Shocker: Moon is 200 Million Years Younger Than Thought

The moon may not be new, but now it is a little newer than previously thought.

(ABC News)- "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.