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Planetary magnetism : investigations of paleomagnetic properties of the Earth and Moon

  • Author(s): Lawrence, Kristin Portle
  • et al.
Abstract

A planetary body's ability to generate a dynamo is dependent on its thermal history and possible energy sources, particularly the convective states of the core and overriding mantle. This dissertation addresses questions regarding the characteristics of magnetic fields for both the Earth and the Moon. The temporal and spatial variations of the Earth's geomagnetic field are investigated using rock magnetic properties (direction and intensity) for the past 5 Ma. This thesis analyzes both published data at \±20\⁰ and provides new paleomagnetic measurements of direction and intensity for a high latitude site ( -78\⁰) to investigate both the longitudinal and latitudinal variations of Earth's field. Investigations of zonal statistical paleosecular variation (PSV) models reveal that recent models are incompatible with the empirical \±20\⁰ directional distributions. The \±20\⁰ latitude data suggest less PSV and smaller persistent deviations from a geocentric axial dipole field during the Brunhes. High dispersion and low intensity observed at - 78\⁰ agrees with the global observation of anti-correlation between dispersion and intensity. Comparisons with temporally-varying dipole magnitude models show that the lack of increased paleointensity at high latitude likely results from temporal sampling bias. Previous paleointensity studies of Apollo era samples suggest strong paleointensities for ancient (>3.6 Ga) lunar rocks and little-to-no strength for younger (<3.6 Ga) rocks. By extrapolation, these results were interpreted as the Moon having an early, short-lived dynamo. However, previous paleointensity measurements do not meet current lab protocols and substantial scatter exists in paleointensities from the putative dynamo period. I analyze published and measure new absolute (thermal and microwave) and relative paleointensities from five Apollo samples spanning from 3.3 to 4.3 Ga to re-examine the hypothesis of an early lunar dynamo. In light of the new experiments and a thorough re-evaluation of existing paleointensities, I show that although some samples with ages of 3.6 Ga to 3.9 Ga are strongly magnetized with occasional stable directions, no observations verifiably demonstrate a primary thermal remanence. In agreement with satellite measurements of crustal magnetism, current paleointensity measurements do not support the existence of a 3.9-3.6 Ga lunar dynamo

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