The original idea was presented in a 2002 paper by University of Washington astronomer John Armstrong, who suggested that material ejected from Earth during the Late Heavy Bombardment (a period about four billion years ago when the Earth was subjected to a rain of asteroids and comets) might be found on the moon.

Armstrong's suggestion was interesting, but whether a meteor ejected from the Earth might arrive intact on the moon remained an open question.

New research by a team under Dr. Ian Crawford and Dr Emily Baldwin of the Birkbeck College School of Earth Sciences used more sophisticated means to simulate the pressures any such terrestrial meteorites might have experienced during their arrival on the lunar surface.

This confirmed Armstrong's hypothesis. In many cases, the pressures could be low enough to permit the survival of biological markers, making the lunar surface a good place to look for evidence of early terrestrial life.

Any such markers are unlikely to remain on Earth, where they would have been erased long ago by more than three billion years of volcanic activity, later meteor impacts, or simple erosion by wind and rain.

Given that material from early Mars has been found in meteorites on Earth, it certainly seems reasonable that material from the early Earth could be found on the moon. Indeed, Armstrong's paper estimated that tens of thousands of tons of terrestrial meteorites may have arrived there during the Late Heavy Bombardment.

However, there is a problem: the moon lacks any appreciable atmosphere. Meteorites arriving on Earth are decelerated by passing through our atmosphere. As a result, while the surface of the meteorite may melt, the interior is often preserved intact. Could a meteorite from Earth survive a high-velocity impact on the lunar surface?

Crawford and Baldwin's analysis, based on commercially available software called AUTDYN, used finite element analysis to simulate the behavior of two different types of meteors impacting the lunar surface.

Armstrong's group performed a crude calculation indicating that pressures experienced by a terrestrial meteorite arriving on the moon probably would not be enough to melt it. Crawford and Baldwin's group simulated their meteors as cubes, and calculated pressures at 500 points on the surface of the cube as it impacted the lunar surface at a wide range of impact angles and velocities.

In the most extreme case they tested (vertical impact at a speed of some 5 kilometers per second), Crawford reports that "some portions" of the simulated meteorite would have melted, but "the bulk of the projectile, and especially the trailing half, was subjected to much lower pressures."

At impact velocities of 2.5 kilometers per second or less, "no part of the projectile even approached a peak pressure at which melting would be expected." He concludes that biomarkers ranging from the presence of organic carbon to "actual microfossils" could have survived the relatively low pressures experienced by the trailing edge of a large meteorite impacting the moon.

Finding terrestrial meteorites on the moon will be challenging. Crawford suggests that the key to finding terrestrial material is to look for water locked inside. Many minerals on Earth are formed in processes involving water, volcanic activity, or both. By contrast, the moon lacks both water and volcanoes.

Minerals formed in the presence of water, called hydrates, can be detected using infrared (IR) spectroscopy. Crawford and his co-authors believe that a high-resolution IR sensor in lunar orbit could be used to detect any large (over one meter) hydrate meteorites on the lunar surface, while a lunar rover with such a sensor "could search for smaller meteorites exposed at the surface."

Other planetary astronomers view the issue more conservatively. Dr. Mike Gaffey of the University of North Dakota Space Studies department argues that while "debris from a large terrestrial impact could have reached the moon... it's highly unlikely that it would be in sufficient concentrations to be seen" using orbital instruments.

He believes that the meteorites would be shattered into small pieces by the impact, and then subjected to a form of lunar weathering due to the solar wind and a continuous rain of micrometeoroids that hit the moon. Instead, he suggests that any surviving material from Earth would be fractured into small pieces embedded in ancient lunar soils; some of which might be exposed at the surface by later meteor impacts.

He believes that the meteorites would be shattered into small pieces by the impact, and then subjected to a form of lunar weathering due to the solar wind and a continuous rain of micrometeoroids that hit the moon. Instead, he suggests that any surviving material from Earth would be fractured into small pieces embedded in ancient lunar soils; some of which might be exposed at the surface by later meteor impacts.

Crawford concedes that point, and suggests that it might be necessary to dig below the surface to find terrestrial meteorites. He adds that collecting samples, observing them on the lunar surface, and picking those that warrant a return to Earth for detailed analysis "would be greatly facilitated by a human presence on the moon."

The last U.S. Astronaut to set foot on the moon, Dr. Harrison Schmitt, was a geologist. If current NASA plans for a return to the moon later in this century are fulfilled, perhaps Dr. Schmitt's successors will search for hydrated rocks, which might unlock the mystery of how life began on the Earth.

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