Lunar surface may hold evidence that asteroids crashed into Earth
Clues to Earth’s earliest days and first microbial inhabitants may survive in an unexpected place: the moon.
Scientists have long debated what happened on the primordial Earth almost 4 billion years ago. Did primitive microbes wriggle within volcano-heated pools of water? Did falling asteroids vaporize oceans and gouge craters the size of small states?
Such questions are terribly hard to answer. The clues have been largely erased by erosion — by rain, wind, tides, plate tectonics, and other natural forces.
But some clues might still exist a quarter of a million miles away, on the frigid, airless surface of the moon. Long ignored by mainstream scientists, the idea has begun to attract some serious attention, including the first serious proposals to go looking for hard evidence.
New calculations by a youthful team of researchers at the University of Washington and Iowa State suggest a strong probability that asteroid impacts could have splashed substantial amounts of terrestrial rock toward the moon, like mud sprayed by a car racing down a dirt road.
Clouds of what the researchers call "terran meteorites" might have sprinkled across the lunar surface. There, in a much less erosive environment than exists on Earth — no wind ever blew and no water ever flowed on the moon — the rocky relics of Earth’s primeval days may endure, awaiting discovery by future astronauts or remotely controlled robotic vehicles.
Hence the three researchers dub the moon "Earth’s attic": a deep-freeze repository for relics of the terrestrial dawn.
The researchers have outlined a plan to test the hypothesis as part of some future lunar-prospecting mission. Details were presented for the first time at a recent astrobiology science conference held at NASA’s Ames Research Center in Mountain View.
After a large impact, the terrestrial rocks "could just fly off Earth and get scooped up by the moon, or go into orbit around the Sun and then later on land on the moon," explained John C. Armstrong of the Center for Astrobiology and Early Evolution at the University of Washington at Seattle.
Perhaps 20 tons of terrestrial rock could be buried over a typical lunar area of about 40 square miles, according to calculations by Armstrong and Llyd E. Wells, also at the Seattle center, and Guillermo Gonzalez, assistant professor in the physics and astronomy department at Iowa State University in Ames, Iowa.
Armstrong is a graduate student in astronomy who expects to receive his doctorate at year’s end. Wells is a biologist and graduate student in oceanography.
The surface of the moon is not completely free of erosion: It is pelted by a steady rain of "micrometeorites" and cosmic rays. The most intact terran rocks are likely to survive within a few feet of the lunar surface, shielded by the overlying rock.
Armstrong said the three men got the idea while "stuck in traffic" near the Ames center in early 2000. Armstrong says they began batting around ideas for space exploration, "and Guillermo said, ‘Say, have you ever thought about what would happen if an asteroid could blast stuff off the Earth and onto the moon?’"
A similar question was asked in the 1960s by a famous chemist, Harold C. Urey, a top adviser to the U.S. space program. His idea drew little attention, though. One reason: It was hard to imagine how material could be violently transferred from one world to another without being destroyed in the process. (To escape Earth gravity, an object must be accelerated to a speed of 7 miles per second or 25,000 miles per hour.)
In recent years, though, scientists have grown accustomed to finding fragments of the moon and Mars on Earth, especially in Antarctica. There, they pluck lunar and Martian meteorites out of the polar ice like kids plucking raisins from raisin pudding.
They know the Mars rocks come from that planet because they contain small pockets of gas whose isotopic contents match those recorded in the Martian atmosphere by the twin Viking robots, which landed on Mars in 1976. Mars meteorites are clear evidence that chunks of one planet can survive a voyage to another.
The most controversial Martian meteorite is known as ALH84001 (ALH stands for the Allan Hills region of Antarctica, where it was found). A few scientists suspect it contains fossils of Martian microbes.
"The moon is strategically located within the inner solar system as a collector of debris," Armstrong said. ‘It has, potentially, collected material from all the terrestrial planets," including Earth, Mars and Venus.
"The Earth meteorites on the moon could provide a geological record of early Earth not available anywhere else in the solar system. . . . While there isn’t a whole lot of Earth stuff up there, some of the Earth material may contain geochemical and biological information such as isotopic signatures, organic carbon, biologically derived molecules and minerals, and maybe even microbial fossils."
Skepticism is expressed by NASA-Ames scientist Dale Cruikshank, a leading figure in the search for organic molecules in space.
"Earth materials probably exist on the moon," he acknowledged, but cautioned that they are probably "hugely diluted in the vast and thick dusty layers that mantle every square inch of our neighbor in space."
Also, any Earth rocks that reached the moon about 4 billion years ago should have been altered by lunar volcanic activity or changed "chemically and mechanically beyond recognition" by other natural means, he said in an e-mail to The Chronicle.
In response, Armstrong agreed that terrestrial materials might be diluted to a scarcity of one to 10 parts per million. Still, even such scarce particles are "not insignificant" and could be identified and studied with advanced scientific methods.
He pointed out that in recent years, scientists have learned a great deal about the evolution of the solar system by studying interstellar dust particles (IDPs), which are literally dust grains that drop to Earth from space. As for lunar vulcanism, Armstrong says it might help, not harm, their proposal because lava "could actually help protect the material from the Earth" from lunar erosive processes such as micrometeorites.
If robots or astronauts return to the moon, how could they distinguish terrestrial meteorites from native lunar rocks? Armstrong’s team is now investigating that question, using small samples of lunar rocks from NASA’s Johnson Space Center in Houston.
One way, they suspect, is by analyzing the rocks’ reaction to ultraviolet light. Ultraviolet light could expose carbonates typically formed in the presence of liquid water, which has long been abundant on Earth.
Also, future explorers might keep their eyes peeled for rocks with burned or "ablated" surfaces. Ablation is a clue that they experienced high friction while shooting through the atmosphere of another planet.
One of the most exciting questions facing space scientists is: Did the inner solar system experience a horrendous "late heavy bombardment" of asteroids 3.8 billion to 4.1 billion years ago? Scientists have debated this question for years.
Terrestrial rocks on the moon might "shed a lot of light on the question of whether there really was a (late) heavy bombardment" at that time, Armstrong said.
If the late heavy bombardment really happened, might it have wiped out any early life? Possibly so, some scientists say.
However, Wells speculates that terrestrial life might have survived the bombardment via an unusual route: brief sojourns in space.
To be specific, asteroid impacts might have hurled rocks with microbes into space. After thousands of years in the deep-freeze of orbit, the rocks might have fallen back to Earth and "re-seeded" the planet with life, Wells says.
If he’s right, then Earth’s first "astronauts" were not Yuri Gagarin and Alan Shepard but, rather, microbes. Knowing that, maybe you’ll show a little more respect for the greenish mold on your shower wall: It looks humble, but its ancestors might have boldly gone where no microbe went before.
San Francisco Chronicle, April 22, 2002.