Science Q & A: Lunar Atmosphere

By C. CLAIBORNE RAY

Q. Did the Moon ever have an atmosphere?

A. It has one now, though it is a very thin and highly dispersed collection
of molecules, not suitable for breathing by Earth creatures.

The existence of a lunar atmosphere was reported in 1933, based on
observation of the Moon using a mask that filtered out moonlight in order to
study the spectrum of light emitted by sodium.

Although sodium is believed to be just a trace in the Moon's atmosphere, it
is studied because it is relatively easy to detect and is used as a marker for
other components, like potassium, neon, argon and helium.

A 1993 study of lunar sodium, by Boston University scientists using improved
instruments, determined that the atmosphere extended at least 5,000 miles
above the Moon's surface.

The molecules, however, are few and far between, only an estimated 10
million per cubic centimeter near the Moon's surface; Earth's atmosphere is
about a billion times as dense.

The sources of the atmosphere are believed to be the release of gases from
within the Moon by moonquakes (a phenomenon called outgassing) and the
loosening of molecules from the surface by the impact of molecules from the
solar wind or by meteorites.

A few moons of other planets have much more impressive atmospheres, like
that of Jupiter's Titan, a thick haze of nitrogen and methane, and that of
Europa, a thin whisp of oxygen.

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Questions of general interest will be answered in this column, but requests
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SCIENCE Q & A is published weekly, on Tuesdays.