The Newly Uncertain Age of the Moon

          The 12 Apollo astronauts collected 842  pounds (382 kg) of rocks and dust during their six lunar landings from 1969 to 1972. Three robotic Luna missions netted Soviet scientists another 10½ ounces (0.3 kg). Since then hundreds, if not thousands of analyses on these precious samples have proved without doubt that, geologically speaking, the Moon is very old and very dead.

But a single pea-size nugget of anortho-site, collected during Apollo 16, now has lunar geochemists in a tizzy. A team led by Lars Borg (Lawrence Livermore National Laboratory) used the best modern techniques to date it, independently by three means, to an accuracy of better than one part in a thousand. The group concludes from it that the Moon has been fibbing about its age of its crust. Instead of being at least 4.43 billion years old, as earlier assays had shown, the group found an age of 4.360 billion years, give or take just 3 million.

A change in the Moon’s age of 70 million years — less than 2% — might not seem like a big deal. But it makes a huge difference in the context of the fast-evolving early solar system.Nearly every lunar researcher now accepts that the Moon formed after a Mars-size protoplanet plowed into the newly formed Earth, splashing white-hot debris into orbit that rapidly coalesced into a sizable satellite. Borg and his team assert that either this happened late, long after the rest of the inner solar system’s major chaotic collisions had quieted down (contrary to the dating of many other lunar samples) — or that the mystery rock represents some kind of large-scale remake of lunar crust that remelted and recrystallized later.

A second satellite of Earth doing a late splat into Moon might play into this story.There’s another way out: force the Moon’s original magma ocean to cool very slowly. Last year, MIT researchers Jennifer Meyer, Linda Elkins-Tanton, and Jack Wisdom proposed that the infant Moon, being much closer to Earth than it is now, would have experienced tidal flexing strong enough to keep it hot and molten.

The Moon’s birth story may be quite complicated. With luck, NASA’s twin GRAIL spacecraft (Gravity Recovery and Interior Laboratory), launched on September 10th, will succeed in mapping the gravitational structure of the lunar interior finely enough to sort out the correct sequence of events. The GRAIL craft will orbit the Moon information from 40 to 140 miles apart, continuously monitoring the distance between them to a precision of 0.2 micron, nearly one part in a trillion. This data will reveal gravitational irregularities that should improve the quality of the Moon’s density maps by a factor of 100 to 1,000 from crust to core, laying bare the Moon’s interior structure.

SOURCE : SKY & TELESCOPE MAGAZINE DECEMBER 2011