WASHINGTON (AP) _ Ask about the moon's atmosphere and most people will respond: It doesn't have one! Like many things people know, that's not quite true.

``Although many people think of the moon as an object which has no atmosphere, it is surrounded by a tenuous envelope of gas atoms,'' explained Urs A. Mall of the Max Planck Institute for Aeronomy in Katlenburg-Lindau, Germany.

Admittedly, the moon's atmosphere isn't much by our standards _ the pressure is about one-billionth that of Earth. But it's there, and Mall is leading a group of scientists harnessing satellite technology to try and figure out what that atmosphere is made of.

``Many people tend to underestimate what we can learn by further investigating the moon, a place so close to home,'' Mall said.

``Of course this atmosphere is very tenuous,'' said Mario Acuna, a specialist in solar-terrestrial physics at the National Aeronautics and Space Administration. ``We want to know what the source is of these gases we find on the moon.''

Acuna, who is not part of Mall's research group, said that since gases escape easily from the moon, there has to be a constant source for them.

Possibilities include vaporization of rocks caused by meteor impacts, moonquakes releasing gases from beneath the surface and heat-generated gases resulting from the uneven solar heating of the moon as it rotates over the period of a month, Acuna said in a telephone interview.

The Apollo manned lunar explorations during the late 1960s and early 1970s identified argon and helium atoms in the space just above the lunar surface. And Earth-based observations have since added sodium and potassium to the list.

But those four elements constitute only about 10 percent of the density of the moon atmosphere, so the team of German and American researchers is using a special satellite instrument to identify other elements that may be present.

So far they have identified very small amounts of oxygen, silicon and aluminum, they report in a paper scheduled for publication in Geophysical Research Letters.

They expect to identify more constituents in November, when the WIND spacecraft _ designed to measure the solar wind emitted from the sun _ will spend time near the moon. The spacecraft carries a special instrument that can identify ions, forms of elements that have gained an electric charge.

When an atom in the moon's atmosphere becomes ionized, it becomes subject to an interplanetary electrical field 1,000 times stronger than the moon's gravity. So, while ordinary forms of the elements would simply drop back to the moon's surface, ions are picked up by this solar wind and carried off, detectable by a special instrument aboard the WIND spacecraft.

One thing they haven't found in the moon's atmosphere so far is nitrogen, the gas that makes up more than three-quarters of the Earth's atmosphere. But that doesn't mean there never was any.

``One has to keep in mind that what the moon originally had as an atmosphere was lost early in its history. The gravitational field of the moon is so weak that it cannot hold most of what may have been there at the beginning,'' Mall said in an interview conducted by electronic mail..

The atoms making up the current atmosphere either originate from the moon or are brought there from the outside, such as by meteoroids, comets of the solar wind, he explained.

Mall called the moon an ``outpost'' for the Earth, allowing the study of complicated problems under somewhat simpler conditions.

``The moon with its weak atmosphere is, in principle, a wonderful place to observe the arrival and impact of micrometeoroids, even when the impacts are not as spectacular as in some contemporary movies,'' he said.