Icarus 215 (2011) Contents lists available at ScienceDirect. Icarus

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1 Icarus 215 (2011) Contents lists available at ScienceDirect Icarus journal homepage: www. elsevier. com/ locate/ icarus The surficial nature of lunar swirls as revealed by the Mini-RF instrument C.D. Neish a,, D.T. Blewett a, D.B.J. Bussey a, S.J. Lawrence b, M. Mechtley b, B.J. Thomson c, The Mini-RF Team a Johns Hopkins University Applied Physics Laboratory, Johns Hopkins Road, MP3-E169, Laurel, MD , USA b School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA c Boston University Center for Remote Sensing, Boston, MA 02215, USA a r t i c l e i n f o abstract Article history: Received 22 December 2010 Revised 22 June 2011 Accepted 25 June 2011 Available online 3 July 2011 Keywords: Moon Moon, Surface Radar observations Lunar swirls are optically bright, sinuous albedo features found on the Moon. The Mini-RF synthetic aperture radar on the Lunar Reconnaissance Orbiter has provided a comprehensive set of X- and S-Band radar images of these enigmatic features, including the first radar observations of swirls on the farside of the Moon. A few general remarks can be made about the nature of the lunar swirls from this data set. First, the average radar properties of lunar swirls are identical to nearby non-swirl regions, in both total radar backscatter and circular polarization ratio (CPR). This implies that average centimeter-scale roughness and composition within the high-albedo portions of the swirls do not differ appreciably from the surroundings, and that the high optical reflectance of the swirls is related to a very thin surface phenomenon (less than several decimeters thick) not observable with X- or S-Band radar. Secondly, bright swirl material appears to be stratigraphically younger than a newly discovered impact melt flow at Gerasimovich D. This observation indicates that the swirls are capable of forming over timescales less than the age of the crater. The Mini-RF data set also provides clues to the origin of the lunar swirls. In at least one case, the presence of an enhanced crustal magnetic field appears to be responsible for the preservation of a highalbedo ejecta blanket around an otherwise degraded crater, Descartes C. The degree of degradation of Descartes C suggests it should not be optically bright, yet it is. This implies that the enhanced albedo is related to its location within a magnetic anomaly, and hence supports an origin hypothesis that invokes interaction between the solar wind and the magnetic anomaly. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Corresponding author. address: catherine.neish@jhuapl.edu (C.D. Neish). Lunar swirls are optically bright, sinuous albedo features observed in both the maria and highlands of the Moon (e.g., El-Baz, 1972; Schultz and Srnka, 1980; Hood and Williams, 1989; Richmond et al., 2003; Blewett et al., 2011). Though generally bright in appearance, lunar swirls often contain lanes of darker material within their looping patterns. Lunar swirls appear to overlay the lunar surface, superposed on top of craters and ejecta deposits, apparently representing diffuse brightening (or darkening) of unmodified terrains (Schultz and Srnka, 1980). The type example the Reiner Gamma Formation is easily distinguished against the dark mare basalt of Oceanus Procellarum (McCauley, 1967). Lunar swirls have been observed in a number of other locales on the Moon, on both the nearside and the farside, in both mare and highland settings (e.g., Schultz and Srnka, 1980; Hood and Williams, 1989; Blewett et al., 2011). Lunar swirls tend to be associated with regions of anomalously high crustal magnetic fields (e.g., Hood et al., 1981). This relationship has led some to speculate that the swirls result from differential space weathering (Hood and Schubert, 1980; Hood and Williams, 1989; Hood et al., 2001). In this scenario, the magnetic anomaly protects the surface from solar-wind ion bombardment, suppressing the soil darkening caused by exposure to the space environment, and making the swirls appear brighter than the surrounding, unshielded surface. Micrometeoroid bombardment is considered to be another agent of space weathering (e.g., Hapke, 2001), so the solar-wind shielding hypothesis requires that solarwind sputtering and/or implantation is a necessary component of space weathering, since micrometeoroids are not affected by the presence of a crustal magnetic anomaly. A more recent idea postulates that the magnetic anomalies associated with the swirls lead to the formation of electric fields, due to the differential penetration of solar wind electrons and protons into the magnetic field (Garrick-Bethell et al., 2011). These electric fields could then selectively attract or repel fine, feldspar-rich electrostatically levitated lunar dust, creating the characteristic bright and dark lanes of the lunar swirls. Still others speculate that the swirls are remnants of collisions with a cometary coma, disrupted comet fragments, or comet-related meteor swarms (Schultz and Srnka, 1980; Pinet et al., 2000; Starukhina and Shkuratov, 2004). In these comet impact-related scenarios, the impact is proposed to expose fresh material from the top /$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi: /j.icarus

2 C.D. Neish et al. / Icarus 215 (2011) Table 1 Summary of past radar observations of lunar swirls. Swirl Wavelength Resolution Observation Reference Reiner Gamma X-Band >1 km No radar enhancement Zisk et al. (1974) (3.8 cm) S-Band 20 m Radar enhancement in central region Campbell et al. (2006) (12.6 cm) P-Band >1 km No radar enhancement Thompson (1974) (70 cm) Descartes highlands X-Band >1 km Radar enhancement Zisk et al. (1972) (3.8 cm) P-Band >1 km No radar enhancement Zisk et al. (1972) (70 cm) <1 m of the lunar surface, enhancing the albedo of the area without causing a major change in the topography. Lunar swirls have been previously studied at visible and nearinfrared (NIR) wavelengths (e.g., Bell and Hawke, 1981, 1987; Pinet et al., 2000; Blewett et al., 2007, 2011). At these wavelengths, swirls are seen to exhibit spectral characteristics similar to immature material (e.g., Blewett et al., 2011). Optically mature regolith tends to be both redder and darker than optically immature regolith, attributed to a greater abundance of nanometer-sized particles of metallic iron that are produced during impact- and sputteringinduced melting and vaporization (e.g., Pieters et al., 1993; Hapke, 2001). Swirls are also observed to have anomalous photometric properties, which diverge from the general anticorrelation between phase function steepness and albedo (Kreslavsky and Shkuratov, 2003; Kaydash et al., 2009). However, the longstanding notion that the Reiner Gamma swirl is strongly forward-scattering (Schultz and Srnka, 1980) has recently been challenged (Opanasenko and Shkuratov, 2004). The photometric function of a regolith is generally linked to regolith properties including a surface s microtexture (e.g., Hapke, 1993). Complementary information can be obtained by observing lunar swirls at radar wavelengths. Radars use an active source to probe the lunar surface at wavelengths of several centimeters to several meters, yielding information about the topography, roughness, and composition of the reflecting surface. Unlike visible-nir remote sensing, radar is capable of probing the near subsurface, to depths roughly 10 times the radar wavelength (Campbell and Campbell, 2006). The relative importance of surface and sub-surface contributions to radar backscatter varies with wavelength. Radar backscatter at shorter wavelengths (X-Band, or 3.8 cm) tends to be dominated by centimeter-sized surface rocks, whereas larger blocks and buried rocks become progressively more important as the wavelength increases to S-, L-, and P-Band (12 cm, 24 cm, and 70 cm, respectively). One useful indicator of surface roughness is the circular polarization ratio (CPR). This value is defined as the ratio of the backscattered power in the same-sense of circular polarization that was transmitted (SC) to the backscattered power in the opposite-sense circular polarization (OC). When an incident circularly polarized radar wave is backscattered off an interface, the polarization state of the wave changes. Thus flat, mirror-like surfaces, dominated by single-bounce reflections, tend to have high OC returns and low CPR values. Rough surfaces, dominated by multiple-bounce reflections, tend to have approximately equal OC and SC returns, with CPR values approaching unity. The backscattered power is also sensitive to composition. Campbell et al. (1997) showed that increasing the loss tangent of the fine fraction of the lunar regolith lowers the observed radar backscatter. Higher titanium abundances in the lunar maria (in the form of the mineral ilmenite, FeTiO 3 ), for example, are correlated with lower radar echo strength (Schaber et al., 1975; Campbell et al., 1997). Several lunar swirls have been observed with Earth-based radars (see Table 1), but there has not yet been a comprehensive, quantitative study of their radar properties. In addition, Earthbased radars are unable to view swirls on the lunar farside, such as the major occurrences found in Mare Ingenii and near the crater Gerasimovich. To obtain a global radar view of the Moon, an orbiting radar is required. The Forerunner Mini-SAR on Chandrayaan-1 (operated from November 2008 to August 2009) was the first spaceborne SAR to orbit the Moon. It carried a single band (12.6 cm) synthetic aperture radar, and focused mostly on polar observations, returning only limited equatorial data (Spudis et al., 2010). The Mini-RF synthetic aperture radar (SAR) (Nozette et al., 2010), which has successfully operated on the Lunar Reconnaissance Orbiter (LRO) spacecraft since June 2009, has acquired SAR observations of all known lunar swirls. Mini-RF can acquire SAR data at both 12.6 cm (S-Band) and 4.2 cm, at spatial resolutions of 30 m (zoom) and 150 m (baseline). (The latter wavelength is formally C-Band, but is usually referred to as X-band for historical reasons.) Mini-RF gives us the opportunity to survey and quantify the radar properties of the lunar swirls, and compare them with the surrounding surface. In this work, we focus on SAR observations of the four strongest magnetic anomalies on the Moon, which are also areas of prominent swirls: Reiner Gamma, Mare Ingenii, Gerasimovich crater, and the Descartes highlands. We determine the radar properties of the high albedo swirl patterns and compare these observations to optical images acquired by the Clementine and LRO spacecraft. We report on those observations in Section 2, and discuss their implications in Section Observations Mini-RF observed areas of prominent swirls at various locations on the Moon. We report on these observations, focusing on comparisons between X- and S-Band SAR data taken by the Mini-RF instrument onboard LRO, the 750-nm optical data acquired by the Clementine UV Vis camera, for which a global lunar mosaic is available (Eliason et al., 1999), and the LRO Narrow Angle Camera (NAC, Robinson et al., 2010), for targeted, high-resolution imagery. We seek to determine whether there is any correlation between optically bright areas of the swirls and radar backscatter. We also consider the CPR of the lunar swirls, to determine the relative roughness of the swirls for comparison with nearby areas that do not have anomalous optical albedo patterns Reiner Gamma Formation Located in Oceanus Procellarum, near 7 N, 59 W, the Reiner Gamma Formation is one of the most prominent lunar swirls on the Moon (Whitaker, 1969). It consists of a bright, elliptical central region, approximately 30 by 60 km in extent, with filamentary

3 188 C.D. Neish et al. / Icarus 215 (2011) Fig. 2. Close-up of the central portion of Reiner Gamma, as viewed (a) at 750-nm by Clementine and (b) in total Mini-RF backscattered power at S-Band. A portion of the swirl is outlined in both images. There is no obvious change in radar properties outside the swirl relative to the optically bright central portion. White dashed lines indicate the region shown in Fig. 3. Fig. 1. The Reiner Gamma swirls, as viewed (a) at 750-nm by Clementine, and by Mini-RF (b) in total backscattered power at S-Band (S1), and (c) the circular polarization ratio at S-Band (overlaid on S1). The CPR values span the color spectrum from purple through red for values between 0 and 1.2. CPR values greater than 1.2 are assigned the color red. White lines outline an area within the swirls (bottom) and outside of the swirls (top). The image spans the range 4 11 N, W in simple cylindrical projection. extensions to the northeast and southwest of the central region (Fig. 1a). The bright central region also contains a lane of dark material concentric to the edge of the bright region. Reiner Gamma was previously imaged with Earth-based radar at X-Band (Zisk et al., 1974), P-Band (Thompson, 1974), and S-Band (Campbell et al., 2006). No significant radar enhancements were observed in the low resolution (several kilometers) X- (3.8 cm) and P-Band (70 cm) data. In higher resolution (20 m) S-Band (12.6 cm) data, Campbell et al. (2006) noted an area of higher SC radar return generally correlated with the bright central portion of Reiner Gamma, but found no evidence for variations associated with its more filamentary high-albedo portions. In the Mini-RF S-Band observations of Reiner Gamma, we note the same area of higher radar return as seen by Campbell et al. (2006), but find that it is not correlated with the bright, central swirl region (Fig. 2). The enhancement in radar backscatter extends both north and south of Reiner Gamma, and appears to be correlated with an increase in the density of small, fresh craters (of size range m) visible in both the radar and NAC images (see Fig. 3). Young craters appear bright in radar due to their rough, blocky ejecta blankets and rough crater walls. The presence of these small craters suggests that the observed increase in radar backscatter observed by Campbell et al. (2006) at Reiner Gamma is due to local changes in crater density seemingly unrelated to the swirl. It has been suggested that the mare surface at Reiner Gamma has been affected by rays or secondaries from the craters Cavalerius or Glushko (formerly named Olbers A) (Hood et al., 1979; Bell and Hawke, 1981, 1987). The clusters of small craters revealed by the NAC images (and seen as diffuse radar brightening in the Mini-RF data) therefore could be related to Cavalerius, Glushko, or other more distant impacts. We compared the radar backscatter at Reiner Gamma to a similarly sized area north of the swirl (Fig. 1), and found that the radar brightening seen at Reiner Gamma is modest at best, representing only a 15% increase (Fig. 4a). Note that the off-swirl region lies outside of the magnetic anomaly, at a field strength of <3 nt, compared to a maximum of 20 nt on-swirl (two-dimensionally filtered field strengths of those measured at 20 km altitude, Hood

4 C.D. Neish et al. / Icarus 215 (2011) Relative Number of Pixels Inside Outside a Total Backscattered Power 1.2 Relative Number of Pixels Inside Outside b Fig. 3. (a) LROC NAC and (b) Mini-RF S-band zoom-mode image of the central portion of Reiner Gamma. Areas of diffuse radar brightening are seen to correlate with an increased density of small craters (arrows). Fresh craters appear bright in radar due to their rough, blocky ejecta blankets. et al., 2001). The normalized reflectance in the Clementine 750-nm mosaic, on the other hand, is 42% higher inside the swirl than outside (0.128 ± versus ± 0.003, determined for an area of 600 km 2 ). The CPRs in the two regions are virtually identical (0.48 ± 0.28 inside the swirl versus 0.49 ± 0.28 outside the swirl, determined for an area of 600 km 2 ), indicating that the roughness of the two areas is, on average, the same on the scale of centimeters to decimeters (Fig. 4b). We have also obtained a limited amount of Mini-RF X-Band data over Reiner Gamma. With a wavelength one-third that of Mini-RF s S-Band, this data is most sensitive to centimeter-scale surface roughness. Calibration observations for the X-Band were only recently acquired, and analysis and development of the X- Band calibration procedure are currently underway. However, a preliminary calibration has been implemented for all data presented here. Two X-Band strips overlap the Reiner Gamma Formation: one X-Baseline strip (at 150 m resolution) and one X-Zoom strip (at 30 m resolution) (Fig. 5). The beam width at X-Band is somewhat narrower than that at S-Band, causing the decrease in intensity seen at the edges of the strips. Nonetheless, we can comment on the radar backscatter near the center of the strips. There is no qualitative change in radar backscatter inside the swirl compared to outside the swirl in the X-Zoom strip, which runs through the center of Reiner Gamma. However, the X-Baseline strip that runs through the western edge of Reiner Gamma is noticeably brighter in the northern portion of the swirl compared to the southern portion of the swirl (indicated by arrows in Fig. 5). To determine the cause of this asymmetry, we compared the X- Baseline image to two overlapping LRO NAC pairs (Fig. 6). A NAC CPR Fig. 4. (a) Histogram of total S-band radar backscatter values and (b) circular polarization ratios for an area inside (black) and outside (gray) the central portion of the Reiner Gamma swirl. The interior and exterior regions of interest are outlined in white in Fig. 1. The radar brightening seen at Reiner Gamma is modest at best, representing only a 15% increase, while the CPRs in the two regions are virtually identical, indicating that the roughness of the two areas is, on average, the same on the scale of centimeters to decimeters. Fig. 5. Mini-RF X-Baseline (left) and X-Zoom (right) radar strips that overlap the Reiner Gamma swirls, seen here at 750-nm by Clementine. A white arrow indicates the bright area in the northern half of Reiner Gamma, while a black arrow indicates the dark area in the southern half of Reiner Gamma. pair taken at low solar incidence angle (34 ) shows the albedo variations in this region. The optical albedo seems to roughly correlate with the brightening/darkening at X-Band, but there are regions where this correlation breaks down (indicated by arrows in Fig. 6a). This argues against a common cause for the enhancement in the optical albedo and the enhancement in the radar

5 190 C.D. Neish et al. / Icarus 215 (2011) Fig. 6. (a) Low incidence angle (34 ) NAC pair overlapping the X-Baseline radar strip of Fig. 5, displayed over the Clementine basemap for context. Arrows indicate regions where optical albedo and radar backscatter do not correlate in intensity. (b) High incidence angle (82 ) NAC pair overlapping the X-Baseline radar strip, displayed over the Clementine basemap for context. An arrow indicates a region of increased X-Band radar backscatter that corresponds to a region of increased crater density, as seen in the NAC image. backscatter. An alternative explanation for the asymmetric radar brightness is suggested by a NAC pair taken at high incidence angle (82 ), which highlights the topographic relief in this region. There appears to be a higher crater density in the northern half of the swirl compared to the southern half of the swirls. Ejecta from craters is rough on the scale of centimeters, and would cause an enhancement in X-Band radar. (For example, see the region marked by an arrow in Fig. 6b.) We conducted a crater count of two regions just east of the X- Band strip, in the NAC pair shown in Fig. 6b. For craters with diameters D > 75 m, we find a crater density of 4.3 craters km 2 in the northern radar bright region and a density of 3.1 craters km 2 in the southern radar dark region. This represents a 40% increase in crater density. For comparison, the X-Band backscatter in the northern bright region is 70% higher than the X-Band backscatter in the southern dark region. Note that this NAC pair does not exactly overlap the X-Band radar strip, so the crater density in the area of the X-Band strip may differ from that presented here. However, it seems reasonable that an increased crater density may be causing the enhanced X-Band backscatter seen in the northern half of Reiner Gamma Mare Ingenii The largest areal distribution of magnetic anomalies (and corresponding swirl features) on the Moon is centered in Mare Ingenii (Schultz and Srnka, 1980; Hood et al., 2001). Mare Ingenii is located on the farside of the Moon, near the antipode of the Imbrium basin (34 S, 164 E). As with Reiner Gamma, bright swirl features show up clearly against the dark mare background (Fig. 7a). Due to its location on the farside of the Moon, Mare Ingenii has never before been imaged by radar. Mini-RF provides the first opportunity to collect data on the radar properties of the mare and its swirls. The initial images returned by Mini-RF show no obvious change in radar properties over the optically bright swirls in Mare Ingenii. Examining adjacent swirl and non-swirl mare regions (Figs. 7b and 8), we find equivalent average values for total backscattered power and circular polarization ratio (Fig. 9). The average CPR inside one prominent swirl is 0.69 ± 0.40; in a similarly sized mare region just northwest of that swirl, it is 0.70 ± For comparison, the normalized reflectance in the Clementine 750 nm mosaic is 34% higher inside the swirl than outside (0.147 ± versus ± 0.007, determined for an area of 200 km 2 ). Similar to the case of Reiner Gamma, the bulk composition and centimeter to decimeter-scale roughness of the areas with anomalous optical albedo appear to be the same as that of the surrounding, ordinary regolith Gerasimovich The strongest magnetic anomaly yet mapped on the Moon is located near Gerasimovich, an 86-km diameter, degraded crater located near the Crisium antipode (23 S, 123 W) (Hood et al., 2001; Richmond et al., 2005; Richmond and Hood, 2008; Blewett et al., 2011). This magnetic anomaly is correlated with bright, swirl-like albedo markings (Fig. 10a). Like Mare Ingenii, Gerasimovich crater is located on the farside of the Moon, and Mini-RF has provided the first radar images of the area. When examined at S-Band, the most prominent feature in the area of the Gerasimovich swirls is a rough (high radar backscatter, high CPR) tongue that extends westward 1 crater radius from the rim of the 26-km diameter crater Gerasimovich D (Fig. 10). We interpret this feature as an impact melt flow. The flow is all but invisible in the high-sun Clementine mosaic of the same area

6 C.D. Neish et al. / Icarus 215 (2011) Unfortunately, the age of this crater is not well constrained, so we cannot place a robust upper limit on the timescale of swirl formation. Initial geologic maps of the area suggested a relatively old age for Gerasimovich D, Upper Imbrian, which would make it >3 Ga old (Wilhelms, 1987). Using Galileo multispectral images of the Moon, McEwen et al. (1993) argued that its color and albedo are more consistent with that of a Copernican-aged crater. They assigned an age of older Copernican for Gerasimovich D, indicating that the crater is <1 Ga old, but older than the crater Kepler. More recently, Grier et al. (2001) assigned relative ages for large craters (D > 20 km) on the Moon based on radial profiles of the optical maturity parameter (OMAT, Lucey et al., 2000). In general, they found that fresh craters have large OMAT values at the rim, which diminish steeply with distance from the crater, while older craters have low OMAT values at the rim and profiles that become indistinguishable from the background relatively close to the crater. They assigned Gerasimovich D an age of older than Copernicus (810 Ma). However, in areas of high magnetic fields (such as the lunar swirls), the maturation process may differ from that experienced by the average lunar surface (see, for example, Blewett et al., 2005, 2011). Solar-wind shielding may preserve high (fresh) OMAT values, making a crater appear younger than a crater of the same age that underwent normal space weathering. Further, the atypical weathering within a magnetic anomaly could maintain high OMAT values in the background regolith, and hence distort the shape of the crater s OMAT profile. These two effects would tend to cause the crater s age to be misclassified by the method used by Grier et al. (2001). In summary, the age of Gerasimovich D is uncertain, but age dating this crater perhaps through crater counting on the impact melt would put an upper limit on the formation time for the swirl Descartes anomaly Fig. 7. The Mare Ingenii swirls, as viewed (a) at 750-nm by Clementine, and by Mini-RF (b) in total backscattered power at S-Band (S1), and (c) the circular polarization ratio at S-Band (overlaid on S1). The CPR color assignments are as in Fig. 1c. White lines enclose an area within the swirls (bottom) and outside of the swirls (top). The image spans the range S, E in simple cylindrical projection. The white dashed box indicates the region shown in Fig. 6. (Fig. 11). In fact, bright swirl material is observed over part of the western edge of the impact melt feature, and appears to be stratigraphically younger than the flow. This observation suggests that the swirls are capable of forming over a timescale less than the age of the crater itself. There is a region of unusually high albedo in the Descartes highlands, located near (11 S, 16 E), 50 km south of the Apollo 16 landing site (Milton, 1968). This optical brightening occurs mainly between the craters Dollond E and Descartes C (Fig. 12a). Blewett et al. (2007) described the Descartes albedo anomaly as an endmember in the continuum of lunar swirl-like markings: a simple diffuse bright area. Like other lunar swirls, the albedo marking is co-located with a crustal magnetic anomaly, the strongest on the nearside of the Moon (Halekas et al., 2001; Richmond et al., 2003; Blewett et al., 2011). Blewett et al. (2005) concluded that the high albedo observed in this region is caused only by maturity differences with the surroundings, and not by an exotic composition, such as highland volcanic material, as suggested by Head and Goetz (1972). This conclusion is consistent with the hypotheses for swirl formation that require interaction of the solar wind with a crustal magnetic field. However, the location of the albedo anomaly between two relatively fresh craters makes it difficult to determine if the anomaly is related to magnetic shielding effects, or rather is caused by an asymmetric set of bright crater rays, emanating from Dollond E and Descartes C. The Descartes region has previously been observed with ground-based radars. Zisk et al. (1972) noted an area of enhanced X-Band (3.8 cm) radar backscatter correlated with the albedo anomaly, but no enhancement at P-Band (70 cm). This observation is indicative of a surface and near-subsurface with an excess of centimeter-sized rocks, consistent with the anomaly s location between two small craters. The Mini-RF S-Band radar backscatter data offer longer wavelength data at higher spatial resolution than the older Earth-based data. The Mini-RF images of the Descartes albedo anomaly do not show noticeable enhancement in radar backscatter, except for the area immediately around the rim of crater

7 192 C.D. Neish et al. / Icarus 215 (2011) Fig. 8. Close-up of two prominent swirls in Mare Ingenii as viewed (a) at 750-nm by Clementine and (b) in total backscattered power at S-Band. The swirls are outlined in both images. There is no obvious difference in radar properties between the optically bright portions of the swirl and the background mare surface. Relative Number of Pixels Relative Number of Pixels Total Backscattered Power Inside Outside CPR Inside Outside Fig. 9. (a) Histogram of total radar backscatter values and (b) circular polarization ratios for an area inside (black) and outside (gray) a prominent swirl in Mare Ingenii. The interior and exterior regions of interest are outlined in white in Fig. 7. There is essentially no difference between the radar properties of the optically bright swirl and the surrounding, ordinary regolith. a b E. If the albedo anomaly is simply the result of two (a) fresh and (b) asymmetric ejecta blankets emanating from Descartes C and Dollond E, it should be reflected in the radar data. To the contrary, the Mini-RF radar images show the ejecta surrounding Dollond E to be symmetric, so the anomalous bright region southeast of Dollond E cannot be explained by an asymmetry in the impact process. More importantly, Descartes C exhibits very little enhancement in backscattered power or CPR (Fig. 12), indicating that it does not possess a fresh ejecta blanket. Descartes C appears to be an older, more degraded crater than Dollond E. This interpretation is supported by a comparison between Descartes C and a crater just to the north, Dollond MB. In radar backscatter and CPR, Dollond MB has a fresher appearance than Descartes C. One would therefore predict that its ejecta blanket would appear optically brighter than the one surrounding Descartes C. However, this is not the case; the optical albedo at Dollond MB is lower than that at Descartes C. The optical anomaly must therefore have another explanation. For example, the cause of the albedo anomaly may be ejecta from the fresher Dollond E crater that has been magnetically shielded from space weathering, or could be a result of levitated dust that has been preferentially deposited between the two craters. Indeed, when the magnetic field magnitude is superposed on the optical data (Fig. 12d), we find that the field maximum (Lon Hood, personal communication) occurs just south of Dollond E, perhaps explaining why only the ejecta south of the crater remains optically bright. (Note that the field magnitude shown in Fig. 12d was produced using the direct-mapping method outlined in Hood (2011), at an altitude of 34 km.) The Mini-RF results presented here cannot test the formation of swirls by collisions of gas and dust within a cometary coma, the fall of fragments of a low-density cometary nucleus, or a meteoroid swarm. Previous studies (Schultz and Srnka, 1980; Pinet et al., 2000; Starukhina and Shkuratov, 2004) predict that these events will result in erosion and deposition at the finest scales, affecting only the upper few millimeters of the lunar surface. Such changes would not be observable with X- or S-Band radar. Dollond E (Fig. 12). This region of high radar backscatter and CPR is clearly associated with a fresh ejecta blanket, with evidence for rays extending radially away from the crater. The Mini-RF data therefore provide new information on the origin of the optically bright region between Descartes C and Dollond 3. Discussion The Mini-RF synthetic aperture radar on LRO has collected a comprehensive set of X- and S-Band radar observations for four prominent lunar swirls. It has also provided the first radar

8 C.D. Neish et al. / Icarus 215 (2011) Fig. 10. The area of the Gerasimovich magnetic anomaly and corresponding high-albedo markings, as viewed (a) at 750-nm by Clementine, and by Mini-RF (b) in total backscattered power at S-Band (S1), and (c) the circular polarization ratio at S-Band (overlaid on S1). The CPR color assignments are the same as Fig. 1c. A white arrow indicates the edge of a newly discovered impact melt flow emanating from Gerasimovich D. The image spans the range S, W in simple cylindrical projection. The dotted box indicates the region shown in Fig. 11. Fig. 11. Close-up of a newly discovered impact melt flow originating at the rim of Gerasimovich D, as viewed (a) by Clementine at 750-nm and (b) by Mini-RF in total backscattered power overlaid by CPR at S-Band. The CPR color assignments are the same as Fig. 1c. The white arrow indicates the southern edge of the flow, which is overprinted by a high albedo optical swirl. observations of swirls located on the farside of the Moon (Mare Ingenii, Gerasimovich), and the first quantitative comparisons of the radar properties between on-swirl and off-swirl regions. Considering the data presented here, a few general observations can be made about the nature of the lunar swirls. First, the average radar properties of lunar swirls are essentially identical to those of nearby non-swirl regions, in both total radar backscatter and circular polarization ratio at S-Band. This suggests that average centimeter to decimeter-scale roughness and composition within the high-albedo portions of the swirls do not differ appreciably from the surroundings. On the Moon there is often a correlation between high optical albedo and high radar backscatter and CPR: fresh crater ejecta are bright (optically) and rough (as sensed by radar). However, the lunar swirls do not adhere to this pattern. Instead, the lunar swirls are optically bright but have no corresponding enhancement in radar backscatter at X-Band (4.2 cm) or S-Band wavelength (12.6 cm). This lack of a prominent radar signature suggests the swirls are a very thin surface phenomenon (less than several decimeters thick) not observable with X- or S-Band radar. Second, as previously noted (Schultz and Srnka, 1980), swirls appear to overprint the terrain, including recent geologic features. For example, the newly discovered impact melt flow at Gerasimovich D is covered by a high-albedo swirl. This observation indicates that the swirls are capable of forming over timescales less than the age of this crater (which is currently poorly constrained). The data set presented here also provides information about the origin of the lunar swirls. In at least one case, the presence of an enhanced crustal magnetic field appears to be responsible for the preservation of a high-albedo ejecta blanket around an otherwise degraded crater, Descartes C. The degree of degradation of Descartes C implies it should not be optically bright, yet it is. This suggests that the enhanced albedo is related to its location within a magnetic anomaly, and is consistent with the preservation of high albedo being caused by atypical space weathering (e.g., Hood and Schubert, 1980) or dust levitation (Garrick-Bethell et al., 2011). There are several distinguishing differences between the solar shielding and the dust levitation models. For example, the dust levitation model predicts the presence of a substantial layer (8 40 cm) of fine dust (<10 lm) at the swirls if they are 4 Gyr old (Garrick- Bethell et al., 2011). Pyroclastic soils, which are also fine-grained (40 lm; Heiken et al., 1974), appear radar dark if they are sufficiently thick (on the order of meters for S-Band radar, and decimeters for X-Band radar; Carter et al., 2009). However, the lack of any perceivable radar darkening at the swirls may not rule out the dust leviation hypothesis, as impact gardening would tend to mix rocks in

9 194 C.D. Neish et al. / Icarus 215 (2011) Fig. 12. The Descartes albedo anomaly, as viewed (a) by Clementine at 750-nm, and (b) by Mini-RF in total backscattered power at S-Band (S1), and (c) the circular polarization ratio at S-Band (overlaid on S1). The CPR color assignments are the same as Fig. 1c. (d) The magnetic field magnitude (B-total) superposed on the Clementine 750 nm data. Contours were produced using the direct mapping method of Hood (2011) at an altitude of 34 km, with a contour interval of 1 nt. The craters Dollond E (DE), Descartes C (DC), and Dollond MB (DMB) are labeled. The image spans the range 9 13 S, E in simple cylindrical projection. with the dust layer over time, causing it to appear like normal regolith, especially if the deposits are relatively thin. However, a decimeter thick dust layer would alter the thermal inertia of the swirls compared to background values. Initial reports by the Diviner radiometer on LRO indicate no difference in thermophysical properties within and outside Reiner Gamma, indicating the surfaces have similar physical textures (Glotch et al., 2010). This is seemingly inconsistent with the dust levitation hypothesis. Measurements of particle fluxes and electric fields at the surface would also help to discriminate between the two hypotheses. It is also possible that exposure of fresh material through a cometary impact or meteoroid swarm could account for the optical brightening. However, we cannot comment on this formation mechanism, as these events are predicted to produce only fine-scale erosion and deposition (less than several millimeters), not observable with X- or S-Band radar. An important question regarding the solar-wind shielding model for the origin of lunar swirls relates to the possible role of crater ejecta input in refreshing these high-albedo surfaces. The correlation between magnetic anomalies and basin antipodes (Lin et al., 1988; Hood et al., 2001; Richmond et al., 2005, Mitchell et al., 2008), suggests that the major magnetic anomalies were emplaced at the time of basin formation. If this is the case, then the magnetic anomalies date to roughly 3.8 Gyr. But are the swirls visible today also 3.8 Gyr old? The swirls observed at Gerasimovich D would suggest that this is not the case, and it seems likely that ejecta from younger outside craters would be occasionally deposited within the magnetically shielded area, allowing the surface to be optically refreshed (Hood et al., 2001). As discussed by Blewett et al. (2011), such refreshment could mean that the degree of solar-wind shielding required to maintain the visibility of a swirl is less than that needed in the absence of the addition of outside material. The lack

10 C.D. Neish et al. / Icarus 215 (2011) of recent ejecta input could explain why no unusual albedo markings are found at several weak and moderate strength magnetic anomalies, whereas other areas of weak or moderate fields do host prominent swirls (Blewett et al., 2011). This observation is not well explained by the dust levitation model, since the proposed brightening agent (fine-grained dust) is continually being created and matured in a steady-state equilibrium, and should presumably operate in a similar manner at all magnetic anomalies. Therefore, it is of interest to assess the degree to which deposition of ray material or formation of secondary craters within a crustal magnetic anomaly has occurred, and to examine whether this ejecta input is associated with the optically bright portions of a swirl. The sensitivity of radar backscatter to surface roughness offers the opportunity to make such an assessment. Fresh craters tend to have bright radar haloes, representing ejecta yet to be substantially weathered by micrometeorite bombardment. These haloes tend to last for 40 þ16 12 Ma for 4 km craters at X-Band (Thompson et al., 1981), and for 730 þ Ma for 4 km craters at S-Band (Bell et al., 2011). There are no bright halo craters near the swirls examined at Reiner Gamma or Mare Ingenii, suggesting either that these albedo anomalies did not require refreshment, or that any refreshment they experienced was not recent. Note that both regions are considered strong magnetic anomalies (field strength of magnetic anomaly at 30-km altitude >15 nt) by Blewett et al. (2011), so recent refreshment may not be necessary. On the other hand, the albedo anomaly in the Descartes highlands may be an example of very near-by refreshment: Dollond E formed immediately adjacent to the magnetic anomaly, depositing fresh, bright ejecta there. A useful test of this hypothesis would be to determine if radar-bright halo craters are preferentially found near moderate and weak magnetic anomalies that exhibit high-albedo swirls (for example Airy and Firsov), but not those anomalies that do not exhibit swirls (for example Hartwig and Stöfler). We plan to explore this hypothesis in future work. 4. Conclusions In summary, we see no evidence of enhanced radar backscatter or CPR in the swirl regions, indicating that there is no increase in centimeter- to decimeter-scale surface roughness within swirls compared to the normal background. We also see no radar evidence for bulk compositional differences between swirl and nonswirl regions, as is observed in some areas of high ilmenite content elsewhere on the Moon. Indeed, the radar backscatter seems relatively unchanged within the swirls. Instead, the swirls appear to be surficial in nature, most consistent with a very thin surface phenomenon (less than several decimeters thick) that lacks a strong signature in X- or S-Band radar. Acknowledgments We thank the LRO project for their efforts in returning the data presented here. Thanks also to M. Robinson for helpful comments on the manuscript, and to L. Hood for his careful review. This work was supported by the LRO project, under contract with NASA, and by NASA Planetary Geology and Geophysics program Grants to D.T.B. (NNX08AL53G and NNX09AQ06G). References Bell, J.F., Hawke, B.R., The Reiner Gamma Formation Composition and origin as derived from remote sensing observations. Proc. Lunar Sci. Conf. 12, Bell, J.F., Hawke, B.R., Recent comet impacts on the Moon The evidence from remote sensing studies. Publ. Astron. Soc. Pacific 99, Bell, S.W., Thomson, B.J., Dyar, M.D., Bussey, D.B.J., Dating fresh lunar craters with Mini-RF. Proc. Lunar Sci. Conf. XLII. 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