KREEP on the Moon

If you go to a conference about lunar geology, sooner or later you'll hear the term "KREEP" bandied about. (And almost as soon as KREEP is mentioned, a bad pun will be made. It's inevitable.) Context will tell you it has something to do with a special kind of lunar rock, but that'll only get you so far. What is KREEP, and why is it important on the Moon?

The simple definition is that KREEP is an acronym for potassium (chemical symbol K), rare earth elements (the ones that are always cut out of the periodic table and drawn in two separate rows of their own, abbreviated REE), and phosphorus (chemical symbol P). Despite their name, the stable rare earths are actually not that uncommon in nature; a few are as common as copper and lead and all are more common than mercury and iodine and way more common than gold and iridium.

Potassium, rare earths, and phosphorus are lumped together in the term KREEP because they tend to occur together in the lunar crust. To explain why they tend to occur together, we have to back up the story to the beginning, when the Moon formed. …

See Planetary Society Weblog for links to further info.