Publication Date:
2020-12-17
Description:
We report a comprehensive study by the UV spectrograph LAMP onboard the Lunar Reconnaissance Orbiter to map the spatial distribution and temporal evolution of helium atoms in the lunar exosphere, via spectroscopy of the HeI emission line at 58.4 nm. Comparisons with several Monte Carlo models show that lunar exospheric helium is fully thermalized with the surface (accommodation coefficient of 1.0). LAMP-derived helium source rates are compared to the flux of solar wind alpha particles measured in situ by the ARTEMIS twin spacecraft. Our observations confirm that these alpha particles (He++) are the main source of lunar exospheric helium, representing 79% of the total source rate, with the remaining 21% presumed to be outgassing from the lunar interior. The endogenic source rate we derive, (1.49 ± 0.08) · 106 cm-2s-1, is consistent with previous measurements but is now better constrained. LAMP-constrained exospheric surface densities present a dawn/dusk ratio of ∼1.8, within the value measured by the Apollo 17 surface mass spectrometer LACE. Finally, observations of lunar helium during three Earth’s magnetotail crossings, when the Moon is shielded from the solar wind, confirm previous observations of an exponential decay of helium with a time constant of 4.5 days.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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