Mathematics – Logic
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmgp43b1049t&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #GP43B-1049
Mathematics
Logic
[1500] Geomagnetism And Paleomagnetism, [5440] Planetary Sciences: Solid Surface Planets / Magnetic Fields And Magnetism, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
Forty years after the Apollo program, the source of the magnetization observed in the lunar crust and in returned samples remains unresolved. The two leading explanations for lunar remanent magnetism are transient magnetic fields created by impact events and the existence of an ancient lunar core dynamo. In order for a lunar sample to present strong evidence that it acquired its primary remanent magnetism from an ancient lunar dynamo, several criteria should be met. First, the sample should not bear any petrologic evidence of shock, implying a peak pressure of ≤ 5 GPa. Then, during demagnetization experiments, the natural remanent magnetization (NRM) ideally should converge to a stable direction, with intensity decreasing towards the origin. The magnitude and direction of primary magnetic remanence should also agree across mutually oriented subsamples. Finally, the initial cooling timescale for the sample should be ≥ ~1 hour in order to effectively rule out impact events as magnetizing field sources. Recent studies on lunar troctolite 76535 (Garrick-Bethell et al. 2009) and ilmenite basalt 10020 (Shea et al. 2010) have demonstrated that these samples meet all of the aforementioned criteria and support the possibility that an ancient lunar dynamo existed at 4.2 Ga and 3.7 Ga, respectively. However, the stable magnetic behaviors observed in 76535 and 10020 appear to represent exceptions rather than the rule when compared to the rest of returned lunar samples. The vast majority of lunar samples subjected to paleomagnetic studies display chaotic NRM behavior, evidenced by non-monotonic decline of magnetic moment and unstable directions of magnetization when undergoing demagnetization experiments (see Brecher, 1976). Before a lunar dynamo can be a fully accepted, it is vital to demonstrate that the poor magnetic behaviors evident in the rest of the lunar sample collection do not indicate the absence of a magnetic field. Here we present results of a study which investigates the alternating field (AF) demagnetization behavior and magnetic recording capacities of mare basalts 15556, 12017 and 15016. All of these samples appear to be unshocked (peak pressures ≤ 5 GPa) and have cooling timescales longer than the expected duration of impact fields. Our results reveal that these three samples display unstable AF demagnetization behavior. The samples also appear to be imprecise paleointensity recorders of fields with intensities under ~ 30 μT (near the lower bound of the Earth’s present field strength), at least when measured using AF demagnetization. Based on these findings, we posit that many lunar samples are intrinsically unable to preserve robust paleointensity records of magnetic fields when analyzed using AF methods. Therefore, their chaotic demagnetization behavior does not rule out the existence of an ancient lunar dynamo.
Buz Jennifer
Gattacceca Jérôme
Grove Timothy L.
Silva M. G.
Tikoo Sonia M.
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