Some scientists believe thatat least one meteorite found in Antarctica preserves evidence of ancient lifeon Mars. Now, work by a team of English scientists reinforces an earlier suggestionthat evidence of life on the early Earth might be found in meteorites on themoon.

The original idea waspresented in a 2002 paper by University of Washington astronomer JohnArmstrong, who suggested that material ejected from Earth during the Late HeavyBombardment (a period about four billion years ago when the Earth was subjectedto a rain of asteroids and comets) might be found on the moon.

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

New research by a team underIan Crawford and Emily Baldwin of the Birkbeck College School of Earth Sciencesused more sophisticated means to simulate the pressures any such terrestrialmeteorites might have experienced during their arrival on the lunar surface. Thisconfirmed Armstrong's hypothesis. In many cases, the pressures could be low enough topermit the survival of biological markers, making the lunar surface a goodplace to look for evidence of early terrestrial life.

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

Crash landings

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

However, there is a problem:The moon lacks any appreciable atmosphere. Meteorites arriving on Earth aredecelerated by passing through our atmosphere. As a result, while the surfaceof the meteorite may melt, the interior is often preserved intact. Could ameteorite 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 elementanalysis to simulate the behavior of two different types of meteors impactingthe lunar surface.

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

In the most extreme casethey tested (vertical impact at a speed of some 11,180 mph, or 5 kilometers persecond), Crawford reports that "some portions" of the simulatedmeteorite would have melted, but "the bulk of the projectile, andespecially the trailing half, was subjected to much lower pressures."

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

Hard to find

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

Minerals formed in thepresence of water, called hydrates, can be detected using infrared (IR)spectroscopy. Crawford and his co-authors believe that a high-resolution IRsensor 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 astronomersview the issue more conservatively. Dr. Mike Gaffey of the University of NorthDakota Space Studies department argues that while "debris from a largeterrestrial impact could have reached the moon ... it's highly unlikely that itwould be in sufficient concentrations to be seen" using orbitalinstruments.

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

Crawford concedes thatpoint, and suggests that it might be necessary to dig below the surface to findterrestrial meteorites. He adds that collecting samples, observing them on thelunar surface, and picking those that warrant a return to Earth for detailedanalysis "would be greatly facilitated by a human presence on the moon."

The last U.S. astronaut toset foot on the moon, Dr. Harrison Schmitt, was a geologist. If current NASAplans 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 themystery of how life began on the Earth.