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Evidence for impact melt sheets in lunar highland smooth plains and implications for polar landing sites
Accepted Manuscript
Evidence for Impact Melt Sheets in Lunar Highland Smooth Plains and Implications for Polar Landing Sites Bruce A. Campbell , Catherine M. Weitz , Jennifer L. Whitten , Gareth A. Morgan PII: DOI: Reference:
S0019-1035(18)30113-1 10.1016/j.icarus.2018.05.025 YICAR 12913
To appear in:
Icarus
Received date: Revised date: Accepted date:
24 February 2018 9 May 2018 25 May 2018
Please cite this article as: Bruce A. Campbell , Catherine M. Weitz , Jennifer L. Whitten , Gareth A. Morgan , Evidence for Impact Melt Sheets in Lunar Highland Smooth Plains and Implications for Polar Landing Sites, Icarus (2018), doi: 10.1016/j.icarus.2018.05.025
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ACCEPTED MANUSCRIPT 1 Highlights:
Smooth plains units of basin ejecta occur over much of the lunar highlands
Many craters 4-20 km in diameter on smooth plains have high interior radar CPR
Melt-rich layers supply fragmental material that is exposed on these crater walls
A melt-rich model for smooth plains has implications for the nature of landing sites
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ACCEPTED MANUSCRIPT 2 Evidence for Impact Melt Sheets in Lunar Highland Smooth Plains and Implications for Polar Landing Sites
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Gareth A. Morgan Planetary Science Institute 1700 East Fort Lowell, Suite 106 Tucson, AZ 85719-2395
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Jennifer L. Whitten Center for Earth and Planetary Studies Smithsonian Institution, MRC 315 PO Box 37012 Washington, DC 20013-7012
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Submitted as a Note to Icarus, 2018
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Catherine M. Weitz Planetary Science Institute 1700 East Fort Lowell, Suite 106 Tucson, AZ 85719-2395
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Bruce A. Campbell Center for Earth and Planetary Studies Smithsonian Institution, MRC 315 PO Box 37012 Washington, DC 20013-7012
ACCEPTED MANUSCRIPT 3 Abstract. Smooth plains units attributed to fluidized basin ejecta occur over much of the lunar highlands. We examined 4-20 km diameter Imbrian- and Eratosthenian-period craters in the southern highlands that have high 12.6-cm wavelength radar backscatter and circular polarization ratios associated with their interior walls but not with their
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proximal ejecta. Of these craters, about 70% occur on or very near deposits of Imbrian highland plains. We propose that melt-rich ejecta from basin-forming impact events, near the top of the stratigraphic section, supplies fragmental material that is exposed in the interior wall regolith of these small craters. These results support previous interpretations
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of 70-cm radar data, which provide insight on the near-surface morphology and rock provenance in proposed smooth plains landing sites such as that of Chandrayaan-2.
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I. INTRODUCTION
The Moon’s polar regions are of considerable interest for future exploration, both for
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the scientific value of areas far from earlier landings (e.g, the most southerly to date, by Surveyor VII, lies on the ejecta blanket of Tycho crater) and the potential for some
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degree of water ice and other volatiles within the regolith. In the south, many low-lying
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areas of crater floors and the surrounding highlands are mantled by light-toned plains units generally attributed to a gas-fluidized or lava-like mixture of solid particles and
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melt from the ejecta of large basins. Eggleton and Schaber (1972) initially proposed this model for the “Cayley Formation” deposits, and noted that any given plains area might comprise overlapping layers from a succession of basin-forming events. Howard et al. (1974) mapped the smooth plains and reinforced the model that Imbrium and Orientale ejecta form many of these units. Wilhelms et al. (1979) describe them as “primary and
ACCEPTED MANUSCRIPT 4 secondary ejecta of Orientale and Imbrium basins and of craters” (their unit Ip), and note that an older, more degraded facies (unit Ntp) is likely due to similar deposits of Nectarian-period basins. More recent analyses of high-resolution images show that a large fraction of the lunar light plains are associated with either the Orientale or Imbrium
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impact events (Meyer et al., 2016, 2018). Because these plains areas are reasonable locations for future landings, it is important to understand their surface and shallowsubsurface properties.
Analysis of 70-cm wavelength Earth-based radar data showed that south polar craters
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with Imbrian smooth plains in their floors (e.g., Amundsen, Shoemaker) host small (fewhundred-meter diameter) craters with anomalously bright 70-cm wavelength radar backscatter. The source of the decimeter-scale and larger rocks in the small-crater ejecta
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was proposed to be near-surface layers rich in impact melt, which yields more fragmental debris during an impact than a deep highland regolith (Campbell and Campbell, 2006).
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The correlation was extended to other occurrences of such plains, both in craters and lowlying terra locales, in the southwestern highlands (Ghent et al., 2008). This work implies
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that many smooth plains contain blocky or lava-flow-like deposits of coalesced impact
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melt, rather than a poorly sintered collection of brecciated fragments. In this work, we use 12.6-cm wavelength Earth-based radar data to study another
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aspect of the smooth plains units. Initial studies of small (4-20 km diameter) craters in the northern and southern polar regions revealed a population of Eratosthenian (1.1-3.2 Ga) and Imbrian-period (3.2-3.85 Ga) craters with high backscattered echoes and enhanced circular polarization ratio (CPR) from their interior walls. While these attributes can be linked with massive ice, the signatures occur in both sunlit and permanently shadowed
ACCEPTED MANUSCRIPT 5 areas (Campbell et al., 2010; Campbell, 2012). There is little radar enhancement from their proximal ejecta blankets due to micrometeorite erosion consistent with most craters of this period (Thompson et al., 1981), so the interior radar signatures are unexpected. The question motivating this study is whether the source of the rocky material on the
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crater walls is related to the impact melt deposits proposed by Campbell and Campbell (2006). It is important to note that our work addresses the geologic cross section within the upper few hundred meters of the current surface, rather than the deep mega-regolith properties studied by Thompson et al. (2009).
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We present a survey of radar-bright craters in the southern highlands, using the 12.6cm Earth-based radar data and the USGS 1:5 M scale geologic maps to assess the relationship between the interior wall morphology and the target terrain. Section II
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describes the radar dataset and the USGS maps used to classify target settings. Section III examines polarimetric radar images of craters over a study area in the southern lunar
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highlands, presents the statistical correlation between craters with bright interior walls and smooth-plains units, and discusses the possibility of distinguishing separate
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populations of pre- and post-Orientale craters. Section IV summarizes our results and
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their implications for the nature of highland plains deposits and future exploration. In particular, we link conclusions drawn from the 4-20 km crater study to the nature of
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plains as seen by 70-cm radar mapping. The 70-cm data allow for a more detailed analysis of localized plains deposits, and are used here to characterize two proposed sites for the Chandrayaan-2 rover, which carries instruments to study surface composition and thermal properties.
ACCEPTED MANUSCRIPT 6 II. Datasets and Methodology We surveyed the radar backscatter properties of all craters 4-20 km diameter within a large region of the central southern lunar highlands (Fig. 1), and compare these observations to USGS geologic maps to determine any correlation with particular units.
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To first order, craters excavate to a maximum depth of about 1/10 the final diameter, or 400 m to 2000 m for our survey. All of the radar enhancements observed for the study craters, however, begin very near the interior rim and thus reflect the properties of material within the upper ~100 m or so of the original target section. The radar data come
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from a nearside Earth-based mapping program using the Arecibo Observatory and Green Bank Observatory radio telescopes. The radar wavelength is 12.6 cm (S-band), and the spatial resolution is 80 m per pixel at four looks (Campbell et al., 2010). By transmitting
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a single circular polarization and receiving both senses (same-sense or SC and oppositesense or OC) of the reflected circular polarization, we may derive a well-calibrated value
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of the circular polarization ratio (CPR=SC/OC). In general, higher CPR values indicate greater topographic roughness at the 12.6-cm scale of the radar wavelength, or a larger
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number of few-cm size (greater than about 1/10 the radar wavelength) and larger rocks on
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or near the surface. Very high values (>1) may occur where these rocks or surface roughness create double-bounce geometries for the radar signal (Campbell, 2012).
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The radar offers a different perspective from orbital visible images. For example, images from the Narrow Angle Camera (NAC) on the Lunar Reconnaissance Orbiter can discriminate features 50-100 cm across (Robinson et al., 2010). These images can identify obvious boulder fields on crater walls and the bright streaks of freshly exposed, immature regolith. There is a considerable scale gap, however, between meter-scale
ACCEPTED MANUSCRIPT 7 boulders and the few-cm diameter rocks that can scatter the 12.6-cm signal. In addition, the radar signal penetrates a meter (at least 10 radar wavelengths) or more in typical highland materials, which have much lower dielectric loss than mafic mare deposits (Carrier et al., 1992). It is thus possible for the radar to observe enhanced echoes and
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CPR values without a clear surface boulder field, as noted in detections of regolithmantled boulders in the Marius Hills (Campbell et al., 2009).
The southern highlands are comprised primarily of heavily cratered pre-Nectarian, Nectarian, and Imbrian-period basin deposits with varying morphology. The maps used
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to characterize the background terrain of each radar-bright crater come from the USGS 1:5 M scale lunar geologic mapping program, which date from 1966 to 1974 and cover latitudes down to 68o S. In general, all of these maps identify highland plains units of
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similar morphology and albedo, but their respective geologic unit names differ: “Ip” in Offield (1971), Pohn (1972), Karlstrom (1974), and Saunders and Wilhelms (1974); “Ips,
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Ipt, Ipc, pIp, or IpIp” in Cumming (1972), Rowan (1971), Mutch and Saunders (1972), Titley (1967), Trask and Titley (1966), and Scott (1972). In their regional map, Wilhelms
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et al. (1979) incorporated all such units into a model of basin ejecta distribution rather
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than the earlier highland volcanism interpretation, grouping plains units as Imbrian (unit Ip) or Nectarian age (unit Ntp). We also separate craters based on the nature of their floor
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materials, some of which are also mapped as Imbrian plains.
III. Results The youngest group of craters are mapped as Copernican period within the USGS quadrangle maps. These craters have optically bright rays, rocky material exposed across
ACCEPTED MANUSCRIPT 8 much of their interiors, and their wall slopes remain steep. They also have strongly enhanced 12.6-cm radar echoes from their proximal ejecta, consistent with abundant rocks in the upper meter or two of the regolith (i.e., at least 10 times the radar wavelength for a low-loss material). These craters offer little guidance on variations in target
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properties, because much of the near-surface rock population still represents the original crater-forming event. In contrast, Eratosthenian and Imbrian period craters exhibit more subdued rims, lower wall slopes, and only modest radar backscatter enhancements from the proximal ejecta blanket (Thompson et al., 1981). Radar echoes from their interior
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walls are enhanced where terraces are exposed, but in general these areas have lower CPR than in a Copernican crater. Our survey focuses on craters of these earlier periods that have enhanced radar backscatter and high CPR from their interior walls.
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We identified 106 Eratosthenian or Imbrian 4-20 km craters with high-CPR wall deposits within the study region (Fig. 1). Craters of this older population are typified by
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Zagut A and Rabbi Levi L (Fig. 2). Selecting the craters with “enhanced” CPR is subjective, but values of 0.6 or more are typical, and these craters are visually distinct in
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the OC images and the CPR (Fig. 2a). LROC-NAC images (Figs. 2b-c) show broad
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boulder fields and immature exposed material starting just below the rim of Zagut A, consistent with the high CPR values. In many instances, however, such boulder fields are
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not evident in high resolution visible images over the full areal extent of the high-CPR signatures, suggesting that the rock population may be dominated by smaller diameters or only evident where slumping has removed a thin overlying regolith. Surface rock abundance maps from the Diviner instrument do show enhancements in the interiors of
ACCEPTED MANUSCRIPT 9 Zagut A and Rabbi Levi L, further indicating a population of debris comparable to or smaller than the resolution of the LROC-NAC (Bandfield et al., 2011). The craters are further broken down by: (a) proximity to (on or very near) Imbrium smooth plains units, and (b) presence of smooth plains material in the floor. The resulting
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four categories (Category 1-criterion (a) alone, Category 2-criterion (a) and (b), Category 3-criterion (b) alone, and Category 4-no correlation with floor or surrounding plains material) are shown by color labeling in Fig. 1. The full dataset of crater locations and characteristics are provided in Table SM-1.
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Our results show that 74 of the 106 (70%) craters with high CPR signatures from their interior walls occur near/on Imbrian smooth plains units (Categories 1 and 2). From that 74, 19 craters also have Imbrian plains (usually unit Ip) mapped in their floors (Category
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2). There are 9 additional craters with smooth floor deposits but no evident nearby surrounding plains units (Category 3). The remaining 23 craters, 22% of the high-CPR
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population, have no obvious floor or surrounding Imbrian plains deposits (Category 4). Within the Category 1 population, there is little evidence of any preference for age group;
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Imbrian and Eratosthenian craters are roughly equal in occurrence.
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The interpretation of the 28 craters (Categories 2 and 3) with smooth plains on their floors depends upon the origin of these floor materials. One possibility is that the deposits
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are formed from impact melt of the crater itself, such that Categories 1 and 2 are essentially identical. A second scenario is that these craters represent an early-Imbrian population that had already formed prior to the Orientale and/or Schrödinger impacts, and thus acquired floor deposits of their ejecta (Fassett et al., 2012). In this case, the rugged wall material may be derived from surrounding melt sheets of older Imbrium or
ACCEPTED MANUSCRIPT 10 Nectarian-period basin-forming impacts. The observed craters might reflect a mix of the two scenarios, but the fact that all Category 2 craters are classified as Imbrian would support the scenario of an older, early-Imbrian population. In either case, the degree of association between high-CPR wall deposits and exterior Imbrian plains is again around
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70% (19 out of 28).
There is thus a relatively strong correspondence between the basin-related smooth plains and the persistence of rocky debris on interior walls of 4-20 km diameter craters. This supports the model proposed in Campbell and Campbell (2006) and Ghent et al.
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(2008) that Imbrian plains have a large component of coalesced melt that is similar to lava flows. The 70-cm radar data used in those studies permit more localized analysis of plains properties because the few-hundred-meter diameter craters are much more
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numerous within small areas.
We use the 70-cm data to provide context for two sites proposed for the Chandrayaan-
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2 landing (Amitabh and Srinivasan, 2018), particularly regarding the possible subsurface stratigraphy and the provenance of surface rocks. Figure 3 shows a 70-cm OC radar
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image of the south polar region, with a color overlay of the CPR values. The correlation
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between occurrences of unit Ip from Wilhelms et al. (1979) and higher CPR values is evident. We also note that some Nectarian crater floor deposits (e.g., in Demonax and
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Helmholtz) mapped as unit Ntp exhibit the same CPR signature, while those in Boguslawsky have no enhancement. Wilhelms et al. (1979) do suggest that limits of photographic coverage and quality may have led to mis-categorization of plains unit ages, so we propose that the radar data reflect the correct assignment of deposits like those in Demonax and Helmoltz as the younger unit Ip.
ACCEPTED MANUSCRIPT 11 In the immediate vicinity of the proposed primary Chandrayaan-2 landing site on smooth plains between Simpelius N and Manzinus C, there is a modest enhancement of the 70-cm CPR over that observed for terra regions just to the southeast (Fig. 3). Smooth plains in this area are mapped as Unit Ntp by Wilhelms et al. (1979), but are more similar
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in radar signature to the Ip materials in the floor of Manzinus. A lander here will encounter a modestly greater surface and near-surface rock population relative to the older surrounding terrae, and some of those surface rocks (along with a fraction of the regolith) will reflect the composition of the underlying, basin-derived melt. The second
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proposed site, northeast of Klaproth crater, is on terrain with low 70-cm CPR values associated with fine-grained, distal ejecta of the Moretus radar-dark “halo” (Ghent et al., 2005). These haloes are of considerable interest as they appear to form on Mars as well
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(Ghent et al., 2010), and a landed investigation would provide a first view of their
IV. Conclusions
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composition and small-scale morphology.
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There is a population of small (4-20 km diameter) Eratosthenian- and Imbrian-period
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craters in the lunar highlands that have enhanced radar backscatter and circular polarization ratio on their interior walls, relative to the average backscatter signatures and
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physical erosion of their exterior ejecta blankets. Based on correlations with smooth deposits mapped as basin-derived plains, we conclude that (a) smooth plains may have a large component of dense, coalesced impact melt in the upper few hundred meters, and (b) craters on these deposits retain boulder-strewn interior walls (and perhaps portions of their floors) due to ongoing meteoritic gardening and mass wasting of the rocky layers
ACCEPTED MANUSCRIPT 12 near the top of the walls. The source of rocky wall debris for the ~30% of craters not associated with mapped Imbrian smooth plains is uncertain, but may reflect smaller, unmapped patches of plains or some other feature of the target site. High-CPR craters with plains on their floors were likely emplaced prior to the Orientale impact event, with
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the wall deposits sourced from Imbrium basin melt sheets. These findings support earlier inferences of abundant fragmental material associated with small (few-hundred-meter diameter) craters in highland smooth plains (Campbell and Campbell, 2006; Ghent et al., 2008).
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The nature of deposits encountered in the exploration of highland smooth plains sites, and interpretations of remote sensing data for the walls of associated craters, are thus strongly affected by the role of Imbrium and Orientale melt-rich basin ejecta across the
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lunar southern hemisphere (e.g., Meyer et al., 2016, 2018). Our results also indicate that a more robust discrimination of Imbrian and Nectarian plains units may be derived from
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the radar data. A landed investigation in smooth plains units, such as the proposed primary Chandrayaan-2 site near the south pole (Amitabh and Srinivasan, 2018), may
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encounter more surface and shallow subsurface debris, with more varied provenance,
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than might be expected of a deep highland regolith. The proposed secondary landing area is within the fine-grained “radar-dark halo” of Moretus ejecta, offering a potentially quite
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different setting from the primary site.
Acknowledgments This work was supported in part by a grant to B.A.C from the NASA LASER Program (NNX-13AL17G). The authors thank the staff of the Arecibo Observatory and the Green
ACCEPTED MANUSCRIPT 13 Bank Observatory for their support in collecting the radar data presented here. The 12.6cm radar data are available on the NASA Planetary Data System (http://pdsgeosciences.wustl.edu/missions/sband/index.htm). Radar data at 70-cm wavelength are
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also on the PDS (http://pds-geosciences.wustl.edu/missions/lunar_radar/index.htm).
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Fig. 1. Locations of 4 to 20 km diameter Eratosthenian- and Imbrian-period craters in the southern lunar highlands with high backscatter and CPR from their interior walls. Colorcoded dots indicate: Category 1-green, Category 2-blue, Category 3-orange, Category 4yellow. Star at lower right indicates location of proposed primary Chandrayaan-2 landing
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site; star at center is proposed secondary site. Black outline encloses region covered by
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12.6-cm wavelength radar images used for this survey, with a dashed line to indicate the southern extent of USGS 1:5 M scale maps. S, T, C, and M note location of Schiller,
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basemap.
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Tycho, Cuvier, and Manzinus craters respectively. LOLA-Kaguya 118m/pixel hillshade
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Fig. 2. Remote sensing data for crater Zagut A (Category 1; 11 km diameter) and nearby high-CPR craters Rabbi Levi A (Category 2) and L (Category 1). (a) 12.6-cm wavelength OC radar map with circular polarization ratio as color overlay. Bright, high-CPR oval features along the south rims of Zagut-A and Rabbi-Levi L are due to double-bounce (longer delay time) echoes from the floor and wall of the crater. Locations of LROCNAC images in (b) and (c) shown by arrows. (b) LROC NAC image of northern wall of
ACCEPTED MANUSCRIPT 19 Zagut A. (c) LROC NAC image of southwestern wall of Zagut A. Note the high albedo of exposed immature material and boulder-strewn areas beginning just below the crater
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Fig. 3. Radar map of the south polar region of the Moon at 70-cm wavelength. Polar
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stereographic projection (lower left ~70 S, 105 E; lower right ~68 S, 100 E; upper center ~65 S, 5 E). Color overlay of circular polarization ratio (CPR) value on opposite-sense
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circular polarization image. Examples of mapped smooth plains (Ip, Ntp) from Wilhelms
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et al. (1979) noted. There is a correlation between occurrences of unit Ip (and some Ntp) and enhanced CPR values (e.g., Klaproth crater at upper left). High-CPR signatures from
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the walls and floor of younger (post-Orientale) craters like Moretus and Schomberger reflect near-surface rocks not yet broken down by micrometeorite bombardment. Arrows denote locations of craters Manzinus C (MC) and Simpelius N (SN) that bracket the proposed Chandrayaan-2 primary landing site: Ch-2 (a). The location of the secondary proposed site is noted at upper left: Ch-2 (b).
Evidence for Impact Melt Sheets in Lunar Highland Smooth Plains and Implications for Polar Landing Sites Bruce A. Campbell , Catherine M. Weitz , Jennifer L. Whitten , Gareth A. Morgan PII: DOI: Reference:
S0019-1035(18)30113-1 10.1016/j.icarus.2018.05.025 YICAR 12913
To appear in:
Icarus
Received date: Revised date: Accepted date:
24 February 2018 9 May 2018 25 May 2018
Please cite this article as: Bruce A. Campbell , Catherine M. Weitz , Jennifer L. Whitten , Gareth A. Morgan , Evidence for Impact Melt Sheets in Lunar Highland Smooth Plains and Implications for Polar Landing Sites, Icarus (2018), doi: 10.1016/j.icarus.2018.05.025
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ACCEPTED MANUSCRIPT 1 Highlights:
Smooth plains units of basin ejecta occur over much of the lunar highlands
Many craters 4-20 km in diameter on smooth plains have high interior radar CPR
Melt-rich layers supply fragmental material that is exposed on these crater walls
A melt-rich model for smooth plains has implications for the nature of landing sites
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ACCEPTED MANUSCRIPT 2 Evidence for Impact Melt Sheets in Lunar Highland Smooth Plains and Implications for Polar Landing Sites
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Gareth A. Morgan Planetary Science Institute 1700 East Fort Lowell, Suite 106 Tucson, AZ 85719-2395
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Jennifer L. Whitten Center for Earth and Planetary Studies Smithsonian Institution, MRC 315 PO Box 37012 Washington, DC 20013-7012
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Submitted as a Note to Icarus, 2018
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Catherine M. Weitz Planetary Science Institute 1700 East Fort Lowell, Suite 106 Tucson, AZ 85719-2395
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Bruce A. Campbell Center for Earth and Planetary Studies Smithsonian Institution, MRC 315 PO Box 37012 Washington, DC 20013-7012
ACCEPTED MANUSCRIPT 3 Abstract. Smooth plains units attributed to fluidized basin ejecta occur over much of the lunar highlands. We examined 4-20 km diameter Imbrian- and Eratosthenian-period craters in the southern highlands that have high 12.6-cm wavelength radar backscatter and circular polarization ratios associated with their interior walls but not with their
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proximal ejecta. Of these craters, about 70% occur on or very near deposits of Imbrian highland plains. We propose that melt-rich ejecta from basin-forming impact events, near the top of the stratigraphic section, supplies fragmental material that is exposed in the interior wall regolith of these small craters. These results support previous interpretations
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of 70-cm radar data, which provide insight on the near-surface morphology and rock provenance in proposed smooth plains landing sites such as that of Chandrayaan-2.
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I. INTRODUCTION
The Moon’s polar regions are of considerable interest for future exploration, both for
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the scientific value of areas far from earlier landings (e.g, the most southerly to date, by Surveyor VII, lies on the ejecta blanket of Tycho crater) and the potential for some
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degree of water ice and other volatiles within the regolith. In the south, many low-lying
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areas of crater floors and the surrounding highlands are mantled by light-toned plains units generally attributed to a gas-fluidized or lava-like mixture of solid particles and
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melt from the ejecta of large basins. Eggleton and Schaber (1972) initially proposed this model for the “Cayley Formation” deposits, and noted that any given plains area might comprise overlapping layers from a succession of basin-forming events. Howard et al. (1974) mapped the smooth plains and reinforced the model that Imbrium and Orientale ejecta form many of these units. Wilhelms et al. (1979) describe them as “primary and
ACCEPTED MANUSCRIPT 4 secondary ejecta of Orientale and Imbrium basins and of craters” (their unit Ip), and note that an older, more degraded facies (unit Ntp) is likely due to similar deposits of Nectarian-period basins. More recent analyses of high-resolution images show that a large fraction of the lunar light plains are associated with either the Orientale or Imbrium
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impact events (Meyer et al., 2016, 2018). Because these plains areas are reasonable locations for future landings, it is important to understand their surface and shallowsubsurface properties.
Analysis of 70-cm wavelength Earth-based radar data showed that south polar craters
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with Imbrian smooth plains in their floors (e.g., Amundsen, Shoemaker) host small (fewhundred-meter diameter) craters with anomalously bright 70-cm wavelength radar backscatter. The source of the decimeter-scale and larger rocks in the small-crater ejecta
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was proposed to be near-surface layers rich in impact melt, which yields more fragmental debris during an impact than a deep highland regolith (Campbell and Campbell, 2006).
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The correlation was extended to other occurrences of such plains, both in craters and lowlying terra locales, in the southwestern highlands (Ghent et al., 2008). This work implies
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that many smooth plains contain blocky or lava-flow-like deposits of coalesced impact
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melt, rather than a poorly sintered collection of brecciated fragments. In this work, we use 12.6-cm wavelength Earth-based radar data to study another
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aspect of the smooth plains units. Initial studies of small (4-20 km diameter) craters in the northern and southern polar regions revealed a population of Eratosthenian (1.1-3.2 Ga) and Imbrian-period (3.2-3.85 Ga) craters with high backscattered echoes and enhanced circular polarization ratio (CPR) from their interior walls. While these attributes can be linked with massive ice, the signatures occur in both sunlit and permanently shadowed
ACCEPTED MANUSCRIPT 5 areas (Campbell et al., 2010; Campbell, 2012). There is little radar enhancement from their proximal ejecta blankets due to micrometeorite erosion consistent with most craters of this period (Thompson et al., 1981), so the interior radar signatures are unexpected. The question motivating this study is whether the source of the rocky material on the
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crater walls is related to the impact melt deposits proposed by Campbell and Campbell (2006). It is important to note that our work addresses the geologic cross section within the upper few hundred meters of the current surface, rather than the deep mega-regolith properties studied by Thompson et al. (2009).
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We present a survey of radar-bright craters in the southern highlands, using the 12.6cm Earth-based radar data and the USGS 1:5 M scale geologic maps to assess the relationship between the interior wall morphology and the target terrain. Section II
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describes the radar dataset and the USGS maps used to classify target settings. Section III examines polarimetric radar images of craters over a study area in the southern lunar
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highlands, presents the statistical correlation between craters with bright interior walls and smooth-plains units, and discusses the possibility of distinguishing separate
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populations of pre- and post-Orientale craters. Section IV summarizes our results and
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their implications for the nature of highland plains deposits and future exploration. In particular, we link conclusions drawn from the 4-20 km crater study to the nature of
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plains as seen by 70-cm radar mapping. The 70-cm data allow for a more detailed analysis of localized plains deposits, and are used here to characterize two proposed sites for the Chandrayaan-2 rover, which carries instruments to study surface composition and thermal properties.
ACCEPTED MANUSCRIPT 6 II. Datasets and Methodology We surveyed the radar backscatter properties of all craters 4-20 km diameter within a large region of the central southern lunar highlands (Fig. 1), and compare these observations to USGS geologic maps to determine any correlation with particular units.
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To first order, craters excavate to a maximum depth of about 1/10 the final diameter, or 400 m to 2000 m for our survey. All of the radar enhancements observed for the study craters, however, begin very near the interior rim and thus reflect the properties of material within the upper ~100 m or so of the original target section. The radar data come
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from a nearside Earth-based mapping program using the Arecibo Observatory and Green Bank Observatory radio telescopes. The radar wavelength is 12.6 cm (S-band), and the spatial resolution is 80 m per pixel at four looks (Campbell et al., 2010). By transmitting
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a single circular polarization and receiving both senses (same-sense or SC and oppositesense or OC) of the reflected circular polarization, we may derive a well-calibrated value
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of the circular polarization ratio (CPR=SC/OC). In general, higher CPR values indicate greater topographic roughness at the 12.6-cm scale of the radar wavelength, or a larger
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number of few-cm size (greater than about 1/10 the radar wavelength) and larger rocks on
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or near the surface. Very high values (>1) may occur where these rocks or surface roughness create double-bounce geometries for the radar signal (Campbell, 2012).
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The radar offers a different perspective from orbital visible images. For example, images from the Narrow Angle Camera (NAC) on the Lunar Reconnaissance Orbiter can discriminate features 50-100 cm across (Robinson et al., 2010). These images can identify obvious boulder fields on crater walls and the bright streaks of freshly exposed, immature regolith. There is a considerable scale gap, however, between meter-scale
ACCEPTED MANUSCRIPT 7 boulders and the few-cm diameter rocks that can scatter the 12.6-cm signal. In addition, the radar signal penetrates a meter (at least 10 radar wavelengths) or more in typical highland materials, which have much lower dielectric loss than mafic mare deposits (Carrier et al., 1992). It is thus possible for the radar to observe enhanced echoes and
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CPR values without a clear surface boulder field, as noted in detections of regolithmantled boulders in the Marius Hills (Campbell et al., 2009).
The southern highlands are comprised primarily of heavily cratered pre-Nectarian, Nectarian, and Imbrian-period basin deposits with varying morphology. The maps used
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to characterize the background terrain of each radar-bright crater come from the USGS 1:5 M scale lunar geologic mapping program, which date from 1966 to 1974 and cover latitudes down to 68o S. In general, all of these maps identify highland plains units of
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similar morphology and albedo, but their respective geologic unit names differ: “Ip” in Offield (1971), Pohn (1972), Karlstrom (1974), and Saunders and Wilhelms (1974); “Ips,
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Ipt, Ipc, pIp, or IpIp” in Cumming (1972), Rowan (1971), Mutch and Saunders (1972), Titley (1967), Trask and Titley (1966), and Scott (1972). In their regional map, Wilhelms
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et al. (1979) incorporated all such units into a model of basin ejecta distribution rather
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than the earlier highland volcanism interpretation, grouping plains units as Imbrian (unit Ip) or Nectarian age (unit Ntp). We also separate craters based on the nature of their floor
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materials, some of which are also mapped as Imbrian plains.
III. Results The youngest group of craters are mapped as Copernican period within the USGS quadrangle maps. These craters have optically bright rays, rocky material exposed across
ACCEPTED MANUSCRIPT 8 much of their interiors, and their wall slopes remain steep. They also have strongly enhanced 12.6-cm radar echoes from their proximal ejecta, consistent with abundant rocks in the upper meter or two of the regolith (i.e., at least 10 times the radar wavelength for a low-loss material). These craters offer little guidance on variations in target
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properties, because much of the near-surface rock population still represents the original crater-forming event. In contrast, Eratosthenian and Imbrian period craters exhibit more subdued rims, lower wall slopes, and only modest radar backscatter enhancements from the proximal ejecta blanket (Thompson et al., 1981). Radar echoes from their interior
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walls are enhanced where terraces are exposed, but in general these areas have lower CPR than in a Copernican crater. Our survey focuses on craters of these earlier periods that have enhanced radar backscatter and high CPR from their interior walls.
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We identified 106 Eratosthenian or Imbrian 4-20 km craters with high-CPR wall deposits within the study region (Fig. 1). Craters of this older population are typified by
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Zagut A and Rabbi Levi L (Fig. 2). Selecting the craters with “enhanced” CPR is subjective, but values of 0.6 or more are typical, and these craters are visually distinct in
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the OC images and the CPR (Fig. 2a). LROC-NAC images (Figs. 2b-c) show broad
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boulder fields and immature exposed material starting just below the rim of Zagut A, consistent with the high CPR values. In many instances, however, such boulder fields are
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not evident in high resolution visible images over the full areal extent of the high-CPR signatures, suggesting that the rock population may be dominated by smaller diameters or only evident where slumping has removed a thin overlying regolith. Surface rock abundance maps from the Diviner instrument do show enhancements in the interiors of
ACCEPTED MANUSCRIPT 9 Zagut A and Rabbi Levi L, further indicating a population of debris comparable to or smaller than the resolution of the LROC-NAC (Bandfield et al., 2011). The craters are further broken down by: (a) proximity to (on or very near) Imbrium smooth plains units, and (b) presence of smooth plains material in the floor. The resulting
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four categories (Category 1-criterion (a) alone, Category 2-criterion (a) and (b), Category 3-criterion (b) alone, and Category 4-no correlation with floor or surrounding plains material) are shown by color labeling in Fig. 1. The full dataset of crater locations and characteristics are provided in Table SM-1.
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Our results show that 74 of the 106 (70%) craters with high CPR signatures from their interior walls occur near/on Imbrian smooth plains units (Categories 1 and 2). From that 74, 19 craters also have Imbrian plains (usually unit Ip) mapped in their floors (Category
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2). There are 9 additional craters with smooth floor deposits but no evident nearby surrounding plains units (Category 3). The remaining 23 craters, 22% of the high-CPR
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population, have no obvious floor or surrounding Imbrian plains deposits (Category 4). Within the Category 1 population, there is little evidence of any preference for age group;
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Imbrian and Eratosthenian craters are roughly equal in occurrence.
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The interpretation of the 28 craters (Categories 2 and 3) with smooth plains on their floors depends upon the origin of these floor materials. One possibility is that the deposits
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are formed from impact melt of the crater itself, such that Categories 1 and 2 are essentially identical. A second scenario is that these craters represent an early-Imbrian population that had already formed prior to the Orientale and/or Schrödinger impacts, and thus acquired floor deposits of their ejecta (Fassett et al., 2012). In this case, the rugged wall material may be derived from surrounding melt sheets of older Imbrium or
ACCEPTED MANUSCRIPT 10 Nectarian-period basin-forming impacts. The observed craters might reflect a mix of the two scenarios, but the fact that all Category 2 craters are classified as Imbrian would support the scenario of an older, early-Imbrian population. In either case, the degree of association between high-CPR wall deposits and exterior Imbrian plains is again around
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70% (19 out of 28).
There is thus a relatively strong correspondence between the basin-related smooth plains and the persistence of rocky debris on interior walls of 4-20 km diameter craters. This supports the model proposed in Campbell and Campbell (2006) and Ghent et al.
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(2008) that Imbrian plains have a large component of coalesced melt that is similar to lava flows. The 70-cm radar data used in those studies permit more localized analysis of plains properties because the few-hundred-meter diameter craters are much more
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numerous within small areas.
We use the 70-cm data to provide context for two sites proposed for the Chandrayaan-
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2 landing (Amitabh and Srinivasan, 2018), particularly regarding the possible subsurface stratigraphy and the provenance of surface rocks. Figure 3 shows a 70-cm OC radar
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image of the south polar region, with a color overlay of the CPR values. The correlation
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between occurrences of unit Ip from Wilhelms et al. (1979) and higher CPR values is evident. We also note that some Nectarian crater floor deposits (e.g., in Demonax and
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Helmholtz) mapped as unit Ntp exhibit the same CPR signature, while those in Boguslawsky have no enhancement. Wilhelms et al. (1979) do suggest that limits of photographic coverage and quality may have led to mis-categorization of plains unit ages, so we propose that the radar data reflect the correct assignment of deposits like those in Demonax and Helmoltz as the younger unit Ip.
ACCEPTED MANUSCRIPT 11 In the immediate vicinity of the proposed primary Chandrayaan-2 landing site on smooth plains between Simpelius N and Manzinus C, there is a modest enhancement of the 70-cm CPR over that observed for terra regions just to the southeast (Fig. 3). Smooth plains in this area are mapped as Unit Ntp by Wilhelms et al. (1979), but are more similar
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in radar signature to the Ip materials in the floor of Manzinus. A lander here will encounter a modestly greater surface and near-surface rock population relative to the older surrounding terrae, and some of those surface rocks (along with a fraction of the regolith) will reflect the composition of the underlying, basin-derived melt. The second
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proposed site, northeast of Klaproth crater, is on terrain with low 70-cm CPR values associated with fine-grained, distal ejecta of the Moretus radar-dark “halo” (Ghent et al., 2005). These haloes are of considerable interest as they appear to form on Mars as well
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(Ghent et al., 2010), and a landed investigation would provide a first view of their
IV. Conclusions
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composition and small-scale morphology.
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There is a population of small (4-20 km diameter) Eratosthenian- and Imbrian-period
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craters in the lunar highlands that have enhanced radar backscatter and circular polarization ratio on their interior walls, relative to the average backscatter signatures and
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physical erosion of their exterior ejecta blankets. Based on correlations with smooth deposits mapped as basin-derived plains, we conclude that (a) smooth plains may have a large component of dense, coalesced impact melt in the upper few hundred meters, and (b) craters on these deposits retain boulder-strewn interior walls (and perhaps portions of their floors) due to ongoing meteoritic gardening and mass wasting of the rocky layers
ACCEPTED MANUSCRIPT 12 near the top of the walls. The source of rocky wall debris for the ~30% of craters not associated with mapped Imbrian smooth plains is uncertain, but may reflect smaller, unmapped patches of plains or some other feature of the target site. High-CPR craters with plains on their floors were likely emplaced prior to the Orientale impact event, with
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the wall deposits sourced from Imbrium basin melt sheets. These findings support earlier inferences of abundant fragmental material associated with small (few-hundred-meter diameter) craters in highland smooth plains (Campbell and Campbell, 2006; Ghent et al., 2008).
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The nature of deposits encountered in the exploration of highland smooth plains sites, and interpretations of remote sensing data for the walls of associated craters, are thus strongly affected by the role of Imbrium and Orientale melt-rich basin ejecta across the
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lunar southern hemisphere (e.g., Meyer et al., 2016, 2018). Our results also indicate that a more robust discrimination of Imbrian and Nectarian plains units may be derived from
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the radar data. A landed investigation in smooth plains units, such as the proposed primary Chandrayaan-2 site near the south pole (Amitabh and Srinivasan, 2018), may
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encounter more surface and shallow subsurface debris, with more varied provenance,
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than might be expected of a deep highland regolith. The proposed secondary landing area is within the fine-grained “radar-dark halo” of Moretus ejecta, offering a potentially quite
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different setting from the primary site.
Acknowledgments This work was supported in part by a grant to B.A.C from the NASA LASER Program (NNX-13AL17G). The authors thank the staff of the Arecibo Observatory and the Green
ACCEPTED MANUSCRIPT 13 Bank Observatory for their support in collecting the radar data presented here. The 12.6cm radar data are available on the NASA Planetary Data System (http://pdsgeosciences.wustl.edu/missions/sband/index.htm). Radar data at 70-cm wavelength are
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also on the PDS (http://pds-geosciences.wustl.edu/missions/lunar_radar/index.htm).
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Fig. 1. Locations of 4 to 20 km diameter Eratosthenian- and Imbrian-period craters in the southern lunar highlands with high backscatter and CPR from their interior walls. Colorcoded dots indicate: Category 1-green, Category 2-blue, Category 3-orange, Category 4yellow. Star at lower right indicates location of proposed primary Chandrayaan-2 landing
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site; star at center is proposed secondary site. Black outline encloses region covered by
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12.6-cm wavelength radar images used for this survey, with a dashed line to indicate the southern extent of USGS 1:5 M scale maps. S, T, C, and M note location of Schiller,
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basemap.
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Tycho, Cuvier, and Manzinus craters respectively. LOLA-Kaguya 118m/pixel hillshade
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Fig. 2. Remote sensing data for crater Zagut A (Category 1; 11 km diameter) and nearby high-CPR craters Rabbi Levi A (Category 2) and L (Category 1). (a) 12.6-cm wavelength OC radar map with circular polarization ratio as color overlay. Bright, high-CPR oval features along the south rims of Zagut-A and Rabbi-Levi L are due to double-bounce (longer delay time) echoes from the floor and wall of the crater. Locations of LROCNAC images in (b) and (c) shown by arrows. (b) LROC NAC image of northern wall of
ACCEPTED MANUSCRIPT 19 Zagut A. (c) LROC NAC image of southwestern wall of Zagut A. Note the high albedo of exposed immature material and boulder-strewn areas beginning just below the crater
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Fig. 3. Radar map of the south polar region of the Moon at 70-cm wavelength. Polar
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stereographic projection (lower left ~70 S, 105 E; lower right ~68 S, 100 E; upper center ~65 S, 5 E). Color overlay of circular polarization ratio (CPR) value on opposite-sense
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circular polarization image. Examples of mapped smooth plains (Ip, Ntp) from Wilhelms
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et al. (1979) noted. There is a correlation between occurrences of unit Ip (and some Ntp) and enhanced CPR values (e.g., Klaproth crater at upper left). High-CPR signatures from
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the walls and floor of younger (post-Orientale) craters like Moretus and Schomberger reflect near-surface rocks not yet broken down by micrometeorite bombardment. Arrows denote locations of craters Manzinus C (MC) and Simpelius N (SN) that bracket the proposed Chandrayaan-2 primary landing site: Ch-2 (a). The location of the secondary proposed site is noted at upper left: Ch-2 (b).