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Lunar Sodium

Ron wrote:

> Boston University astronomers announced today the discovery
> of an enormous tail of sodium gas stretching to great distances
> from the moon.

> T e n  y e a r s  ago, groundbased telescopes revealed
> that sodium gas (Na) was in the lunar atmosphere, ...

Hello List and Good Night,

The Moon's Atmosphere (Sky & Telescope, June 1989, p. 589):

The Moon is commonly thought of as a barren, airless world. While it
certainly is desertlike, the Moon does have an atmosphere - extremely
thin, but present nevertheless. Measurements by the Apollo astronauts
revealed that the Moon's "air" amounts to only about a million atoms per
cubic centimeter, more than a trillion times thinner than Earth's at sea
level. This is so tenuous that it hardly qualifies as an atmosphere;
hence astronomers refer to it as an exosphere.
While instruments found argon, neon, and helium on the dark side and the
possibility of methane and ammonia at sunrise, the composition of the
daytime exosphere remained a mystery. The lunar landers polluted the
areas where the astronauts placed surface detectors, making readings
worthless. No other gases have been directly detected; only upper limits
of their concentrations were calculated.
Now two NASA scientists have discovered sodium and potassium on the
Moon's day side. Andrew E. Potter and Thomas H. Morgan (Johnson Space
Center) made more than seven hours of spectroscopic observations using
telescopes at McDonald and National Solar observatories. By looking just
off the edge of the Moon's disk, they recorded the elements' telltale
emissions - the sodium D lines at 5890.0 and 5895.9 angstroms and a
potassium feature at 7699 angstroms. Another group led by Ann L. Tyler
(University of Arizona) has since confirmed the sodium observation.
Potter and Morgan carefully checked that the emissions did not arise on
Earth, such as from the glow of twilight or light pollution by distant
streetlamps. In each case, terrestrial emission was below the threshold
of detection.
Potter and Morgan write in Science last year that the ratio between the
sodium and potassium gases is close to that of lunar rocks. This
suggests that vaporization by small meteoroid impacts is chiefly
responsible for introducing these elements into the lunar exosphere.
Secondary sources include sunlight and solar wind that knock atoms off
the surface.


Best wishes,

Bernd

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