Space.com has a story about the formation of the moon.  A recent paper in the journal Proceedings of the National Academy of Scieces lends more evidence for the “Giant-Impact  Hypothesis” of the creation of the moon.  

Scientists have found fresh evidence in lunar rocks showing that the moon was likely formed after a Mars-sized planet crashed into the proto-Earth more than 4 billion years ago.

A NASA-led team examined moon rocks brought back to Earth by Apollo astronauts more than 50 years ago. Investigating the samples with advanced tools not available to researchers in the 1960s and 1970s, the team found further evidence of the “giant impact theory” by focusing on the amount and type of chlorine in the rocks, a new study reports.

The researchers discovered the moon has a higher concentration of “heavy” chlorine compared to Earth, which sports more “light” chlorine. The terms “heavy” and “light” refer to versions of the chlorine atom, known as isotopes, that contain different numbers of neutrons in their nuclei.