The Delivery of Water to the Lunar Mantle by Late Planetesimal Accretion (Invited)

Details The Delivery of Water to the Lunar Mantle by Late Planetesimal Accretion (Invited) The Delivery of Water to the Lunar Mantle by Late Planetesimal Accretion (Invited)

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p41a..07b&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #P41A-07

Physics

[5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties, [6250] Planetary Sciences: Solar System Objects / Moon

Scientific paper

The final stages of planet formation in the inner Solar System are thought to have culminated in enormous planetary collisions, such as the hypothesized ‘giant impact’ origin for the Earth and Moon that occurred ~50-100 My after the formation of the first Solar System solids. The giant impact event probably triggered a final phase of core formation on these worlds, with global magma oceans effectively stripping the terrestrial and lunar mantles of highly siderophile elements (HSE; Re, Os, Ir, Ru, Pt, Rh, Pd, Au), which have extremely high metal-silicate partition coefficients. Studies of mantle-derived terrestrial peridotites and derivative lunar mantle melts, however, show that the terrestrial and lunar mantles have elevated absolute, and approximately chondritic relative abundances of highly siderophile elements (HSE). We argue this material was most likely delivered by continued planetesimal accretion via HSE-rich impactors within tens of My of core formation termination, with subsequently-accreted materials mixed into each mantle by convection. This process, often called the “late veneer” but here termed late accretion, delivered > 0.4% Earth masses to the terrestrial mantle and produced an Earth/Moon mass input ratio of ~1,000. Using Monte Carlo models, we found that this high ratio most likely came from planetesimal populations dominated by massive impactors. Specifically, if the late accretion population had the form dN ∝ D-q dD (i.e., dN is the number of planetesimals of diameter D within bin dD), the power law index of the projectiles was q < 2 for 200 < D < 4000 km. Interestingly, q ~ 2 populations are also found in planetesimal size distributions derived from evidence taken near 1 AU (e.g., D > 250 km asteroids in the inner/central main belt with semimajor axis < 2.8 AU, the population of non-saturated ancient martian impact basins with 700 < D < 2000 km) as well as from new planetary accretion models that allow planetesimals to be “born big” via turbulent concentration mechanisms in the gas disk. Using a q ~ 2 size distribution and HSE constraints, we predict the largest late accretion impactors to strike the Earth and Moon, on average, were D = 2,500-3,000 km and 250-300 km, respectively. If true, it is possible that the same projectile that delivered most of the Moon's HSE may have also have provided it with water. The Moon's interior was once thought to be largely dry, with bulk water estimates of less than 1 part per billion (ppb). New sample measurements, however, suggest the water content in the lunar mantle is between 200 and several thousand ppb. Assuming that our inferred D = 250-300 km lunar projectile could reach and mix itself into a spherical shell that is 100 to 500 km deep within the Moon, and that the projectile had a minimum bulk water content of 0.05-0.2 wt% (i.e., conservative values similar to those measured from “dry rocks” like ordinary and enstatite chondrites), we estimate that 400-3000 ppb water could be delivered to the lunar interior by late accretion. This finding could help sidestep a difficult problem, namely explaining how the water in the lunar mantle could migrate from a “wet” primordial Earth to a growing Moon through a very hot and largely vaporized protolunar disk.

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