Moon Holds Earth's Ancient Secrets
Robert Roy Britt
Tons of rocks and dust long ago blasted from Earth by asteroid impacts lay on the Moon's surface and could hold secrets to our home planet's early history and the origin of life.
John Armstrong sees the Moon as Earth's attic, and he figures we should go back and fetch some of the valuable goods stored there. The information is not available anywhere else, he and other astronomers agree.
Armstrong, of the University of Washington in Seattle, led a new study that concludes the Moon ought to be littered with terrestrial debris -- some 11,000 pounds within a few inches of the surface of every square lunar mile. He told SPACE.com that retrieving some of it would be the quickest and least expensive way to learn more about the solar system.
"We are talking about finding material from the very early Earth," Armstrong explained. "Samples of the Earth 3.9 to 4.0 billion years ago could tell us a lot about the state of the early atmosphere, what the crust and surface were like, and possibly even when life began to evolve."able
A close-up view of Apollo 15 lunar sample no. 15415 in the Lunar Receiving Laboratory (LRL).
Astronauts David R. Scott, right, commander of the Apollo 15 mission, gets a close look at the sample referred to as the "genesis rock" in the Lunar Receiving Laboratory. Scientist-Astronaut Joseph P. Allen, left, looks on.
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There might also be Venusian rocks on the Moon, say Armstrong and his colleagues, Llyd Wells of the University of Washington and Iowa State University's Guillermo Gonzalez. No rock from Venus has ever been found, nor is it likely that any will ever be retrieved from its toasty surface. Gathering one up would likely reveal a wealth of information about Venus, astronomers say.
A paper detailing the study will be published later this year in the journal Icarus.
Late Heavy Bombardment
No one has set foot on the Moon since 1972, the end of an era of exploration in which Apollo astronauts brought back 842 pounds (382 kilograms) of material from the lunar surface. Among the more important things learned from the lunar dust and rock is that unlike here on Earth, the stuff on the surface of the Moon is incredibly old, a record of what was going on in this neck of the solar system some 4 billion years ago, just a few hundred million years after the solar system formed.
Earth's surface is continually recycled, folded deep inside the planet by the same forces that generate earthquakes and volcanoes. The Moon, on the other hand, has almost none of this tectonic activity.
Scientists already knew that rocks from Mars have been blasted into space and ended up on Earth. They have found some.
However, few researchers have seriously looked into the same scenario for terrestrial rock being booted to the Moon. Armstrong and his colleagues realized that this transfer of material should have occurred at a frenzied pace up until about 3.8 billion years ago, when a period called the Late Heavy Bombardment is thought to have ended. No material of this sort has ever been identified, however, and in fact the extent and timing of the bombardment itself is not known with certainty.
The bulk of terrestrial rock that's been shot to the Moon would likely be pebble-sized or dust, having been pulverized by the initial impact. "However, there is a chance that larger rocks survived the trip," Armstrong said.
A mission to gather material would be tricky.
A robotic rover could sift lunar dust and analyze its chemistry, hunting for stuff diluted to just seven parts per million. Because the Moon is mostly dry, the robot would look for water-bearing minerals. Also, asteroids that have hit the Moon on their own would litter it with material high in water and metal, so the robot would have to look for wet stuff that's low in metals -- and possibly from Earth.
Armstrong said finding larger terrestrial rocks on the Moon would be harder and likely require sending humans.
Back to the Moon
Researchers know that getting back to the Moon is a tough sell these days.
"It is commonly held that we've already sort of 'done' the Moon," Armstrong points out. "However, science was not the main driver of the Apollo mission, and we have so much left to learn."
He said a lot of planetary science is based on knowledge gleaned from Apollo missions.
"This [new study] gives us a compelling reason to go back -- to look at the Moon as a window to early Earth," he said. "But going to the Moon is the fastest and cheapest way to learn more about our solar system."
Finding stuff on the Moon that came from Earth would also verify the whole premise of the new study, Armstrong said. This would allow researchers to accurately date the period of bombardment they suspect would have put the rocks there in the first place.
Kevin Zahnle, a scientist at NASA's Ames Research Center who was not involved in the study, agreed that there might be terrestrial material on the lunar surface.
"They [the rocks] may tell us something about the presence or absence of continents [on Earth] 4 billion years ago," Zahnle said. "There is a very small chance that one might find a rock that shows unambiguously that there was life on Earth before 4 billion years ago."
Scientists are presently debating when life began on Earth. The most commonly believed time frame, based on reasonably solid evidence, is 3.8 billion years ago. Some scientists argue that life is not that old, however, while others think it began earlier. The answer bears on the prospects for life having originated on other planets.
Zahnle said further investigation into the potential fruitfulness of a new lunar mission ought to start with an examination of lunar samples kept in a vault at the Johnson Space Center in Houston.
More than 2,400 separate bits of the Moon are sequestered there, ranging from sand grains to basketball-sized rocks. Other scientists have proposed such work, Zahnle said, but all requests have been rebuffed.
Armstrong and his colleagues are also seeking access to the vault but have had no luck. Meanwhile, they've obtained several grams of fine lunar particles from another researcher.
"This gives us thousands if not millions of grains to test some of our ideas about how to go about searching for this stuff," Armstrong said. "I have no illusions about finding Earth material in that sample, but it gives us a place to hone our techniques."
Ultimately, Zahnle said, the "profound information" about Earth's past that might be gleaned rummaging through Earth's attic "is probably impossible to obtain in any other way," than by going to the Moon. "Its not that the Moon is a good place to look. It's rather that the Moon is the only place to look.”