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The results from the analysis of volatile material released by the LCROSS probe, that impacted in the crater Cabeus near the lunar pole on 9th October 2 years ago, have been confirmed (the cloud produced is visible in the above image).
Amongst the most interesting results are those obtained by Rosemary Killen (NASA's Goddard Space Flight Center in Greenbelt, Md) and her team, who used the Kitt Peak National Observatory (Tucson, Arizona), to observe the event spectroscopically at high resolution with the McMath-Pierce solar telescope!
From the large quantity of water liberated by the impact (that had remained hidden as ice in the permanent shade of Cabeus) traces of silver, calcium, magnesium, and mercury, as well as 1-2 kg of sodium were detected (the exact value depending on the temperature reached by the LCROSS impact).
The impact released about 100 times more water than sodium, so that the mass fraction of sodium is actually very similar to that of the terrestrial oceans; about 1%. Given that somewhat less sodium was expected, it will be interesting to understand what mechanism resulted in such a high concentration in the lunar ice.
There are various plausible explanations: it could have been present in comets that impacted on the Moon, liberated from the lunar rock during asteroid impacts or deposited by the solar wind. In regions sheltered from the glare of the Sun the sodium atoms could then have accumulated, along with other elements, to form the "dirty" ice vaporised by LCROSS.
The best way to determine the origin of the elements detected in the vapour cloud is to measure their isotope ratios and compare them with those of other bodies in the solar system, such as comets and asteroids. Isotopes of a given element are chemically identical but have different numbers of neutrons in their nuclei.
The ratios of the various isotopes in a body depend on how it was formed, and so are a clear signature of any family of objects.
At the moment it looks like the best candidates to explain the elemental mixture detected by Killen and colleagues are comets, as their sodium/water ratio is around 2%, while asteroid impacts do not exceed 0.4%. The definitive answer should come from the study of the extremely tenuous lunar atmosphere, and is the objective of the Lunar Atmosphere and Dust Environment Explorer (LADEE), due for launch in 2013.
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