Science / Medicine : Astronomers Find Moon Has a Tail

From Times Staff and Wire Reports

Astronomers have found that the moon has a tail.

Like the luminous plumes that stretch out from comets, a glowing 15,000-mile tail of sodium atoms streams from the moon, blown away from the sun by the solar wind--the constant flow of particles, including protons and electrons, that stream out from the sun. The tail is not visible to the naked eye, but instruments can see the faint orange glow of sodium.

"We went out looking for it and were delighted to find it," astronomer Michael Mendillo of Boston University said recently. "It was remarkable that on our first night of observations we saw this extended feature."

Robert Brown, a senior astronomer at the Space Telescope Science Institute in Maryland, was first to detect a sodium cloud surrounding Io, one of Jupiter's moons, in the early 1970s.

"The really fantastic thing," Brown said, "is that we're now seeing three solar system bodies with sodium atmospheres." In addition to the moon and Io, sodium also surrounds Mercury.

"The scientific community still isn't sure of the source of these sodium atoms," Mendillo said. It is probable, he said, that sodium atoms are being blasted loose as rocks on the moon's surface are hit by tiny meteorites. Others argue that sodium is liberated by solar wind particles or by photons.

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