APOLLO SPECIAL: IT'S THE SOLAR SYSTEM, STUPID
by Dana Mackenzie
New Scientist
July 10, 2009
Original LinkWhile the world watched in fascination as Neil Armstrong and Buzz Aldrin gambolled about on the moon, planetary scientists had their eyes on a different prize. For them, the value of the mission lay in the cargo the mission aimed to bring home, and the astronauts did not disappoint. By the time Armstrong and Aldrin climbed into the lunar module for the last time, they had collected 22 kilograms of moon rocks, enough to fill a small suitcase.
Five more Apollo crews brought the total collection of moon rock to 382 kilograms of material, made up of some 2200 individually numbered samples. Three uncrewed Russian landers recovered a further 300 grams of soil.
The rocks were billed at the time as a scientific treasure and they didn't disappoint either. "Our ideas about planetary formation and evolution had to be rewritten from scratch after Apollo," says geologist Paul Spudis of the Lunar and Planetary Institute in Houston, Texas.
Apollo samples decisively laid to rest many myths about the moon. Nobel prizewinner Harold Urey, one of the first great advocates of lunar exploration, had predicted the moon was made of primitive meteoritic material. He was wrong. Some of the rocks looked a lot like Earth rocks, notably the dark basalts that give the lunar maria, or "seas", their distinctive hue. Others were quite different, such as the ubiquitous jumbled pieces of rock called breccias, which have been smashed up and welded together by millions of years of meteorite impacts.
Many of the clues that the lunar rocks contained have taken years to decode, and some of the conclusions are still hotly debated. A huge surprise was the evidence that the early moon was covered by a deep ocean of molten rock. The moon's mountainous regions are dominated by anorthosite, a rare rock on Earth that forms when light, aluminium-rich minerals float to the top of a lava pool. If anorthosites are everywhere on the moon, then its entire surface must once have been a magma ocean, and this prompts a puzzling question: where did the energy to produce this magma ocean come from?
These days, the smart money is on the idea that it was the result of a cataclysmic event about 50 million years after the solar system began to form, when the Earth was in its infancy. According to this hypothesis, proto-Earth ran into a Mars-sized planet, and debris from the collision entered orbit around Earth where it rapidly coalesced to form the moon.
This "giant impact" scenario has led to a radical re-evaluation of the history of the early solar system. Before Apollo, planetary scientists saw the collection of objects orbiting the sun as a clockwork mechanism in which collisions were rare and insignificant. Now it is recognised as being a far more dynamic environment, in which planets can shuffle around, collide or can be ejected altogether. The history of all the inner planets has been shaped by collisions, and nowhere is that history more visible than the moon.
Another surprise was that the rocks from the moon's largest impact craters show that all the craters are roughly the same age, formed 3.8 to 4 billion years ago. It's unlikely that this could be a coincidence. The moon -- and by extension, Earth -- must have been subjected to a devastating barrage half a billion years after the solar system was formed. To cause this, something big must have been going on back then in the outer solar system, but what? Curiously, this episode in the solar system's history, which has come to be known as the late heavy bombardment, ended at about the same time as the first signs of life appeared on Earth. Did it in fact create the conditions under which life could evolve? That, too, remains a matter of speculation.
Without the samples brought back from the moon for chemical analysis and isotopic dating, we might never have made these key discoveries about our planet's history. So do the Apollo rocks harbour any more secrets? All 2200 samples have been studied, and Randy Korotev, a lunar geochemist at Washington University in St Louis, Missouri, says this means there is unlikely to be anything world-shaking left to discover from them. They may yet hold some more subtle secrets, however. "We are constantly developing better tools and asking better questions," Korotev says.
In particular, the instruments for dating mineral samples have become more sophisticated, enabling researchers to determine the age of ever smaller samples, such as tiny mineral grains within a rock.
In the past two years, these techniques have prompted a rethink of some key dates in lunar history. A team at the Swiss Federal Institute of Technology dated the formation of the moon's magma oceans -- and, by inference, the moon itself -- to 20 to 30 million years later than we thought, to about 4.5 billion years ago (Nature, vol 450, p 1206). And Alexander Nemchin of Curtin University of Technology in Perth, Western Australia, with five colleagues, dated a lunar zircon at 4.417 billion years old, which pins down the likely time when the last of the magma oceans solidified (Nature Geoscience, vol 2, p 133).
What the Apollo samples will never do is answer some of the remaining big-picture questions. What will we find on the far side -- the half of the moon's surface we can never see from Earth? Can we put together a detailed history of the lava flows that formed the basalts of the lunar seas? Can we find any samples from deep inside the moon?
These are all seen as powerful reasons for returning to the moon. The big picture requires more samples, more data and lots more context. "There's no lack of targets and scientific questions," says Gary Lofgren, curator of NASA's lunar rock collection at Johnson Space Center in Houston. "It's not just about the moon but about the solar system's history. That's the lesson that we have learned from Apollo."