Band-limited Bouguer gravity identifies new basins on the Moon

Authors

  • W. E. Featherstone,

    Corresponding author
    1. Western Australian Centre for Geodesy and Institute for Geoscience Research, Curtin University of Technology, Perth, Western Australia, Australia
    • Corresponding author: W. E. Featherstone, Western Australian Centre for Geodesy and Institute for Geoscience Research, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia. (w.featherstone@curtin.edu.au)

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  • C. Hirt,

    1. Western Australian Centre for Geodesy and Institute for Geoscience Research, Curtin University of Technology, Perth, Western Australia, Australia
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  • M. Kuhn

    1. Western Australian Centre for Geodesy and Institute for Geoscience Research, Curtin University of Technology, Perth, Western Australia, Australia
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Abstract

[1] Spectral domain forward modeling is used to generate topography-implied gravity for the Moon using data from the Lunar Orbiter Laser Altimeter instrument operated on board the Lunar Reconnaissance Orbiter mission. This is subtracted from Selenological and Engineering Explorer (SELENE)-derived gravity to generate band-limited Bouguer gravity maps of the Moon so as to enhance the gravitational signatures of anomalous mass densities nearer the surface. This procedure adds evidence that two previously postulated basins on the lunar farside, Fitzgerald-Jackson (25°N, 191°E) and to the east of Debye (50°N, 180°E), are indeed real. When applied over the entire lunar surface, band-limited Bouguer gravity reveals the locations of 280 candidate basins that have not been identified when using full-spectrum gravity or topography alone, showing the approach to be of utility. Of the 280 basins, 66 are classified as distinct from their band-limited Bouguer gravity and topographic signatures, making them worthy of further investigation.

1 Introduction

[2] Understanding the structure and evolution of the Moon has benefitted greatly from satellite-based topographic mapping and satellite-based gravimetry. For instance, there are (i) asymmetry between the nearside and the farside hemispheres with a ~1.9 km offset between the center of figure (from topography) and the center of mass (from gravimetry) [Smith et al., 1997], (ii) the early detection of “mascons” [Muller and Sjogren, 1968], and (iii) information on the long-wavelength isostatic compensation state of the lunar crust [e.g., Wieczorek, 2007]. Overviews of the various lunar satellite missions are given in, e.g., Floberghagen [2002], Matsumoto et al. [2010], Sinha et al. [2010], Vondrak et al. [2010], and Zuber et al. [2013].

[3] Early satellite-based studies of the farside had previously been hampered by the inability to track lunar-orbiting satellites from the Earth due to the synchronous rotation and revolution of the Moon. This was redressed by the Selenological and Engineering Explorer (SELENE) lunar gravity mission [e.g., Namiki et al., 2009], which has already revealed several new features on the farside. Most of these are large-scale basins because smaller structures could not be discriminated from noise in the high-degree spherical harmonic coefficients of the SGM100h gravity model [e.g., Matsumo et al., 2010]. In addition, small-scale structures can be obscured by the long-wavelength gravitational signatures.

[4] In this article, we instead compute band-limited Bouguer gravity (BGG) from the newer SGM100i gravity model [Goossens et al., 2011] and LOLA (Lunar Orbiter Laser Altimeter) topography [Smith et al., 2010]. This reveals basins that were previously masked when using full-spectrum Bouguer gravity from SELENE alone [cf. Matsumoto et al., 2010] or topography alone [cf. Frey, 2011]. We present three case studies to exemplify: (i) the masking when using full-spectrum Bouguer gravity on its own, (ii) validation of the band-limited approach using an already known lunar basin, and (iii) adding more evidence for the presence of two farside basins [Fitzgerald-Jackson (25°N, 191°E) and to the east of Debye (50°N, 180°E)] using band-limited Bouguer gravity. Sensitivity analyses are conducted by varying the topographic mass density used in the forward modeling and the degrees of band limitation, indicating these identifications to be robust.

[5] The band-limited Bouguer gravity technique is then extended to the entire lunar surface, corroborating the presence of small-scale (~100 km to ~300 km in diameter) basins: Some are already known, some are probable, and some are uncertain but remain candidates for future investigations.

2 Methods and Data

[6] Namiki et al. [2009], Matsumoto et al. [2010], and several others generally use full-spectrum Bouguer gravity, where the spherical harmonic summations of gravity and topography begin at degree n = 2. The method used in this paper starts the summations in equations (1) and (4) at arbitrarily higher degrees (n1 > 2), thus enhancing the medium- and shorter-wavelength signals that are generated by near-surface mass anomalies (section 2.3). However, both methods are inevitably restricted by the maximum reliable degree of the lunar gravitational model and noise in the high-degree coefficients.

[7] The use of band-limited data in the planetary sciences is not novel, however. For instance, Frey et al. [1996] correlated band-limited free-air gravity with topography-implied gravity in spherical harmonic bands on Mars. Han [2008] used high-pass-filtered free-air gravity in the context of localized spherical harmonic functions on the Moon. Zuber et al. [2013] emphasized shorter-scale Bouguer gravity signatures through high-pass filtering, also on the Moon. However, band-limited Bouguer gravity is not used routinely in practice. Finally, we note that the band-limited approach is conceptually quite similar to regional-residual separation that has been applied to Bouguer gravity on Earth [e.g., Griffin, 1949; Nettleton, 1954].

2.1 Band-Limited Gravity From Lunar Gravitational Models

[8] Spherical harmonic syntheses of SGM100i are used to generate band-limited lunar gravity disturbances, which are synonymously termed radial derivatives of the gravitational potential in the planetary science literature [e.g., Wieczorek, 2007, p. 6]; this is

display math(1)

where GM = 4902.80080 × 109 m3 s−2 [Goossens et al., 2011] is the product of the universal gravitational constant G and the lunar mass M for SGM100i, r is the selenocentric radius to the computation point, R = 1,738,000 m is the SGM100i model's reference radius, n1 and n2 denote the lower and upper degrees of the band-limited syntheses, inline image and inline image are the fully normalized coefficients of SGM100i (from http://www.miz.nao.ac.jp/rise-pub/en), inline image are the fully normalized associated Legendre functions, and θ and λ are, respectively, the colatitude and longitude of the computation point. The (n + 1) term in equation (1) delivers gravity disturbances, whereas (n − 1) delivers gravity anomalies, which will be discussed further below. SGM100i is provided to degree nmax = 100, but Goossens et al. [2011] recommend that it only be used to degree 70 because of noise in the higher-degree coefficients. As such, all syntheses herein are limited to n2 = 70.

[9] For computational speed and consistency with other works on planetary gravimetry, we have evaluated equation (1) at the surface of the SGM100i model's reference sphere of radius r = R = 1,738,000 m. The use of some constant reference radius r, often set equal to the gravity model's radius R, follows common practice in the planetary sciences [e.g., Konopliv et al., 2001; Wieczorek, 2007; Matsumoto et al., 2010]. We acknowledge that this raises the problem of gravity continuation when r is inside the topographic masses. Alternatively, gravity can be evaluated at the surface of the topography [cf. Hirt, 2012], thus avoiding the need to take into account these additional continuation terms [e.g., Sjöberg, 2007]. We experimented with both cases, and it did not affect the spatial mapping of the basins.

[10] The subtle difference between gravity anomalies and gravity disturbances is described by, e.g., Hackney and Featherstone [2003a]. In short, the disturbance is the difference between model gravity and reference gravity evaluated at exactly the same point, and the anomaly is the difference evaluated at different points. However, the use of anomalies or disturbances has little consequence here, as we are seeking to locate the basins, rather than apply the subtleties of geodetic approaches and terminology to the planetary sciences. Hereafter, we simply use the term “gravity” to denote the radial derivatives of the gravitational potential at r = R from equation (1).

2.2 Band-Limited Gravity From Lunar Topography

[11] The lunar topography model used herein comes from the 2010 release produced by the LOLA instrument on board the Lunar Reconnaissance Orbiter (LRO) mission [Vondrak et al., 2010]. The LRO configuration delivers an ~18 m along-track and an ~1.8 km across-track spacing at the equator [Smith et al., 2010]. The inline image and inline image coefficients were taken from http://pds-geosciences.wustl.edu/lro/lro-l-lola-3-rdr-v1/lrolol_1xxx/data/lola_shadr/ (version lro_ltm02_sha.tab), which are based on around 1 year of LOLA observations (start date: 13 July 2009; end date: 20 August 2010) (G. Neumann, personal communication, 2013). While the maximum spherical harmonic degree available is 719, it has been truncated to degree 70 so as to be consistent with the maximum reliable degree of SGM100i.

[12] Band-limited spectral domain forward modeling was used to generate gravity from the LOLA lunar topography. We have validated this spectral approach with brute force numerical integration of band-limited topography, with both approaches giving comparable results. The fully normalized spherical harmonic coefficients of the gravitational potential of the topography are obtained from [e.g., Rummel et al., 1988; Wieczorek and Phillips, 1998]

display math(2)

where pmax is the order of the series expansion, M is the lunar mass (7.347 × 1022 kg, derived from GM/G for SGM100i), ρ is the assumed mass density of the lunar topography, and inline image and inline image are derived from spherical harmonic analysis of the surface function

display math(3)

where H is the height of the LOLA topography relative to the reference sphere of radius R = 1,737,400 m. The 600 m difference in reference radii between SGM100i and LOLA results in a direct current (DC) shift of Bouguer shell gravity of 134.28 mGal, computed from 4πGρH. However, this constant term does not affect the spatial mapping of the basins. The coefficients from equation (2) are converted to band-limited topography-implied gravity using

display math(4)

[13] The approximation error arising from truncating the series expansion in equation (2) decreases with increasing order pmax. Wieczorek [2007] found that pmax = 4 [and nmax ≤ 90] gives rise to maximum errors of <1 mGal anywhere on the Moon, which is about two orders of magnitude less than the band-limited Bouguer gravity (sections 3 and 4). In this study, we have evaluated equation (2) for pmax = 5, which makes the approximation error even smaller. Equation (2) also relies on a constant mass density assumption, which we shall investigate later (section 3) by evaluating it for two likely end-members of the lunar topographic mass density.

[14] The band-limited Bouguer gravity is then computed as equation (1) minus equation (4) but only for the same values of n1 > 2 and n2 ≤ nmax. Equations (1) and (4) also omit the degree-1 term of the topography which results from the ~1.9 km offset between the Moon's center of mass and center of figure, but this manifests as a very long wavelength signal that we wish to remove to enhance the detailed gravity signatures.

2.3 Selection of Bandwidths

[15] We have used two criteria to select the bandwidths of the Bouguer gravity used for the identification of lunar basins: (i) the limiting relation of Bowin [1983], which gives the deepest point mass that can generate a surface spherical harmonic signature of a particular degree n; and (ii) the typical spatial scale of the basins versus the maximum reliable degree of the gravity model, which is n2 = 70 in the case of SGM100i. However, interpretation of gravity data is inherently plagued by nonuniqueness. For instance, a lens-shaped mass anomaly near the surface can generate the same gravitational signature as a deeper point mass.

[16] Bowin's [1983] limiting relation is given by

display math(5)

where Zn is the maximum depth of the point mass that generates a surface feature of degree n (and R is the mean spherical radius of the Moon). There is also a relation between the degree n of a feature and its spatial scale s of a spherical body; this is

display math(6)

[17] For instance, starting the summations in equations (1) and (4) at n1 = 18 senses spatial scales shorter than ~300 km and point masses no deeper than ~100 km. In equation (6), we also distinguish between s and the minimum resolvable wavelength λmin.

[18] This band-limited approach also lessens the influence of assumptions about lunar isostatic compensation on our mapping. For instance, Sugano and Heki [2004] assert that there is no isostatic compensation for lunar basins with diameters up to 300 km. This is contradicted somewhat by Reindler and Arkani-Hamed [2001], who state that “most intermediate-size lunar craters show some degree of compensation.” However, the aim of this investigation is more concerned with the identification and spatial location of basins rather than interpretations of their isostatic state.

3 Exemplar Studies

[19] As stated, the computation of lunar gravity is most often performed over all spatial scales, e.g., to the maximum available expansion of the model. We therefore first present full-band Bouguer gravity derived from SGM100i and LOLA (lro_ltm02_sha.tab) as a slightly updated replication of the farside results in Matsumoto et al. [2010] and Namiki et al. [2009]. That is, equations (1) and (4) are evaluated from n1 = 2 to n2 = 70, the degree to which SGM100i contains full power [Goossens et al., 2011]. In this initial example, the topography-implied gravity has been computed using a constant mass density assumption of 2700 kg m−3, which is taken as the most representative topographic mass density of the farside topography from pre–Gravity Recovery and Interior Laboratory (GRAIL) values given in Huang and Wieczorek [2012], but a sensitivity analysis incorporating the more recent GRAIL-derived mass densities [cf. Wieczorek et al., 2013] will be presented later in this section.

[20] Figure 1a shows full-banded gravity from SGM100i to n2 = 70 (equation (1)), Figure 1b shows the topography-implied gravity, spectrally forward modeled from the LOLA topography over the same bands (equations (2)(4)), and Figure 1c shows their difference which is the full-banded Bouguer gravity. Comparing Figure 1c with Matsumoto et al. [2010, Figure 12] shows that two farside features are better resolved by the SGM100i gravity model: Fitzgerald-Jackson (25°N, 191°E) and what could possibly be a basin to the east of Debye at 50°N, 180°E. These two features can only just be discriminated in Matsumoto et al. [2010, Figure 12] with the benefit of hindsight, but the noise in SGM100h and its expansion to degree 100 cause a cantaloupe effect that renders them uncertain in Matsumoto et al. [2010].

Figure 1.

(a) Full-band SGM100i gravity, (b) full-band topography-implied gravity, and (c) full-band Bouguer gravity. Units in mGal. All panels show the lunar farside.

[21] To achieve improved mapping of basins where the planetary gravity field and topography have been observed, we propose the use of band-limited Bouguer gravity because it is capable of emphasizing the signatures of regional and near-surface mass density anomalies (section 2.3). The benefits of this strategy are exemplified here for SGM100i gravity mapping of selected features over the lunar farside. As a first case study example, we use SGM100i and topography-implied gravity in the spectral band between degrees n1 = 18 and n2 = 70. This corresponds to spatial scales from ~300 km to ~80 km (equation (5)) and a limiting depth of the generating mass anomalies of ~100 km (equation (6)). Of course, other parameters can be selected according to the analyst's choice.

[22] Figure 2c shows that the band-limited Bouguer approach enhances the gravity signatures of many farside basins occurring at spatial scales less than ~300 km. Notably, the clear signature of the massive South Pole-Aitken Basin in Figure 1c is absent from Figure 2c, showing the high-pass spatial filtering effect of the band-limited approach. It also much enhances the signatures of Fitzgerald-Jackson (25°N, 191°E) and the basin at 50°N, 180°E. Most basins are characterized by central positive gravity highs, surrounded by annular gravity lows. They are also more distinct in comparison to the SGM100i gravity alone (cf. Figures 1a, 2a, and 2c). Hence, band-limited gravity emphasizes the basin signatures much better than the full-spectrum Bouguer gravity maps.

Figure 2.

(a) Band-limited SGM100i gravity, (b) band-limited topography-implied gravity, and (c) band-limited Bouguer gravity. Units in mGal. The spectral band is n1 = 18 to n2 = 70, corresponding to spatial scales between ~300 km and ~80 km and a limiting depth of ~100 km. All panels show the lunar farside.

[23] In the remainder of this section, we focus on three farside basins: Hertzsprung (centered at 1.5°N, 128.5°W), Fitzgerald-Jackson (centered at 25°N, 191°E), and the basin to the east of Debye centered at 50°N, 180°E. We use Hertzsprung to validate the band-limited technique because it is a well-established impact basin on the more challenging farside. Huang et al. [2009] and Frey [2011] postulated the presence of Fitzgerald-Jackson (named TOPO-41 in Frey [2011, p. 57]) from analysis of farside lunar topography only. Also, based only on topographic data, Frey [2011] postulated a likely basin to the east of Debye (named TOPO-22 in Frey [2011, p. 57]). We believe that the anomalous mass feature identified in our study at 50°N, 180°E is the corresponding gravity field signature, thus providing further evidence for both being real basins.

[24] Figure 3 shows zoomed-in plots of the full-resolution LOLA topography and the band-limited (degrees n1 = 18 to n2 = 70) Bouguer gravity for the three basins considered here. For Hertzsprung, the presence of the basin is evident in both the topography (Figure 3a) and the band-limited gravity (Figure 3b). The band-limited gravity exhibits a circular central gravity high surrounded by a negative gravity annulus (circular gravity high-low) that is correlated spatially with the topographic signature. As this is a well-established basin, this shows that the band-limited technique is capable of detecting basins.

Figure 3.

(a, c, e) LOLA topography in meters and (b, d, f) band-limited Bouguer gravity in mGal for the three selected farside regions: Hertzsprung is shown in Figures 3a and 3b, Fitzgerald-Jackson is shown in Figures 3c and 3d, and the basin near Debye is shown in Figures 3e and 3f. The spectral band is n1 = 18 to n2 = 70, corresponding to spatial scales between ~300 km and ~80 km and a limiting depth of ~100 km.

[25] For Fitzgerald-Jackson, Figure 3c shows that the basin is not as clearly defined by the LOLA topography alone. However, Figure 3d shows a mass anomaly that correlates spatially with a broad topographic low. There is less evidence of a circular gravity high-low structure that was so clear for Hertzsprung. As stated, Huang et al. [2009] and Frey [2011] did not use gravity data to identify this basin. However, when both data sets are considered together, we believe that they provide stronger evidence of the presence of a real basin.

[26] The basin near Debye is hardly discernible from the LOLA topography alone (Figure 3e), with masking caused by the many smaller-scale basins scattered throughout this region. On the other hand, a mass anomaly is clearer from the band-limited gravity, which also shows a circular gravity high-low signature. Given that the band-limited technique has proven effective over Hertzsprung and Fitzgerald-Jackson, we infer that it has correctly identified this as a basin.

3.1 Sensitivity Analyses

[27] The lunar maps presented in Figures 1-3 use a constant topographic mass density for the topography-implied gravity of 2700 kg m−3 as the mean of pre-GRAIL mass densities over many farside regions [Huang and Wieczorek, 2012]. We therefore conducted a sensitivity analysis based on the end-members of more recently published lunar topographic mass densities [Wieczorek et al., 2013; Huang and Wieczorek, 2012; Kiefer et al., 2012]. Figure 4 shows that the topographic mass density between the end-members of 2400 kg m−3 to 2900 kg m−3 makes no difference to the spatial mapping of these three basins; each is mapped to exactly the same location irrespective of the mass density chosen. One minor exception is that the lower density estimate more clearly identifies the annular gravity low around Hertzsprung (Figure 4a).

Figure 4.

Band-limited Bouguer gravity (a, c, e) based on a mass density of 2400 kg m−3 and (b, d, f) based on 2900 kg m−3. Hertzsprung is shown in Figures 4a and 4b, Fitzgerald-Jackson is shown in Figures 4c and 4d, and the basin near Debye is shown in Figures 4e and 4f. The spectral band is n1 = 18 to n2 = 70, corresponding to spatial scales between ~300 km and ~80 km and a limiting depth of ~100 km.

[28] We also conducted sensitivity analyses to the degree of band limiting. Table 1 shows the spatial scales resolved (equation (6)) and the limiting depths according to Bowin's relation (equation (5)). These bands have been chosen somewhat arbitrarily but only to show the effect of the bandwidth on the identification of the basins. Figure 5 shows that the higher the starting degree n1 of the summations in equations (1) and (4), the lower the amplitude of the signature of the basins, thus lessening the method's resolving power. This is particularly the case for Fitzgerald-Jackson, so caution needs to be exercised for higher degrees of filtering. Nevertheless, the identification of the basins remains quite robust for the lower range of degrees of filtering.

Table 1. Spatial Resolution and Limiting Depth Corresponding to Different Spherical Harmonic Degrees (Computed From Equations (6) and (5))
DegreeSpatial Resolution (km)Limiting Depth (km)
5~1100~430
10~550~190
25~220~70
36~150~50
70~80~25
Figure 5.

Band-limited Bouguer gravity for various spectral bands. (top) Hertzsprung, (middle) Fitzgerald-Jackson, and (bottom) the basin near Debye. The spatial scales and limiting depths for each band are given in Table 1.

4 Classification of Lunar Basin Locations

[29] We next use band-limited Bouguer gravity and topography to perform a classification of previously reported lunar basins as listed or postulated by Wood [2004], Frey [2011], Wieczorek and Le Feuvre [2009], and Matsumoto et al. [2010]. Based on the sensitivity analyses (section 3.1), we use the band-limited Bouguer gravity (n1 = 18 to n2 = 70) and a topographic mass density of 2700 kg m−3, corresponding to spatial scales (half wavelength) of ~300 km down to ~80 km. This is now applied over the entire lunar surface.

[30] The classification is based on three indicators: (i) the presence of a circular gravity high-low structure in the band-limited Bouguer gravity, (ii) the range (maximum minus minimum) of band-limited Bouguer gravity over the supposed basin, and (iii) the presence of a topographic rim structure. We interpret a circular gravity high-low structure in the band-limited Bouguer gravity as an indication of a regional mass density anomaly that largely follows the same spatial pattern. A large range indicates a significant mass density anomaly. The third indicator is included because most already known basins also show a topographic signature [cf. Frey, 2011].

[31] Each indicator is assigned four somewhat subjectively determined numerical values: (0) not seemingly present, (1) present to some limited extent, (2) present to some considerable extent, and (3) quite clearly present. From these three indicators, two ratings are derived: Rating 1 is based on the sum of all indicators, including the topographic signatures (i, ii, and iii); Rating 2 is based on the sum of only the two band-limited Bouguer gravity-related indicators (i and ii). Similar to the approach of Frey [2011], each rating is then assigned to one of three classes in order to classify the supposed basin locations: doubtful, possible, and distinct.

[32] Most of the 122 known or postulated basins listed in Table 2, including our three case study basins (section 3), are classified as either distinct or possible. Our classification supports the presence of several basins proposed by Frey [2011] that exhibit anomalous thin crustal thickness but no topographic signature (classified here as distinct or possible). Table 2 also lists several basins postulated by Frey [2011] based on their topographic signature only, but which do not exhibit a clear signature in the band-limited Bouguer gravity and are thus classified as doubtful.

Table 2. Classification of Previously Reported Lunar Basins Based on Band-Limited Bouguer Gravitya
Basin IdentificationbBand-Limited Bouguer GravityReferencesh
NameSymbolLat (°)Lon (°)D (km)grcrdtreRating 1fRating 2g 
  1. a

    The contents are ordered in terms of the basins' ratings from distinct to doubtful.

  2. b

    Parameters taken from Frey [2011, Tables 1–3], if not indicated otherwise. Longitude is given as eastern longitude. Latitude and longitude values have been truncated to one decimal place.

  3. c

    Gravity ring structure: (0) not present; (1) present to some extent; (2) present to a considerable extent; (3) clearly present.

  4. d

    Range (max minus min) over the basin: (0) <50 mGal; (1) 50–100 mGal; (2) 100–150 mGal; (3) >150 mGal.

  5. e

    Topographic rim structure: (0) not visible; (1) visible to some extent; (2) visible to a considerable extent; (3) clearly visible.

  6. f

    Rating 1 (based on the sum of pr, r, and tr): (0–2) doubtful; (3–6) possible; (7–9) distinct.

  7. g

    Rating 2 (based on the sum of pr and r): (0–1) doubtful; (2–4) possible; (5–6) distinct.

  8. h

    References: 1, Frey [2011]; 2, Wood [2004]; 3, Matsumoto et al. [2010]; 4, Wieczorek and Le Feuvre [2009].

  9. i

    Coordinates differ by at least 2 arc degrees to that given by Frey [2011]: coordinates given by Wood [2004] for Humboldtianum (59.0°N/82.0°E), Imbrium (35.0°N/343.0°E), Poincaré (57.0°S/146.0°E), Ingenii (43.0°S/165.0°E), and Tranqillitatis (7.0°N/30.0°E); coordinates given by Wieczorek and Le Feuvre [2009] for Humboldtianum (58.0°N/83.0°E), Imbrium (38.0°N/340.0°E), Poincaré (57.0°S/164.0°E), Ingenii (43.0°S/165.0°E), and Tranqillitatis (7.0°N/30.0°E).

  10. j

    TOP0-24 and TOPO-30 correspond to Dirichlet-Jackson and Cruger-Sirsalis, respectively, as listed by Wieczorek and Le Feuvre [2009].

  11. k

    Name and basin identification as provided by Wood [2004].

  12. l

    Location corresponds to TOP0-30.

  13. m

    Masked by a strong signal from a nearby basin.

  14. n

    Not present in the band-limited Bouguer gravity as long spatial scales have been removed.

CrisiumCr17.558.510603339Distinct6Distinct1,2,4
OrientaleOr−20.0265.09303339Distinct6Distinct1,2,3,4
Mendel-RydbergMR−50.0266.06303339Distinct6Distinct1,2,3,4
HumboldtianumiHm61.084.06003339Distinct6Distinct1,2,3,4
Freundlich-SharonovFS18.5175.06003339Distinct6Distinct1,2,3,4
HertzsprungHe1.5231.55703339Distinct6Distinct1,2,3,4
NectarisNe−16.034.08603328Distinct6Distinct1,2,4
SmythiiSm−2.087.08403328Distinct6Distinct1,2,4
HumorumHu−24.0320.58203328Distinct6Distinct1,2,4
ApolloAp−36.0209.05053328Distinct6Distinct1,2,3,4
MoscovienseMo25.0147.04453328Distinct6Distinct1,2,3,4
Coulomb-Sartoncs52.0237.05303317Distinct6Distinct1,2,3,4
TOP0-30 (Cruger-Sirsalis)jT30−15.8293.43803317Distinct6Distinct1,4
Amundsen-GanswindtAG−81.0120.03553317Distinct6Distinct1,2,4
CTA-25C2511.4350.13303317Distinct6Distinct1
Schiller-ZucchiusSZ−56.0315.53253317Distinct6Distinct1,2,4
CTA-10C10−25.2122.33243317Distinct6Distinct1
TOP0-22T2250.0179.83143317Distinct6Distinct1
No name givenk,lNN2−20.0290.03003317Distinct6Distinct2
SerenitatisSe27.019.07402338Distinct5Distinct1,2,4
CTA-26C2626.5188.55332338Distinct5Distinct1
KorolevKo−4.5203.04402338Distinct5Distinct1,2,3,4
TOP0-24 (Dirichlet-Jackson)jT2413.8201.74272338Distinct5Distinct1,3
SchrodingerSc−75.0134.03202338Distinct5Distinct1,2,3,4
TOP0-41T4124.8191.93172338Distinct5Distinct1
Schrodinger-ZeemankSZe−81.0195.02502338Distinct5Distinct2
ImbriumiIm33.0342.011602327Distinct5Distinct1,2,4
TOP0-13T13−35.8148.13282327Distinct5Distinct1
PlanckPI−57.5135.53252327Distinct5Distinct1,2,3,4
TOP0-18T18−19.2160.98052316Possible5Distinct1
TOP0-3T355.033.75102316Possible5Distinct1
GrimaldiGr−5.0292.04302316Possible5Distinct1,2,4
CTA-2C214.23.44192316Possible5Distinct1
LorentzLo34.0263.03603216Possible5Distinct1,2,3,4
TOP0-15T15−64.8150.33522316Possible5Possible1
PoincaréiPo−57.5162.03402316Possible5Distinct1,2,3,4
CTA-23C2312.6306.63042316Possible5Distinct1
MilnekMi−31.0113.02623126Possible4Possible2,4
lnsularumIn9.0342.06002305Possible5Distinct1,2,4
CTA-16C1650.8195.54912305Possible5Distinct1
CTA-22C221.8299.84012305Possible5Distinct1
CTA-17C1740.1210.83622305Possible5Distinct1
CTA-1C11.51.23282305Possible5Distinct1
CTA-24C240.4314.73232305Possible5Distinct1
No name givenkNN150.0165.04501337Distinct4Possible2
lngeniiiLg−34.0163.05601326Possible4Possible1,2,3,4
FecunditatisFe−4.052.09901315Possible4Possible1,2,4
TranquillitatisiTr7.040.08001315Possible4Possible1,2,4
NubiumNu−21.0345.06901315Possible4Possible1,2,4
TOP0-10T1057.8117.46031315Possible4Possible1
CTA-21C2161.9286.04682215Possible4Possible1
TOP0-1T159.12.94381315Possible4Possible1
TOP0-20T2039.6176.44321315Possible4Possible1
CTA-27C2718.4341.64092215Possible4Possible1
TOP0-19T19−0.2170.73921315Possible4Possible1
CTA-7C747.595.83891315Possible4Possible1
TOP0-12T12−16.3138.83291315Possible4Possible1
TOP0-9T9−50.8116.73212215Possible4Possible1
Mutus-VIacqMV−51.521.06901304Possible4Possible1,2,4
Balmer-KapteynBK−15.569.05502204Possible4Possible1,2,4
CTA-19C19−34.7245.84671304Possible4Possible1
No name givenkNN330.0165.03301304Possible4Possible2
MendeleevMe6.0141.03301236Possible3Possible1,2,3,4
BirkhoffBi59.0213.03301236Possible3Possible1,2,3,4
BaillyBa−67.0292.03002136Possible3Possible1,2,4
ComptonkCo56.0104.01750336Possible3Possible2
TOP0-34T34−44.0303.83171225Possible3Possible1
TOP0-32T3220.4297.912531204Possible3Possible1
TOP0-11T1150.1124.78241214Possible3Possible1
TOP0-17T1714.4156.56001214Possible3Possible1
CTA-6C629.180.54571214Possible3Possible1
TOP0-14T14−5.5149.64461214Possible3Possible1
TOP0-2T2−15.67.04371214Possible3Possible1
CTA-12C12−36.8128.63601214Possible3Possible1
TOP0-33T33−38.2298.05001203Possible3Possible1
CTA-15C15−15.3190.64901203Possible3Possible1
CTA-14C1476.6142.67441113Possible2Possible1
CTA-20C2067.8247.55011113Possible2Possible1
Werner-AiryWA−24.012.05000213Possible2Possible1,2,4
TOP0-21T21−71.6177.83770213Possible2Possible1
Bailly-NewtonkBN−73.0303.03301113Possible2Possible2
CTA-13C1315.9135.13151113Possible2Possible1
TOP0-8T8−26.9103.43142013Possible2Possible1
AustraleAu−51.594.58800202Doubtful2Possible1,2,4
Keeler-HeavisideKH−10.0162.07800202Doubtful2Possible1,2,3,4
TOP0-23T23−57.1197.96961102Doubtful2Possible1
MarginisMa20.084.05802002Doubtful2Possible1,2,4
Flamsteed-BillyFB−7.5315.05700202Doubtful2Possible1,2,4
TOP0-37T3759.2337.74701102Doubtful2Possible1
CTA-11C1127.1369.03131102Doubtful2Possible1
Sikorsky-RittenhouseSR−68.5110.03101102Doubtful2Possible1,2,4
AntoniadikAn−69.0188.01400134Possible1Doubtful2
TOP0-6T6−32.887.54740112Doubtful1Doubtful1
TOP0-28T2829.6245.73921012Doubtful1Doubtful1
TOP0-7T7−34.298.53890112Doubtful1Doubtful1
TOP0-31T3142.1294.49730101Doubtful1Doubtful1
TOP0-4T4−46.967.09420101Doubtful1Doubtful1
Tsiolkovskiy-StarkTS−15.0128.07000101Doubtful1Doubtful1,2,4
TOP0-25T25−57.4222.76880101Doubtful1Doubtful1
Lomonosov-FlemingLF19.0105.06200101Doubtful1Doubtful1,2,4
AI-Khwarismi KingAK1.0112.05900101Doubtful1Doubtful1,2,4
CTA-18C1818.6236.65391001Doubtful1Doubtful1
Sylvester-NansenkSN83.045.05000101Doubtful1Doubtful2
CTA-3C3−24.64.34060101Doubtful1Doubtful1
No name givenkNN445.055.03500101Doubtful1Doubtful2
No name givenkNN560.0139.04000101Doubtful1Doubtful2
Pingre-HausenPH−56.0278.03000101Doubtful1Doubtful1,2,4
No name givenkNN655.0330.07000011Doubtful0Doubtful2
CTA-4C4−83.432.93190011Doubtful0Doubtful1
CTA-5C542.370.43090011Doubtful0Doubtful1
Grissom-WhiteGW−44.0199.06000000Doubtful0Doubtful1,2,4
CTA-9C923.2118.23120000Doubtful0Doubtful1
TOP0-40T4015.8347.4771mmmmmmm1
TOP0-16T1627.1150.3626mmmmmmm1
TOP0-38 (inside Imbrium)T3837.8341.2616mmmmmmm1
TOP0-35T35−7.7322.2451mmmmmmm1
TOP0-5T516.668.0428mmmmmmm1
TOP0-26T26−14.9240.8410mmmmmmm1
TOP0-27T27−10.4243.8325mmmmmmm1
ProcellarumPr26.0345.03200nnnnnnn1,2
South Pole-AitkenSPA−56.0180.02500nnnnnnn1,2,4
CTA-8C819.9106.81764nnnnnnn1

[33] Using the band-limited Bouguer gravity, we identify positive signals with a classification of possible or distinct and a minimum resolvable spatial scale of ~80 km. These have (i) a circular gravity high-low structure to some extent, (ii) a range of >50 mGal, and (iii) a topographic rim structure to some extent. If no part of a topographic rim structure is evident, only the strong gravity signals with a range of >100 mGal are included. From all the so-identified signals, we exclude those that are centered over the basin locations listed in Table 2 but do retain signals that are close to them. This resulted in a total of 280 band-limited Bouguer gravity signals that indicate locations of significant mass density surpluses (Figure 6 and Appendix A). Table 3 lists only the 66 locations that are classified as distinct by at least one of the two above ratings; the remainder is listed in Appendix A.

Figure 6.

Spatial distribution of all newly identified locations of band-limited Bouguer gravity signatures. The spectral band is n1 = 18 to n2 = 70, corresponding to spatial scales between ~300 km and ~80 km and a limiting depth of ~100 km. The locations, full names, and descriptions are given in Appendix A. Rectangular projection.

Table 3. Identification of New Locations of Significant Band-Limited Bouguer Gravity Signals That Are Classified as Distinct in One or Both of Ratings 1 and 2 (See Appendix A for a Complete List)
Basin IdentificationaBand-Limited Bouguer GravityCommentsg
NameSymbolLat (°)Lon (°)D (km)grbrctrdRating 1eRating 2f
  1. a

    Name reflects band-limited Bouguer gravity (BGG). Longitude is given as eastern longitude. Rim diameter D is approximated to the nearest 20 km and is based on the spatial extent of the topographic signature or on the gravity signature in case no topographic signature is present (e.g., tr = 0).

  2. b

    Gravity ring structure: (0) not present; (1) present to some extent; (2) present to considerable extent; (3) clearly present.

  3. c

    Range (max minus min) over the basin: (0) <50 mGal; (1) 50–100 mGal; (2) 100–150 mGal; (3) >150 mGal.

  4. d

    Topographic rim structure: (0) not visible; (1) visible to some extent; (2) visible to a considerable extent; (3) clearly visible. The letter c indicates that the gravity ring structure is centred over the topographic rim structure, while the letters nc indicate that it is not centred.

  5. e

    Rating 1 (based on the sum of pr, r, and tr): (0–2) doubtful; (3–6) possible; (7–9) distinct.

  6. f

    Rating 2 (based on the sum of pr and r): (0–1) doubtful; (2–4) possible; (5–6) distinct.

  7. g

    Cluster indicates multiple impacts with partly overlapping gravity and topography signals. Reference to existing impact basins indicates their closeness to the identified signal; names and abbreviations used refer to those in Table 2.

BBG-67B6737.2219.1320333nc9Distinct6DistinctPart of cluster 15 south of CTA-17
BBG-91B9134.2106.3180333c9Distinct6DistinctBetween clusters 12 and 20
BBG-128B1285.247.7460332c8Distinct6DistinctBetween Tr and Fe
BBG-228B228−48.1175.6220332c8Distinct6Distinct 
BBG-66B6637.7208.6180332c8Distinct6DistinctPart of cluster 15 south of CTA-17
BBG-52B5245.6153.2240233c8Distinct5DistinctPart of cluster 13 north of Mo
BBG-47aB47a43.2100.9200323nc8Distinct5DistinctPart of cluster 12 south of CTA-7
BBG-100B1007.422.9460331c7Distinct6DistinctWithin impact basin of Tr
BBG-96B9621.0350.0400331nc7Distinct6DistinctPart of cluster 21; close to Pr, CTA-27, and TOPO-40
BBG-127B127−6.526.9420331c7Distinct6DistinctLocated partly within impact basin of Ne
BBG-117B11710.0245.5320331c7Distinct6Distinct 
BBG-94B9426.5268.5300331c7Distinct6DistinctSouth of Lo
BBG-209B209−23.7350.1400232c7Distinct5DistinctSoutheast of Nu
BBG-198B198−28.4250.9280232c7Distinct5DistinctPart of cluster 38 between Or and CTA-19
BBG-23B2352.4114.5260232c7Distinct5DistinctPart of cluster 6 around TOPO-10
BBG-73B7346.5275.8260232nc7Distinct5DistinctPart of cluster 18
BBG-40B4036.115.8260232c7Distinct5DistinctWithin the impact basin of Se
BBG-213B213−32.7354.5260322c7Distinct5Distinct 
BBG-227B227−41.6158.2260232nc7Distinct5DistinctSouthwest of ln
BBG-6B676.0316.4220232c7Distinct5DistinctPart of cluster 2
BBG-21B2161.0106.8200232c7Distinct5DistinctPart of cluster 6 around TOPO-10
BBG-277B277−73.795.0160232c7Distinct5DistinctPart of cluster 53 southwest of SR
BBG-278B278−85.6277.7100322c7Distinct5Distinct 
BBG-81B8144.6328.6280133nc7Distinct4PossiblePartly within impact basin of Im
BBG-47bB47b39.999.1280223c7Distinct4PossiblePart of cluster 12 south of CTA-7
BBG-32B3257.9260.5220223nc7Distinct4PossiblePart of cluster 8
BBG-115B11513.2189.6220223nc7Distinct4PossiblePart of cluster 26 southeast of FS
BBG-118B11816.9271.5200223c7Distinct4Possible 
BBG-27bB27b57.1197.5180223nc7Distinct4PossiblePart of cluster 7; between Bi and CTA-16
BBG-54B5438.6141.8140223nc7Distinct4PossiblePart of cluster 13 north of Mo
BBG-83B8323.840.7360231c6Possible5DistinctBetween Se and Cr
BBG-163B163−18.813.4360231c6Possible5DistinctBetween Ne, WA, and TOPO-2
BBG-162B162−9.912.8320231c6Possible5DistinctBetween Ne, WA, and TOPO-2
BBG-113B1138.3169.7200321c6Possible5Distinct 
BBG-43B4339.229.7160231c6Possible5DistinctPart of cluster 11
BBG-26B2664.4184.0120321nc6Possible5DistinctPart of cluster 7
BBG-203B203−36.0274.04002305Possible5DistinctBetween Or and MR
BBG-158B158−13.7331.63602305Possible5DistinctPart of cluster 30 around TOPO-30, north of Hu
BBG-205B205−30.9306.23602305Possible5DistinctSouthwest of Hu
BBG-129B129−3.043.73402305Possible5DistinctPart of cluster 28 around Fe
BBG-137B137−1.8168.43202305Possible5DistinctPart of cluster 29 around TOPO-19
BBG-161B161−14.2352.43202305Possible5DistinctPart of cluster 31; northeast of Nu
BBG-130B130−7.051.43002305Possible5DistinctPart of cluster 28 around Fe
BBG-172B172−16.9154.73002305Possible5DistinctPart of cluster 33, band including TOPO-18
BBG-97B9728.2358.82802305Possible5DistinctPart of cluster 21; joins cluster 10
BBG-173B173−21.3159.62802305Possible5DistinctPart of cluster 33, band including TOPO-18
BBG-42B4244.232.52602305Possible5DistinctPart of cluster 11
BBG-174B174−18.7165.42602305Possible5DistinctPart of cluster 33, band including TOPO-18
BBG-165B165−29.328.62602305Possible5DistinctBetween Ne and WA
BBG-192B192−31.4146.82602305Possible5DistinctPart of cluster 36, north of TOPO-13
BBG-22B2258.2122.12402305Possible5DistinctPart of cluster 6 around TOPO-10
BBG-122B12220.7312.52402305Possible5Distinct 
BBG-168B168−16.689.32402305Possible5DistinctPart of cluster 32 south of Sm
BBG-171B171−19.4137.32402305Possible5DistinctSouth of TOPO-12
BBG-109B10921.2160.12202305Possible5DistinctBetween Mo and FS
BBG-191B191−25.4145.32202305Possible5DistinctPart of cluster 36
BBG-183B183−37.01.72202305Possible5Distinct 
BBG-74B7446.0287.62002305Possible5DistinctPart of cluster 18
BBG-121B1218.8309.42002305Possible5DistinctSouth of CTA-23
BBG-193B193−33.6153.22002305Possible5DistinctPart of cluster 36, north of TOPO-13
BBG-63B6326.2164.71802305Possible5DistinctPart of cluster 14 between Mo and TOPO-20
BBG-101B10114.130.31802305Possible5DistinctPart of cluster 23; between Se and Cr
BBG-238B238−50.0299.21802305Possible5DistinctPart of cluster 44 between MR and SZ
BBG-112B1129.5163.21602305Possible5DistinctPart of cluster 25 around TOPO-17
BBG-236B236−45.3289.41603205Possible5DistinctPart of cluster 44 between MR and SZ
BBG-56B5641.3156.91402305Possible5DistinctPart of cluster 13 north of Mo

[34] Of the 280 band-limited Bouguer gravity signals examined, 174 are colocated with a complete or part of a topographic rim structure, providing some more confidence that they are indeed basins. The locations and diameters D listed in Table 3 and Appendix A are based on either the topographic rim, if present, or the location and spatial extent of the band-limited Bouguer gravity signal. The extracted diameters range from ~100 km to ~760 km with the majority (246 of 280) less than ~300 km, slightly larger than the resolving power of the band-limited Bouguer gravity (~80 km). Many of the basins classified in Table 3 and Appendix A are close to or partly located within existing basins or form part of a cluster of basins. These weaker signals are only revealed when using band-limited Bouguer gravity but obscured when using full-banded Bouguer gravity.

5 Summary and Conclusions

[35] Three case studies over the farside of the Moon have demonstrated the ability of band-limited Bouguer gravity to identify and map lunar basins that are not detected so clearly with full-spectrum Bouguer gravity or topography alone [cf. Huang et al., 2009; Namiki et al., 2009; Matsumoto et al., 2010; Frey, 2011]. The band-limited Bouguer gravity enhances the signatures of small-scale structures by suppressing long wavelengths that can hamper localized investigations. This has revealed signatures of two distinct mass concentrations on the lunar farside. The Fitzgerald-Jackson (25°N, 191°E) gravity signature is also partly visible as a topographic feature but can now be better classified as a basin with inner mass excess. The gravity signature to the east of Debye (50°N, 180°E) lacks an obvious corresponding topographic signature, albeit identified by Frey [2011] as a candidate basin. The band-limited gravity signature adds evidence that this is indeed a real basin. The positive band-limited gravity signatures at their centers indicate mass excesses with respect to their surrounds, which could reflect mantle uplift postimpact [cf. Neumann et al., 1996; Wieczorek et al., 2006].

[36] After showing the band-limited approach to be a robust tool for identifying candidate basins on the more challenging farside for some selected bandwidths and end-members of the likely lunar topographic mass density, we have applied it over the entire lunar surface. This was done for spectral bands n1 = 18 to n2 = 70, corresponding to spatial scales between ~300 km and ~80 km and a limiting depth of ~100 km. A combination of indicators, including the topography-only signature, was used to determine whether the candidate basin was distinct or possible. Of the 280 candidate basins, 66 have been classified as distinct. We have deliberately restricted this study to the identification and mapping of the basins rather than attempting an interpretation of their origins and relation to lunar history; this is left for future work.

6 Note Added During Review

[37] The embryonic part of this work was carried out and submitted before results from the GRAIL mission became available in December 2012. We submitted a first version back in October 2011 and a revision in November 2012, both describing band-limited Bouguer gravity for improved mapping of lunar gravity signatures and substantiating the two farside basins. Preliminary results or papers from the GRAIL mission were thus not available to us to draft our manuscript. Only after the release of preliminary GRAIL results, we were able to incorporate different topographic mass density estimates in section 3, but which did not alter our conclusions. The GRAIL Bouguer gravity map published by Zuber et al. [2013, Figure 1B] shows signatures of 100 mGal (or more) in amplitude at 25°N, 191°E and 50°N, 180°E, hence providing post facto independent evidence of the basins.

Appendix A: Full List of Band-Limited Bouguer Gravity Basins

This appendix provides a list of all new 280 locations of band-limited Bouguer gravity signals that are classified as either distinct or possible (Table A1).

Table A1. Identification of All New Locations of Band-Limited Bouguer Gravity Signals That Are Classified as Either Distinct or Possible in Ratings 1 and 2

Basin IdentificationaBand-Limited Bouguer GravityCommentsg
NameSymbolLat (°)Lon (°)D (km)grbrctrdRating 1eRating 2f
  1. a

    Name reflects band-limited Bouguer gravity (BGG). Longitude is given as eastern longitude. Rim diameter D is approximated to the nearest 20 km and is based on the spatial extent of the topographic signature or on the gravity signature in case no topographic signature is present (e.g., tr = 0).

  2. b

    Gravity ring structure: (0) not present; (1) present to some extent; (2) present to considerable extent; (3) clearly present.

  3. c

    Range (max minus min) over the basin: (0) <50 mGal; (1) 50–100 mGal; (2) 100–150 mGal; (3) >150 mGal.

  4. d

    Topographic rim structure: (0) not visible; (1) visible to some extent; (2) visible to a considerable extent; (3) clearly visible. The letter c indicates that the gravity ring structure is centred over the topographic rim structure, while the letters nc indicate that it is not centred.

  5. e

    Rating 1 (based on the sum of pr, r, and tr): (0–2) doubtful; (3–6) possible; (7–9) distinct.

  6. f

    Rating 2 (based on the sum of pr and r): (0–1) doubtful; (2–4) possible; (5–6) distinct.

  7. g

    Cluster indicates multiple-impact region with partly overlapping gravity and topography signals. Reference to existing impact basins indicates their closeness to the identified signal; names and abbreviations used refer to those in Table 2.

BBG-1B183.5139.71802204Possible4Possible 
BBG-2B277.977.1160222c6Possible4PossibleCluster 1
BBG-3B374.099.3140121c4Possible3PossibleCluster 1
BBG-4B478.4199.4160221c5Possible4Possible 
BBG-5B572.2210.7140221c5Possible4Possible 
BBG-6B676.0316.4220232c7Distinct5DistinctCluster 2
BBG-7B772.5307.5260122c5Possible3PossibleCluster 2
BBG-8B869.0318.3160122c5Possible3PossibleCluster 2
BBG-9B973.718.9140122nc5Possible3PossibleCluster 3
BBG-10B1071.533.7140121c4Possible3PossibleCluster 3
BBG-11B1170.619.380122nc5Possible3PossibleCluster 3
BBG-12B1266.915.7160121nc4Possible3PossibleCluster 3
BBG-13B1365.422.6140121nc4Possible3PossibleCluster 3
BBG-14B1469.8148.6140121c4Possible3PossibleCluster 4
BBG-15B1568.6159.8160132c6Possible4PossibleCluster 4
BBG-16B1668.0172.5120121c4Possible3PossibleCluster 4
BBG-17B1756.846.5140111c3Possible2PossibleCluster 5 north of NN4
BBG-18B1856.058.3120123nc6Possible3PossibleCluster 5 north of NN4
BBG-19B1950.250.2160121c4Possible3PossibleCluster 5 north of NN4
BBG-20B2049.261.0180122c5Possible3PossibleCluster 5 north of NN4
BBG-21B2161.0106.8200232c7Distinct5DistinctCluster 6 around TOPO-10
BBG-22B2258.2122.12402305Possible5DistinctCluster 6 around TOPO-10
BBG-23B2352.4114.5260232c7Distinct5DistinctCluster 6 around TOPO-10
BBG-24B2461.6146.1140113c5Possible2PossibleClose to NN5
BBG-25B2557.4151.3140111c3Possible2Possible 
BBG-26B2664.4184.0120321nc6Possible5DistinctCluster 7
BBG-27aB27a60.2191.3160222nc6Possible4PossibleCluster 7
BBG-27bB27b57.1197.5180223nc7Distinct4PossibleCluster 7; between Bi and CTA-16
BBG-28B2852.2206.31602204Possible4PossibleCluster 7; between Bi and CTA-16
BBG-29B2963.6227.6120221c5Possible4Possible 
BBG-30B3069.5244.71402204Possible4PossibleCluster 8
BBG-31B3163.8250.1160121c4Possible3PossibleCluster 8
BBG-32B3257.9260.5220223nc7Distinct4PossibleCluster 8
BBG-33B3357.9278.2140123nc6Possible3Possible 
BBG-34B3452.6316.5340132nc6Possible4PossibleJoins with BBG-75 of cluster 18
BBG-35B3554.0336.8180121c4Possible3PossibleCluster 9; south of TOPO-37
BBG-36B3656.1344.3140121c4Possible3PossibleCluster 9; south of TOPO-37
BBG-37B3751.09.8200121nc4Possible3PossibleCluster 10 connected to TOPO-1
BBG-38B3844.78.1260122c5Possible3PossibleCluster 10 connected to TOPO-1
BBG-39B3939.64.3220131c5Possible4PossibleCluster 10 connected to TOPO-1
BBG-40B4036.115.8260232c7Distinct5DistinctWithin the impact basin of Se
BBG-41B4140.722.6160221c5Possible4PossibleCluster 11
BBG-42B4244.232.52602305Possible5DistinctCluster 11
BBG-43B4339.229.7160231c6Possible5DistinctCluster 11
BBG-44B4442.581.5180132c6Possible4PossibleCluster 12 south of CTA-7
BBG-45B4545.092.92401304Possible4PossibleCluster 12 south of CTA-7
BBG-46B4639.292.31802204Possible4PossibleCluster 12 south of CTA-7
BBG-47aB47a43.2100.9200323nc8Distinct5DistinctCluster 12 south of CTA-7
BBG-47bB47b39.999.1280223c7Distinct4PossibleCluster 12 south of CTA-7
BBG-48B4841.1115.6160212c5Possible3Possible 
BBG-49B4941.1132.5200121c4Possible3PossibleCluster 13 north of Mo
BBG-50B5046.6142.9160122nc5Possible3PossibleCluster 13 north of Mo
BBG-51B5152.7152.5120121c4Possible3PossibleCluster 13 north of Mo
BBG-52B5245.6153.2240233c8Distinct5DistinctCluster 13 north of Mo
BBG-53B5347.9160.01401203Possible3PossibleCluster 13 north of Mo
BBG-54B5438.6141.8140223nc7Distinct4PossibleCluster 13 north of Mo
BBG-55B5541.0146.6120123nc6Possible3PossibleCluster 13 north of Mo
BBG-56B5641.3156.91402305Possible5DistinctCluster 13 north of Mo
BBG-57B5741.0164.91402204Possible4PossibleCluster 14 between Mo and TOPO-20
BBG-58B5838.7172.81801304Possible4PossibleCluster 14 between Mo and TOPO-20
BBG-59B5938.3179.81401203Possible3PossibleCluster 14 between Mo and TOPO-20
BBG-60B6032.0161.91802204Possible4PossibleCluster 14 between Mo and TOPO-20
BBG-61B6132.1169.02201304Possible4PossibleCluster 14 between Mo and TOPO-20
BBG-62B6232.4175.92001304Possible4PossibleCluster 14 between Mo and TOPO-20
BBG-63B6326.2164.71802305Possible5DistinctCluster 14 between Mo and TOPO-20
BBG-64B6434.8188.71602204Possible4PossibleCluster 14 between Mo and TOPO-20
BBG-65B6531.6205.3220121c4Possible3Possible 
BBG-66B6637.7208.6180332c8Distinct6DistinctCluster 15 south of CTA-17
BBG-67B6737.2219.1320333nc9Distinct6DistinctCluster 15 south of CTA-17
BBG-68B6852.0215.8120123c6Possible3PossibleCluster 16 between Bi and CTA-17
BBG-69B6947.9214.01602204Possible4PossibleCluster 16 between Bi and CTA-17
BBG-70B7034.8248.0200221c5Possible4PossibleCluster 17 between CS and Lo
BBG-71B7140.6251.5220122c5Possible3PossibleCluster 17 between CS and Lo
BBG-72B7245.3261.5220122nc5Possible3PossibleCluster 17 between CS and Lo
BBG-73B7346.5275.8260232nc7Distinct5DistinctCluster 18
BBG-74B7446.0287.62002305Possible5DistinctCluster 18
BBG-75B7551.6296.11801203Possible3PossibleCluster 18; joins with BBG-34
BBG-76B7633.9285.81801304Possible4PossibleCluster 19
BBG-77B7728.8292.11801203Possible3PossibleCluster 19
BBG-78B7841.9309.7200131c5Possible4Possible 
BBG-79B7933.4314.7440121c4Possible3Possible 
BBG-80B8036.1327.4360131nc5Possible4PossiblePartly within impact basin of Im
BBG-81B8144.6328.6280133nc7Distinct4PossiblePartly within impact basin of Im
BBG-82B8245.4342.1400131c5Possible4PossiblePartly within impact basin of Im
BBG-83B8323.840.7360231c6Possible5DistinctBetween Se and Cr
BBG-84B8432.750.2260131c5Possible4PossibleNorth of Cr
BBG-85B8531.962.8280131c5Possible4PossibleNorth of Cr
BBG-86B8629.771.12401304Possible4PossibleNorth of Cr
BBG-87B8725.993.6180213nc6Possible3PossibleCluster 20; north of LF
BBG-88B8831.3100.0180113c5Possible2PossibleCluster 20; north of LF
BBG-89B8926.0104.9240121c4Possible3PossibleCluster 20; north of LF
BBG-90B9025.5111.1200111c3Possible2PossibleCluster 20; north of LF
BBG-91B9134.2106.3180333c9Distinct6DistinctBetween clusters 12 and 20
BBG-92B9234.2118.5200222c6Possible4Possible 
BBG-93B9321.3261.9280121c4Possible3Possible 
BBG-94B9426.5268.5300331c7Distinct6DistinctSouth of Lo
BBG-95B9521.0333.92601304Possible4PossibleWest of CTA-27
BBG-96B9621.0350.0400331nc7Distinct6DistinctCluster 21; close to Pr, CTA-27, and TOPO-40
BBG-97B9728.2358.82802305Possible5DistinctCluster 21; joins cluster 10
BBG-98B987.98.32401304Possible4PossibleCluster 22; between CTA-1 and CTA-2
BBG-99B995.34.12601304Possible4PossibleCluster 22; between CTA-1 and CTA-2
BBG-100B1007.422.9460331c7Distinct6DistinctWithin impact basin of Tr
BBG-101B10114.130.31802305Possible5DistinctCluster 23; between Se and Cr
BBG-102B10216.335.22201304Possible4PossibleCluster 23; between Se and Cr
BBG-103B10317.340.82201304Possible4PossibleCluster 23; between Se and Cr
BBG-104B10411.475.82601304Possible4PossibleBetween Cr and Sm
BBG-105B10513.495.52202204Possible4Possible 
BBG-106B10610.1116.52402204Possible4PossibleCluster 24
BBG-107B10715.0120.0240222c6Possible4PossibleCluster 24
BBG-108B10813.0127.6240121c4Possible3PossibleCluster 24
BBG-109B10921.2160.12202305Possible5DistinctBetween Mo and FS
BBG-110B11015.3154.61601304Possible4PossibleCluster 25 around TOPO-17
BBG-111B11114.4160.61601304Possible4PossibleCluster 25 around TOPO-17
BBG-112B1129.5163.21602305Possible5DistinctCluster 25 around TOPO-17
BBG-113B1138.3169.7200321c6Possible5Distinct 
BBG-114B11411.2183.22002204Possible4PossibleCluster 26 southeast of FS
BBG-115B11513.2189.6220223nc7Distinct4PossibleCluster 26 southeast of FS
BBG-116B11618.7221.52002204Possible4PossiblePart of a cluster east of TOPO-24
BBG-117B11710.0245.5320331c7Distinct6Distinct 
BBG-118B11816.9271.5200223c7Distinct4Possible 
BBG-119B1199.2276.0260221c5Possible4Possible 
BBG-120B12014.5286.32401203Possible3Possible 
BBG-121B1218.8309.42002305Possible5DistinctSouth of CTA-23
BBG-122B12220.7312.52402305Possible5Distinct 
BBG-123B1239.1328.1260121c4Possible3PossibleCluster 27 west of ln
BBG-124B1244.3333.8260221c5Possible4PossibleCluster 27 west of ln
BBG-125B125−4.75.9280121nc4Possible3PossibleSouthwest of CTA-1
BBG-126B126−1.412.8280121c4Possible3PossibleSouthwest of CTA-1
BBG-127B127−6.526.9420331c7Distinct6DistinctLocated partly within impact basin of Ne
BBG-128B1285.247.7460332c8Distinct6DistinctBetween Tr and Fe
BBG-129B129−3.043.73402305Possible5DistinctCluster 28 around Fe
BBG-130B130−7.051.43002305Possible5DistinctCluster 28 around Fe
BBG-131B1311.460.8240121c4Possible3PossibleNortheast of Fe
BBG-132B1323.670.3420131c5Possible4PossibleBetween Cr and Sm
BBG-133B1334.8102.4340131c5Possible4PossibleBetween Sm and AK
BBG-134B1341.3123.62602204Possible4PossibleBetween AK and Me
BBG-135B135−0.8131.62002204Possible4PossibleBetween AK and Me
BBG-136B1363.5167.22201304Possible4PossibleCluster 29 around TOPO-19
BBG-137B137−1.8168.43202305Possible5DistinctCluster 29 around TOPO-19
BBG-138B138−5.3176.51602204Possible4Possible 
BBG-139B139−5.0183.42001203Possible3Possible 
BBG-140B140−6.2192.22001203Possible3PossibleWest of Ko
BBG-141B141−10.0209.01602204Possible4PossibleSoutheast of Ko
BBG-142B142−12.1214.82402204Possible4Possible 
BBG-143B143−5.9224.02201203Possible3PossibleSouthwest of He
BBG-144B144−13.3250.73201304Possible4PossibleEast of TOPO-26 and TOPO-27
BBG-145B145−6.7254.12401203Possible3PossibleNorthwest of Or
BBG-146B146−3.2259.02001203Possible3PossibleNorth of Or
BBG-147B147−3.6265.7280131c5Possible4PossibleNorth of Or
BBG-148B1480.3268.92801304Possible4PossibleNorth of Or
BBG-149B149−4.6273.52601203Possible3PossibleNorth of Or
BBG-150B150−9.1278.72401203Possible3PossibleNortheast of Or
BBG-151B151−16.9282.22801304Possible4PossibleBetween Or and TOPO-30
BBG-152B15212.2296.62001203Possible3Possible 
BBG-153B153−12.9308.5420131c5Possible4PossibleBetween Cr, Hu, TOPO-30, and TOPO-35
BBG-154B154−20.2304.3340131c5Possible4PossibleBetween TOPO-30 and Hu
BBG-155B155−11.4324.52201203Possible3PossibleCluster 30 around TOPO-30, north of Hu
BBG-156B156−4.7326.22401203Possible3PossibleCluster 30 around TOPO-30, north of Hu
BBG-157B157−9.4335.82402204Possible4PossibleCluster 30 around TOPO-30, north of Hu
BBG-158B158−13.7331.63602305Possible5DistinctCluster 30 around TOPO-30, north of Hu
BBG-159B159−1.5350.92601203Possible3PossibleCluster 31
BBG-160B160−7.7348.7260121c4Possible3PossibleCluster 31
BBG-161B161−14.2352.43202305Possible5DistinctCluster 31; northeast of Nu
BBG-162B162−9.912.8320231c6Possible5DistinctBetween Ne, WA, and TOPO-2
BBG-163B163−18.813.4360231c6Possible5DistinctBetween Ne, WA, and TOPO-2
BBG-164B164−25.619.63401203Possible3PossibleBetween Ne and WA
BBG-165B165−29.328.62602305Possible5DistinctBetween Ne and WA
BBG-166B166−20.855.0400131c5Possible4Possible 
BBG-167B167−27.181.4240123c6Possible3Possible 
BBG-168B168−16.689.32402305Possible5DistinctCluster 32 south of Sm
BBG-169B169−17.896.0180123nc6Possible3PossibleCluster 32 south of Sm
BBG-170B170−11.6104.8280123nc6Possible3PossibleCluster 32 south of Sm
BBG-171B171−19.4137.32402305Possible5DistinctSouth of TOPO-12
BBG-172B172−16.9154.73002305Possible5DistinctCluster 33, east-west band, around TOPO-18
BBG-173B173−21.3159.62802305Possible5DistinctCluster 33, east-west band, around TOPO-18
BBG-174B174−18.7165.42602305Possible5DistinctCluster 33, east-west band, around TOPO-18
BBG-175B175−22.2175.03001203Possible3PossibleCluster 33, east-west band
BBG-176B176−21.9184.9220132c6Possible4PossibleCluster 33, east-west band
BBG-177B177−22.9191.8220122c5Possible3PossibleCluster 33, east-west band
BBG-178B178−18.9194.2260132c6Possible4PossibleCluster 33, east-west band, southeast of CTA-15
BBG-179B179−19.2198.8260132c6Possible4PossibleCluster 33, east-west band
BBG-180B180−24.6200.9180222c6Possible4PossibleCluster 33, east-west band
BBG-181B181−21.3207.3200222c6Possible4PossibleCluster 33, east-west band
BBG-182B182−21.3233.1760122nc5Possible3Possible 
BBG-183B183−37.01.72202305Possible5Distinct 
BBG-184B184−39.765.2160121c4Possible3PossibleCluster 34
BBG-185B185−35.469.9220121c4Possible3PossibleCluster 34
BBG-186B186−35.776.8200121c4Possible3PossibleCluster 34
BBG-187B187−38.098.3200121c4Possible3PossibleCluster 35, southeast of TOPO-17
BBG-188B188−34.6104.0200121c4Possible3PossibleCluster 35, southeast of TOPO-17
BBG-189B189−37.1108.0200122c5Possible3PossibleCluster 35, southeast of TOPO-17
BBG-190B190−36.2120.71801203Possible3Possible 
BBG-191B191−25.4145.32202305Possible5DistinctCluster 36
BBG-192B192−31.4146.82602305Possible5DistinctCluster 36, north of TOPO-13
BBG-193B193−33.6153.22002305Possible5DistinctCluster 36, north of TOPO-13
BBG-194B194−38.1179.3260123nc6Possible3PossibleCluster 37 between ln and Ap
BBG-195B195−33.3186.8220121c4Possible3PossibleCluster 37 between ln and Ap
BBG-196B196−35.3193.9220131c5Possible4PossibleCluster 37 between ln and Ap
BBG-197B197−40.9189.4200123c6Possible3PossibleCluster 37 between ln and Ap
BBG-198B198−28.4250.9280232c7Distinct5DistinctCluster 38 between Or and CTA-19
BBG-199B199−32.9254.1200021c3Possible2PossibleCluster 38 between Or and CTA-19
BBG-200B200−36.3252.81801304Possible4PossibleCluster 38 between Or and CTA-19
BBG-201B201−37.9259.22001304Possible4PossibleCluster 38 between Or and CTA-19
BBG-202B202−35.2262.32201304Possible4PossibleCluster 38 between Or and CTA-19
BBG-203B203−36.0274.04002305Possible5DistinctBetween Or and MR
BBG-204B204−29.3280.32401203Possible3PossibleSoutheast of Or
BBG-205B205−30.9306.23602305Possible5DistinctSouthwest of Hu
BBG-206B206−38.3311.12001304Possible4PossibleSouthwest of Hu
BBG-207B207−39.8320.72201203Possible3PossibleSouth of Hu
BBG-208B208−25.2337.72201203Possible3PossibleBetween Hu and Nu
BBG-209B209−23.7350.1400232c7Distinct5DistinctSoutheast of Nu
BBG-210B210−34.3332.0320121nc4Possible3PossibleCluster 39 southeast of Nu
BBG-211B211−40.0334.4200121c4Possible3PossibleCluster 39 southeast of Nu
BBG-212B212−38.8342.2180121c4Possible3PossibleCluster 39 southeast of Nu
BBG-213B213−32.7354.5260322c7Distinct5Distinct 
BBG-214B214−45.92.71801203Possible3Possible 
BBG-215B215−42.834.42202204Possible4Possible 
BBG-216B216−58.355.9200121nc4Possible3PossibleCluster 40
BBG-217B217−54.763.7220121c4Possible3PossibleCluster 40
BBG-218B218−54.376.1180221nc5Possible4PossibleCluster 40
BBG-219B219−47.693.32002204Possible4PossibleNorthwest of Au
BBG-220B220−48.3114.0160122c5Possible3PossibleCluster 41 around TOPO-9
BBG-221B221−53.0114.7180121c4Possible3PossibleCluster 41 around TOPO-9
BBG-222B222−54.8122.5160131c5Possible4PossibleCluster 41 around TOPO-9
BBG-223B223−49.0131.81402204Possible4PossibleCluster 42
BBG-224B224−45.3133.91602204Possible4PossibleCluster 42
BBG-225B225−40.1138.7240121c4Possible3PossibleSoutheast of CTA-12
BBG-226B226−45.9147.4200121c4Possible3Possible 
BBG-227B227−41.6158.2260232nc7Distinct5DistinctSouthwest of ln
BBG-228B228−48.1175.6220332c8Distinct6Distinct 
BBG-229B229−47.0216.5140221c5Possible4Possible 
BBG-230B230−39.7228.8180132c6Possible4PossibleCluster 43 between Ap and MR
BBG-231B231−44.0232.6200132c6Possible4PossibleCluster 43 between Ap and MR
BBG-232B232−44.2238.7220132c6Possible4PossibleCluster 43 between Ap and MR
BBG-233B233−48.7233.3160132c6Possible4PossibleCluster 43 between Ap and MR
BBG-234B234−50.6241.2180132c6Possible4PossibleCluster 43 between Ap and MR
BBG-235B235−53.6235.6180132c6Possible4PossibleCluster 43 between
BBG-236B236−45.3289.41603205Possible5DistinctCluster 44 between MR and SZ
BBG-237B237−51.2291.1220121c4Possible3PossibleCluster 44 between MR and SZ
BBG-238B238−50.0299.21802305Possible5DistinctCluster 44 between MR and SZ
BBG-239B239−55.0293.9180121c4Possible3PossibleCluster 44 between MR and SZ
BBG-240B240−60.6297.9140121c4Possible3PossibleCluster 44 between MR and SZ
BBG-241B241−54.8337.12001203Possible3PossibleCluster 45
BBG-242B242−52.4344.11801203Possible3PossibleCluster 45
BBG-243B243−65.220.7160121c4Possible3PossibleCluster 46
BBG-244B244−71.026.0140121c4Possible3PossibleCluster 46
BBG-245B245−69.237.3180121nc4Possible3PossibleCluster 46
BBG-246B246−63.839.5120221c5Possible4PossibleCluster 46
BBG-247B247−63.591.6300221c5Possible4PossibleCluster 47 between Au and SR
BBG-248B248−59.4104.3200221c5Possible4PossibleCluster 47 between Au and SR
BBG-249B249−62.0117.21201203Possible3PossibleBetween SR and Pl
BBG-250B250−59.9148.0140131c5Possible4PossibleCluster 48 between Pl and Po
BBG-251B251−64.7143.2160121nc4Possible3PossibleCluster 48 between Pl and Po
BBG-252B252−67.2150.3120121c4Possible3PossibleCluster 48 between Pl and Po
BBG-253B253−60.2184.01601203Possible3PossibleSouth of SA
BBG-254B254−57.9206.0160121nc4Possible3PossibleCluster 49 southeast of TOPO-23
BBG-255B255−61.5203.1140121c4Possible3PossibleCluster 49 southeast of TOPO-23
BBG-256B256−66.1199.3240221c5Possible4PossibleCluster 49 southeast of TOPO-23; joins cluster 50
BBG-257B257−73.0202.8160121c4Possible3PossibleCluster 50 southeast of TOPO-21; joins cluster 49
BBG-258B258−78.0208.7100121c4Possible3PossibleCluster 50 southeast of TOPO-21
BBG-259B259−82.1216.1100121c4Possible3PossibleCluster 50 southeast of TOPO-21
BBG-260B260−83.1185.91401203Possible3PossibleCluster 50 southeast of TOPO-21
BBG-261B261−77.9183.4140121nc4Possible3PossibleCluster 50 southeast of TOPO-21
BBG-262B262−74.7170.9180121c4Possible3PossibleCluster 50 southeast of TOPO-21
BBG-263B263−60.9258.51801203Possible3PossibleCluster 51 southwest of MR
BBG-264B264−63.6249.7240121nc4Possible3PossibleCluster 51 southwest of MR
BBG-265B265−67.0240.7160121c4Possible3PossibleCluster 51 southwest of MR
BBG-266B266−72.3246.81001304Possible4PossibleCluster 51 southwest of MR
BBG-267B267−76.6242.2801304Possible4PossibleCluster 51 southwest of MR
BBG-268B268−71.2274.0140222c6Possible4Possible 
BBG-269B269−61.0277.4200221c5Possible4PossibleSouth of PH
BBG-270B270−62.7320.71601203Possible3PossibleSoutheast of SZ
BBG-271B271−69.7344.1100221c5Possible4Possible 
BBG-272B272−62.1358.1140121c4Possible3Possible 
BBG-273B273−77.936.1120122c5Possible3PossibleCluster 52
BBG-274B274−74.851.1180122c5Possible3PossibleCluster 52
BBG-275B275−81.459.51001203Possible3PossibleCluster 52
BBG-276B276−76.579.6160222c6Possible4PossibleCluster 53 southwest of SR
BBG-277B277−73.795.0160232c7Distinct5DistinctCluster 53 southwest of SR
BBG-278B278−85.6277.7100322c7Distinct5Distinct 
BBG-279B279−78.6308.7260132c6Possible4PossibleCluster 54
BBG-280B280−77.6330.21601203Possible3PossibleCluster 54

Acknowledgments

[38] We would like to thank the Australian Research Council for funding through discovery project grants DP0663020 and DP120102441. We also thank the model producers for making their lunar gravity field and topography models freely available. We are grateful to three anonymous reviewers for their thorough reviews and attention to detail, and to the Editor, Mark Wieczorek, for handling our manuscript and suggesting the whole-of-Moon analysis. Some of the figures were produced using GMT [Wessel and Smith, 1998]. This is the Institute for Geoscience Research (TIGeR) publication 471.