Tracing the glow of a lunar tail.

Earth's moon continues to serve up surprises. In 1988, scientists detected traces of sodium and potassium atoms above the lunar surface, suggesting that the moon has a definite atmosphere, albeit an extremely thin one. Now, by looking specifically for sodium atoms in the moon's vicinity, a team of researchers has discovered that the moon's sodium atmosphere stretches out into a long tail pointing away from the sun.

"The moon has the appearance of a comet," says astronomer Michael Mendillo of Boston University.

To find the moon's tail, Mendillo and his co-workers used a specially modified telescope that can focus on certain wavelengths of light emitted by sodium atoms. They had first used this instrument to determine the size of the sodium cloud surrounding Io, a satellite of Jupiter (SN: 6/9/90, p.359).

One set of images depicting the sunward edge of the bright crescent moon shows sodium extending about 7,000 kilometers above the lunar surface, or five times the moon's radius. Images of the opposite, dark edge reveal much fainter sodium emissions, but these come from a region at least 21,000 kilometers long. Taken together, the observations suggest that the moon's sodium atmosphere qualitatively resembles that of a comet, featuring a bright coma centered on the moon and an extended tail stretching away from the sun (see diagram).

The researchers also found that the intensities of the sodium emissions were lower in the middle of the tail than near its sides. This indicates that the moon casts a shadow down its tail, which reduces the number of sodium atoms present. However, even at its thickest, the moon's evanescent sodium atmosphere probably contains only a few dozen atoms per cubic centimeter -- far more tenuous than a comet tail.