheres
Published on Mar 13, 2006 at 8:11 pm.
No Comments.
Filed under Sun, moon.
The Moon is an airless world. The Moon’s gravity is insufficient for it to hold onto an atmosphere of any consequence. There are a few atoms hanging around, and lunar scientists talk about a “lunar atmosphere,†but it is so thin that it shouldn’t really count as an atmosphere at all. In fact, you get a thicker gas by pumping on a vacuum chamber with our best vacuum pumps!
The Sun is a slightly different story. We also talk about the solar atmosphere to describe its outermost layers. Again, the term is not really the best that we could come up with, but it is easier to live with than the lunar atmosphere. The apparent visible surface of the Sun is called the photosphere. It shines because it is hot, about 5800K The photosphere rapidly thins with altitude. As you travel outwards from the center of the Sun, the temperature drops. This is to be expected, as the source of energy for the Sun is the thermonuclear fusion at its core. What may come as surprise, though, is that the temperature increases with altitude above the photosphere. Above the photosphere is a layer of the Sun that is much thinner than the photosphere, but also much hotter. This layer is too thin to shine as bright as the photosphere, so it is very difficult to see. It shines brightly, though, in the emission spectrum of hydrogen. One particular hydrogen spectral line, the Balmer alpha line, is frequently used to look at this layer, as it shines brighter at this one color than does the photosphere. However, during an eclipse, just as the photosphere is covered, or just as it is about to be uncovered, this elusive solar layer is visible to the naked eye. It appears red, so this layer is called the chromosphere (literally, the colored sphere). The outermost part of the Sun is called the corona. The corona is very thin, almost a vacuum, in fact. This is one reason that it does not shine brightly, despite being so hot. The corona can be as hot as one million degrees. It shines brightly in X-rays. However, during an eclipse, the photosphere is blocked from view, so it does not overwhelm the outer solar layers. The thin gasses of the corona scatter sunlight passing through it, so it shines with a ghostly glow around the Sun. Collectively, the photosphere, the chromosphere, and the corona are referred to as the solar atmosphere.
You might ask why the chromosphere and the corona are hotter than the photosphere, and this is a good question. We think that mechanical forces, shockwaves and such, may be a source of heat for the chromosphere. The corona may be heated by magnetic effects, solar flares for example. We know that small red dwarf stars, such as UV Ceti, are very active magnetically, and they have coronae that are much hotter than our own Sun’s corona.Â
All of the above can be found in most any introductory astronomy textbook. What these books don’t have, though, is some of the trivia surrounding these facts.Â
As you can imagine, the idea of the chromosphere and corona being part of the Sun would be hard to grasp if you have never seen it. They are only visible to Earthlings during a total solar eclipse. Such eclipses are rare, and often there are lots of other things to observe, so the corona and chromosphere are often missed, particularly by lay observers. As for a lunar atmosphere, once the idea that celestial bodies are not some mystical things, and that Earth is a planet, it is natural to attribute Earth-like attributes to the other planets, and the Moon. So, when the smooth dark lava plains on the Moon were observed, they were naturally called seas. It seemed perfectly reasonable that if Earth had an atmosphere, then so did the Moon. In fact, Johannes Kepler even wrote what may be one of the first science fiction stories about beings living on the Moon. While John Herschel was observing the sky from the southern hemisphere, reporters in the US made up fantastical stories about creatures that he was observing on the Moon. People actually believed the stories, since the idea of a lunar atmosphere and lunar life seemed perfectly reasonable at the time. The great astronomer John Flamsteed observing a solar eclipse at the beginning of the 18th Century saw the chromosphere. Instead of realizing it to be a part of the Sun, though, he attributed it to the Moon. He thought that he was seeing sunlight refracted through the lunar atmosphere, much as it is during twilight on Earth. Even Edmund Halley made a similar mistaken conclusion when observing chromospheric features during an eclipse. The lunar atmosphere idea really got a boost in the mid 18th Century from the famous mathematician Leonhard Euler (famous to physics, mathematics, and engineering students for his Euler’s angles, and the Euler formula for complex numbers). Euler observed that just at the beginning and end of totality, the edges of the crescent sun seemed to be momentarily indistinct. He attributed this to distortion due to the lunar atmosphere (more likely he was seeing diffraction effects from his observing instrument). He even went so far as to assert that perhaps not just the chromosphere was a lunar atmospheric feature, but perhaps so was the corona. He even went so far as to calculate the density of the lunar atmosphere, coming up with a number nearly many times more dense than Earth’s atmosphere.
Reading this report put the Croatian mathematician Roger Boscovich onto the search for the lunar atmosphere. Boscovich, a Jesuit priest, was serving as a papal science advisor in Rome at the time. The Vatican had come a long way in the century since opposition to Galileo and his observations — now there was an astronomer on staff! He pooled his observations with others and published a treatise on the lunar atmosphere, giving several arguments about why it could not have an appreciable atmosphere, and most certainly could not have an atmosphere as thick as many were claiming. For one thing, observing the Moon through a telescope, the shadows of mountains and craters appeared very distinct and black. If the Moon had a thick atmosphere, the shadows would be gray, and they would have fuzzy edges. Furthermore, as the Moon moves through the sky, it occasionally passes in front of stars. This event is called an occultation.  Observing stars being occulted, or reappearing from being occulted, Boscovich determined that the stars remained at constant brightness and clarity right until the moment that they disappeared, and that reappeared instantly as the Moon passed, without any change in intensity or clarity. If the Moon had a thick atmosphere, then the stars should gradually fade and get indistinct as they were occulted, and they should gradually become more distinct and brighter as they reappeared. Furthermore, a thick lunar atmosphere would make the stars appear redder as they were seen through thicker atmosphere the closer they appeared to the lunar surface. This, too, was not seen. Also, a very thick atmosphere, as was being suggested, would even appear to bend the light from the star, making it appear to change position slightly just before it disappeared or just after it reappeared. This, too, was not seen. So, he concluded that there was, in fact, no lunar atmosphere. The Moon, unlike the Earth, was a dead airless world.
font>So, here you have a bit more astronomical trivia.
-Astroprof
