The moon may have less atmosphere than your local fast food outlet, but astronomers would still like to know its ingredients. Now, a spacecraft has intercepted atoms of oxygen, aluminum, and silicon that apparently escaped from the moon's feeble gravity. Further sampling of these charged particles should illuminate the source of the moon's atmosphere, researchers report in an upcoming Geophysical Research Letters.
Although most schoolbooks say otherwise, the moon does have an atmosphere, although it's exceedingly thin--perhaps 100 trillion times less dense than Earth's. This air continually drifts into space, forming what astronomers call an "exosphere." Instruments aboard Apollo 17 spotted the gases helium and argon there in 1972. Since then, Earth-based telescopes have identified only two more constituents, sodium and potassium. But those four elements account for just 10% of the measured bulk of the lunar atmosphere. In a big step toward accounting for the rest of the atmosphere, a satellite that studies particles streaming from the sun now has spied at least three more elements drifting from the moon, confirming hints of observations from two previous satellites.
The spacecraft, called WIND, loops around Earth in petal-shaped orbits that often bring it near the moon. Between 1995 and 1997, an instrument on WIND snared hundreds of ions, or charged particles, that scientists traced to the moon based on their paths in space. Oxygen was the most common, followed by aluminum, silicon, and possibly phosphorus. Astronomers think most of these gases enter the exosphere from the moon's interior or have been steadily knocked off the lunar surface by particles from the sun. "Interplanetary probes can track these ions and help us detect moonlike atmospheres from afar," says study co-author Antoinette Galvin of the University of New Hampshire in Durham. Starting in November, she notes, WIND will swing closer to the moon for several months to collect more ions.
The results are "believable and expected," says planetary scientist S. Alan Stern of the Southwest Research Institute in Boulder, Colorado. "The moon is the touchstone for understanding the boundaries between surfaces and exospheres in the solar system," Stern says, especially at Mercury and Jupiter's satellites Io and Europa. The expected Leonid meteor storm in November may stir up more lunar particles, he notes, for WIND to catch during its flybys.