The Moon lacks a dense atmosphere in which gases can interact to create weather systems or undergo chemical reactions, but it does posses an exosphere. This is a zone around its solid surface within which individual atoms and molecules can be found moving freely, but seldom (if ever) interacting. This exosphere extends about 5 lunar radii from the center of the Moon on the Sunward side, and 15-20 lunar radii on the far side, suggesting that material is being lost from the exosphere into space on this side, presumably as a result of the solar winds (though the Moon is protected from these somewhat by the Earth’s magnetosphere). Individual sodium atoms (which are a convenient substitute for total exosphere density as they are easy to study from Earth using spectrographic techniques) within the lunar exosphere have been shown to have an average speed of around 2 km per second, suggesting that these are being produced as a result of some high energy process.
Previous studies have suggested that the distribution of sodium atoms in the lunar exosphere is related to the solar zenith angle, and therefore likely to be driven by the ionization of lunar regolith by solar photons or solar wind impact. However studies of the interaction between the lunar exosphere and the Leonid Meteor Shower in 1997 and 1999 suggested that the density and temperature of the exosphere increased during the shower, suggesting that impacts by micro-meteorites might be liberating atoms from the regolith, but follow-up studies of the Geminid and Quadratid Meteor Showers in 1999 failed to repeat this.
In a paper available ahead of publication on the website of the journal Planetary and Space Science from 20 March 2014 and on the online arXiv database at Cornell University Library on 8 April 2014, a team of scientists led by Aleksei Berezhnoy of the Sternberg Astronomical Institute at Moscow State University describe the results of a study of the lunar exosphere near the Lunar North Pole during the Perseid Meteor Shower on 13 and 14 August 2009, using the Multi Mode Cassegrain Spectrometer on the 2 m Zeiss Telescope at the Terskol Branch of the Institute of Astronomy of the Russian Academy of Sciences at Kabardino-Balkaria Repulblic in the North Caucacus.
Berezhnoy et al. observed a marked increase in sodium atom density in the Lunar Exosphere at the Lunar North Pole during the Persied Meteor Shower. Attempts to model the cause of this suggested that the most likely cause was an impact cloud produced by multiple micro-meteorite strikes, though a similar cloud produced by a single large meteorite could not be ruled out. Berezhnoy et al. feel that this confirms the role of meteorite showers in the formation of the Lunar Exosphere, but also suggest that more studies of this kind are needed.
The Moon on 13 (left) and 14 (right) August 2009, showing the potential imact region (stippled) as well as the area targeted by the 2 m Zeiss Telescope and the Lunar North Pole. Berezhnoy et al. (2014).
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