Icarus 215 (2011) 186–196
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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, 11100 Johns Hopkins Road, MP3-E169, Laurel, MD 20723-6099, USA
School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
c
Boston University Center for Remote Sensing, Boston, MA 02215, USA
b
a r t i c l e
i n f o
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
a b s t r a c t
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.
! 2011 Elsevier Inc. All rights reserved.
1. Introduction
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 differen⇑ Corresponding author.
E-mail address: [email protected] (C.D. Neish).
0019-1035/$ - see front matter ! 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.icarus.2011.06.037
tial 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
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Table 1
Summary of past radar observations of lunar swirls.
Swirl
Wavelength
Resolution
Observation
Reference
Reiner Gamma
X-Band
(3.8 cm)
S-Band
(12.6 cm)
P-Band
(70 cm)
>1 km
No radar enhancement
Zisk et al. (1974)
20 m
Radar enhancement in central region
Campbell et al. (2006)
>1 km
No radar enhancement
Thompson (1974)
X-Band
(3.8 cm)
P-Band
(70 cm)
>1 km
Radar enhancement
Zisk et al. (1972)
>1 km
No radar enhancement
Zisk et al. (1972)
Descartes highlands
<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, FeTiO3), 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 3.
2. 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.
2.1. 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
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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, 54–
61"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 !50–150 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
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Relative Number of Pixels
1.2
1.0
Inside
0.8
Outside
a
0.6
0.4
0.2
0.0
0.1
0.2
0.3
0.4
Total Backscattered Power
0.5
Relative Number of Pixels
1.2
1.0
Inside
0.8
Outside
0.6
0.4
0.2
0.0
0.0
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 ± 0.011 versus 0.090 ± 0.003, determined for an area of
!600 km2). 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 km2), 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 XBand 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 XBaseline image to two overlapping LRO NAC pairs (Fig. 6). A NAC
b
0.2
0.4
0.6
0.8
1.0
1.2
1.4
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
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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 XBand 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.
2.2. 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 ± 0.36. For comparison, the normalized reflectance in the
Clementine 750 nm mosaic is 34% higher inside the swirl than outside (0.147 ± 0.013 versus 0.110 ± 0.007, determined for an area of
!200 km2). 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.
2.3. 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
C.D. Neish et al. / Icarus 215 (2011) 186–196
191
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.
2.4. 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 30–38"S, 160–170"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
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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
1.2
1.0
Inside
0.8
Outside
0.6
0.4
0.2
0.0
0.1
0.2
0.3
0.4
Total Backscattered Power
1.2
Relative Number of Pixels
a
1.0
Inside
0.8
Outside
0.5
b
0.6
0.4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
CPR
1.0
1.2
1.4
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.
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
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.
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
C.D. Neish et al. / Icarus 215 (2011) 186–196
193
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 18.5–25.5"S, 120–125"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 (GarrickBethell 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
194
C.D. Neish et al. / Icarus 215 (2011) 186–196
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, 14–18"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
C.D. Neish et al. / Icarus 215 (2011) 186–196
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 40þ16
"12 Ma for 4 km craters at X-Band
(Thompson et al., 1981), and for 730þ690
"270 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).
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