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Physical Constraints on Impact Melt Properties from Lunar Reconnaissance Orbiter Camera Images ⇑ Brett W

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Physical Constraints on Impact Melt Properties from Lunar Reconnaissance Orbiter Camera Images ⇑ Brett W Icarus 219 (2012) 665–675 Contents lists available at SciVerse ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Physical constraints on impact melt properties from Lunar Reconnaissance Orbiter Camera images ⇑ Brett W. Denevi a, , Steven D. Koeber b, Mark S. Robinson b, W. Brent Garry c, B. Ray Hawke d, Thanh N. Tran b, Samuel J. Lawrence b, Laszlo P. Keszthelyi e, Olivier S. Barnouin a, Carolyn M. Ernst a, Livio L. Tornabene f a Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA b School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85251, USA c Planetary Science Institute, Tucson, AZ 85719, USA d Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822, USA e Astrogeology Science Center, US Geological Survey, Flagstaff, AZ 86001, USA f Center for Planetary Science and Exploration, University of Western Ontario, London, ON, Canada article info abstract Article history: Impact melt flows exterior to Copernican-age craters are observed in high spatial resolution (0.5 m/pixel) Received 11 February 2011 images acquired by the Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC). Revised 12 March 2012 Impact melt is mapped in detail around 15 craters ranging in diameter from 2.4 to 32.5 km. This survey Accepted 18 March 2012 supports previous observations suggesting melt flows often occur at craters whose shape is influenced by Available online 24 March 2012 topographic variation at the pre-impact site. Impact melt flows are observed around craters as small as 2.4 km in diameter, and preliminary estimates of melt volume suggest melt production at small craters Keywords: can significantly exceed model predictions. Digital terrain models produced from targeted NAC stereo Moon images are used to examine the three-dimensional properties of flow features and emplacement setting, Moon, Surface Cratering enabling physical modeling of flow parameters. Qualitative and quantitative observations are consistent with low-viscosity melts heated above their liquidii (superheated) with limited amounts of entrained solids. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction volume of melt at craters <10 km is generally insufficient for it to coalesce and flow without being choked by fragmental debris Hypervelocity impacts by asteroids, comets, and other inter- (Hawke and Head, 1977). The exterior distribution of melt around planetary debris release large amounts of energy, often destroying a crater was shown to be affected by preexisting topographic vari- the impactor and fragmenting, transporting, melting, and in some ations at the impact site that result in lows in the crater rim, cases vaporizing portions of the target (Gault et al., 1968). The through which melt can preferentially escape (Hawke and Head, cumulative impact history of the lunar surface results in soils (reg- 1977), as well as impact angle, with melt found preferentially in olith) that are comprised of large fractions of impact-produced the down-range direction (Howard and Wilshire, 1975; Hawke glass, and even the smallest impacts generate glass-lined craters and Head, 1977). on rocks at the micron to centimeter scale (e.g. Roddy et al., Features associated with flows of impact melt are of particular 1977). In this study we focus on the macroscopic (meter- to hecto- interest because the morphology and dimensions of such flows meter-scale) morphologic attributes of crater deposits interpreted can yield insights into properties of the melt, including its rheology to have formed from material melted by impacts that formed cra- (from which temperature and clast content can be inferred), as ters kilometers to tens of kilometers in diameter. For craters of this well as flow rates and cooling rates through analogy with terres- scale, impact melts typically occur as thin coatings (veneers <1 m trial lava flows (e.g. Walker, 1973; Hulme, 1974; Pinkerton and thick), as ponds on crater floors, terraces, and exterior surfaces, Wilson, 1994; Cashman et al., 1999; Gregg and Fink, 2000; Harris and as channels and lobes formed through down-slope movement and Rowland, 2009; Hauber et al., 2011). of melt (Howard and Wilshire, 1975; Hawke and Head, 1977). The Some portion of melt created by impacts is thought to be heated hundreds of degrees above its liquidus, a condition often referred to as superheated. The high temperatures of melt are inferred from ⇑ Corresponding author. impact-produced shock pressures of tens to hundreds of GPa, E-mail address: [email protected] (B.W. Denevi). which result in correspondingly high temperatures (Stöffler, 0019-1035/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2012.03.020 666 B.W. Denevi et al. / Icarus 219 (2012) 665–675 Table 1 Summary of craters for which impact melt flows were mapped. Oblique column indicates whether the ejecta pattern indicates an oblique impact, topography column indicates whether preexisting topography at the impact site (such as an older impact crater) influenced the crater shape. Crater name or Crater Latitude Longitude Oblique Topography Number Flow Maximum Average NAC NAC images used location diameter (°) (°E) of flows area flow slope (°) coverage (km) (sq. length (%) km) (km) S of Ingalls G 2.4 24.2 209.9 No Yes 1 0.2 0.9 4.5 80 M153863408, M158578867 Hesiodus E 3.0 À27.9 344.6 No No 3 0.0 0.4 1.3 85 M109332207, M131751750, M144722138 S of Steno N 3.0 30.0 161.0 No Yes 1 3.5 3.4 5.0 95 M108182454, M112902715, M130592665, M128231242 Near Kovalsky 4.5 À21.0 100.0 No Yes 1 0.1 0.7 9.3 100 M106218135, M113297625, M119197340, M121558772, M126276067, M128637465, M161652188 Rim of Gibbs 4.8 À17.5 85.2 No Yes 5 2.0 1.5 4.3 85 M103954588, M115754243, M128732564, M139348293, M146425186, M152315509, M152322295 Litchtenberg B 4.8 33.3 298.5 No No 13 0.2 1.0 7.0 70 M168604140, M153286591, M109636737, M120257109, M137949855 NE of 8.7 13.2 127.6 Yes No 9 3.6 2.3 9.6 70 M103674814, M110757216, M123737802, Meshcherskiy M136708426, M143784896, M149683464, M159121513, M165007796 Near Sklodowska 9.8 À17.0 93.9 Yes Yes 1 0.5 1.5 2.8 90 M108618565, M121599564, M126316879, M128678281, M136933002, M141648673, M149900065, M152261237, M161692980 O’Day M 12.4 À31.6 157.2 No Yes 5 2.2 3.0 18.8 70 M105845279, M110561202, M112924264, M115286121, M118823947, M121185457, M123540237, M134158057, M143595833, M164818681 Mandel’shtam F 15.0 5.1 166.1 Yes No 6 12.8 4.3 5.5 95 M105787191, M110506225, M115224385, M115231170, M136457598, M141180043, M151786761, M154148046, M158863498, M163579812, M164756882, M167111745, M167118530, M169473135, M174190392, M176541713, M176548460 Rim of Korolev X 16.4 1.1 200.5 No Yes 1 51.3 12.3 3.8 85 M105557749, M110275526, M118539609, M125618115, M145664820, M145671603, M148024805, M112638407, M176319163 Byrgius A 18.6 À24.5 296.2 No Yes 7 12.8 4.5 11.7 95 M102573276, M102580439, M107293947, M109651403, M109658189, M112007213, M112014030, M114369897, M114376684, M117901138, M117907930, M120262823, M120269605, M122631094, M124992970, M127341132, M127347886, M133248775, M135610086, M142686769, M145040947, M145047737, M147401544, M148570061, M148576875, M150931237, M153285478, M153292309, M155647355, M158001042, M158007798, M160362819, M166270828 Proclus 26.9 16.1 46.9 Yes Yes 5 0.6 1.4 18.0 85 M104204440, M104211600, M106568866, M106576039, M108930569, M113643184, M113649971, M116004716, M121898878, M126622820, M134876523, M139598775, M141960238, M144314177, M146675452, M152573983, M159644209, M161998965, M165537215 Weiner F 30.0 41.0 150.0 No Yes 1 197.8 21.5 0.0 75 M103538324, M108250104, M108256905, M108263674, M110620925, M112977209, M115332250, M118866043, M118872789, M121220727, M121227545, M123582321, M123589104, M125944637, M125951441, M128306064, M130660682, M130667502, M130674265, M136558673, M136565448, M136572231, M141287909, M143635132, M143641913, M149534747, M149541568, M151889185, M151902753, M154250505, M154257291, M156605386, M156612169, M161320867, M161334451, M164858005, M164871560, M171943021 Necho 32.5 À5.2 123.3 No Yes 2 113.6 17.3 6.1 95 M103703826, M106060009, M106067165, M108427492, M110784724, M113147897, M115502787, M115509575, M119041553, M119048343, M121403021, M121409805, M123757801, M123764608, M126120247, M126127019, M128488433, M130849797, M134374642, M134381429, M134388215, M136735908, M139096931, M139103726, M141458450, M143812371, M146167001, B.W. Denevi et al. / Icarus 219 (2012) 665–675 667 Table 1 (continued) Crater name or Crater Latitude Longitude Oblique Topography Number Flow Maximum Average NAC NAC images used location diameter (°) (°E) of flows area flow slope (°) coverage (km) (sq. length (%) km) (km) M154426096, M154432886, M156780821, M156787618, M159148294, M161496407, M161503176, M165035267, M165035301, M167396918 Fig. 1. Examples of lunar impact melt flows. North is up in all images. (A) Unnamed crater SE of Das G (29.0°S, 227.3°E, Image M118362649L). (B) Giordano Bruno, 5.5 km from the rim (35.4°N, 102.9°E, Image M101476840L). (C) Unnamed 9-km crater (13.6°N, 234.5°E, Image M118315948L). (D) Unnamed crater, 4.8 km in diameter, on the rim of Gibbs crater. Flow is 340 m from the rim (17.5°S, 85.2°E, NAC images for this and subsequent craters are listed in Table 1). 1971; O’Keefe and Ahrens, 1975), and from studies of terrestrial tions of impact melt flows combined with physical models permit impact melts where refractory minerals such as zircon (ZrSiO4) qualitative and quantitative assessments of the properties of melts have undergone partial melting or decomposed to baddeyelite during their emplacement shortly after the impact event. The (ZrSiO2) and silica (SiO2), indicating temperatures >1775 °C (e.g. observations and measurements presented in this work provide El Goresy, 1965). There is evidence for superheated impact melts the means to evaluate the modes of emplacement of melt flows on the Moon as well, where the volume percent of clasts and nat- around impact craters and assess the possibility that large volumes ure of clast digestion in impact melt-bearing rocks indicates tem- of melt are superheated during the impact event.
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