Mathematics – Logic
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmgp34a..04g&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #GP34A-04
Mathematics
Logic
[1517] Geomagnetism And Paleomagnetism / Magnetic Anomalies: Modeling And Interpretation, [5440] Planetary Sciences: Solid Surface Planets / Magnetic Fields And Magnetism, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
Bright swirl-shaped regions on the Moon have remained as one of the most enigmatic lunar geologic features. Generally, lunar swirls have high albedo, low optical maturity, and often exhibit dark, optically mature lanes that interweave with brighter areas. Swirls are associated with the strongest lunar magnetic anomalies, some of which are antipodal to large basins. No completely satisfactory model for swirl formation has emerged. The most promising model suggests that the magnetic anomalies associated with swirls stand off the solar wind and prevent darkening of the underlying soil [1]. Under this model, dark lanes are produced by enhanced fluxes of deflected protons. Because the focusing width of these deflected protons is limited by the proton gyroradius, we can use the width of dark lanes to estimate the near-surface magnetic field at lunar swirls. At Reiner Gamma swirl the width of some lanes approach ~500 m. Assuming a gyroradius of 250 m and typical solar wind parameters, the near-surface fields must be ~12,000 nT, which is >35 times the strongest field measured by the Apollo surface magnetometers. If we model the 12,000 nT fields as due to near-surface magnetized disks with size scales similar to those of the dark lanes, their required magnetization is >20 A/m. However, the most magnetic lunar samples measured have a magnetization of only ~1 A/m. Therefore, Reiner Gamma is either associated with unusually magnetic near-surface material, or the solar wind standoff hypothesis and its dark lane formation mechanism is incomplete. Either of these conclusions is interesting, but we will assume the latter scenario. We do so in part because the standoff model has other difficulties. For example, magnetic fields do not shield out micrometeoroids, which also darken lunar soil. Furthermore, we have identified unique spectral properties of swirl interiors. Plots of band strength (950/750 nm) vs. albedo (750 nm) form parallel clusters, but they are continuously displaced in both band strength and albedo as the distance from the swirl increases. This pattern is not expected for changes in weathering state, but is similar to what is observed across mare units of different composition, suggesting swirls are not due to solar wind weathering differences, at least as we presently understand the process. An alternative hypothesis to the standoff model is motivated by the observation that lunar crustal magnetic anomalies produce weak electric fields when they interact with the solar wind [2]. The electric field is produced by charge separation due to the differential penetration of electrons and protons into the magnetic field. Such electric fields may be responsible for moving fine, electrostatically lofted dust [3] within swirl regions, thereby changing the spectral properties of the swirl. We modeled this process based on the electric field strengths inferred from charged particle measurements at the Apollo 12 and 14 magnetic anomalies, and found that small but significant amounts of fine dust may have moved out of swirl regions continuously for billions of years. This hypothesis can be further tested by better spectral and topographic data. [1] Hood, L.L. & Williams, C.R. (1989) Proc. Lunar Sci. Conf. 19th, 99. [2] Neugebauer, M. et al. (1972) Planet. Space. Sci. 20, 1577. [3] Criswell, D.R. (1972) Proc. Lunar Sci. Conf. 3rd, 2671.
Garrick-Bethell Ian
Head James W.
Pieters Carlé M.
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