Geophysical Constraints on the Existence and Size of a Lunar Metallic Core: A Reassessment Based on Initial Lunar Prospector Data

Details Geophysical Constraints on the Existence and Size of a Lunar Metallic Core: A Reassessment Based on Initial Lunar Prospector Data Geophysical Constraints on the Existence and Size of a Lunar Metallic Core: A Reassessment Based on Initial Lunar Prospector Data

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lpico.957...14h&link_type=abstract

Origin of the Earth and Moon, Proceedings of the Conference held 1-3 December, 1998 in Monterey, California. LPI Contribution N

Physics

Geophysics

Geophysics, Lunar Composition, Lunar Core, Moon, Selenology, Lunar Geology, Geomagnetic Tail, Lunar Prospector, Magnetometers, Lunar Magnetic Fields

Scientific paper

The mass of a possible Fe-rich lunar metallic core is a basic constraint on lunar bulk composition and, hence, lunar origin models. Initial Lunar Prospector data provide additional evidence relating to this question in two ways. First, an improved gravity solution has yielded a refined moment of inertia factor of C/MR2 = 0.3932 +/- 0.0002. This result is approximately consistent with upper limits assumed in earlier assessments and density modeling that indicates the possible existence of an Fe-rich core with a radius in the range of 200-450 km. Second, Lunar Prospector magnetometer data can yield measurements of the residual induced magnetic dipole moment of the Moon after extended exposure (hours or more) to a uniform magnetic field during passages through the near-vacuum environment of the geomagnetic tail. The induced moment is directly relatable to the radius of a highly electrically conducting core. Similar measurements have previously been reported using data from the Apollo 15 and 16 subsatellite magnetometers. Here, we report initial attempts to measure the lunar-induced moment in the geomagnetic tail using data from the Lunar Prospector magnetometer. During the April 1998 tail-lobe passage, the Lunar Prospector orbit plane was nearly parallel to the Sun-Moon line and was therefore optimally oriented for measurements of the induced moment. Careful editing and averaging of magnetic field date rotated into a coordinate system defined by the mean tail-lobe field orientation yields evidence for an induced moment with an amplitude that corresponds to a conducting core radius of 450 100 km. Data from the November tail-lobe passage will be necessary to confirm this preliminary result and may be available by the time of the meeting. Also, model calculations will be presented to estimate the minimum implied conductivity of the electrically conducting core.

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