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

Icarus 219 (2012) 665–675

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Icarus

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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,

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 27.9 344.6 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 21.0 100.0 No Yes 1 0.1 0.7 9.3 100 M106218135, M113297625, M119197340, M121558772, M126276067, M128637465, M161652188 Rim of Gibbs 4.8 17.5 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 17.0 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 31.6 157.2 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 24.5 296.2 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 5.2 123.3 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. peratures >1450 °C for a liquidus temperature of 1310 °C (Simonds et al., 1976). Evidence for such superheating may also exist in impact melt 2. Impact melt deposits observed in LROC NAC images flow morphology. Observations of melt ponds and veneers can help elucidate the total volume of melt and its distribution and ballistic We examine well preserved impact melt flows associated emplacement (Bray et al., 2010) at a given impact site; observa- with Copernican craters (Fig. 1), and map in detail the melt flows 668 B.W. Denevi et al. / Icarus 219 (2012) 665–675

exterior to 15 craters using LROC NAC images (Table 1). Impact melt deposits in nine of the fifteen craters were previously identified by Hawke and Head (1977); six were identified from Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images and extend the size range down to craters 2.4 km in diameter. All craters in our study have at least 70% NAC coverage. NAC images are acquired by two separate 5064-pixel line scan imagers, which are mounted on the spacecraft such that their fields-of-view (FOV) overlap by 135 pixels. This configuration allows images obtained simultaneously from the two cameras to be mosaicked together to cover a wider swath (the combined FOV is 5.7° wide). The detectors have 7-lm pixels and the focal length of each system is 700 mm, resulting in an instantaneous field of view of 10 lrad and a pixel scale of 0.5 m from LRO’s primary 50-km orbit (Robinson et al., 2010). Multiple NAC images were investigated per crater, often allowing comparison of images at complementary illumination conditions. In order to assess the influence of surrounding topography and impact angle on melt distribution, we use LROC Wide Angle Camera (WAC) monochrome images at pixel scales of 100 m. WAC images were selected with illumination geometries favorable for viewing morphology (large incidence angles, where incidence angle is the solar illumination angle measured from the surface normal), as well as with illumination favorable for viewing albedo variations (small incidence an- Fig. 2. A crater 3.0 km in diameter located south of crater Steno N at 30.0°N, gles). The global lunar digital terrain model (DTM) with a 161.0°E. Impact melt (outlined in red) has flowed over 3 km from the parent crater sampling scale of 100 m (GLD100) (Scholten et al., 2011) provides rim; also note the smooth deposit that formed from impact melt that pooled on the topographic information and allows the slope along each melt flow crater floor. to be measured. These datasets provide an excellent resource for extending previous observations to smaller craters, studying the

Fig. 3. Examples of the effects of preexisting topography at the impact site on melt flow distribution. Flows are highlighted in red, dotted white lines indicate the approximate underlying crater rim where not obvious. (A) Crater O’Day M (31.6°S, 157.2°E; 12.4 km in diameter) formed on the rim of O’Day; melt from O’Day M flowed into O’Day. (B) A 2.4-km crater south of Ingalls G, where melt flowed into and ponded in a topographic low (24.2°N, 209.9°E). (C) A 16.4-km crater (1.1°N, 200.5°E) that formed on the rim of Korolev X; melt flowed from this crater into Korlev X. (D) A 4.8-km crater that formed on the rim of Gibbs (a detail of which is shown in Fig. 1D). All exterior melt flows moved away from Gibbs’ interior, opposite the low in the crater rim crest. B.W. Denevi et al. / Icarus 219 (2012) 665–675 669

two-dimensional gaussian and paraboloid crater profiles (Melosh, 1989), assuming the flat floor in the measured profile is due to pooling of impact melt. This estimated melt volume is 12 times higher than predicted by models of impact melt production based on analytical and hydrocode methods (Cintala and Grieve, 1998; Pierazzo et al., 1997; Watters et al., 2009). Assuming an average flow thickness of 1 m (LOLA profiles show the flow is convex, with a maximum thickness of 2–3 m near its center; no NAC stereo imagery has yet been acquired for this crater), the total volume of melt observed at this crater increases by an additional 0.003 km3 and the total volume of melt is 1.5% of the displaced crater volume. Of the fifteen craters studied here, four have ejecta patterns indicative of an oblique impact (Mandel’shtam F, a crater north- west of Sklodowska, Proclus, and a crater northeast of Meshcher- skiy; Table 1). We interpret the impact direction to be from the center of the zone of avoidance in the ejecta distribution which occurs for craters with an impact angle of <30° from the horizontal (Gault and Wedekind, 1978). For these oblique craters, the flows Fig. 4. Impact melt flows on the exterior of Lichtenberg B (33.3°N, 298.5°E; 4.8 km are generally downrange of the impact in three of the four cases, in diameter). Lichtenberg B does not display an asymmetric ejecta pattern consistent with previous work (Howard and Wilshire, 1975; indicative of an oblique impact, and surviving topography adjacent to the crater Hawke and Head, 1977). However, flows are also found close to suggests that no large topographic variations existed at the pre-impact site. the edge of the zone of avoidance (Mandel’shtam F, Proclus) or Thirteen individual flows are observed outside the crater rim, the most of any crater mapped in this work, and the flows are distributed around much of the crater. The without a clear relationship to impact angle (Proclus). Eleven of far eastern side of this crater has not been imaged with the NAC at favorable the fifteen craters were affected to varying degrees by preexisting illumination conditions; WAC coverage is shown instead. topographic variation at the impact site – they formed atop a break in slope due to the presence of an older crater – and exhibit irreg- ular shapes with one portion of the rim lower than others. Several topography of impact sites, and assessing the emplacement, mor- are extreme examples, where a portion of the crater wall collapsed phology, and physical properties of impact melt flows. (Fig. 3). In most of these extreme cases, melt flowed through the Observations of impact melt in LROC NAC images suggest that collapsed wall into the topographic low, consistent with observa- relatively large impact melt flows can occur around craters as tions of Hawke and Head (1977). However, for a crater that formed small as 2.4 km in diameter; six of the craters in this study were on the rim of Gibbs, flows are predominantly observed opposite 5 km in diameter or smaller. An example is an unnamed 3-km cra- the collapsed wall (Fig. 3D). Of the eleven craters affected by ter at 30.0°N, 161.0°E, where melt has pooled within the floor and topography, six have flows that moved into the topographic low, flowed over one crater diameter from the rim (Fig. 2). The total vol- two have flows opposite the low in the crater rim, and three have ume of melt within this crater is estimated to be 0.04 ± 0.01 km3. flows with no discernable relationship to crater rim height. While The melt volume is determined by comparing the difference it does appear that melt flows more commonly occur when the between the crater profile as measured by the LRO Lunar cater shape is influenced by topography or the impact angle was Orbiter Laser Altimeter (LOLA) (Zuber et al., 2010) and best-fit oblique (impact angle <30°), these factors are not required for

Fig. 5. Examples of large volumes of impact melt that ﬂowed as broad, ﬂuid sheets. (A) Melt from the 16.4-km crater that formed on the rim of Korolev X (also seen in Fig. 3C). (B) Melt east of the 32.5-km crater Necho (5.2°S, 123.3°E). Scale bar applies to both panels. 670 B.W. Denevi et al. / Icarus 219 (2012) 665–675

Fig. 6. Comparisons of the stratigraphic relationships between impact melt and fragmental ejecta. (A) Impact melt flows that clearly overlie fragmental ejecta (higher reflectance) at an 8.7-km crater northeast of Meshcherskiy (13.2°N, 127.6°E). (B) Crater south of Ingalls G (also seen in Fig. 3B) showing the relationship between blocky ejecta and melt is unclear; blocks either landed on top of the melt flow, coincident in time, or melt flowed around or rafted blocks. (C) An example of an impact melt flow that appears to emanate from blocky ejecta, suggesting melt and ejecta were emplaced simultaneously (Crater on the rim of Gibbs, also seen in Figs. 1D and 3D). impact melt flows to occur. Two of the craters (Lichtenberg B and transient crater was still collapsing and ejecta was being emplaced Hesiodus E; Table 1) show no asymmetry of ejecta blanket or crater – a turbulent environment in which the surrounding terrain and shape, and have prominent exterior melt flows. Lichtenberg B in underlying slope were still changing. In yet other cases, melt flows particular has numerous flows distributed around much of the cra- appear to originate from within blocky crater ejecta (e.g. Fig. 6C). In ter rim (Fig. 4). such situations the melt was likely ejected as a combined melt- Many of the flows observed have flow lobes and channels that clast mass, where the volume and temperature of the melt were are broadly similar in appearance to terrestrial lava flows (e.g. high enough to allow the molten material to coalesce and flow, Fig. 1). Several cases (e.g. those found at Necho and a crater that leaving block-rich ejecta behind. formed atop Korolev X) have broad, smooth, sheet-like flows (Fig. 5). These flows appear to have been very fluid, flowing in a broad front before significant cooling occurred at the margins. 3. Impact melt flows at Mandel’shtam F The majority of flows observed begin at or near their parent crater rim. Cases where flows begin immediately at the crater rim are As a case study, we examine the morphology and emplacement consistent with an origin as melt that lined the transient crater settings of impact melt deposits and flows at one of the craters in cavity (Melosh, 1989) and was emplaced across the interior-rim our survey, Mandel’shtam F (Fig. 7), a 15-km crater formed in far- boundary. Little slumping is expected for the majority of simple side feldspathic highlands terrane (5.2°N, 166.2°E) (Jolliff et al., craters we observed, but in the case of complex craters, the initial 2000). The estimated ferrous iron content of the target material depositional location of melt flows that originate near the rim may is 4 wt% (Lawrence et al., 2002), consistent with a composition have been slumped into the crater interior. of anorthosite to noritic-anorthosite. The crater displays a sharp LROC NAC images generally confirm the superposition of melt and raised rim with concentric fractures and high-reflectance rays, on top of fragmental ejecta (e.g. Fig. 6A) noted previously (Howard attributes consistent with a youthful, Copernican age (Wilhelms, and Wilshire, 1975; El-Baz, 1972), which suggests that the melt 1987). Its asymmetric distribution of ejecta (Fig. 7A) suggests an typically originates deep within the excavated portions of the tar- oblique impact from the northeast. The majority of the ejected get and is emplaced late in the excavation stage of the crater’s for- melt volume is located outside the western rim of the crater mation. In such cases the properties of the melt as it flowed are (Fig. 7C), and melt distribution does not appear to have been influ- representative of conditions shortly (seconds) after the impact enced by the topographic low caused by a large crater to the south event. We find other cases in which the stratigraphic relationship of Mandel’shtam F (Fig. 7B). is less clear. In some cases large blocks are observed within the In order to characterize the three-dimensional configuration of boundaries of the melt flows and it is not obvious whether the melt the impact melt flow features and the surfaces surrounding them, flowed around these blocks, the blocks were rafted along with the targeted off-nadir NAC image pairs (M115231170 and flow, or they landed on top of the flow (e.g. Fig. 6B). If blocks M115224385, convergence angle 28.8°) of the western crater rim superpose the melt flows, these flows likely occurred even as the and exterior of Mandel’shtam F were acquired in summed mode B.W. Denevi et al. / Icarus 219 (2012) 665–675 671

(Robinson et al., 2010) with a pixel scale of 1.0 m and an average pronounced levees on the unconfined slope and essentially no le- incidence angle of 71°. A DTM at sampled at 3 m was derived from vees on the topographically confined northern boundary. Where these observations using the SOCET SET Toolkit (Miller and Walker, the topographic gradient shallows to 7°, the flow front broadens 1993). The scene’s overall slope was controlled to LOLA topo- (250 m) and levee width increased (Fig. 8). The final 500 m of graphic profiles of the region to remove any tilt introduced to the flow is unchannelized, suggesting that flow in this region the model from inaccuracies in pointing knowledge and to provide was not sustained for a sufficient duration for a preferred pathway absolute elevation control (Tran et al., 2010). Errors in relative ele- to develop. Flow 2 (Fig. 8)is1.8 km in length with wider channels vations derived from the DTM are on the order of ±1 m, lengths and (65–100 m) than Flow 1. Along the steep slope (11°) beginning widths measured from 1.0-m/pixel images are considered valid to approximately 500 m from the rim, the flow begins as two chan- ±1 m. nels, each 10–12 m deep with poorly defined flow margins and narrow, discontinuous levees. After a transition in slope from 11° 3.1. Flow morphology and morphometry to 7°, the two channels merge into one. Within this portion of the flow, the deposit within the channel is nearly as thick as the le- We focus on two Mandel’shtam F flows that have well-defined vees are tall (7–15 m). flow features for which measurements were obtained from the The channelized morphologies and the emplacement setting of NAC DTM (Fig. 8). Both appear to originate from a 5 to 10 m-thick Flows 1 and 2 are consistent with melt that was ejected at low hor- melt-rock deposit that extends to the rim. Flow 1 (Fig. 8)is izontal velocity, landed near the rim, and flowed predominantly 2.1 km long and has a well-defined medial channel that begins under the influence of gravity. These flows overlie an earlier (but 250 m from the crater rim. Its channel is widest (70 m) in its from the same impact event) emplaced broad (0.6–1.0 km) melt proximal reach and tapers distally (to 25 m). Flow thicknesses deposit that extends an additional 2.5 km from their termini were measured by assuming a straight profile between the topog- (Fig. 8). The melt in this region displays multiple streamlines raphy to either side of the flow as a baseline level; thicknesses approximately parallel to flow direction, consistent with melt range from 8 to 26 m (Table 2). The melt moved down gradient moving at a higher velocity being slowed in places by topographic around the southern tip of a region of high ground and on a rela- obstacles such as boulders. The DTM also shows that melt flowed tively steep slope (17°) where the levees grew asymmetrically with up and over gentle slopes (<3°). These observations are consistent

Fig. 7. The crater Mandel’shtam F (5.1°N, 166.1°E; 15 km diameter). (A) WAC image with illumination from overhead showing an ejecta pattern consistent with an impact direction from the northeast. (B) WAC image of the same area with illumination conditions that highlight morphology; the topographic low to the south does not appear to have influenced the distribution of melt around Mandel’shtam F. (C) Sketch map overlain on NAC images of Mandel’shtam F showing melt distribution. Green indicates melt pooled in the bottom of the crater. Yellow shows the approximate distribution of melt veneers; in many places the borders are hard to determine. Blue highlights the distribution of discrete flows on exterior impact melt sheets. Many of the southern-most flows appear to originate at or near the crater rim but were only mapped where clear flow morphology can be observed. (D) NAC images showing the majority of impact melt outside the crater rim and locations of panels E and F. (E) Short, thick flows of melt that appear to originate further from the rim. (F) Portions of channelized Flows 1 and 2. 672 B.W. Denevi et al. / Icarus 219 (2012) 665–675 with this portion of the melt flowing at a higher velocity, as would (Scarfe et al., 1983; Shaw, 1969), indicating anorthositic melts will be expected for melt ejected farther from the rim at a shallower an- be hundreds of degrees hotter than typical terrestrial basalts of like gle and with a higher component of horizontal velocity (Melosh, viscosity. Thus for morphologies such as those observed at Man- 1989). del’shtam F, which share many morphologic attributes with terres- Impact melt flows such as Flows 1 and 2 are morphologically trial basaltic flows, this morphologic similarity is consistent with similar to some terrestrial volcanic flows, especially near-vent clas- broadly comparable viscosities and thus higher temperatures for togenic (spatter-fed) lava flows (e.g. Sumner, 1998 and references the case of anorthositic melts. therein). Lava flow morphology is determined by properties of the melt (e.g. viscosity, temperature, composition) as well as erup- 3.2. Physical modeling tion parameters and environmental conditions (e.g. Walker, 1973; Hulme, 1974; Pinkerton and Wilson, 1994; Cashman et al., 1999; A first-order comparison of relative flow properties can be Gregg and Fink, 2000; Zimbelman and Gregg, 2000; Harris and accomplished using simple physical models. Assuming the flow be- Rowland, 2009; Hauber et al., 2011). The similarity of the morpho- haves as a Bingham fluid, yield strength (sy), the internal strength logic attributes of the Mandel’shtam F flows to commonly ob- of a fluid that developed to stop a flow from moving, can be calcu- served volcanic flow features suggests first order similarities in lated using measurements from the DTM with two methods: melt properties that control morphology, such as viscosity. Typical highland soils, such as those sampled at the Apollo 16 landing site, sy ¼ qgd sinðhÞð1Þ are relatively low in silica (45% SiO2; mafic/ultramafic). In fact, their silica contents are similar to, or in some cases lower than, d2 those of lunar mare basalts, and lower than most terrestrial basalts s ¼ qg ð2Þ y w (Basaltic Volcanism Study Project 1981). The viscosity of anorthositic melt is comparable to that of basaltic melt near their respec- where q is the density of the melt, g is gravitational acceleration tive liquidii (1200–1300 °C basaltic; 1400–1500 °C anorthositic) (1.62 m/s2), d is the thickness of the flow, h is the underlying slope,

Fig. 8. Flows 1 and 2 at Mandel’shtam F. Top: Flows 1 and 2 are outlined in white, white arrows show flow direction. A sheet-like flow of impact melt is also outlined in yellow, with yellow arrows indicating flow direction and dotted lines showing streamlines within the flow. Red arrows indicate the rim of Mandel’shtam F. Bottom: NAC- derived DTM is overlain on the orthorectified NAC images, locations of measurements in Table 2 are indicated along Flows 1 and 2. B.W. Denevi et al. / Icarus 219 (2012) 665–675 673 and w is the total width of the flow (Hulme, 1974, 1982; Moore approximately 10–1000 Pa s (Keszthelyi and Pieri, 1993; Gregg et al., 1978). It should be noted that these simple models are over- and Keszthelyi, 2004)). simplifications for actual lava flows (e.g. Keszthelyi and Pieri, 1993). For example, Eq. (2) comes from a calculation that assumes that the 4. Discussion lava in the center of the channel will stand higher than the levees. This is typically not the case for channelized lava flows and was Previous work postulated that impact melt flows are generally probably not the case for the intra-channel deposit of Flow 2 while limited to craters >10 km in diameter, with the volume of melt pro- it was moving. However, if applied uniformly, the equations can be duced at smaller craters thought to be insufficient to coalesce and used to roughly compare the properties of different flows (but can- flow (Hawke and Head, 1977). High-resolution NAC images allow not be compared to laboratory measurements of the rheology of sil- for a closer examination of impact melt flows in association with icate melts). small Copernican craters. We observe melt flows around craters We adopt the composition of Apollo 16 sample 61221 (Rose as small as 2.4 km in diameter, and six of the craters <5 km in et al., 1973), a highlands soil with a similar FeO content our study have exterior melt flows. The high volumes of melt im- (4.6 wt%), as a proxy for the Mandel’shtam F site. From this compo- plied by these flows and the high measured melt volume for the sition, the corresponding melt density is calculated (Lange and Car- 3.0-km crater in Fig. 2 suggest that melt production can be consid- michael, 1987) to be 2640 kg/m3 at 1400 °C. The melt density erably higher than predicted by models. Higher than average melt shows little change across the expected temperature range and volumes may be due to impacts which occurred at the high end of thus varying the density has a small affect on calculated yield the velocity distribution (5–40 km/s) expected for projectiles strength compared to other parameters. Slope, flow thickness, striking them Moon (e.g. Le Feuvre and Wieczorek, 2008); second- and width are measured from the DTM and orthorectified images ary impacts of this size would not be expected to produce melt. (Table 2). These parameters vary along the flows; yield strengths Continued examination of NAC images can help improve models are evaluated using measurements at 10 points each along Flows of melt production for small craters. 1 and 2 (Fig. 8B, Table 2). We assume a homogeneous clast-free The distribution of melt flows around craters in our study is melt. consistent with impact angle playing a secondary role to topo- Eqs. (1) and (2) give yield strength estimates of 3.0 102– graphic influences in determining the spatial distribution of impact 2.5 104 Pa for Flow 1, with an average of 7.8 103 Pa and melt relative to the parent crater (Hawke and Head, 1977). Melt is 9.2 102–1.9 104 for Flow 2, with an average of 6.7 103 Pa (Ta- often observed exterior to lows in the crater rim, when the influ- ble 2). We find no consistent trend in yield strength along each ence of preexisting topography on crater shape is extreme (e.g. flow, and much of the variation observed may be due to the diffi- Fig. 3A–C), though in some cases flows are observed opposite lows culty in determining widths along portions of the flows and/or er- (e.g. Fig. 3D) or apparently randomly oriented. The lack of a consis- rors in measured flow thickness or slope. Compared to yield tent pattern in melt flow orientation suggests that in the dynamic strengths calculated for terrestrial flows using the same methods, environment of crater formation, no one variable consistently these values are on average two orders of magnitude lower than dominates. those for rhyolitic and andesitic flows (1.5–2.4 105 Pa) and with- The lava-flow-like morphologies of impact melt flows observed in the range calculated on average for Hawaiian basalts (3.3 103– at many of the craters in our study, besides being aesthetically 3.3 104 Pa) (Moore et al., 1978; Fink and Zimbelman, 1986). The stunning, provide an opportunity to estimate the fluid properties similarities in yield strengths suggests that the rheology of the of the melt shortly after impact. Physical modeling of flows at Man- Mandel’shtam F flows was comparable to terrestrial basaltic lava del’shtam F suggests rheologies similar to terrestrial basaltic flows flows (i.e., melts with negligible yield strength and a viscosity of and implies high temperatures for the anorthositic melt at the site.

Table 2 Flow parameters and yield strength calculated from Eqs. (1) and (2).

Distance along ﬂow (m) Width (m) Thickness (m) Slope (°) Yield strength 1 (Pa) Yield strength 2 (Pa) Flow 1 1 216 26 0.3 4.4 102 1.3 104 157 174 18 0.3 3.0 102 7.9 103 460 168 18 16.7 2.2 104 8.1 103 724 141 21 16.7 2.5 104 1.3 104 922 225 16 16.7 1.9 104 4.8 103 1087 276 8 6.5 3.8 103 9.8 102 1222 246 14 6.5 6.7 103 3.4 103 1342 237 10 6.5 4.8 103 1.8 103 1471 246 15 6.5 7.2 103 3.9 103 1540 258 13 6.5 6.2 103 2.8 103 Average 219 16 8.3 9.6 103 6.0 103

Flow 2 1 366 24 2.5 4.5 103 6.6 103 198 315 16 11.0 1.3 104 3.4 103 342 246 10 11.0 8.1 103 1.7 103 492 216 15 11.0 1.2 104 4.4 103 657 219 23 11.0 1.9 104 1.0 104 843 159 1 6.8 7.5 103 6.0 103 963 225 7 6.8 3.5 103 9.2 102 1161 141 17 9.2 1.2 104 8.7 103 1272 248 9 6.8 4.5 103 1.4 103 1482 154 10 6.8 5.0 103 2.7 103 Average 229 15 8.3 8.8 103 4.6 103 674 B.W. Denevi et al. / Icarus 219 (2012) 665–675

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