Science: The Moon’s Atmosphere

A favorite diversion of amateur astronomers is to watch the moon eclipsing a star. When the star touches the moon’s jagged edge, it winks out all at once with no preliminaries. Even the delicate instruments of professional astronomers cannot detect the slightest trace of dimming or wavering. But if an astronomer on the moon were to watch the earth eclipsing a star, he would see a different performance. The star would grow dim and reddish like the setting sun, and its light would be bent by refraction in the earth’s atmosphere, making the star appear to shift its position.

The absence of such changes in stars eclipsed by the moon has long been offered as evidence that the moon has no atmosphere, or at least none that could be detected by instruments that use light. This negative report has now been changed slightly. In Britain’s New Scientist, Physicist Bruce Elsmore of Cambridge University tells how the new technique of radio astronomy has detected and measured a very thin gas that surrounds the moon.

The Helpful Crab. The radio astronomers of Cambridge’s famous Cavendish Laboratory started with the assumption that if the moon has any atmosphere at all, the atoms of gas in it will be ionized (split into electrically charged particles) by sunlight, just as they are in the thin upper fringe of the earth’s atmosphere. Such an ionized gas will bend radio waves, and the amount of bending will give by calculation the density of the charged particles.

A fine opportunity came when the moon was scheduled to eclipse the Crab nebula, which is the 4th strongest concentrated source of radio waves in the sky. Watching with a radio telescope, the astronomers noted when the waves from the nebula were cut off by the moon. They reappeared on the other side in about one hour. Calculations showed that the nebula’s radio waves had been bent very slightly: about 13 sec. of arc.

Soap-Bubble Film. The density of an atmosphere detected in this way depends partly on what gases it contains, and the radio waves give no such information. Elsmore points out that the moon’s gravitation is too feeble to hold comparatively light gases like the oxygen and nitrogen in the earth’s atmosphere. Any gas molecules that hang around the moon for long must be much heavier. But the moon may have in addition a temporary atmosphere made of helium and argon given off by radioactivity in the moon’s rocks and of other light gases escaping from the moon’s interior or contributed by the vaporization of meteors hitting the surface. Elsmore figures that if the moon’s atmosphere is half permanent (heavy) and half temporary (light), it will be something like one five-trillionth (2 x 10¯¹³) as dense as the earth’s atmosphere.

This barely detectable wisp of gas will offer colonists on the moon no shelter from solar X rays, meteors or other unpleasant features of space. If the earth’s atmosphere were compressed to the density of steel, it would form an armor plating 49 in. thick, but the moon’s meager atmosphere, compressed in the same way, would be only one-millionth as thick as the thinnest soap-bubble film.