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Register a new userThe Local-time variations of Lunar Prospector epithermal-neutron data
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L.F.A. Teodoro
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We assess local-time variations of epithermal-neutron count rates measured by the Lunar Prospector Neutron Spectrometer. We investigate the nature of these variations and find no evidence to support the idea that such variations are caused by diurnal variations of hydrogen concentration across the lunar surface. Rather we find an anticorrelation between instrumental temperature and epithermal-neutron count rate. We have also found that the measured counts are dependent on the temperatures of the top decimeters of the lunar subsurface as constrained by the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment temperature measurements. Finally, we have made the first measurement of the effective leakage depth for epithermal-neutrons of ~20 cm.
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The timeline of the lunar bombardment in the first Gy of the Solar System remains unclear. Some basin-forming impacts occurred 3.9-3.7Gy ago. Many other basins formed before, but their exact ages are not precisely known. There are two possible interpretations of the data: in the cataclysm scenario there was a surge in the impact rate approximately 3.9Gy ago, while in the accretion tail scenario the lunar bombardment declined since the era of planet formation and the latest basins formed in its tail-end. Here, we revisit the work of Morbidelli et al.(2012) that examined which scenario could be compatible with both the lunar crater record in the 3-4Gy period and the abundance of highly siderophile elements (HSE) in the lunar mantle. We use updated numerical simulations of the fluxes of impactors. Under the traditional assumption that the HSEs track the total amount of material accreted by the Moon since its formation, we conclude that only the cataclysm scenario can explain the data. The cataclysm should have started ~3.95Gy ago. However we show that HSEs could have been sequestered from the lunar mantle due to iron sulfide exsolution during magma ocean crystallization, followed by mantle overturn. Based on the hypothesis that the lunar magma ocean crystallized about 100-150My after Moon formation, and therefore that HSEs accumulated in the lunar mantle only after this time, we show that the bombardment in the 3-4Gy period can be explained in the accretion tail scenario. This hypothesis would also explain why the Moon appears so depleted in HSEs relative to the Earth. We also extend our analysis of the cataclysm and accretion tail scenarios to the case of Mars. The accretion tail scenario requires a global resurfacing event on Mars ~4.4Gy ago, possibly associated with the formation of the Borealis basin, and it is consistent with the HSE budget of the planet.
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