Reiner Gamma Swirl: A Remarkable Science And (Partially?) Safe-Haven Nearside Site
Resolving key science questions surrounding Reiner Gamma (and other lunar swirls) may open new, beneficial options for long-term human presence on the Moon. Insufficient information is derived from orbit, however, and in situ analyses are required. Lunar swirls are enigmatic wispy light-dark albedo features that have no inherent topography, are independent of local composition, but are intimately associated with strong local magnetic anomalies [e.g., 1]. A ‘mini-magnetosphere’ that deflects solar wind particles and perhaps concentrates them nearby has been detected from orbit for a prominent swirl on the lunar farside . Since solar wind (SW) interaction with the surface is believed to produce the surficial OH/H2O observed pervasively across the Moon, the ‘mini-magnetosphere’ is predicted to inhibit the production of OH at swirls (and perhaps concentrate it elsewhere). Indeed, near-infrared spectra of multiple swirls obtained from orbit document lower OH for the high albedo component [3, 4, 5], confirming the ‘mini-magnetosphere’ SW protective link for swirls in general. The origin of lunar swirls is unknown. The association with strong magnetic anomalies is particularly perplexing. Hypotheses range from processes that occurred early in lunar history [e.g., 6] when a lunar dynamo was active , as a product of the basin forming epoch [e.g., 8], or as a result of recent comet interactions at 1 AU [e.g, 9]. The interwoven cause(s) and effect(s) of swirls with their local magnetic properties can only be studied very close to or on the surface. Not only are swirls important lunar-specific science targets, but their unique ‘mini-magnetosphere’ environment hints these locations might provide some advantage for long-term human activities on the Moon. Example Science/Exploration Questions: a) What are the magnitude, scale, and depth of the magnetic anomalies on the surface? What are their origins? b) How and why does the regolith differ across dark-light regions and surrounding soil? What is the dust environment above the surface? Are diurnal variations observed? c) How does the solar wind flux at the surface vary with location and magnetic field strength? How is it affected by Earth’s magneto-tail and solar wind plasma pressure? d) What is the spatial pattern of OH across the surface and local magnetic conditions? Are there temporal time-of-day variations in OH abundance? Is OH concentrated in the ‘dark lanes’? e) What is the radiation environment across a swirl and does it vary? A strategy to address these questions as a precursor to potential long-term human exploration activities can be performed robotically on or near the surface. Potential approaches range from simple (cubesat) exploratory missions to more complete surface documentation (long-term MER-like rover). Although always desirable, sample return is not immediately necessary. The scale of swirls such as Reiner Gamma (in smooth basaltic terrain) allows many of the above questions to be addressed in a ~15 km traverse during one lunar day, but being able to survive the night and complete a ~150 km traverse is obviously a better strategy (see figures). References:  Blewett, D. T., et al., (2011), J. Geophys. Res., 116, E02002.  Wieser, et al., (2010) Geophys. Res. Lett., 37, L05103.  Kramer, G. Y., et al. (2011), J. Geophys. Res., 116, E00G18.  Pieters, C. M. and I. Garrick-Bethell (2015), Lunar Planet. Sci. 46, 2120.  Li, S. and R. Milliken (2017) Sci. Adv. 3, e1701471  Wieczorek et al., (2012) Science 335, 1212-1215.  Weiss, B. P. and S. M. Tikoo (2014), Science, 346.  Hood, L. L. et al. (2013), J. Geophys. Res. Planets, 118, 1265–1284,  Bruck Syal and Schultz (2015) Icarus 257: 194-206 Figure 1: LRO WAC mosaic of the Reiner Gamma swirl region obtained under high sun conditions. The entire region is smooth basaltic terrain with no topography except local impact craters and the Marius Hills volcanic complex in the NE corner. Figure 2: LRO shaded relief mosaic of the same Reiner Gamma region illustrating the featureless smooth basaltic terrain of the region and the Marius Hills complex. Figure 3: Same Reiner Gamma region with superimposed magnetic field strength measured from orbit by Lunar Prospector in 1998-99. Example traverses (white lines) are shown for a multi-lunar-day mission that would map the surface properties across Reiner Gamma swirl and many of the wispy outlying swirl areas.