Characterization of Lunar Swirls as a Function of Magnetic Anomaly Strength: Evidence for Cause and Effect

Objectives: The objective of this proposal is to characterize the similarities and differences in the regolith properties of lunar swirls. These similarities and differences can be used to help distinguish between the three formation mechanisms for lunar swirls. Lunar swirls are sinuous high- and low-reflectance regions associated with localized magnetic anomalies [1,2]. The three hypothesis explaining their origin include: (1) shielding from solar wind irradiation by the magnetic anomaly which impedes one of the space weathering processes active on the lunar surface [3,4], (2) sorting of soil grains through electromagnetic dust levitation which would preferentially accumulate feldspar-rich dust [5] or small grains with a lower component of nanophase iron [6], and (3) formation from cometary impacts in which the remanent magnetic field is generated by the impact event [7].

Methods: Each of the formation processes would produce different regolith properties. In the shielding hypothesis, those regions that are shielded would contain a lower quantity of nanophase iron, the grains would have thinner radiation-damage produced rims (affecting scattering properties), and there would be no size sorting of regolith components. In the electromagnetic transport hypothesis, there would be a lower quantity of grains with nanophase iron, radiation-damaged rims would be ubiquitous between the high- and low-reflectance regions, and there would be evidence of size sorting with a larger component of fine grains. In the cometary impact hypothesis, there would be no difference in nanophase iron content, radiation-damaged rims would be ubiquitous between the high- and low-reflectance regions, and there would be no evidence of size sorting within the regolith. These regolith properties can be characterized using a combination of photometric and spectroscopic modeling. Using Hapke’s model the differences in the scattering properties of the regolith can be quantified [8,9]. Spectral mixing models can identify the size and quantity of submicroscopic/nanophase components [10]. Using lunar shape models to derived incidence, emission, and phase angles that account for topography, Lunar Reconnaissance Orbiter (LRO) color images, and Moon Mineralogy Mapper (M3), we will apply Hapke’s model to characterize the scattering properties of the various components of the swirl regions. Using spectral mixing models we will examine the quantity, size, and composition of the regolith constituents. We will use these methods to characterize and map similarities and differences between various swirl regions across the lunar surface.

Relevance: Understanding the formation mechanism for lunar swirls and the processes that associate them with magnetic anomalies is key to understanding the Earth-Moon system and the processes that create and modify the lunar surface.

References: [1] Hood, L.L. and Schubert, G., (1980) Science, 208, 49–51. [2] Hood, L.L. and Williams, C.R., (1989) LPSC 19, 99–113. [3] Halekas, J.S. et al. (2008) PSS, 56, 941–946. [4] Wieser M., et al. (2010) GRL, 37, L05103. [5] Garrick-Bethell, I. et al. (2011) Icarus, 212, 480–492. [6] Pieters, C.M. et al., (2014) LPSC 45, Abs. 1408. [7] Schultz, P.H. and Srnka, L.J., (1980) Nature, 284, 22–26. [8] Hapke, B., (2012) Theory of Reflectance and Emittance Spectroscopy, 2nd. Cambridge University Press, Cambridge, UK. [9] Hapke, B., (2012) Icarus 221, 1079–1083. [10] Lucey, P. and Riner, M., (2012) Icarus 212.