2011 Annual Science Report
University of Hawaii, Manoa Reporting | SEP 2010 – AUG 2011
Lunar Water, Volatiles, and Differentiation
Recent discoveries of water in the Moon have important implications for how and when water was delivered to Earth. One way of investigating this is to determine how much water the Moon had when it formed. We do this by searching for water in rocks rich in trace elementsSo far our results indicate that either Earth experienced a second gain of water after the moon formed, or there was an as-yet unexplained loss from the proto-lunar disk.
We have concentrated on the two major prongs of this project, the amount of water in the Moon and the extent of loss from the proto-lunar disk. We have been studying lunar felsites, which formed from highly evolved magmas and are good candidates for high water contents. Petrologic data indicate that these rocks formed in small intrusions, hence are sufficient pressure to retain water if it was present. Small grains of the phosphate mineral apatite contain either no detectable water (actually in the form of OH molecules) or just above detection limits. The one with detectable water, however, had D/H above the terrestrial value (delta D of about 300-400 vs 0 for Earth), suggesting either a different source of water than the bulk of terrestrial water or loss from the proto-lunar disk. The other area was to study water and volatile loss from the proto-lunar disk, done in collaboration with Steve Desch (ASU). Water loss during lunar formation is extensive but not complete (98% loss), and D and H can be fractionated. Elemental loss may not be as complete, however, suggesting that the low lunar volatile contents could be inherited from the moon-forming impactor. Our analysis also suggests that addition of water and volatile elements shortly after lunar formation is also a possibility.