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Outgassing events in which volatiles are vented from deep in the lunar interior to the surface, specifically, radon, nitrogen, carbon monoxide, and carbon dioxide, may be the result of low level volcanic or tectonic events on the moon, and are hypothesized to be the source of the tenuous lunar atmosphere. Knowing when and where these events occur may help us understand more about lunar tectonics and how the moon's atmosphere was formed.
Carbon and Nitrogen are both vital elements which will have to be imported from earth or from carbonaceous asteroids. If outgassing events can be predicted it may be possible to utilize this valuable resource.
The Alpha Particle Spectrometer aboard Lunar Prospector was designed to detect these events by tracking the alpha particles emitted during the decay of radon 222 and polonium 226 (two increments immediately preceded by Ra226 in the decay series of U238). The APS data was obscured by solar activity during the mission, but may be viewable once the effects of solar activity have been filtered out. This task has not been done.
Ground Penetrating Radar studies have been made with wavelengths penetrating 10-20 meters via the Arecibo and Green Bank radio observatories, but deeper and higher resolution scans are likely needed to detect further sub-surface gas pockets. Radar dark areas in the 70cm band, termed "halos", have been detected around many impact craters. An especially large halo has been detected around Aristarchus crater and encompasses much of the Aristarchus plateau. Aristarchus is the major source of recent outgassing events detected via Radon 222 activity. It has been suggested that the Aristarchus halo may be the result of subsurface ice deposits, but this has not been confirmed. Aristarchus is known to be an active outgassing site.
Many observers have reported witnessing so called "Transient lunar phenomena" including many amateur Astronomers. The crew of Apollo 11 reported observing a TLP at the crater Aristarchus.
Although the Moon is relatively inert geologically, the lunar atmosphere continues to be replenished by lunar outgassing events.
- NASA Lunar Prospector Project - Lunar Geology
- NASA Lunar Prospector Project - Alpha Particle Spectrometer Results
- Encyclopedia Astronautica - Lunar Prospector
- Lunar and Planetary Science XXXVIII (2007) - Probing Lunar Volatiles: Initial Ground-Based Results (PDF file)
- Wikipedia: Lunar Prospector
- Wikipedia: Transient lunar phenomenon - Outgassing
- University of Iowa: Radon Occurrence and Health Risk (PDF file)
- U238 Radioactive Decay
- US Environmental Protection Agency - Uranium-238 Decay Chain