Seismic detection of the layers of the lunar core (Invited)

Details Seismic detection of the layers of the lunar core (Invited) Seismic detection of the layers of the lunar core (Invited)

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmdi33b..03w&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #DI33B-03

Physics

[1026] Geochemistry / Composition Of The Moon, [6250] Planetary Sciences: Solar System Objects / Moon, [7200] Seismology

Scientific paper

Despite recent studies that have advanced many areas of lunar science through the analysis of Apollo-era seismic data, important deficiencies remain in our understanding of the lunar interior, especially with regard to the structure of the innermost Moon. Various indirect geophysical measurements, including moment of inertia, lunar laser ranging, and electromagnetic induction provide supporting evidence for the presence of a core, but differ on key characteristics such as its radius, composition, and state (solid versus molten). Constraining the detailed structure of the lunar core is necessary to improve our understanding of the present-day thermal structure of the interior and the history of a lunar dynamo, as well as the origin and thermal and compositional evolution of the Moon. We analyze Apollo deep moonquake seismograms using terrestrial array processing methods to search for the presence of reflected and converted energy from the lunar core. Beginning with stacks of individual event seismograms from the known distribution of deep moonquake clusters, we apply a polarization filter to account for the effects of seismic scattering that typically obscure all but the main P- and S-wave arrivals. The filtered traces are then shifted to the predicted arrival of a core phase (such as PcP, a P-wave that reflects off the core-mantle boundary) and stacked to enhance subtle arrivals associated with the Moon’s core. Our results indicate the presence of a solid inner and fluid outer core, overlain by a partial-melt-containing boundary layer, consistent with current indirect geophysical estimates of core and deep mantle properties. The tidal Love number k2 of our preferred model is in agreement with results obtained from the Lunar Laser Ranging experiment. The modeled layers are consistently observed among stacks from four classes of reflections: P-to-P, S-to-P, P-to-S, and S-to-S. Relative model sizes of the inner and outer core indicate that the lunar core is approximately 60 percent crystallized. It is likely, based on the phase diagrams of iron alloys and the presence of partial melting above the core, that the lunar core contains less than 6 weight percent of lighter alloying components, consistent with a volatile-depleted lunar interior.

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