In November 1998, when the annual Leonid meteor shower coincided with the New Moon, astronomers working in Texas noticed an unexpectedly high concentration of sodium atoms localized above the Earth. This 'Na spot' was identified as a stream of atoms escaping from the Moon. Now, a comprehensive study by Majd Matta and colleagues of the brightness of this spot over 31 consecutive lunar months has provided clues as to how sodium atoms escape the Moon's surface (Icarus http://dx.doi.org/10.1016/j.icarus.2009.06.017; 2009).

Credit: © NASA / JPL

The Moon is known to have a thin and transient atmosphere made up of atoms that have been released from the lunar surface. One of the species detected so far is sodium, which can be picked up using Earth-based spectroscopic techniques. These measurements have shown that the sodium atmosphere extends to about 8,700 km — five times the lunar radius, RM — on the side of the Moon closest to the Sun and to about 20 RM on the dark side. This comet-like sodium tail is attributed to solar radiation pushing the sodium atoms away from the Sun.

All-sky observations made between 18–20 November 1998 at the McDonald Observatory in Fort Davies, Texas, revealed an intense spot of 589 nm light — the same wavelength as sodium D lines — in a 3° × 3° area of the night sky. This Na spot was seen near the antisolar point and only on the three nights of the New Moon, when the Earth is roughly aligned with the Sun and the Moon. The explanation was that the Moon's sodium tail was being focused by the Earth's gravitational field. It was also postulated that the spot was made particularly bright by an increase in the rate of sodium escape from the Moon's surface during the Leonid meteor shower, which had been at its most intense a couple of days earlier (it takes the sodium atoms roughly two days to travel the distance between the Moon and the Earth).

Matta and colleagues, using the El Leoncito Observatory in Argentina, spent almost two and a half years (from April 2006 to September 2008) monitoring how the intensity of this spot varies. Their aim was to determine whether increases in intensity coincided with any other form of astronomical activity, and thereby provide a better understanding of the mechanism by which the sodium atoms escape from the lunar surface.

Contrary to what was first thought, the team found little correlation between meteoric activity at the Moon and the lunar-tail brightness. There was also little connection between the Na-spot intensity and the flux of either solar-wind protons, which are thought to sputter sodium atoms from the surface, or solar photons, which could lead to photon-stimulated desorption. Matta et al. suggest that no single mechanism drives sodium expulsion exclusively, but stress that the ambiguity may be due to the absence of any appreciable meteor storms in the period of the study.