"It is up there. I don't know how common it is, but somewhere up there is at least one fist-sized chunk of rock with Earth's name on it," said John Armstrong, a University of Washington doctoral student in astronomy and astrobiology. "It would probably take hundreds of years of human habitation to find it and recognize it, but it's there."

"Up there" is the moon, what Armstrong and his colleagues refer to as Earth's attic. And while he doesn't expect to actually find a large chunk of Earth rock, he believes there is likely a wealth of planetary debris in the form of fine particles on the moon's surface.

In an upcoming edition of Icarus, an international journal of solar system science, Armstrong and colleagues Llyd Wells, a UW graduate student in oceanography, and Guillermo Gonzalez, an assistant physics and astronomy professor at Iowa State University, argue that humans should seriously consider returning to lunar exploration. Any mission, they say, should include a search for fossils of some of Earth's earliest microbial life.

In its very early history, 3.8 billion to 3.9 billion years ago, comets and asteroids constantly bombarded the Earth. Some of those bodies hit with such force that chunks of the planet's surface were ejected beyond the pull of its gravity. The three astrobiologists believe some of that ejected material went directly to the lunar surface, while other material went into orbit and some gradually fell to the moon.

Finding rocks, or even particles, from that period could be invaluable in understanding how life on Earth came to be as it is today, since any fossils found likely would be from a time long before life on Earth developed great complexity and diversity.

Because of the moon's position in the inner solar system, in theory it has collected material from all the planets. Earth matter probably is most abundant, since it is closest to the moon, but Armstrong, Wells and Gonzalez expect that fallout from Mars and Venus also is abundant enough that it could be recovered. They have calculated that, on average, perhaps 22 tons of Earth material is spread over every 38 square miles of the moon.

If they are right, Armstrong said, that means about 10 parts per million of lunar material originated on Earth. However, it would not necessarily be identifiable as Earth material. And while material blasted directly to the moon probably came down on the side facing Earth, any material that ended up in orbit could have been deposited anywhere on the lunar surface.

"One thing we're still debating is where would be the best place to look," Armstrong said.

The scientists believe Earth material from that long ago probably became buried over time, so one strategy they have devised is to look for recent craters in which old rocks have been excavated. They also are searching through lunar samples already returned to Earth, on the off chance those samples might contain rocks that originated on the planet. They would be able to tell by isotope ratios or by determining whether materials in the rocks were formed in water -- something that cannot occur on the moon.

At the very least, Armstrong hopes to find dust from pulverized Earth rocks, material that can be dated and can give clues about the history of bombardment by asteroids and meteors and how that affected evolution. If materials are more intact, they might contain interesting elements that would shed light on the early Earth's environment and how life existed at that time. Less likely, he said, would be the discovery of rare, volatile elements that would tell about the early atmosphere. The least likely find -- and the one most prized -- would be microbiological fossils from 3.9 billion years ago.

"The part I like about this is that it makes the moon a window on the early Earth," Armstrong said. "It also would give us access to samples you can't get anywhere else at the moment."

Currently there are no plans for a U.S. mission to the moon, though other nations are considering the possibility. It has been 30 years since the National Aeronautics and Space Administration's last manned mission, and three years since the controlled crash of the robotic Lunar Prospector spacecraft in a search for water on the moon.

Armstrong believes a new lunar mission would benefit science on several levels and could help prepare and test systems for eventual Martian exploration. To search for material of Earth origin, he suggests the inclusion of a rover that could systematically pass over a specific area, conduct tests and prepare samples to be launched back to Earth. The only way to find the Earth rocks, he said, is to find a way to look specifically for them.

"If you find one of these things, it's going to be hard to convince someone that it's of Earth origin," he said. "But if you find more than one, then you can begin to classify them as a group of materials and begin to make a more convincing case."

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