Journal of Geophysical Research: Planets

Geology of the King crater region: New insights into impact melt dynamics on the Moon

Authors


Abstract

[1] New geologic sketch maps and digital elevation models for King crater and the surrounding lunar farside highlands were created from Lunar Reconnaissance Orbiter Camera (LROC) Wide-angle Camera (WAC) and Narrow Angle Camera (NAC) images. NAC images reveal that high volume impact melt accumulations exhibit evidence of dynamic processes during and following emplacement that resulted in downwarped zones, and other morphologic anomalies visible at the 50 cm pixel scale. Among the most significant of these forms are negative relief features, some of which may represent evidence for near-surface caverns, offering points of access to subsurface environments and possible shelter from surface hazards. Other negative relief features may represent regions of extension and separation in response to possible subsurface drainage, together with isostatic readjustments, contraction, and/or compaction, in the cooling impact melt. Crater counts on the continuous ejecta blanket suggest a Late Eratosthenian to Copernican age for King crater, which is older than the estimate of Young (1977), but consistent with those of others.

1. Introduction

[2] Since its discovery during the Lunar Orbiter program in the 1960s, King crater (5.0°N, 120.5°E) has remained a site of fascination to the lunar science and exploration communities as one of only a few complex craters that is also geologically recent (Late Eratosthenian to Copernican age) [El-Baz, 1972; Howard, 1972; Oberbeck et al., 1972; Heather and Dunkin, 2003; Gruener and Joosten, 2009]. The pre-existing topography likely had a significant impact on the final form of King crater and its ejecta (Figure 1) [Hawke and Head, 1977]. Of particular interest is the behavior of impact melt during and following accumulation within the crater and nearby topographically low areas. Originally thought to be volcanic in origin [e.g., Strom and Fielder, 1968, 1971], impact melt has since been recognized as resulting from the energy of impact [e.g., Howard and Wilshire, 1975; Hawke and Head, 1977; Melosh, 1989]. One large deposit of melt is found in the adjacent Al-Tusi crater (King Y) to the north-northwest of King crater (referred to hereafter as the Al-Tusi pond). This unit comprises an area of approximately 300 km2 of ponded melt, with an estimated volume of ∼23 km3 (see section 3.4). Understanding the occurrence and behavior of impact melt provides fundamental insights into the thermal dynamics of cratering processes [e.g., Abramov et al., 2012]. In addition, visual evidence presented herein for subsurface voids within melt deposits is important for the future of exploration and habitation of the Moon. Such voids could serve as subsurface shelters suitable for long-term protection from surface hazards on the Moon, and possible points of access to environments of high scientific value that are normally hidden to orbital remote sensing assets [Robinson et al., 2012].

Figure 1.

WAC mosaic regional context image. White and red boxes define extent of WAC and NAC geologic sketch maps, respectively.

[3] In this paper, we focus on the behavior of the Al-Tusi pond melt deposit following accumulation and during subsequent cooling and modification, using morphometric and topographic measurements made from Lunar Reconnaissance Orbiter Camera (LROC) [Robinson et al., 2010] images. The King crater area and melt pond surfaces were imaged from a nominal 50 km altitude at pixel scales of 100 m, and up to 0.5 m, for the Wide-angle Camera (WAC) and Narrow Angle Cameras (NAC), respectively. These images were used to create geologic sketch maps for the King crater region, including the Al-Tusi pond. The new images permit improvement upon previous mapping efforts [e.g.,Wilhelms and El-Baz, 1977], while corroborating former observations [El-Baz, 1972; Howard, 1972; Oberbeck et al., 1972; Heather and Dunkin, 2003]. In addition, regions of image overlap with sufficient parallax permit the generation of Digital Elevation Models (DEMs) at 100 m/pxl and 2 to 5 m/pxl, from the WAC and NAC images, respectively, using the techniques of Scholten et al. [2012], Tran et al. [2010], and Burns et al. [2012]. WAC image overlap has provided repeat stereo observations for the entire illuminated surface of the Moon, enabling production of a near-global DEM. NAC DEMs are computed after joining LROC stereo models with Lunar Orbiter Laser Altimeter (LOLA) altimetric profiles to maximize precision. A DEM of the Al-Tusi pond was also created from SELenological and ENgineering Explorer (SELENE/Kaguya) Terrain Camera [Haruyama et al., 2008] data for comparison purposes. By highlighting subtle surface details, the DEMs provide significant insight into morphometry (and thus geologic processes), and assist with defining geologic unit contacts and stratigraphic relationships. Finally, we performed crater counts using NAC and WAC images both on the Al-Tusi melt deposit, and on the continuous ejecta blanket, to estimate a formation age for King crater.

2. Geologic Sketch Mapping and Digital Elevation Modeling

2.1. Methods, Conventions and Assumptions

[4] Experiment Data Records (EDRs; raw LROC images) were calibrated, map projected, and mosaicked using United States Geological Survey (USGS) Integrated Software for Images and Spectrometers (ISIS) [Anderson et al., 2004]. Using planetary mapping guidelines and unit color conventions [e.g., Greeley and Batson, 1990; Tanaka et al., 2009], geologic sketch maps were then produced from WAC and NAC image mosaic base maps in Adobe Photoshop. Not all planetary mapping conventions are observed, however. For example, the inner and outer facies of the continuous ejecta blanket of large craters are often defined by the transition from hummocky to radial landforms [e.g., Wilhelms, 1987], not sharp breaks in slope. In the case of King crater, however, we identify a clear break in slope along the southeastern portion of the proximal ejecta blanket (see the DEM discussed in section 2.4), so we chose to use this break to define the transition from proximal to distal ejecta facies. Unit identifications were thus based on texture, albedo, topography, visibility of contact with neighboring units, and geologic significance. When necessary, several images with different solar incidence angles were used to help identify contacts and other diagnostic landforms.

[5] Impact crater diameters were measured on the Al-Tusi melt deposit from NAC images and on the continuous ejecta blanket of King using a WAC mosaic, by fitting circles to three points along each crater rim using a circle-drawing tool in IDL, and established techniques [e.g.,Hartmann, 1966; Crater Analysis Techniques Working Group, 1979; Neukum, 1983; Hiesinger et al., 2000]. Crater size-frequency distributions (CSFDs) were plotted and fit with CraterStats [Michael and Neukum, 2010], using the production function and lunar chronology of Neukum et al. [2001].

[6] Because the Al-Tusi melt deposit has been designated as a region of interest for future lunar exploration [e.g.,Gruener and Joosten, 2009], units within Al-Tusi pond were further considered in regard to their potential trafficability and habitability. This engineering analysis is particularly relevant to the negative relief features within the Al-Tusi pond (seesection 3.3).

2.2. Wide-Angle Camera Geologic Sketch Map

[7] The WAC geologic sketch map covers an area of ∼34,340 km2 centered on a point just within the northwestern crater wall (Figure 2a, and white box in the Figure 1 context image). The mapped area is offset to the west of the King crater center to account for the asymmetrical distribution of ejecta deposits around the crater. These deposits include dunes, imbricated deceleration lobes, ballistically emplaced deposits, and other forms of fluidized debris surge deposits (represented by fine lineations in the map; Figure 2a) [e.g., Howard, 1972]. The full extent of discontinuous ejecta associated with the King impact remains undefined, but begins approximately at the four corners of the WAC map area. Mapped units, their descriptions, and letter designations are presented as Table 1, with a key as Figures 2b and 2c. Figure 2dis a cross-section prepared from a WAC DEM surface trace from point A to A′.

Figure 2.

(a) Geologic sketch map (transparent with WAC mosaic base map), (b) opaque geologic sketch map, with (c) unit key and (d) cross section. Unit descriptions and letter designations are presented in Table 1. The cross section surface trace is defined by the WAC DEM with a vertical exaggeration of approximately 3.8x.

Table 1. Geologic Sketch Map Unit Descriptions
UnitDescription
Recent craters (rC)small circular features that clearly post-date all deposits associated with the King impact event.Interpretation:relatively recent impact craters. No crater in this category is larger than ∼7 km in diameter. Ejecta blankets for craters in this category are not mappable as contiguous units. Craters smaller than one kilometer were not mapped because of their high number and often-irregular morphologies within the main melt pond.
Viscoid forms (Vf)positive relief landforms associated with ponded melt, often showing clear signs of viscous flow, deformation, and accumulation within the Al-Tusi pond deposit; and frequently occurring as lobate ridges and other topographic constructs.Interpretation: Late stage emplacements of impact melt deposits.
Crater floor (Cf)irregular morphologies and textures across the crater floor. Notable features within this unit include anomalous circular mounds, occurring individually or in coalesced groups, irregular hummocks or knobby deposits, and ropy viscoid forms. Interpretation:melt-dominated material pooled on the crater floor. Deposit also incorporates ejecta and slumped wall fragments.
Ponded impact melt (Mp)flat, horizontal beds of moderately uniform texture, most conspicuously within Al-Tusi crater, but also locally within topographic lows both inside and outside of the King crater walls.Interpretation: impact melt deposits associated with the King event. Most areas of ponded melt exhibit evidence for rapid flow of low viscosity capable of achieving equipotential stabilization, while others show signs of more viscous flow (see Vf).
Terraced crater walls (Cw)terraces within King crater having a notable topographic low or breach along the north-northwest boundary with Al-Tusi crater (melt pond area).Interpretation:fault-bounded crater walls. Local occurrences of ponded melt can be found among the terraces in this area. Fault traces are indicated as fine lineations.
Slumped crater walls (Cs)deposits interior to the crater and associated with the walls, but which do not show fault-bounded terraces. These deposits are concentrated along the southern and south-southeastern rim and superpose material that appears contiguous with the Mons Ganau portion of the asymmetric central peak.Interpretation: mass wasting deposits slumped from the crater walls.
Central peak (Cp)fork- or Y-shaped assemblage of three mountains in center of crater basin.Interpretation:uplifted central crater peak. The three sub-peaks have been named Mons Ganau (base of Y), Mons Dieter (western Y tong), and Mons André (eastern Y tong) by the International Astronomical Union (IAU) Working Group for Planetary System Nomenclature (WGPSN) in 1976. This unit includes lobate accumulations at the base and superposed on the crater floor deposits, interpreted as slump aprons.
Distal ejecta (Ed)continuous deposits defined as beginning at a break in slope that delineates the extent of proximal ejecta. Pre-existing terrain is often detectable through the deposit. This material transitions to discontinuous deposits with distance from the crater rim, which may or may not also exhibit unique morphologies as either ballistically emplaced debris, or ground-hugging, fluidized deposits. Some of these latter forms are indicated in the WAC morphologic map by lineations.Interpretation: distal impact ejecta. Precise transition boundaries between continuous and discontinuous deposits are not discernible within the four corners of the WAC geomorphic map, and so are mapped contiguously.
Proximal ejecta (Ep)deposits obscuring pre-impact topography and tending to display a sharp break in slope where they meet the unit defined as representing distal ejecta deposits.Interpretation: continuous impact ejecta deposits associated with the immediate proximity of the crater.
Pre-existing craters (pC)circular features, often possessing degraded outlines and other topographic details visible through the distal ejecta. Interpretation: craters lying beneath King ejecta deposits. Craters in this category range between ∼7 and 30 km in diameter. Many are described as subdued to highly subdued by Wilhelms and El-Baz [1977].

2.3. Narrow Angle Camera Geologic Sketch Map

[8] The King crater NAC geologic sketch map is centered on the Al-Tusi pond, where NAC images show several distinctive types of surface features, allowing investigations of melt accumulation rates, cooling history, and post-emplacement movement (Figure 3). In addition to flow features and viscoid forms observed in Apollo 10 and 16, and Lunar Orbiter images [El-Baz, 1972; Howard and Wilshire, 1975; Hawke and Head, 1977; Heather and Dunkin, 2003], the Al-Tusi pond deposit is characterized by positive relief features (such as hills and domes), craters with anomalous morphologies (including hummocky floors, irregular outlines, boulder associations, and high depth/diameter ratios [e.g.,Schultz and Spencer, 1979]), and negative relief features (discussed further in section 3.3). Locations for the most apparent of these are shown in Figure 4. A positive-relief ‘viscoid forms’ (Vf) unit was resolvable from lower relief melt deposits at NAC scale, but not at WAC scale (seeTable 1, and Figures 2c and 3c). Other positive relief features within the pond are mapped as part of the proximal ejecta unit, as they often exhibit signs of being draped by melt (see section 3.2), and so probably represent ejecta that underlies the melt.

Figure 3.

(a) Narrow Angle Camera mosaic base map, (b) transparent geologic sketch map with NAC mosaic base map, and (c) opaque geologic sketch map.

Figure 4.

Al-Tusi impact melt deposit anatomy on NAC mosaic base. Green zones are positive relief features, lavender highlights are negative relief features, and orange traces are flow features. Note inlets at western shores and their relationship to flow features.

2.4. Digital Elevation Models

[9] The crater wall low point along the north-northwest rim, sharp break in slope within the continuous ejecta deposit to the southeast of the crater, and the flatness of the melt in Al-Tusi crater, are conspicuous in the colorized WAC DEM superposed on a WAC basemap (Figure 5a). Both WAC DEM contours and LOLA altimetric data show the eastern two thirds (approximately) of the Al-Tusi pond surface to be nearly level at the 20 m contour interval (Figure 5b; LOLA data not shown). The maximum elevation difference between the average eastern flat pond surface and the western end is ∼240 m.

Figure 5.

Wide-angle Camera Digital Terrain Model. (a) Regional; (b) the Al-Tusi pond at 20 m contour interval. The units are in meters with the melt pond surface set artificially at 0 m.

[10] The melt pond NAC DEM reveals kilometer-scale topographic depressions, showing evidence of sagging or downwarping in the surface crust (Figure 6). Vertical displacement within the central portions of these depressions from their inferred original horizontal and equipotential surfaces ranges from 15 to 20 m. The perimeters of these downwarped zones correlate moderately with the occurrence of negative relief features (see section 3.3 and Figure 6).

Figure 6.

Narrow Angle Camera Digital Elevation Model. Elevations are referenced to the standard lunar geodetic vertical datum. Note downwarped zones (dark blue) and the occurrence of negative relief features (yellow highlights) near their perimeters.

3. Impact Melt Distribution and Characteristics

[11] Figure 7shows details of impact melt anatomy within the Al-Tusi pond that formed during and following melt emplacement. The following discussions address each of the feature categories.

Figure 7.

Examples of impact melt occurrence; north is up in each NAC frame. The Al-Tusi pond features include Figures 7a–7e. (a) Viscoid form near Al-Tusi pond western inlet; NAC frame M130863593L/R. (b) Melt-coated positive relief feature and (c) negative relief feature; both in M136756054R. (d) Example of positive relief feature crosscut by negative relief features; M128509025R. (e) Rock bridge; M113168034R. (f) Draped ejecta outside Al-Tusi crater; M128509025R.

3.1. Flow Features

[12] As discussed by previous workers [e.g., El-Baz, 1972; Heather and Dunkin, 2003], curvilinear landforms occurring as lobate ridges and other topographic constructs, often showing signs of deformation and flow, are found associated with the Al-Tusi melt deposits (Figure 4). Most of these ridges are grouped near the western boundary and west-central interior regions, and are generally absent within the eastern and northeastern portions of the pond. Many occur in groups or clusters, and tend to be concave in the direction of the pond margin. The most conspicuous of these features are mapped as ‘viscoid forms’ (Vf) in the NAC geologic sketch map (seeFigure 3). Other flow features do not lend themselves well to mapping because of their small size or because their gradational contacts with other melt morphologies makes contact definition difficult. Higher terrain occupies the western third of the pond (evident in the WAC DEM contour map; Figure 5b), corresponding to several inlets where melt has solidified into viscoid landforms along the pond perimeter (e.g., Figure 7a).

3.2. Positive Relief Features

[13] Positive relief features ranging from <10 m to ∼3.4 km in diameter occur within the Al-Tusi pond as irregular to subcircular mounds (seeFigure 4). The largest of these, a conspicuous mound in the northern half of the pond, has approximately 9.7 km2of plan view surface area. Most positive relief features do not present clearly demarcated contacts with the surrounding melt deposits, but rather gradational topographic transitions. Except for this difference in topography, the positive relief features share textural characteristics of melt in the Al-Tusi pond (Figure 7b). Some mounds even exhibit fractured surfaces with similar characteristics to the negative relief features described in section 3.3 below (Figure 7d). These features could represent either pre-existing underlying topography or large blocks ejected during the formation of King that were subsequently draped with and/or submerged in the melt deposit.

3.3. Negative Relief Features

[14] Also shown in Figure 4 are the locations of negative relief features. The most common of these appear as linear canyons and sinuous valleys ranging from ∼1 m to over 2 km in length. Their outlines range from sharply defined to subtle (Figure 7c), and can even grade into undisturbed topography. Floor depths from shadow measurements have been determined for 92 negative relief features. These range from a few meters up to 20 m, with an average of 7 m. Some negative relief features occur circumferentially to the edges of positive relief features, creating a moat-like zone. Still others crosscut positive relief features, as discussed insection 3.2 (Figure 7d). Where certain morphologic criteria are met, it is possible to separate features involving possible extension from those resulting from melt withdrawal and collapse into the resulting void spaces. For example, a rock bridge spans the width of its associated negative relief feature (Figure 7e). If this negative relief feature was the result of extensional cracking, a bridge spanning the cavity would be impossible. Additional features can be found which share a similar appearance in outline to the feature in Figure 7e, and may also be the results of collapse into subsurface voids, but which do not have an obvious rock bridge to make a compelling case. Mechanisms for void creation are explored in section 5.1.

3.4. Calculated Impact Melt Volumes

[15] The volume of the Al-Tusi deposit was estimated by subtracting the original surface of Al-Tusi crater from the current pond surface, defined as the pond area bounded by the steep-walled slopes. Since the original floor of the crater is obscured, we estimate it by assuming that the slopes surrounding the pond and the original crater floor surface were smoothly connected topographically. Elevation data at 472 points both outside and within the pond area combined in a merged WAC and NAC DEM are used in the surface reconstruction. The reconstructed surface is modified by 42 additional control points selected from positive relief features within the pond. Both depth and volumetric determinations employ a multiquadratic radial basis function (Python script scipy.interprolate.rbf; v.0.9.0) to interpolate between known elevations along the pond perimeter and the Al-Tusi crater floor surface morphology assumption. Radial basis functions are techniques for interpolating between scattered data points [e.g.,Buhmann, 2003]. This method yielded ∼23 km3as the total estimated pond volume, and ∼92 m as the average pond thickness. Because of uncertainties inherent in determining the exact shape of the pre-existing Al-Tusi crater floor, both numbers assume smooth (uncluttered) crater floor surfaces, and therefore represent maximum estimates. The volume estimation is also affected by DEM resolution and quality. We also calculated a volume using the same method and sampling points as described above, but using a Kaguya DEM (sampled at 10 m/pxl), and obtained a comparable ∼24 km3 as the total volume, and ∼96 m as the average thickness. Both thickness estimates are roughly two thirds of that estimated by Heather and Dunkin [2003] using crater excavation methods. The calculated volume by this method includes not only molten material, but solid debris, including submerged ejecta blocks, that may have been incorporated into the melt pond. Although the volume of solid debris in the melt is ultimately unknowable with available data, the areal percent fraction of mapped positive relief features within the eastern two thirds of the pond (see Figure 4) can be extrapolated to the third dimension and used as a lower limit for the ejecta block volume. Reducing the Al-Tusi melt pond volume by this value (estimated at ∼8 percent), yields a calculation of ∼21 km3.

[16] The WAC DEM was also used to estimate the melt volume within the King crater floor (unit Cf in Figures 2a and 2b). The areal extent of unit Cf is simply multiplied by the depth estimate of 60 m provided by Heather and Dunkin [2003] to arrive at this value, calculated at ∼51 km3. This value should also be reduced by the percent fraction of solid material incorporated within the melt during emplacement [e.g., El-Baz, 1972] (together with probable shallowing near the margins of the deposit), but since the solid fraction has not been differentiated from the melt unit, we will simply treat 51 km3 as a maximum volume.

3.5. Other Impact Melt Occurrences

[17] In addition to the Al-Tusi melt deposit, many melt-derived deposits were identified in NAC images based on their occurrence, appearance of high fluidity, presence of cracks and fractures, stratigraphic relationships to other rock units, and absence of an obvious volcanic source. These are found 1) as thin veneers over ejecta deposits (Figure 7f); 2) as melt puddles superposed over and among fault terraces in the King crater walls; and 3) pooled within the King crater floor (refer to geologic sketch maps; Figures 2 and 3). The floor deposits exhibit a variety of features, ranging from circular mounds to viscoid forms with chaotic, blocky and/or ropy textures. Melt ponding locally within the crater wall terraces demonstrates that post-transient crater fault slumping occurred while melt remained free-flowing. NAC images show herringbone patterns (indicating rapid flow), sharp contacts, and level topographies (indicating hydraulic equilibration), within these deposits that are consistent with very low viscosity emplacement.

4. Results of Crater Size-Frequency Distribution Measurements

[18] We counted 9,198 craters in a 23 km2area of the Al-Tusi melt pond NAC mosaic (Figure 8a) which yield an absolute model age of 385 +51/−53 Ma with an N(1) of 3.23 × 10−4 (Figure 8b). Craters with diameters less than about 60 m are in equilibrium, as they run parallel to the standard equilibrium curve. Measurements on the proximal ejecta blanket, excluding the Al-Tusi melt deposit and crater interior, are based on 1,411 craters above the 5 pixel threshold of 500 m, across an area of 9,420 km2 (Figure 9a), and give an absolute model age of 992 +87/−89 Ma and N(1) of 8.31 × 10−4 (Figure 9b).

Figure 8a.

Five-kilometer-square count area for NAC crater size frequency distribution (CSFD; white box).

Figure 8b.

CSFD measurements of the Al-Tusi melt pond using a NAC mosaic age of about 385 Ma.

Figure 9a.

Count area for WAC crater size frequency distribution (light gray toned region). Note exclusion of the Al-Tusi melt pond and King crater interior, and focus on proximal ejecta blanket (equivalent to unit Ep inFigure 2).

Figure 9b.

Crater size-frequency distribution measurements on NAC data yielded an absolute model age of ∼385 Ma for the Al-Tusi melt pond. Measurements on WAC data give an absolute model age of ∼1 Ga for the ejecta blanket of King. Comparison with WAC-derived counts of Copernicus and Tycho craters [Hiesinger et al., 2012], indicates that King is slightly older than Copernicus.

5. Discussion

5.1. Al-Tusi Melt Deposit

[19] While impact melt appears to have entered Al-Tusi crater from all directions along its perimeter, the primary flows are located along the southwestern, western, and northwestern margins. None of the flow features have the characteristics of volcanic flow lobes, which is another indication that the pond is comprised of impact melt, and not late-stage volcanic deposits. The conspicuous groupings of some viscoid forms and flow features are suggested to reflect different arrival times during melt accumulation, possibly even representing pulsed melt introduction [Heather and Dunkin, 2003; Bray et al., 2010]. Topographic patterns across the pond thus reflect the differences between early stage, relatively low-viscosity accumulation of melt, and late-stage, relatively high-viscosity accumulation. An equipotential surface was probably achieved during a period of hot, low-viscosity accumulation, producing the flat, eastern portion of the pond.

[20] Sagging within the eastern portion of the pond surface interpreted from the NAC DEM probably occurred as the result of adjustments within the melt deposit both during and following cooling (see discussion below). The absence of obvious faulting in these surfaces is consistent with plastic deformation. Interpreting the negative relief features along the perimeters of the downwarped zones as extension/separation cracks [e.g., Howard, 1972] is also consistent with plastic deformation, as lateral separation might be anticipated where a surface crust had formed. Some component of horizontal contraction probably also contributes to the occurrence of these features.

[21] Based on the thermodynamic modeling of Lange and Carmichael [1990], a 4.6 percent volumetric contraction is anticipated for a pure calcic plagioclase melt upon cooling from its liquidus temperature to a solid. Assuming this value to be comparable to melts of noritic anorthosite composition, and that the melt was not superheated far beyond the liquidus temperature, a ∼5 percent contraction translates to only ∼10 m of vertical displacement from cooling alone for a melt depth of ∼200 m, and still less for regions of thinner melt [Lange and Carmichael, 1990]. However, the actual deformation depends, at a minimum, on 1) the initial melt temperature, 2) the difference in contraction between adjacent melt columns, not the column thickness alone, and 3) the purity of melt with respect to any incorporated clastic fragments. The later two factors are likely to moderate the impact of vertical contraction on topographic modification. Since the actual displacement ranges from 15 to 20 m for the downwarped areas, additional mechanisms are required to account for the observations.

[22] As mentioned in section 3.3, the suggestion of localized melt withdrawal from beneath the chilled surface layer of the cooling melt deposit is consistent with the observation of the rock bridge and other possible collapse features. One possible mechanism is melt penetration into subsurface void spaces within the megaregolith by gravity-driven infiltration [e.g.,Riller et al., 2010]. Such large-scale porosity might conceivably develop during brecciation, ejecta block emplacement, and the subsurface readjustment that accompanied the King crater impact event. However, melt infiltration into fractures presents temperature, viscosity, and volumetric problems that are difficult to reconcile with a melt's tendency to quench within the small pore volumes of typical cold bedrock targets.

[23] Another scenario involves localized readjustment of melt confined to Al-Tusi crater resulting from a combination of 1) contraction from cooling, with 2) compaction, 3) localized drainage of residual melt into porous ejecta, and/or 4) isostatic readjustment of the region from impact-induced compression. The possibility of 5) late-stage melt arrival or 6) late-stage arrival of solid ejecta into the melt, adjusting the pond surface elevation, must also be considered.

[24] Figure 10illustrates the effect of variable contraction from cooling on topography schematically in cross section. In this model, the floor has been covered with a blocky ejecta deposit of irregular thickness prior to melt emplacement. Both the regional occurrence of similar deposits outside Al-Tusi crater, and positive relief features within the melt pond, make this a reasonable assumption. The pond naturally develops a crust at its surface during an initial cooling stage, which thickens as it cools. Contraction from cooling is greatest in places where the melt deposit is thickest, made irregular by the non-uniform distribution of pre-existing deposits on the crater floor. The result is a patchy arrangement of depressions, each centered on the locus of greatest downwarping within its respective zone. The greatest stress in the crust during sagging would naturally occur at the high points of the ejecta-strewn floor, which could act as fulcrums to the sagging crust, focusing torque and facilitating extensional separation. Again, contraction alone could not account for the full volumetric change observed in the NAC DEM, but may initiate areas of structural weakness that become further modified by other mechanisms, such as additional sagging in response to melt removal, or impact-induced faulting. For example, if fresh ejecta deposits on the crater floor were of sufficiently high porosity and permeability, and with a minimum of tortuosity, some melt might conceivably enter this material.

Figure 10.

Suggested Al-Tusi pond evolution sequence. (a) Low viscosity flow achieves equipotential surface across eastern half of the pond. Note irregular floor from pre-existing ejecta deposits and regions of greater pond thickness (yellow arrows). (b) During cooling, zones of greater thickness contract more than surrounding regions, resulting in downwarped areas. Black arrows show points where crusted surface experiences extension and separation, producing linear negative relief features. Minor adjustments (discussed in text) within volume of melt permit localized, near-surface cavern formation (subject to collapse at future times) and further modification of downwarped areas. (c) Schematic shows a completely solidified volume of melt with an irregular surface. Figure is vertically exaggerated.

5.2. Overall Impact Melt Volume

[25] The combined volume of the two largest accumulations of melt associated with King crater (the Al-Tusi pond and King floor) is estimated at ∼72 km3. This value is considerably less than that calculated for an impact of King crater's energy. Cratering models suggest that a complex crater of King's size would typically have a ∼48 km diameter transient crater on the Moon [Melosh, 1989]. According to impact melt production models, approximately 300 km3 of melt is anticipated from a 45° impact in an anorthositic target with a transient crater 48 km in diameter [Abramov et al., 2012]. Based on the occurrence of melt-draped ejecta across much of the proximal ejecta blanket (seesection 3.5), particularly in the northwest quadrant, it is likely that this discrepancy is accounted for by non-pooled melt deposits within this and other units. Some melt may also be lost to large fracture systems within the target rock subsurface [e.g.,Riller et al., 2010]. Without being able to calculate the volume of material present as draped melt, or melt admixed with ejecta deposits, it is difficult to make a direct comparison with model estimates.

5.3. Crater Size-Frequency Distribution Measurements

[26] Accurate model age determinations for King crater and other young lunar impact features [e.g., Hiesinger et al., 2012] are important for assessing post-heavy bombardment impact rates, and for correlating global lunar stratigraphy. Our absolute model age determination for the Al-Tusi melt deposit of ∼385 Ma (Figure 8b) is consistent with the determination of Young [1977]from the same area. However, these ages are incongruent with WAC image-derived counts on the proximal ejecta blanket, which show King crater to be ∼1 Ga in age (Figure 9b). Such differences have also been observed at other Copernican craters; for example, Jackson crater [van der Bogert et al., 2010], Tycho and Copernicus craters [Hartmann, 1968; Strom and Fielder, 1968; Hiesinger et al., 2012], and in earlier work on King crater by Schultz and Spencer [1979]. Possible causes for this age discrepancy are discussed by Hiesinger et al. [2012][2012] and van der Bogert et al. [2010], including differences in target properties between melt and ejecta deposits that result in differing final crater diameters, and the preferential occurrence of self-secondary craters on ejecta blankets. Multiphase volcanism is excluded as an origin for the age differences because thermal and impact modeling preclude the occurrence of late volcanism at Copernican-aged craters [Hiesinger et al., 2012]. Moreover, the melt deposits at King are clearly associated with the King impact event. Small craters on the Al-Tusi melt deposit also often exhibit anomalous morphologies [e.g.,Schultz and Spencer, 1979], and can be difficult to measure accurately.

[27] Because ejecta blankets are used as marker horizons in lunar stratigraphy and they are correlated with radiometric and exposure ages of lunar samples, it is reasonable to compare the absolute model ages of different crater ejecta blankets [Hiesinger et al., 2012]. The CSFD for the ejecta blanket of King crater is slightly older than Copernicus crater, and is significantly older than that of Tycho (Figure 9b). If King crater is only slightly older than Copernicus crater, then its optical maturity index (OMAT) [e.g., Lucey et al., 2000] should be similar to that of Copernicus. Although the ejecta rays of Giordano Bruno and Necho craters intersect at King crater [e.g., El-Baz, 1972], which could conceivably adjust OMAT values, we find that the Al-Tusi pond and the floor of Copernicus show OMAT indices of 0.17 and 0.18, respectively, both with a standard deviation of 0.01. The similarity of these values is consistent with our age estimate based on crater counts, placing King at Late Eratosthenian to Copernican in age.

5.4. Future work

[28] Anticipated future work will involve continued NAC imaging of select negative relief features within the Al-Tusi pond under a variety of lighting conditions and camera slew angles. This will improve opportunities to confirm the presence of subsurface voids in a manner similar to that conducted for steep-walled pits in mare deposits [Robinson et al., 2012]. These frames will also be used to complete the NAC DEM beyond the Al-Tusi pond perimeter, across the regional occurrence of melt, and within the King crater interior. Extended coverage of melt accumulations on the crater floor will also enable a comparison study of melt-related features between deposits within the crater and those in the Al-Tusi pond. In addition, differences between Al-Tusi crater and King crater subsurface bedrock structure, residual heat flux, and unmelted rock content may have affected melt-related processes (emplacement, dynamics, and evolution) differently between the two environments. Future NAC images and other data products will help in this comparison.

6. Conclusions

[29] Understanding impact melt pond dynamics aids in assessing the occurrence of impact melt on the Moon and helps to characterize its relationships to parent craters and their features. New WAC and NAC image mosaics and their resulting geologic sketch maps and DEMs assist by providing a detailed anatomy of surface and near-surface features, together with insights into their geologic evolution during and following melt accumulation. Among the more significant findings for King crater are the following.

[30] Early stage King crater impact melt accumulations within Al-Tusi crater achieved equipotential equilibration within the eastern two thirds of the ponded material; late stage arrivals of melt were more viscous, and contributed to variable topography, primarily at the western end of the pond.

[31] Topographic irregularities within the hydraulically stabilized eastern portion of the Al-Tusi pond provide evidence for contraction and/or compaction, in concert with local drainage of melt while still in a partially molten state, possibly augmented by isostatic adjustments, initiating extension and separation of the pond surface crust along downwarped zone perimeters.

[32] Drainage resulted in near-surface, localized void spaces, and the ceilings of some of these voids collapsed, perhaps providing points of access to intact caves. The true interconnectedness of potential subsurface passage networks remains unknown. Consideration of King crater's potential subsurface environments as both 1) shelters from surface hazards; and 2) geologic time capsules preserving hidden characteristics of impact melt emplacement, possible volatile accumulation, and other features of scientific importance during future human or robotic exploration, is recommended.

[33] We estimate a Late Eratosthenian to Copernican age for King crater. The anomalous young age for the melt pond derived from crater counting statistics is interpreted to be the result of effects other than a true age difference.

Acknowledgments

[34] We are indebted to our reviewers for their helpful suggestions and constructive critiques. NASA?s Lunar Reconnaissance Orbiter project funded much of this work. C. H. van der Bogert and H. Hiesinger were supported by the Deutsches Zentrum f?r Luft-und Raumfahrt.

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