The footprints left on the moon 20 years ago by Neil Armstrong and Edwin "Buzz" Aldrin should be visible for another half-million years, says a chemist who studies lunar erosion rates.

"The moon is a quiet place, lacking an atmosphere and therefore free of wind and rain," said James R. Arnold, a "cosmochemist" at the University of California, San Diego.But even without an atmosphere, the moon's surface is slowly eroded - by bombardment from meteorites and cosmic rays. Arnold said they eventually will obliterate the footprints left on the lunar surface when Apollo 11's Armstrong and Aldrin became the first humans on the moon in July 1969.

Arnold and his colleagues studied moon rocks retrieved during six Apollo missions to develop techniques to measure the rates at which meteorites and solar and galactic cosmic rays erode lunar rocks and soils.

The information "bears directly on many questions about the evolution of our solar system, including the Earth," he said. His findings were presented during the recent annual meeting of the American Association for the Advancement of Science.