Near-ultraviolet and near-infrared space-weathering trends at mare and highland swirls

Introduction: Solar-wind ion bombardment is one of the agents responsible for altering the optical properties of materials that are exposed on the surfaces of airless bodies. Micrometeoroid impacts are also thought to play an important role in such optical alteration, which is termed "space weathering" [e.g., 1, 2], although the relative contribution to space weathering by the solar wind and micrometeoroids is not fully understood and can vary from place to place in the Solar System [e.g., 3] and on the Moon [4, 5]. Typical lunar space weathering involves introduction of a strong positive ("red") spectral slope from the visible to near-infrared (NIR), an overall decrease in reflectance, and weakening of Fe2+ absorption bands near 1000 and 2000 nm. The effects of lunar-style space weathering in the ultraviolet (UV) were described by Hendrix and Vilas [6]. The near-UV (NUV) slope at wavelengths shortward of 415 nm is steep in fresh material and shallows (becomes "bluer") in more mature material [6, 7]. The optical alteration is caused by the accumulation of submicroscopic blebs and coatings of iron (SMFe) on and within regolith particles [1, 8]. These NIR and NUV effects are illustrated in Fig. 1, using laboratory spectra for lunar soils and crushed lunar rocks (from RELAB and also Wagner et al. [9]). We plot the NIR ratio (1550-nm/700-nm, a measure of continuum slope) vs. the NUV ratio (321nm/415-nm). Fig. 1 shows that soils have steep (red) NIR continua and shallow (blue) NUV ratios. The reverse is true of the rocks, which have shallow NIR continua but steep NUV slopes. However, the low-Fe highland samples (black triangles) show little variation in the NUV ratio as a function of maturity [7]. Lunar Swirls and Anomalous Weathering: Areas of magnetized crustal rocks (magnetic anomalies) have been mapped globally by Lunar Prospector [e.g., 10, 11], and magnetic anomalies are known to decrease the flux of solar-wind ions reaching the lunar surface [e.g., 12–14]. Hence, it might be expected that a magnetically shielded area could experience atypical space weathering [15]. Indeed, the unusual high-albedo markings called lunar swirls [e.g., 16–20] are all collocated with magnetic anomalies. We have undertaken an analysis of spectral trends in order to gain further insight into the nature and origin of swirls and the causes of lunar space weathering. Spacecraft Observations: Prior studies have examined lunar swirls with data from the Clementine UVVis camera [e.g., 4, 18, 19, 21–23], the Clementine NIR camera [19], M3 [24–26], Kaguya Multiband Imager (MI) [23], and in the far-UV using LAMP [27, 28]. LROC NUV images have proven to be especially useful for mapping swirls [7, 20]. We perform an analysis similar to that of Hemingway et al. [4], who defined regions of interest (ROIs) on and around the high-reflectance portions of mare swirls in Clementine images in order to characterize the behavior in terms of the 950-nm/750-nm reflectance ratio and 750-nm reflectance. Those workers found that the swirl maturity trajectory in this spectral space differs from the trend in non-swirl surfaces. We use the NIR continuum ratio (1550-nm/700-nm, from topographically corrected Kaguya MI [25]) to focus on the effects of SMFe on continuum slope. This ratio is favored over the 950nm/750-nm ratio, which is a function of both the continuum slope and the strength of the 1000-nm band. We simultaneously examine swirls in the NUV using LROC WAC [26]. Results: Figs. 2 and 3 demonstrate that separate normal and anomalous (swirl) trends are evident in the NUV and NIR for the Reiner Gamma (RG) swirl. Bright swirl pixels lie along trends that are shallower than that of the mature background, attesting to unusual optical alteration within the swirl. The swirl pixels also have higher NIR and NUV ratios than predicted for normal (fresh) material of the same albedo.