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I. Antonenko Difficult to Correlate to Topography

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I. Antonenko Difficult to Correlate to Topography PLANETARY INSTITUTE OF TORONTO Re-Examining the Identification of Methods A total of 60 DHC's were identified the literature [2-11]. Only craters identified specifically by Dark-Haloed Impact Craters: name or on good quality images were consid- Introduction New Criteria for Modern Data Sets ered, because many studies [e.g. 2, 3, 4] only Dark-haloed impact craters (DHC's) are used to show DHC's in high sun angle images, which are identify and characterize hidden mare deposits I. Antonenko difficult to correlate to topography. (cryptomaria), expanding our understanding of Each crater was a located and measured using volcanism and thermal history of the Moon [1]. Planetary Institute of Toronto, [email protected] JMARS Moon software [12]. The craters were Historically, DHC’s have been identified by: then evaluated using various data sets [13-18]. • Low albedo halos • Symmetrical halos Craters were examined for: • Mature ejecta • Absence of impact melts • low albedo halo Wt% FeO around a crater of decidedly impact origin [2]. 20 • halo symmetry Much new lunar data have been obtained since 12 • halo maturity then, so it is important to revisit these criteria. • halo iron composition 10 • presence of mafic spectra 8 • distinct impact topography 0 Lunar Reconnaissance Orbiter • absence of impact melt features Narrow Angle Camera M139191423RC Image centre: 108.53E, 21.17 Clementine FeO over Clementine 750 nm Band Lunar Reconnaissance Orbiter Wide Angle Camera 10 km 10 km Image centre: 109.3E, 20.5 Image centre: 109.3E, 20.5 About 37% of the DHC’s studied have non-symmetrical low Iron anomalies were found at every DHC where FeO data albedo haloes, like this one (red arrow). Note how the lack of was available. The iron signature generally matches albedo topographic information in the high sun angle Clementine (red arrow). Elevated iron levels are sometimes even seen data makes orientation difficult (compare this image to the inside the crater (black arrow). The circular, highly sym- 200 m one on the right, which shows the same area at low sun in metrical, and bowl-shaped topography of the crater is Topography was also observed in LOLA Impact melt flow features were rarely identified LROC WAC data). Yellow box shows view to the left. clearly seen in the low sun LROC WAC image. data. Kapteyn B profile shows steep sides around any of the DHC’s, but melts may be ubiq- and a flat floor (from dashed line below). uitous. Almost all DCH’s exhibit smooth dark patches around their rims in high resolution Two craters had no dark halos, but showed very distinct FeO anomalies, and basalt-like spectra. NAC images. These are thought to be indicative Topography of all the DHC's in this study was indicative of impact origin. Wt% FeO of impact melt veneers [e.g. 19]. 20 However five small craters (1-2.5 km in di- ameter) did not register in the LOLA data. 12 10 OMAT 0.5 8 0 0.2 Clementine 750 nm Band Clementine FeO over LROC WAC 0 20 km 20 km Image centre: 71.0E, -15.6 Image centre: 71.0E, -15.6 Basalt spectra were noted Clementine 750 nm Band Kapteyn B was identified by [10] as having a Distinct FeO anomalies are observed on the rim [3, 4, 7, 9-11] for half the Image centre: 111.2E, 17.8 5 km Clementine Optical Maturity over LROC WAC dark halo. This study found the albedo around of Kapteyn B, indicating the presence of mafic DHC's. Kapteyn B spectra 5 km Image centre: 341.7E, 6.9 In Clementine data, the low albedo halo and the crater to be consistent with the surroundings. materials. Dashed line shows profile location. [10] (highlighted in green). Only half the DHC's were surrounded by crater morphology of this DHC can be seen. mature soils, based on optical maturity (OMAT) maps [16]. Copernicus H (shown here), along with 13% of the DHC’s, is sur- Discussion Implications rounded by completely immature soils. Well-established DHC's do not conform to histori- Identifying DHC's using mostly compositional cal criteria. Halo symmetry, maturity, and even low data has significant implications for estimating albedo are shown to be variable properties of cryptomaria. For example, basalts exposed on Table of DHC Observations DHC’s. Even the absence of melt requirement is crater walls are compositionally detectable, but Crater Longitude Latitude Diameter Dark Halo Halo Maturity FeO Mafic Crater Identifier and # (deg E) (km) Symmetry Anomaly Spectra Reference questionable, since high resolution images suggest they do not show whether the basalts have been ex- 1 7.57 14.40 1.72 LO, Clem, WAC Yes No Yes E of Manilius, [2] LOLA contours over LROC WAC 2 10.41 6.13 1.56 LO, Clem, WAC Yes No Yes N of Agrippa, [11] 3 13.93 9.76 6.75 LO, Clem, WAC Patchy Yes Yes Julius Caesar B, [11] 5 km 4 17.28 2.75 17.52 LO, Clem, WAC Yes Moderately Yes [11] Dionysius, [2,11] Image centre: 71.0E, -15.6 that many fresh crater rims may have melt veneers. cavated beyond the crater rim [1]. Each crater type 5 17.59 5.13 0.89 LO, Clem Yes Moderately Yes in Ariadaeus E, [11] 6 55.78 -10.44 8.76 Clem, WAC Mostly Moderately Yes Langrenus D/Al- Marrakushi, [5] 7 58.30 -6.80 6.60 Clem Mostly Moderately Yes Langrenus KA, [5] provides different information about the depth and 8 60.14 -5.64 13.25 Clem, WAC Yes Yes Yes Langrenus C/Acosta [5] But, because the crater is so small, it falls The most consistent and reliable DHC criteria was 9 63.63 -20.80 4.80 Clem, WAC Yes Moderately Yes [10] [10] #10, [6] 10 67.71 -14.16 1.82 Clem Mostly Yes Yes [10] #21 11 68.26 -15.30 1.18 Clem Mostly Moderately Yes [10] #20 12 68.29 -14.08 1.36 Clem Yes Yes Yes [10] #22 between the LOLA sampling grid points. thickness of the underlying basalt layers. More 13 68.52 -22.12 5.86 Clem Mostly Yes Yes [10] #1 found to be the presence of high iron content or 14 68.66 -23.12 3.45 Clem Yes Yes Yes [10] #2 15 69.15 -18.92 1.13 No Yes [10] #14 16 69.29 -23.31 5.28 Clem Yes Yes Yes [10] #3 work is needed to learn how to distinguish between 17 69.57 -23.21 2.76 Clem, WAC Yes Moderately Yes [10] [10] #4 mafic minerals associated with impact topography. 18 69.61 -20.22 1.51 Clem Yes Moderately Yes [10] #12 19 69.69 -23.04 1.01 Clem, WAC Yes Moderately Yes near [10] #4 20 69.84 -20.12 1.95 Clem Directed Moderately Yes [10] [10] #13 such craters in order to accurately estimate crypto- 21 70.45 -23.66 1.86 Clem, WAC Yes Moderately Yes [10] [10] #5 22 70.50 -27.46 5.85 Clem Directed Yes Yes Legendre N, [10] #8 23 70.71 -13.3 3.25 Clem Mostly Yes Yes [10] #25 24 70.73 -23.46 1.12 Clem Yes Moderately Yes [10] [10] #6 mare volumes. 25 70.98 -15.60 39.67 Yes Yes [10] Kapteyn B, [10] #19 26 71.94 -25.05 2.88 Clem Yes n/a n/a near [10] #7 27 72.30 -24.32 1.50 Clem Yes Yes Yes [10] #9 28 72.32 -18.27 2.69 Clem Yes Moderately Yes [10] #24 29 93.74 8.02 9.37 LO, Clem, WAC Yes n/a n/a [9] [9] #16 References 30 93.83 11.53 5.35 LO, Clem, WAC Yes n/a n/a [9] [9] #17 31 94.38 7.76 9.19 Clem Directed Yes Yes [9] [9] #15 [1] Antonenko I. (1999), Thesis, Brown Univ. p309. [2] Schultz P.H. and Spudis P.D. 32 94.48 8.47 17.90 LO, Clem, WAC Yes Yes Yes [9] [9] #14 33 96.83 -39.17 7.27 Clem, WAC Yes Moderately Yes N of Jenner, [2] 34 98.04 19.06 27.40 Clem Patchy Yes Yes [9] [9] #11 (1979), Proc LPSC 10, 2899-2918. [3] Hawke B.Ray and Bell J.F. (1981), Proc. Lunar 35 98.53 -34.82 6.39 AS8-12-2192 Yes n/a n/a NW of Gernsback, [2] 36 101.22 -38.81 2.25 Clem Yes Moderately Yes S of Gernsback J, [2] Planet. Sci. 12B, 665-678. [4] Bell J.F. and Hawke B.Ray (1984), JGR 89, B8 6899-6910. 37 104.81 20.23 6.81 LO, Clem, WAC Yes Moderately Yes [9] S of Malyy,[2],[9] #7 38 106.94 22.57 9.00 Clem Mostly Yes Yes [9] [9] #1 [5] Hodges C.A. (1973), Lunar Map I-739, USGS. [6] Hawke B.Ray and Spudis P.D. 39 107.66 24.18 13.31 Clem, WAC Mostly Yes Yes [9] [9] #3 40 107.88 22.77 4.76 Clem, WAC Mostly Moderately Yes [9] NE of Malyy G, [2],[9] Future Work #2 (1980), Proc. Conf. Lunar Highlands Crust, 467-481. [7] Hawke B.Ray et al. (1993), GRL 41 108.61 21.39 11.46 LO, Clem Directed Yes Yes [9] [9] #8 42 109.57 -39.75 10.92 Clem Yes Yes Yes NW of Pogson, [2] 20, 419-422. [8] Blewett D.T.
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