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1 Planetary and Space Science 59 (2011) Contents lists available at ScienceDirect Planetary and Space Science journal homepage: Eminescu impact structure: Insight into the transition from complex crater to peak-ring basin on Mercury Samuel C. Schon a,n, James W. Head a, David M. H. Baker a, Carolyn M. Ernst b, Louise M. Prockter b, Scott L. Murchie b, Sean C. Solomon c a Department of Geological Sciences, Brown University, Providence, RI 02912, USA b Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA c Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA article info Article history: Received 6 August 2010 Received in revised form 4 January 2011 Accepted 7 February 2011 Available online 19 February 2011 Keywords: Mercury Peak ring Impact cratering Crater Basin MESSENGER abstract Peak-ring basins represent an impact-crater morphology that is transitional between complex craters with central peaks and large multi-ring basins. Therefore, they can provide insight into the scale dependence of the impact process. Here the transition with increasing crater diameter from complex craters to peak-ring basins on Mercury is assessed through a detailed analysis of Eminescu, a geologically recent and well-preserved peak-ring basin. Eminescu has a diameter (125 km) close to the minimum for such crater forms and is thus representative of the transition. Impact crater sizefrequency distributions and faint rays indicate that Eminescu is Kuiperian in age, geologically younger than most other basins on Mercury. Geologic mapping of basin interior units indicates a distinction between smooth plains and peak-ring units. Our mapping and crater retention ages favor plains formation by impact melt rather than post-impact volcanism, but a volcanic origin for the plains cannot be excluded if the time interval between basin formation and volcanic emplacement was less than the uncertainty in relative ages. The high-albedo peak ring of Eminescu is composed of bright crater-floor deposits (BCFDs, a distinct crustal unit seen elsewhere on Mercury) exposed by the impact. We use our observations to assess predictions of peak-ring formation models. We interpret the characteristics of Eminescu as consistent with basin formation models in which a melt cavity forms during the impact formation of craters at the transition to peak ring morphologies. We suggest that the smooth plains were emplaced via impact melt expulsion from the central melt cavity during uplift of a peak ring composed of BCFD-type material. In this scenario the ringed cluster of peaks resulted from the early development of the melt cavity, which modified the central uplift zone. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction The planet Mercury provides an important record of impact crater forms, from small simple craters through large multi-ring basins, and, more generally, a distinctive opportunity for assessing models for the crater formation process under conditions of high impact velocity. Mercury may also provide a particularly important point of comparison to the record of impact craters and basins on the Moon (Neukum et al., 2001). Until recently, however, information on Mercury s impact crater population was limited. Only 45% of Mercury was imaged at close range during the Mariner 10 flybys (Murray et al., 1974; Murray, 1975); additional areas were imaged by Earth-based radar but at varying n Correspondence to: Department of Geological Sciences, Brown University, Box 1846, Providence, RI 02912, USA. Tel.: ; fax: address: samuel_schon@brown.edu (S.C. Schon). resolution (Harmon et al., 2007). The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, which will be the first probe to orbit Mercury (Solomon et al., 2001; Solomon, 2003), returned near-global imaging coverage from three Mercury flybys in 2008 and 2009 (Solomon et al., 2008). In this paper, we employ MESSENGER data to assess the onset of the transition from complex craters to peak-ring basins by an analysis of the 125-km-diameter Eminescu impact structure centered at 10.81N, E, on Mercury (Fig. 1). The morphology and impact crater size-frequency distributions for Eminescu show that Eminescu is younger than most other basins on Mercury. On the basis of images, geological mapping, and impact crater distributions, we assess the possibility of post-impact volcanism in the basin, we consider the implications of scaling relationships between melt volume and crater volume for impact features of this size, and we review formation models that endeavor to account for the onset of peak rings in the transition from complex craters to peak-ring basins /$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi: /j.pss

2 1950 S.C. Schon et al. / Planetary and Space Science 59 (2011) Fig. 1. (A) Eminescu (10.81N, E) is a young, 125-km-diameter ringed peak-cluster basin (Baker et al., this issue) imaged for the first time by the MESSENGER spacecraft. (B) Geologic map of Eminescu. The following units are mapped in the basin interior: peak-ring massifs, bright annuli surrounding the peak-ring massifs, dark smooth interior plains, light smooth plains, and a rough interior facies. The peak-ring massifs and bright annuli comprise the bright crater-floor deposits (BCFDs) within Eminescu. Wall slumps and terraces are well preserved and have associated deposits of ponded impact melt. Crater wall slumps are designated by lines denoting the headwall scarps. Inset shows an enlargement of the peak-ring region. Mosaic of MESSENGER MDIS images EN M, EN M, EN M, and EN M. Finally, we summarize a scenario for emplacement of the smooth plains within the basin via impact melt expulsion from a central melt cavity during cavity collapse and peak ring uplift. 2. Peak-ring basins The general trend in the form of impact features with increasing diameter, from simple bowl-shaped craters, to larger complex craters (Fig. 2), to still larger peak-ring basins, and finally to the largest multi-ring basins (Fig. 3) has long been recognized (Hartmann and Wood, 1971; Gault et al., 1975; Wood and Head, 1976; Cintala et al., 1977; Spudis, 1993). However, the transitions between these forms and their relative abundances vary among planetary bodies, which permits investigation of the relative influence of key parameters in the cratering process by exploring impact crater morphologies and populations in different environments. From images obtained during the Mariner 10 flybys, Pike (1988) grouped impact craters on Mercury into seven morphological classes. Crater nomenclature has not always been consistent in the literature; for instance, similar suites of morphological characteristics have been variously termed central-peak basins, peak-ring basins, protobasins, and two-ring basins. At issue has been the application of the basin designation to crater forms transitional between complex craters and multi-ring basins. The term peak-ring basin is employed here for descriptive simplicity and in line with terminology used in a recent comprehensive survey of these features on Mercury (Baker et al., this issue; Fig. 3A). Peak-ring basins are defined by Baker et al. (this issue) as craters with a single raised inner ring that is entirely separate from the crater walls, or a circular arrangement of nearly continuous massifs and no central peak. Basins with peak rings and central peaks are termed protobasins. Ringed peak-cluster basins are characterized by a ring-like arrangement of peak elements, with no central peak, and occur at smaller rim-crest diameters than peak-ring basins and with much smaller ring diameters (Baker et al., this issue; Fig. 3B). Although preservation and subsequent modification vary among individual examples, peak-ring basins generally exhibit other characteristics associated with smaller complex craters, such as wall terraces and smooth flat floors (Fig. 2). Eminescu (Fig. 3) is smaller than all 31 two-ring basins identified by Pike (1988), the smallest of which (Ahmad Baba) has a diameter of 132 km. MESSENGER images have revealed an additional 44 peakring basins and 17 newly discovered protobasins (Baker et al., this issue), confirming that Mercury hosts the largest population of such basins in the solar system (Wood and Head, 1976). The survey of Baker et al. (this issue) revealed a distinct sub-group of peak-ring basins (Fig. 3), the ringed peak-cluster basins, with smaller inner rings of semi-continuous massifs but lacking central peaks. Eminescu is among the largest such basins (Fig. 3B) and occurs near the minimum or onset diameter for peak-ring basins. The excellent state of preservation of Eminescu (Fig. 1) makes the basin an ideal focus of study for improving our understanding of the transitions within peak-ring morphologies on Mercury and testing hypotheses for peak-ring basin formation (Baker et al., this issue, and references therein). 3. Eminescu We have examined images from the narrow-angle camera (NAC) on MESSENGER s Mercury Dual Imaging System (MDIS) (Hawkins et al., 2007) to document the nature of the interiors of fresh craters on Mercury at diameters near that for the onset of peak-ring morphology. Eminescu is the morphologically freshest basin cataloged by Baker et al. (this issue) and lies at the size transition between ringed peak-cluster basins and peak-ring basins. Stereo photogrammetry from MESSENGER images shows that Eminescu has a depth, the difference between the mean

3 S.C. Schon et al. / Planetary and Space Science 59 (2011) Fig. 2. Typical complex craters on Mercury exhibit crater wall and floor morphologies similar to Eminescu. These craters have terraced walls and flat floors. Smooth and rough floor textures are observed in varying proportions on the crater floors. Central-peak morphologies range from a single peak (e.g., D) to disaggregated peak elements. (A) Crater at N, E, 42 km diameter; (B) crater at S, E, 62 km diameter; (C) crater at 9.551N, E, 67 km diameter; (D) crater at S, E, 107 km diameter. From NAC mosaics of images obtained during MESSENGER s first (14 January 2008) and third (29 September 2009) Mercury flybys. North is up for all images. elevations of the crater rim crest and crater floor, of approximately 3100 m (Oberst et al., 2010; Preusker et al., this issue). This depth is consistent with the limited dataset on crater depths and diameters assembled by Pike (1988) and shown in Fig. 4. Additional stereo data (Oberst et al., 2010; Preusker et al., this issue) and topographic profiles obtained with the Mercury Laser Altimeter (Cavanaugh et al., 2007; Zuber et al., 2008; Barnouin- Jha et al., 2009) will expand substantially the dataset of crater depths and diameters for Mercury during the orbital phase of the MESSENGER mission. Eminescu has a continuous ejecta deposit that extends about one crater radius from the rim crest. Distinct chains of secondary craters, and more distal secondary clusters, some of which are associated with faint ray segments, are arrayed radial to Eminescu (Fig. 5). Extensive, generally east-west-trending, high-albedo rays in the vicinity are associated with Xiao Zhao crater, located to the east. The scalloped rim crest of Eminescu is crisp, and the rim itself, composed of structural uplift and ejecta, stands above the surrounding terrain. Substantial slumping and terracing have modified the crater walls (Fig. 1A). Mapping of slump block contacts shows that between two and five major faults are observed in the crater wall, which extends inward from the rim by about 0.3 crater radii (Fig. 1A). These wall characteristics are typical of craters of this size on Mercury (Smith and Hartnell, 1978; Malin and Dzurisin, 1978; Pike, 1988). Smaller ( km-diameter) complex craters on Mercury have similar terraced wall morphologies and interiors displaying both smooth plains and primary crater floor roughness (Fig. 2). These comparisons indicate that Eminescu has not been extensively flooded, in contrast to some lunar craters and basins (e.g., Head, 1982), in agreement with the fact that rim-to-floor depth is in line with other unflooded impact features of comparable size (Fig. 4). This observation suggests that crater wall morphology does not itself distinguish the transition from complex craters to peak-ring basins. Eminescu has a high-albedo peak ring defined by a circle of discrete massifs and massif clusters approximately 26 km in diameter (Fig. 1). Surrounding individual peak-ring massifs are

4 1952 S.C. Schon et al. / Planetary and Space Science 59 (2011) Crater Depth (km) S I M P L E C R A T E R S C O M P L E X immature EMINESCU Crater Diameter (km) Fig. 4. Crater depth versus diameter for Mercury (from Pike, 1988). Measurements for simple craters (solid circles), modified simple craters (open triangles), immature-complex craters (open squares), and mature-complex craters (solid squares) are from Pike (1988). The depth for Eminescu (gray diamond) was derived from MESSENGER stereo observations (Oberst et al., 2010; Preusker et al., this issue). Fig. 3. (A) Three peak-ring forms are transitional between complex craters and multi-ring basins (Baker et al., this issue): peak-ring basins, protobasins, and ringed peak-cluster basins. Peak-ring basins have a nearly continuous peak ring and no central peak, whereas protobasins have a peak ring and central peak. Ringed peak-cluster basins lack central peaks but have a semi-continuous peak ring. (B) Ring diameter versus crater rim-crest diameter for these three transitional basin forms (Baker et al., this issue). The positions of Eminescu and Raditladi are indicated by yellow stars. smooth, high-albedo regions of material that extend radially for 1 2 km from the massifs and form annuli. The outer margins of these annuli often coalesce with each other (Fig. 1A). Dark, smooth floor deposits can be followed continuously both inside and outside the peak ring. The contact between the annular unit (peak-ring annuli) and the lower-albedo crater floor material inside and outside the peak ring is sharp at the limit of resolution (Fig. 1, inset). The floor of Eminescu is subdivided into three map units on the basis of albedo and texture (Fig. 1B). One is the dark smooth plains unit noted above, which has sharp contacts with all adjacent map units. Exterior to the peak-ring annuli, this unit appears to surround small circular or quasi-circular features of higher albedo and positive topography (i.e., kipukas), similar to the peak-ring massifs and annular material. Contacts between these features and the surrounding dark smooth plains also appear distinct (Fig. 1). A light smooth plains unit on the basin floor has distinctly higher albedo than the dark plains unit but a similarly smooth texture. The same smooth texture is also observed in some complex craters (Fig. 2). Farther from the crater center are mottled and rougher surface textures that comprise the rough interior facies (Fig. 1), which are reminiscent of primary crater floor roughness (Head, 1975) that is also observed in complex craters (Fig. 2). The rough interior facies contains interspersed regions of smooth texture (Fig. 1A) that are similar in appearance to ponded impact melt documented in lunar craters by many workers (e.g., Hawke and Head, 1977). From color images obtained with the MDIS wide-angle camera (WAC) during the first Mercury flyby, Robinson et al. (2008) defined three areally extensive spectral units: low-reflectance material (often observed to be excavated from depth), moderateto high-reflectance smooth plains (e.g., the interior plains of Caloris), and a unit intermediate in reflectance that includes much of the area mapped as heavily cratered terrain from Mariner 10 images. The surface in the vicinity of Eminescu is this spectrally intermediate terrain. A bright crater-floor deposit (BCFD) unit, of much more limited extent, was also defined by Robinson et al. (2008) on the basis of spectral characteristics. BCFDs are spectrally distinct from fresh crater ejecta and were identified at isolated, geographically separated locations such as Sander, a 50-km-diameter complex crater within the Caloris basin, and the peak ring of the Raditladi basin. The annular and peak-ring units mapped in Eminescu correspond to the BCFD material of Robinson et al. (2008). AnalysesofMariner 10 data also identified similar bright crater interior deposits (Dzurisin, 1977; Schultz, 1977; Denevi and Robinson, 2008). However, hypotheses for the origin of these deposits invoking severely shock-altered crustal material (Schultz, 1977) and localized physiochemical alteration associated with impact-related fractures (Dzurisin, 1977) have remained speculative due to the limited data available. Blewett et al. (2010) concluded that BCFDs constitute a distinct composition, genetically unrelated to lunar swirls or space weathering phenomena. Ernst et al. (2010) analyzed impact crater excavation of buried stratigraphic units and found that the BCFDs in the cases they examined represented excavation of a lithology at

5 S.C. Schon et al. / Planetary and Space Science 59 (2011) Fig. 6. The Raditladi basin (centered at 271N, 1191E) is a young, 250-km-diameter peak-ring basin with a prominent 125-km-diameter peak ring and a semi-circular pattern (70 km in diameter) of extensional graben on the central floor (Prockter et al., 2009; Head et al., 2009b). depth that had been exposed by impact cratering events. Our investigation of Eminescu provides context and new geologic constraints on the occurrence of this potential unit and suggests that here it represents impact exposure of a buried lithology in the Eminescu region. 4. Relative ages Mercury s geologic timescale (Fig. 7) is divided into five periods (Spudis, 1985; Spudis and Guest, 1988) that are comparable to the five divisions of the lunar timescale (e.g., Wilhelms, 1987) and similarly based on the formation of large basins and prominent younger craters. The Mercury timescale was developed with Mariner 10 data, and many questions have remained since that mission regarding the early geologic history and planetary evolution of Mercury, in particular regarding the prevalence and lifetime of surface volcanism (e.g., Head et al., 2000, 2007). The MESSENGER flybys have provided images from which volcanic plains were conclusively identified (Head et al., 2008; Head et al., 2009a), and that identification permitted evaluation of plains chronology with crater size-frequency distributions (Strom et al., 2008, this issue). After the first flyby, volcanic plains were definitively identified within and exterior to the 1500-kmdiameter Caloris basin that formed following the end of heavy Fig. 5. Chains of secondary craters and discontinuous ray/crater cluster segments associated with Eminescu. The bright ray/crater cluster segments are concentrated on smoother plains terrain, which suggest that they are compositionally distinct from their surroundings and therefore may be more persistent than rays discernible only because of soil immaturity (Hawke et al., 2004). The preservation of these rays indicates that Eminescu is Kuiperian in age, consistent with crater size-frequency data (Fig. 8). The lack of rays identifiable as due to soil immaturity indicates that Eminescu is at least as old as the time required for space weathering to alter the reflectance to background levels. Mosaic of NAC images; north is at the top.

6 1954 S.C. Schon et al. / Planetary and Space Science 59 (2011) Moon Mercury COPERNICAN KUIPERIAN 1.0 Age (Ga) 2.0 ERATOSTHENIAN MANSURIAN IMBRIAN NECTARIAN PRE-NECTARIAN CALORIAN TOLSTOJAN PRE-TOLSTOJAN Fig. 8. Crater size-frequency distributions for the interior units, walls, and continuous ejecta of Eminescu are compared with similar data for the Raditladi basin (Eminescu: number of craters counted¼40, area¼37,800 km 2 ; Raditladi: number of craters counted¼178, area¼53,500 km 2 ). These data are displayed using a relative or R plot, which is referenced to crater-size frequency distribution defined by a power law of slope 3. Vertical position measures crater density, with cratered surfaces reaching an empirical saturation density at R (Strom et al., 2008). Although absolute ages are difficult to determine, the Raditladi basin may have formed at 1 Ga(Strom et al., 2008) and Eminescu is resolvably younger. Crater size-frequency distribution data from the dark smooth plains unit of Eminescu are not distinguishable from those for the continuous ejecta deposit. Fig. 7. Comparison of crater stratigraphic timescales on Mercury and the Moon. The timescale for Mercury (Spudis, 1985; Spudis and Guest, 1988) is based on five poorly dated stratigraphic systems that are considered broadly analogous to those that form the basis for the lunar timescale (Wilhelms, 1987). With no returned samples from Mercury, absolute ages (e.g., of the boundaries between periods) on Mercury are uncertain and therefore this comparison to the lunar record is only approximate. bombardment and that defines the base of the Calorian system (Murchie et al., 2008; Head et al., 2008; Strom et al., 2008). Imaging during the third MESSENGER flyby revealed a 290-kmdiameter peak-ring basin (Rachmaninoff) with unambiguous evidence of post-impact volcanic plains on the interior floor (Prockter et al., 2010). Spectrally distinct smooth plains, within the basin and embaying the ejecta deposit, are chronologically separable from the age of the basin and are associated with a likely vent structure (Prockter et al., 2010). This basin is substantially younger than the previously identified volcanic provinces on Mercury, perhaps as young as late Mansurian (Fig. 7) (Prockter et al., 2010). However, Rachmaninoff and its interior volcanic plains are older than Raditladi (Fig. 6), a 250-km-diameter peak-ring basin with a prominent peak ring of BCFD material that is also likely Mansurian in age (Prockter et al., 2009, 2010). The peak ring of Raditladi (Fig. 6) lacks extensive bright annuli of BCFD material similar to the annular deposits surrounding the peak-ring massifs in Eminescu (Fig. 1A). Crater-size frequency distributions from the continuous ejecta and the interior of Eminescu show that Eminescu is unambiguously younger than Raditladi (Fig. 8). Crater size-frequency distributions observed on the dark smooth plains unit in the central portion of the basin are not distinguishable from crater sizefrequency distributions found on the other interior units and continuous ejecta deposit. Therefore, the dark smooth plains are not chronologically separable from basin formation on the basis of their crater-retention age. Although Eminescu is younger than comparable basins, we cannot constrain well its absolute age. The bright-rayed crater Kuiper defines the youngest stratigraphic system on Mercury, the Kuiperian (Spudis, 1985; Spudis and Guest, 1988) (Fig. 7). Kuiperian-age deposits are restricted to rayed craters because analysis of Mariner 10 data showed no evidence of volcanic or tectonic activity during this period (Spudis and Guest, 1988; Neukum et al., 2001; Head et al., 2007). Since the time of this definition of the Kuiperian, analyses of lunar deposits have shown that crater rays are not homogenous features. Rather, lunar crater rays have been divided into three types: immaturity rays, compositional rays, or a combination of immaturity and compositional rays (Hawke et al., 2004).

7 S.C. Schon et al. / Planetary and Space Science 59 (2011) Immaturity rays are visually apparent due to the presence of fresh high-albedo material that is compositionally similar to its surroundings but has not been exposed to space weathering processes (e.g., Hapke, 2001) for a sufficient time to have its reflectance reduced to background levels. Compositional rays are visible because of the contrast between high-albedo ray material of highlands mineralogy, for example, and background material of lower reflectance, typically maria (Hawke et al., 2004). Whereas immaturity rays are sensitive to degradation by space weathering and are generally young, compositional rays tend to be more persistent and can therefore be older features (Hawke et al., 2004; Werner and Medvedev, 2010). We interpret the ray segments of Eminescu (Fig. 5) as consistent with the preservation of compositional rays. In Fig. 5, two rays appear distinctly on regions of smooth plains where they are associated with clusters and chains of secondary craters from Eminescu. Rays from Eminescu are difficult to distinguish, however, and are generally not observed on cratered terrain closer to the crater. Bright rays nearly perpendicular to Eminescu secondary crater chains (Fig. 5) are from Xiao Zhao crater. We interpret these rays as immaturity rays given the brightness of the Xiao Zhao rays, superposition relationships, and their preservation on the cratered terrain. Therefore, although Eminescu meets the strict definition for inclusion in the Kuiperian period, any immaturity rays from Eminescu have apparently been space weathered compared to those from Xiao Zhao crater. These relationships suggest that the age of Eminescu is intermediate between the characteristic degradation times of immaturity rays and compositional rays on Mercury (Blewett et al., 2007). Investigations of rayed crater systems and ray degradation during the orbital phase of the MESSENGER mission should help to resolve uncertainties in the Mansurian Kuiperian boundary. Crater retention-age criteria should be established for period boundaries, and ray preservation should be investigated with high-resolution spectra obtained with the Mercury Atmospheric and Surface Composition Spectrometer (McClintock and Lankton, 2007) to quantify optical maturity differences and compositional variations between rays and their surroundings (e.g., Fischer and Pieters, 1994; Lucey et al., 2000; Noble and Pieters, 2003). Eminescu contains strong contrasts in albedo (Fig. 1A) between the dark smooth plains unit and other basin interior units, which could result from either distributed impact melt or post-impact volcanic activity. The bright material surrounding the Eminescu peak ring extends outward from the peak massifs by greater distances (1 2 km) than talus and scree associated with lunar central peaks, none of which have a similar annulus (Hale and Head, 1979; Pieters et al., 1994). This geometry suggests that both the peak ring and annular material were embayed by the dark smooth plains. However, no evidence of volcanic vents has been observed in Eminescu. For comparison, the lunar protobasin Antoniadi (Fig. 9) has a peak ring and diminutive central peak that were embayed by mare basalt. The Antoniadi mare plains (2.58 Ga) are among the youngest mare units on the Moon (Haruyama et al., 2009) and substantially postdate formation of the crater (Wilhelms et al., 1979). Eminescu is demonstrably younger than Raditladi (Fig. 8) and therefore postdates the extensive Calorian-age plains volcanism that has been documented elsewhere on Mercury (Robinson and Lucey, 1997; Head et al., 2008; Prockter et al., 2010). In contrast to the mare deposits of Antoniadi, the dark smooth plains unit of Eminescu is not chronologically separable from the formation of the basin on the basis of crater retention ages. Although the peak ring and annuli appear embayed by the dark smooth plains unit, there is no clear morphological or crater chronological evidence that those plains are volcanic deposits. As described in greater Fig. 9. Clementine mosaic of lunar protobasin Antoniadi (69.51S, 1871E, 143 km diameter), which contains a diminutive central peak and a peak ring of massifs. Dark mare plains are concentrated between the central peak and the peak ring but also extend peripherally to the east and west. Dating by Haruyama et al. (2009) suggests that this mare unit has an emplacement age of 2.58 Ga (Eratosthenian), which is younger than the Upper Imbrian age of Antoniadi (Wilhelms et al., 1979). detail in the next section, we suggest that these plains, as well as the brighter plains, are both impact melt. However, a volcanic origin for the dark smooth plains cannot be excluded if the interval between basin formation and volcanism was sufficiently short as to lie within the uncertainty in relative age determination from crater size-frequency distributions. 5. Conceptual model for the formation of Eminescu Geologic mapping of Eminescu (Fig. 1) provides the basis to propose a conceptual scenario for the basin-forming event derived from general impact cratering principles (e.g., Melosh, 1989). Geological observations (e.g., Roddy et al., 1977), numerical modeling (e.g., Ahrens and O Keefe, 1977), and laboratory impact experiments (Gault et al., 1974; Schultz, 1987) have elucidated the primary mechanics of impact cratering, and there is general consensus on the sequential crater-formation stages of contact and compression, excavation, and collapse and modification (see Melosh, 1989; Spudis, 1993). On the basis of these principles, in the case of Eminescu, as with other craters of this size, hypervelocity impact of a projectile into the substrate resulted in the formation of a displaced zone and an excavation zone (Gault et al., 1968), which together formed a transient cavity (Fig. 10). Peak levels of shock stress and impact heating in the central portion of the transient cavity caused melting of the target rock and a central concentration of impact melt. Evolution of the growing transient cavity caused streaming of a portion of the melt out along the growing melt-rock interface, with pure impact melt from the interior cavity mixing with rock debris dislodged and ejected from the growing cavity. The boundary between completely melted material and highly shocked but unmelted material is represented by a zone of peak heating just below the liquidus of the target material. A variety of evidence (e.g., Melosh, 1989; Grieve and Cintala, 1992; Pierazzo et al., 1997) indicates that the

8 1956 S.C. Schon et al. / Planetary and Space Science 59 (2011) Transient Cavity Configuration (During Impact) BOLIDE melt expulsion Final Configuration (Post-Impact) EXCAVATED ZONE A1 VAPOR B2 A2 MELT B1 DISPLACED ZONE TRANSIENT CAVITY Fig. 10. Central peak and peak-ring formation and the non-linear growth of the volume of impact melt with increasing crater volume, after Cintala and Grieve (1994,1998). In this scenario, if a central peak is present, such a feature is derived from the maximum depth of melting below the impact point (shown by the uplift of B1 to B2 to form a central peak). Peak-ring massifs originate from off-axis locations relative to the central melt zone (shown by the lesser uplift of A1 to the level of A2 required to form a peak ring). Rebound and modification of the melt cavity lead to melt expulsion and drainage of residual melt through breaks in the peak ring (e.g., Fig. 1). For peak-ring basins, the formation of a specific central peak is inhibited in this scenario by the growth of the melt cavity and by the transition of the zone of peak shock stress to a hemisphere, ultimately forming a peak ring. Our observations are consistent with the view that the development of a melt cavity within the uplift zone led to the production of the ringed morphology of Eminescu and other ringed peak-cluster basins (Baker et al., this issue). These ringed peak clusters follow the diameter trend of central peaks in complex craters (Fig. 3B) by this view, because the developing melt cavity modifies the uplift zone, in contrast to larger basins, where the melt cavity dominates this zone and can reach the base of the transient cavity (Head, 2010). volume of melt in the interior of the cavity increases with crater dimensions faster than crater volume. The projectile that formed Eminescu impacted a substrate composed of spectrally intermediate terrain (Robinson et al., 2008) and formed a transient cavity that grew laterally out to a diameter somewhat less than the current diameter of 125 km. Structural uplift of the rim occurred by the downward, lateral, and radial movement of material in the displaced zone (Fig. 10). The ejecta expelled from the transient cavity thinned with radial distance from the transient cavity rim crest; proximal ejecta was continuous on the crater rim and discontinuous farther out, forming secondary crater chains, clusters, and rays in more distal regions. Within the transient crater, the melt cavity continued to grow to greater depths, penetrating into and exposing the BCFD spectral unit. In our preferred interpretation, the BCFD spectral unit is a specific lithology, occurring at depth, which was exposed during formation of the Eminescu peak ring in a fashion similar to the exposure of buried lithologies by impact craters documented by Ernst et al. (2010). In the later stages of the cratering event, by the scenario developed here, collapse of the edge of the melt cavity caused uplift of the peak ring to form the peaks and associated annuli of BCFD (Fig. 10). The kipukas of BCFD are distributed away from the ringed peak cluster on the southern, eastern, and western part of the crater floor; their distance from the peak ring suggests that the melt cavity penetrated to a substantial depth into the BCFD material. In this same stage of the cratering event, downward and inward rotation of the transient cavity rim crest occurred along listric faults, forming a step-like series of terraces (e.g., Gault et al., 1968; Melosh, 1989) currently comprising the crater wall (Fig. 1). This collapse of the transient cavity rim crest enlarged the diameter to its current value of 125 km. Cavity wall debris mixed with the impact melt and the toes of slump blocks to produce the primary floor roughness of the rough interior facies (Fig. 1). The dark smooth plains and the bright smooth plains that make up a major portion of the crater floor are interpreted to be different facies of impact melt (e.g., Hawke and Cintala, 1977). The outer bright smooth plains are interpreted to correspond to impact melt deposits in similar positions within terrestrial craters (e.g., Grieve and Cintala, 1992) where the melt was mixed with substantial amounts of fragmental debris and cooled and solidified rapidly. The dark smooth plains are interpreted to represent the more pure impact melt deposits characteristic of terrestrial crater interiors; often these deposits are sufficiently thick to undergo differentiation (e.g., Grieve et al., 1991; Stöffler et al., 1994; Therriault et al., 2002; Zieg and Marsh, 2005; Spray and Thompson, 2008). What is the relationship of the peak ring and the dark smooth plains, interpreted to be impact melt? Dark smooth plains line the floor of the interior of the ringed peak cluster, embay the BCFD annuli, and extend through gaps in the peaks out to the surrounding crater floor, where they embay other kipukas of BCFD smaller than peaks of the ringed peak cluster. On the basis of the position of Eminescu (yellow star in Fig. 3B) along the extension of the central-peak diameter relationship for complex craters, and at the uppermost end of the trend of the ringed peak-cluster basins (Fig. 3B), we interpret Eminescu to represent a type example of the transition from complex craters to peak-ring basins. On the basis of cratering theory (e.g., Melosh, 1989; Grieve and Cintala, 1992; Pierazzo et al., 1997; Cintala and Grieve, 1998), impact melt should be concentrated in the central part of the cavity and then be redistributed during cavity collapse. In oblique impacts, the melt region may be shifted downrange, but the melt volume will remain comparable for impact angles down to 301 from the horizontal (Pierazzo and Melosh, 2000). We interpret the distribution of the dark smooth facies to be the result of the uplift of the innermost crater floor to form the ringed peak cluster and the concurrent expulsion of melt from the innermost part of the cavity onto the adjacent crater floor, draining through gaps in the ringed peak cluster. The present volume of the interior of the Eminescu peak ring (from the average height of the peakring massifs to the level of the inner floor), which should approximately correspond to the volume of the melt cavity in this scenario (Fig. 10), is equivalent to a thickness of plains surrounding the peak ring on the Eminescu floor (for an area of 1670 km 2 ) of about 100 m. This result is consistent with the distribution of the dark plains unit and its embayment relationships by the scenario described here (Fig. 1).

9 S.C. Schon et al. / Planetary and Space Science 59 (2011) Discussion of peak-ring basin formation processes Although the primary mechanics of impact cratering are known, debate remains regarding the formation process responsible for peak-ring basins. Early work, informed by lunar exploration, suggested scaling relations based on gravitational acceleration (e.g., Hartmann, 1972) or kinetic energy (e.g., Head, 1978) that might govern the onset of peak-ring basins. More recent work has focused on theories of acoustic fluidization and hydrodynamic collapse of central-uplift materials (Melosh, 1979) and so-called differential melt scaling, a power-law description for the increase in melt volume as a fraction of crater volume with increasing crater size (e.g., Grieve and Cintala, 1992). Here we focus on assessing predictions of impact-melt scaling for the transition to peak-ring basins as manifested in the geology of Eminescu and our interpretive scenario developed in the previous section. A continuous transition from complex craters with central peaks to peak-ring basins has been suggested on the basis of observations of craters on Earth (Grieve and Cintala, 1992), Venus (Alexopoulos and McKinnon, 1994), and the Moon (Wood and Head, 1976). A continuum of central uplift features (Fig. 3) marks the transition from complex crater to basin, ranging from single central peaks, to multiple central peaks, and finally to peak rings (sometimes with central peaks). Such a continuum, at face value inconsistent with the discrete classification scheme of Pike (1988), calls for a more detailed approach to the onset and formation of peak-ring basins with increasing crater size. Many workers have explored relationships between central peak area and crater diameter and between ring diameter and crater diameter (e.g., Head, 1978; Pike, 1988; Alexopoulos and McKinnon, 1994; Baker et al., this issue). Pike (1988) distinguished two-ring basins from protobasins and, on the basis of differing ratios of inner-ring and rim-crest diameters, declared them to be different types of two-ring structures that should not be commingled in statistical analyses. In contrast, Alexopoulos and McKinnon (1994) rejected this distinction and considered ring-diameter ratios as consistent with a morphological continuum between central-peak craters and peak-ring basins. They presented data showing that ratios of the diameters of crater rim to peak ring decrease with increasing crater diameter for craters on Mercury, the Moon, Mars, and Venus (Alexopoulos and McKinnon, 1994). In this context we can assess the ratio of peak-ring diameter to crater-rim diameter (0.21) of Eminescu. This low ratio is below Pike s average ratios (with 95% confidence intervals) for two-ring basins ( ) and protobasins ( ). Comparison to the data presented by Alexopoulos and McKinnon (1994) also indicates that Eminescu s ring/rim ratio is an extreme value. However, Baker et al. (this issue) have shown that Eminescu is not unusual for its classification as a ringed peak-cluster basin (Fig. 3; yellow star). Although the protobasin category of Pike (1988) contains features with central peak elements in addition to inner rings, none of these rings are as completely developed as the peak ring of Eminescu, and none are markedly smaller in diameter. How can the onset diameter (for peak-ring basins, Fig. 3) and development of Eminescu s peak ring be used in conjunction with the mapped interior units to assess formation models for peakring basins? Eminescu is below the onset diameter observed for peak-ring basins on the Moon (Wood and Head, 1976) but above the diameter range for peak-ring basins on Venus (Alexopoulos and McKinnon, 1994), consistent with Mercury s intermediate value of surface gravitational acceleration (3.7 m/s 2 ). Although gravity is the major factor in determining crater dimensions, it is only a minor influence on the volume of melt generated during a hypervelocity impact (Cintala and Grieve, 1994). A body of work developed by Cintala and Grieve has focused on characterizing the non-linear scaling between impact melt volume and crater volume (Grieve and Cintala, 1992, 1997; Cintala and Grieve, 1994, 1998) and the potential implications of this scaling for crater form (Fig. 10). Their work, incorporating theoretical, field, and experimental studies (Grieve and Cintala, 1992), as well as the work of others (e.g., Pierazzo et al., 1997), shows that larger impact events lead to the generation of disproportionately more impact melt than smaller events because melt generation is a function of kinetic energy, whereas crater dimensions are strongly influenced by gravity. The impact of a given size object on Mercury or the Moon would generate dimensionally similar craters because of the counterbalancing effects of higher gravity and higher impact velocity at Mercury, but the impact would generate approximately twice as much melt on Mercury (Cintala, 1992). There are several other geologic effects arising from the increasing fractional melt volume with increasing crater size. The composition and thermal evolution of impact melt deposits will differ, because larger volumes of impact melt have less contact with crater walls, incorporate fewer lithic clasts, and therefore cool more slowly (Cintala and Grieve, 1998). The depth of melting also influences the depth of origin of central uplift features (Fig. 10). Grieve and Cintala (1992) suggested the growing fractional volume of melt as a possible mechanism for the transition from central-peak craters to peak-ring basins (see also Cintala and Grieve, 1991). They also noted that beneath the zone of complete melting, the target is weakened and undergoes partial melting, physical changes that may be manifested in central uplift features (Grieve and Cintala, 1992, 1997). The non-linear relation between melt volume and crater volume form the basis for the peak-ring basin formation scenario of Head (2010) called the nested melt-cavity model. In that scenario, the depth of greatest melting exceeds the depth of the transient cavity for impact features of sufficient size, resulting in the development of a so-called nested melt cavity within the displaced zone underlying the transient cavity. Deepening and expansion of the melt cavity during the cratering event prevents the formation of a single central peak. Collapse of the transient crater results in the formation of a peak ring surrounding the former melt cavity (Head, 2010). In our interpretation of Eminescu, interior smooth plains units are impact melt sheets, and the rough interior facies is partially composed of impact melt breccia. Our analysis is not a complete test of the nested melt-cavity model. However, our mapping of the ringed peak cluster and distribution of impact melt is consistent with retardation of central peak development with increased depth of melting as shown in Fig. 10 (Cintala, 1992; Grieve and Cintala, 1992, 1997; Head, 2010). Therefore, we suggest that Eminescu and other ringed peak-cluster basins follow the trend on depth-diameter plots of central-peak craters (Fig. 3) because uplift in these craters and basins is similar, but greater depths of melting at the sub-impact point than for central-peak craters lead to small melt cavities (Fig. 10). These small melt cavities are contained within the central uplift region and are responsible for the formation of ringed peak clusters rather than traditional central peaks in these features (Baker et al., this issue). In this conceptual model, the dark smooth plains found inside the peak ring (Fig. 1) extend peripherally around the bright deposits due to an expulsion and drainage of impact melt from a central pooling as a result of inward and upward movement of underlying material to form the peak ring during collapse and modification of the transient crater. This scenario is consistent with peak-ring emplacement as envisioned by Grieve and Cintala (1992) and Head (2010) (Fig. 10). This study is consistent with predictions of impact melt scaling and the nested melt-cavity model for basin evolution on Mercury (Grieve and Cintala, 1992, 1997; Cintala and Grieve, 1994, 1998;

10 1958 S.C. Schon et al. / Planetary and Space Science 59 (2011) Head, 2010; Baker et al., this issue). However, rigorous testing of this scenario must await improved topographic, compositional, and imaging observations from the MESSENGER orbital mission phase. Additional geological effects predicted by melt scaling will provide fruitful lines of inquiry during MESSENGER s orbital mission phase at which time the provenance of the BCFD should be further investigated and the potential occurrence of volcanic plains in Eminescu should be reevaluated, especially with high-resolution topography data (Cavanaugh et al., 2007; Zuber et al., 2008). 7. Conclusions Eminescu is a 125-km-diameter ringed peak-cluster basin newly observed in flyby data from the MESSENGER mission. Its size and nature provide insight into the transition between complex craters and peak-ring basins. The peak ring and surrounding bright material are composed of bright crater floor deposits (BCFDs), a unit seen elsewhere on Mercury that we interpret as a buried lithology that was exposed by the Eminescu impact event. Dark smooth plains found on the floor of Eminescu have an impact crater size-frequency distribution similar to deposits coeval with the Eminescu impact and thus are interpreted to be impact melt, rather than products of post-impact volcanic flooding. However, volcanic emplacement at a time after basin formation less than the relative time uncertainty in crater size-frequency distributions cannot be excluded. On the basis of the size and morphology of the ringed peak cluster, and comparisons to other crater transitional forms (Fig. 3), we interpret the formation of Eminescu as being consistent with an impact melt scaling (nested melt cavity) model of basin formation. The interior of the ringed peak cluster represents the formation of a central melt cavity at a crater size just below that at which distinctive peak rings develop in larger impact structures (Fig. 3). Compositional rays from Eminescu are preserved on adjacent smooth plains, but immaturity rays are not preserved. Eminescu exhibits good morphological preservation, and crater counts support the inference that it is Kuiperian in age and likely the youngest basin on Mercury. New data from MESSENGER orbital observations will help to provide further tests of this and other models for the transition from complex crater to peak-ring basin on Mercury. Acknowledgments We thank Jay Dickson, Caleb Fassett, Seth Kadish, and Laura Kerber for help with data preparation. Thanks also to Time Scale Creator (Huang and Ogg, 2008), The authors gratefully acknowledge the MESSENGER operations teams for spacecraft operations and the MESSENGER Geology Discipline Group for helpful reviews. Mark Cintala and an anonymous reviewer provided detailed and constructive appraisals that have improved the manuscript. This work was partly supported by the NASA Earth and Space Fellowship Program under grant NNX09AQ93H. The MESSENGER project is supported by the NASA Discovery Program under contracts NASW to the Carnegie Institution of Washington and NAS to the Johns Hopkins University Applied Physics Laboratory. References Ahrens, T.J., O Keefe, J.D., Equations of state and impact-induced shock-wave attenuation on the Moon. In: Roddy, D.J., Pepin, R.O., Merrill, R.B. (Eds.), Impact and Explosion Cratering. Pergamon Press, New York, pp Alexopoulos, J.S., McKinnon, W.B., Large impact craters and basins on Venus, with implications for ring mechanics on the terrestrial planets. In: Dressler, B.O., Grieve, R.A.F., Sharpton, V.L. (Eds.), Large Meteorite Impacts and Planetary Evolution. Geological Society of America, Boulder, Colo., pp (Special Paper 293). Baker, D.M.H., Head, J.W., Prockter, L.M., Schon, S.C., Blewett, D.T., Ernst, C.M., Denevi, B.W., Solomon, S.C. The transition from complex crater to peak-ring basin on Mercury: new observations from MESSENGER flyby data and constraints on basin formation models. Planet. Space Sci., this issue. Barnouin-Jha, O.S., Zuber, M.T., Oberst, J., Preusker, F., Smith, D.E., Neumann, G.A., Solomon, S.C., Hauck, S.A., Phillips, R.J., Head, J.W., Prockter, L.M., Robinson, M.S., Assessing the relationship between crater depth and diameter on Mercury with topographic measurements by MESSENGER. Lunar Planet. Sci. 40 (abstract 1638). Blewett, D.T., Hawke, B.R., Lucey, P.G., Robinson, M.S., A Mariner 10 color study of Mercurian craters. J. Geophys. Res. 112, E doi: / 2006JE Blewett, D.T., Denevi, B.W., Robinson, M.S., Ernst, C.M., Purucker, M.E., Solomon, S.C., The apparent lack of lunar-like swirls on Mercury: implications for the formation of lunar swirls and for the agent of space weathering. Icarus 209, doi: /j.icarus Cavanaugh, J.F., Smith, J.C., Sun, X., Bartels, A.E., Ramos-Izquierdo, L., Krebs, D.J., McGarry, J.F., Trunzo, R., Novo-Gradac, A.M., Britt, J.L., Karsh, J., Katz, R.B., Lukemire, A.T., Szymkiewicz, R., Berry, D.L., Swinski, J.P., Neumann, G.A., Zuber, M.T., Smith, D.E., The Mercury Laser Altimeter instrument for the MESSENGER mission. Space Sci. Rev. 131, doi: /s Cintala, M.J., Impact-induced thermal effects in the lunar and Mercurian regoliths. J. Geophys. Res. 97, doi: /91je Cintala, M.J., Grieve, R.A.F., Impact melting and peak-ring basins: interplanetary comparisons. Lunar Planet. Sci. 22, Cintala, M.J., Grieve, R.A.F., The effects of differential scaling of impact melt and crater dimensions on lunar and terrestrial craters: some brief examples. In: Dressler, B.O., Grieve, R.A.F., Sharpton, V.L. (Eds.), Large Meteorite Impacts and Planetary Evolution. Geological Society of America, Boulder, Colo., pp (Special Paper 293). Cintala, M.J., Grieve, R.A.F., Scaling impact melting and crater dimensions: implications for the lunar cratering record. Meteorit. Planet. Sci. 33, Cintala, M.J., Wood, C.A., Head, J.W., The effects of target characteristics on fresh crater morphology: preliminary results for the Moon and Mercury. Proc. Lunar Sci. Conf. 8, Denevi, B.W., Robinson, M.S., Mercury s albedo from Mariner 10: implications for the presence of ferrous iron. Icarus 197, doi: / j.icarus Dzurisin, D., Mercurian bright patches: evidence for physio-chemical alteration of surface material? Geophys. Res. Lett. 4, Ernst, C.M., Murchie, S.L., Barnouin, O.S., Robinson, M.S., Denevi, B.W., Blewett, D.T., Head, J.W., Izenberg, N.R., Solomon, S.C., Roberts, J.H., Exposure of spectrally distinct material by impact craters on Mercury: implications for global stratigraphy. Icarus 209, doi: /j.icarus Fischer, E.M., Pieters, C.M., Remote determination of exposure degree and iron concentration of lunar soils using VIS NIR spectroscopic methods. Icarus 111, Gault, D.E., Guest, J.E., Murray, J.B., Dzurisin, D., Malin, M.C., Some comparisons of impact craters on Mercury and the Moon. J. Geophys. Res. 80, Gault, D.E., Quaide, W.L., Oberbeck, V.R., Impact cratering mechanics and structures. In: French, B.M., Short, N.M. (Eds.), Shock Metamorphism of Natural Materials. Mono Book Corporation, Baltimore, Md., pp Gault, D.E., Quaide, W.L., Oberbeck, V.R., Impact cratering mechanics and structures. In: Greeley, R., Schultz, P.H. (Eds.), A Primer in Lunar Geology. NASA Ames Research Center, Moffett Field, Calif., pp Grieve, R.A.F., Cintala, M.J., An analysis of differential impact melt-crater scaling and implications for the terrestrial cratering record. Meteoritics 27, Grieve, R.A.F., Cintala, M.J., Planetary differences in impact melting. Adv. Space Res. 20, doi: /s (97) Grieve, R.A.F., Stöffler, D., Deutsch, A., The Sudbury structure: controversial or misunderstood? J. Geophys. Res. 96, 22,753 22,764. doi: / 91JE Hale, W., Head, J.W., Central peaks in lunar craters: morphology and morphometry. Proc. Planet. Lunar Sci. Conf. 10, Hapke, B., Space weathering from Mercury to the asteroid belt. J. Geophys. Res. 106, 10,039 10,073. doi: /2000je Harmon, J.K., Slade, M.A., Butler, B.J., Head, J.W., Rice, M.S., Campbell, D.B., Mercury: radar images of the equatorial and midlatitude zones. Icarus 187, doi: /j.icarus Hartmann, W.K., Interplanet variations in scale of crater morphology Earth, Mars, Moon. Icarus 17, doi: / (72)90036-x. Hartmann, W.K., Wood, C.A., Moon: origin and evolution of multi-ring basins. Earth Moon Planets 3, doi: /bf Haruyama, J., Ohtake, M., Matsunaga, T., Morota, T., Honda, C., Yokota, Y., Abe, M., Ogawa, Y., Miyamoto, H., Iwasaki, A., Pieters, C.M., Asada, N., Demura, H., Hirata, N., Terazono, J., Sasaki, S., Saiki, K., Yamaji, A., Torii, M., Josset, J.-L., Long-lived volcanism on the lunar farside revealed by SELENE terrain camera. Science 323, doi: /science Hawke, B.R., Cintala, M.J., Impact melts on Mercury and the Moon. Bull. Am. Astron. Soc. 9, 531 (abstract 9.13).