Lunar Internal Structure From Reflected and Converted Core Phases

Details Lunar Internal Structure From Reflected and Converted Core Phases Lunar Internal Structure From Reflected and Converted Core Phases

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p51b0483b&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #P51B-0483

Physics

6200 Planetary Sciences: Solar System Objects, 6250 Moon (1221), 7299 General Or Miscellaneous

Scientific paper

Seismic constraints on lunar structure come primarily from the analysis of surface impacts and both shallow and deep moonquakes. These have been used to estimate the thickness of the lunar crust, and the structure of the mantle down to approximately 1000 km depth. However, a direct seismic constraint on the size of the lunar core is difficult to obtain due to several factors. Core phases from naturally-occurring deep moonquakes are not observed on seismograms from the Apollo instruments, in part due to the strong scattering of seismic energy in the lunar crust. The scattering coda associated with the main P and S arrivals prohibit the immediate identification of secondary phases, even on stacked seismograms. Depending on the choice of lunar structure model, a variety of phase arrivals from deep moonquakes could theoretically be detected at the four Apollo seismic stations. We attempt to determine whether further manipulation of the Apollo seismic data will permit the identification of core phases. We will employ a time-domain method of resolving secondary phase arrivals from the Apollo data, using stacked deep moonquake seismograms which include recently discovered additional events. Because the P-wave coda associated with most deep moonquakes is very long, later arrivals are masked. Deconvolution of a given moonquake signal by its P-wave arrival may therefore enhance arrivals from reflected (e.g. PcP) and converted (e.g. PKP) core phases. These can be compared to travel times and amplitudes predicted from ray theory in order to constrain the structure of the lunar core. In addition, we asses the likelihood that a future lunar seismometer could detect core phases. The seismometer currently in development for the ESA mission ExoMars, if modified for use on the lunar surface, would permit measurements 100 times more sensitive than the Apollo instruments. For a given core radius, we can determine which locations on the lunar surface would receive core phases from the known distribution of deep moonquake sources. A seismic station at the proposed location of the south pole lunar base could detect both PcP and PKP from many deep source regions, in addition to ScS, SKS, ScP, PcS, and both the direct P and S phases.

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