The Mysteries of the Moon's Atmosphere
The moon is a barren landscape when viewed at night. Nowhere do you see traces of life or color but a bland grey with moments of black. Okay, so maybe that is too bleak of a picture to paint for the moon. It is actually an awesome place with many surprises like volcanic activity and even water. And it has an atmosphere too, but it is not quite like ours.
Most scientists in the past felt that the moon had nothing that could sustain an atmosphere. Radio astronomers looked at the edge of the moon as the sun moved from behind it and found that if a lunar atmosphere existed it would have a max pressure of 1/10,000,000,000 of a pascal. The gravity of the moon would be strong enough to hold onto it but it would not take much for it to dissipate. But what would such an atmosphere be? The prevailing thought at the time was solar wind from the sun but we would need data from the surface of the moon if any theories were to be proven (Stern 37).
And so the Apollo missions were our different approach to getting that data. Several of the astronauts reported a glow along the horizon of the moon, calling it “Lunar Horizon Glow.” Besides a visual report, astronauts left special instruments designed by scientists in the hopes of measuring any sign of an atmosphere including 9 spectrometers and 5 pressure gauges. At first, it seemed like nothing was found of merit from them and even Apollo 17 hunted for solar wind (hydrogen, helium, carbon, and xenon) on the surface with a UV spectrometer but again no dice. However, alpha particle spectrometers from Apollo 15 and 16 later detected small amounts of radon and polonium gases that seemed to be emitted from the surface of the moon. Scientists believe it comes from decaying uranium inside the moon, but a gas on the surface was still an interesting find and the first hints of something more (37).
The Data Rolls In
Slowly, data began to trickle in that gave a deeper picture of the atmospheric nature of the moon. Surface detectors from Apollo 12 and 14 showed that an average of 100,000 particles per cubic centimeter were in their vicinity during the lunar night. In fact, as the night progressed, ion detectors from Apollo’s 12, 14, and 15 all saw fluctuations in the levels of several particles but mainly in neon and argon. On top of that, the Apollo 17 mass spectrometer found argon-40, helium-4, nitrogen, oxygen, methane, carbon monoxide and carbon dioxide, and changes in both argon and helium as the solar wind flowed from the sun. However, the Lunar Atmospheric Composition Experiment (LACE) found that argon levels also changed as seismic activity did and peaked at 40,000 particles per cubic centimeter. This seems to indicate that argon may come from within the moon, just like the radon and polonium. So why did argon change with the solar wind then? Pressure from the stream of particles pushed the argon along the surface, scientists suspect. Clearly, the moon does not have a traditional atmosphere but gases are present on its surface, despite the low levels and fluctuations. But what else is present? (Stern 38, Sharp)
After sodium and potassium were found on Mercury, scientists wondered if any was on the moon. After all, both objects share many similarities in composition and appearance. Drew Patten and Tom Morgan (the scientists who found the Mercury gases) used a sensitive and large telescope, the 2.7-meter Mc-Donald Observatory, in 1987 to gather data about those potential elements. They did indeed find them on the moon but in low concentrations: sodium is concentrated at an average of 201 particles per cubic centimeter while potassium is at 67 particles per cubic centimeter! (Stern 38)
Now, how can we quantify the atmosphere in terms of altitude? We need a scale height, or the vertical distance it takes the moon’s atmosphere to decrease by a third (and with density and pressure closely related to altitude, we gain even more insights). Now, the scale height is affected by the molecular energy aka collisions of particles which increase kinetic energy. If the atmosphere was solely based on solar wind, one would expect the scale height to be 50-100 kilometers with a temperature of 100 degrees Kelvin. But data seems to indicate that the scale height is likely 100’s of kilometers, which corresponds to a temperature of 1000-2000 Kelvin! To add to the mystery, the surface of the moon has a max temperature of 400 Kelvin. What causes such a spike in heat? Sputtering, perhaps. This is when photons and solar wind strike the surface and free atoms from their molecular bonds, escaping upward with an initial temperature of 10 million Kelvin (38).
Final Closing Facts
If you take the entire atmosphere of the moon, it weighs a mere 27.5 tons and is fully replaced every few weeks. In fact, the average density of gas molecules at the moon’s surface is 100 molecules per cubic centimeter. To compare, Earth’s is 1*10^18 molecules per cubic centimeter! (Stern 36, Sharp) And I have no doubt that with the moon even bigger surprises await. Why, the atmosphere has even been postulated to help with the water cycle of the moon! Stay tuned, fellow readers…
Sharp, Tim. “Atmosphere of the Moon.” Space.com. Space.com, 15 Oct. 2012. Web. 16 Sept. 2015.
Stern, Alan. “Where The Lunar Winds Blow Free.” Astronomy Nov. 1993: 36-8: Print.
© 2015 Leonard Kelley
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