"The moon's a harsh mistress," goes the song.  It does not surrender its secrets easily.  Scientists have thought it was 4.6 billion years old — but now Lars Borg of Lawrence Livermore National Lab, writing in today's edition of the journal Nature, says the real number is more like 4.4 billion.  That's a difference of 200 million years.

This matters, say scientists.  It's not just an argument about the age of rocks.  It has to do with how the world came to be, how quickly, and how it affects our existence today.  Scientists theorized that the moon probably formed when the solar system was young and filled with debris, and something — an object roughly the size of Mars — crashed into Earth and sent molten chunks in all directions.  Over the next several hundred million years, the debris coalesced, cooled, and formed the moon that orbits us today.

The oldest piece of the moon we have — or so scientists have believed — was brought back by astronauts John Young and Charles Duke of Apollo 16, in April 1972.  The age, based on the decay of chemical isotopes inside, was estimated at 4.56 billion years, and two other Apollo samples have been dated at 4.47 billion years — impossible if Borg and his colleagues are correct.

"It's not as ancient as we might think," Borg told the AP.  Either the moon is younger than past studies suggest, he said, or everyone's been wrong about how it formed.  The prevailing theory was that the early solar system was so hot, with so many space rocks crashing into each other, that the moon (and Earth too) had an ocean of molten rock, which only cooled as the debris cleared. (Sid Perkins of the journal Science has a good summary HERE.)

Other scientists have their doubts about Borg's theory. Erik Asphaug of University of California, Santa Cruz, called the conclusion "a little bit fancy for my taste." Maria Zuber of MIT called it "very puzzling."

NASA is agnostic on the matter.  Its lunar lab curator said "these things change. Instruments are getting better and ages are more reliable as they are redone."

"The moon's a harsh mistress," goes the song.  "The sky is made of stone."   (Above: Crescent moon seen from the International Space Station, September 2010.  Click on image to enlarge)