Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon Greenwood et al (Go Wes!)

Water on the Moon? Lunar water helps astronomers understand the Earth—Moon system Water has been found in lunar volcanic glasses and apatite… But where did this water come from?

Important Terms SIMS: Secondary-ion mass spectrometry SCAPS: Stacked complementary metal-oxide-semiconductor-type active pixel sensor VSMOW: Vienna standard mean ocean water D/H Ratio: ratio of Deuterium (“heavy H”) and Hydrogen Mare: low-lying lunar plain, filled with some lunar rocks; darker (basaltic) Highlands: higher plains; lighter (anorthositic)

Points No indigenous water was discovered in Apollo program samples Bad: some samples from Apollo mission were terrestrially contaminated Good: ion microbe measurements have shown water in apatite from mare and highlands rocks in uncontaminated samples

Apollo 11 High Ti-mare basalt D/H of apatite grans in thin sections (10022,12 and 10044,644) similar although 10044,12 is from 40 years ago; 10044,644 was made weeks before SIMS analyses Apollo 12 Low Ti-mare basalts and is pigeonite basalt associated with mesostasis; highest water content measured is olivine basalt associated with prolonged cooling; very low water content Apollo 17 High-Ti mare basalt D/H and water content similar to Apollo 14 High-Al mare basalt D/H is very different from other lunar basalt—cannot be distinguished from Earth water Apollo 12 Breccia with granite Black with lithology similar to KREEP Apollo 14 Crystalline Matrix Breccia 14305,94 -Very low δD value and consistent with terrestrial adsorbed water Apollo ,303 Pristine Alkali Anorthosite Clast c2 -Uncertain water content but has lower water content than standard terrestrial apatite minerals

Figure 1: Backscatter electron image and SCAPS 1 H image of apatite grain 5 of 10044,12 Supplementary Table 5: δD and H 2 O content (wt.%) of lunar apatite

Some Info. δD = {[(D/H) sample / (D/H) VSMOW ] − 1} × 1000 Earth’s water ranges from ~ −500‰ to ~ +100‰ with most in −200‰ to +50‰ range Mean δD value of 0‰ is ~ the same as ocean water Meteoric Water Line: linear progressive D depletion as H 2 O condenses near the poles “Latitudinal dependence of δD (and 18 O/ 16 O) values of water”

Figure 2: δD (‰) versus H2O (wt.%) of lunar apatite measured in this study.

Figure 3: δD plot of the solar system

Conclusions Changes in D/H ratios of apatite-containing samples indicate the presence of water throughout the Moon’s magmatic history...  Water on the Moon could have come from lunar mantle, solar wind protons, and comets Comets delivered a significant amount of water to the Earth—Moon system after Moon-forming impact