Planetary and Space Science 59 (2011) 1949–1959

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Planetary and Space Science

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Eminescu impact structure: Insight into the transition from complex craterto 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, USAb Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USAc Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA

a r t i c l e i n f o

Article history:

Received 6 August 2010

Received in revised form

4 January 2011

Accepted 7 February 2011Available online 19 February 2011

Keywords:

Mercury

Peak ring

Impact cratering

Crater

Basin

MESSENGER

33/$ - see front matter & 2011 Elsevier Ltd. A

016/j.pss.2011.02.003

espondence to: Department of Geological

6, Providence, RI 02912, USA. Tel.: +1 401 863

ail address: samuel_schon@brown.edu (S.C. S

a b s t r a c t

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 size-

frequency 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 impactcrater forms, from small simple craters through large multi-ringbasins, and, more generally, a distinctive opportunity for asses-sing models for the crater formation process under conditions ofhigh impact velocity. Mercury may also provide a particularlyimportant point of comparison to the record of impact craters andbasins on the Moon (Neukum et al., 2001). Until recently,however, information on Mercury’s impact crater populationwas limited. Only �45% of Mercury was imaged at close rangeduring the Mariner 10 flybys (Murray et al., 1974; Murray, 1975);additional areas were imaged by Earth-based radar but at varying

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chon).

resolution (Harmon et al., 2007). The MErcury Surface, SpaceENvironment, GEochemistry, and Ranging (MESSENGER) space-craft, which will be the first probe to orbit Mercury (Solomonet al., 2001; Solomon, 2003), returned near-global imaging cover-age from three Mercury flybys in 2008 and 2009 (Solomon et al.,2008). In this paper, we employ MESSENGER data to assess theonset of the transition from complex craters to peak-ring basinsby an analysis of the 125-km-diameter Eminescu impact struc-ture centered at 10.81N, 114.11E, on Mercury (Fig. 1).

The morphology and impact crater size-frequency distribu-tions for Eminescu show that Eminescu is younger than mostother basins on Mercury. On the basis of images, geologicalmapping, and impact crater distributions, we assess the possibi-lity of post-impact volcanism in the basin, we consider theimplications of scaling relationships between melt volume andcrater volume for impact features of this size, and we reviewformation models that endeavor to account for the onset of peakrings in the transition from complex craters to peak-ring basins.

Fig. 1. (A) Eminescu (10.81N, 114.11E) 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 EN0108826917M, EN0108826922M, EN0108826982M, and

EN0108826987M.

S.C. Schon et al. / Planetary and Space Science 59 (2011) 1949–19591950

Finally, we summarize a scenario for emplacement of the smoothplains within the basin via impact melt expulsion from a centralmelt cavity during cavity collapse and peak ring uplift.

2. Peak-ring basins

The general trend in the form of impact features with increas-ing diameter, from simple bowl-shaped craters, to larger complexcraters (Fig. 2), to still larger peak-ring basins, and finally tothe 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 transitionsbetween these forms and their relative abundances vary amongplanetary bodies, which permits investigation of the relativeinfluence of key parameters in the cratering process by exploringimpact crater morphologies and populations in different environ-ments. From images obtained during the Mariner 10 flybys, Pike(1988) grouped impact craters on Mercury into seven morpholo-gical classes. Crater nomenclature has not always been consistentin the literature; for instance, similar suites of morphologicalcharacteristics have been variously termed central-peak basins,peak-ring basins, protobasins, and two-ring basins. At issue hasbeen the application of the ‘‘basin’’ designation to crater formstransitional between complex craters and multi-ring basins. Theterm peak-ring basin is employed here for descriptive simplicityand in line with terminology used in a recent comprehensivesurvey of these features on Mercury (Baker et al., thisissue; Fig. 3A). Peak-ring basins are defined by Baker et al. (thisissue) as craters with a single raised inner ring that is entirelyseparate from the crater walls, or a circular arrangement of nearlycontinuous massifs and no central peak. Basins with peak ringsand central peaks are termed protobasins. Ringed peak-clusterbasins are characterized by a ring-like arrangement of peakelements, with no central peak, and occur at smaller rim-crest

diameters than peak-ring basins and with much smaller ringdiameters (Baker et al., this issue; Fig. 3B).

Although preservation and subsequent modification varyamong individual examples, peak-ring basins generally exhibitother 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 of132 km. MESSENGER images have revealed an additional 44 peak-ring basins and 17 newly discovered protobasins (Baker et al., thisissue), confirming that Mercury hosts the largest population ofsuch basins in the solar system (Wood and Head, 1976). Thesurvey of Baker et al. (this issue) revealed a distinct sub-group ofpeak-ring basins (Fig. 3), the ringed peak-cluster basins, withsmaller inner rings of semi-continuous massifs but lacking centralpeaks. Eminescu is among the largest such basins (Fig. 3B) andoccurs near the minimum or onset diameter for peak-ring basins.The excellent state of preservation of Eminescu (Fig. 1) makes thebasin an ideal focus of study for improving our understanding ofthe transitions within peak-ring morphologies on Mercury andtesting hypotheses for peak-ring basin formation (Baker et al., thisissue, 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 offresh craters on Mercury at diameters near that for the onset ofpeak-ring morphology. Eminescu is the morphologically freshestbasin cataloged by Baker et al. (this issue) and lies at the sizetransition between ringed peak-cluster basins and peak-ringbasins. Stereo photogrammetry from MESSENGER images showsthat Eminescu has a depth, the difference between the mean

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 12.421N, 226.641E, 42 km diameter; (B) crater at 15.541S, 275.071E, 62 km diameter; (C) crater at 9.551N, 234.211E, 67 km diameter; (D) crater at 29.261S,

135.061E, 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.

S.C. Schon et al. / Planetary and Space Science 59 (2011) 1949–1959 1951

elevations of the crater rim crest and crater floor, of approxi-mately 3100 m (Oberst et al., 2010; Preusker et al., this issue).This depth is consistent with the limited dataset on crater depthsand diameters assembled by Pike (1988) and shown in Fig. 4.Additional stereo data (Oberst et al., 2010; Preusker et al., thisissue) and topographic profiles obtained with the Mercury LaserAltimeter (Cavanaugh et al., 2007; Zuber et al., 2008; Barnouin-Jha et al., 2009) will expand substantially the dataset of craterdepths and diameters for Mercury during the orbital phase of theMESSENGER mission.

Eminescu has a continuous ejecta deposit that extends aboutone crater radius from the rim crest. Distinct chains of secondarycraters, and more distal secondary clusters, some of which areassociated with faint ray segments, are arrayed radial to Eminescu(Fig. 5). Extensive, generally east-west-trending, high-albedo raysin the vicinity are associated with Xiao Zhao crater, located tothe 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 havemodified the crater walls (Fig. 1A). Mapping of slump blockcontacts shows that between two and five major faults areobserved in the crater wall, which extends inward from the rimby about 0.3 crater radii (Fig. 1A). These wall characteristics aretypical of craters of this size on Mercury (Smith and Hartnell, 1978;Malin and Dzurisin, 1978; Pike, 1988). Smaller (40–100 km-diameter)complex craters on Mercury have similar terraced wall morphologiesand interiors displaying both smooth plains and primary crater floorroughness (Fig. 2). These comparisons indicate that Eminescu has notbeen extensively flooded, in contrast to some lunar craters and basins(e.g., Head, 1982), in agreement with the fact that rim-to-floor depthis in line with other unflooded impact features of comparable size(Fig. 4). This observation suggests that crater wall morphology doesnot itself distinguish the transition from complex craters to peak-ringbasins.

Eminescu has a high-albedo peak ring defined by a circle ofdiscrete massifs and massif clusters approximately 26 km indiameter (Fig. 1). Surrounding individual peak-ring massifs are

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 transi-

tional basin forms (Baker et al., this issue). The positions of Eminescu and Raditladi

are indicated by yellow stars.

2001001010.1

S I M P L E C

R A T E R SC O M P L E X

EMINESCU

immature

Crater Diameter (km)

Cra

ter

Depth

(km

)

0.01

0.1

1

6

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).

S.C. Schon et al. / Planetary and Space Science 59 (2011) 1949–19591952

smooth, high-albedo regions of material that extend radiallyfor 1–2 km from the massifs and form annuli. The outer marginsof these annuli often coalesce with each other (Fig. 1A). Dark,smooth floor deposits can be followed continuously both insideand outside the peak ring. The contact between the annular unit(peak-ring annuli) and the lower-albedo crater floor materialinside 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 thebasis of albedo and texture (Fig. 1B). One is the dark smooth plainsunit noted above, which has sharp contacts with all adjacent mapunits. Exterior to the peak-ring annuli, this unit appears tosurround small circular or quasi-circular features of higher albedoand positive topography (i.e., kipukas), similar to the peak-ringmassifs 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 distinctlyhigher albedo than the dark plains unit but a similarly smoothtexture. The same smooth texture is also observed in somecomplex craters (Fig. 2). Farther from the crater center are mottledand rougher surface textures that comprise the rough interiorfacies (Fig. 1), which are reminiscent of primary crater floorroughness (Head, 1975) that is also observed in complex craters(Fig. 2). The rough interior facies contains interspersed regions ofsmooth texture (Fig. 1A) that are similar in appearance to pondedimpact 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-reflectancematerial (often observed to be excavated from depth), moderate-to high-reflectance smooth plains (e.g., the interior plains ofCaloris), and a unit intermediate in reflectance that includesmuch of the area mapped as heavily cratered terrain fromMariner 10 images. The surface in the vicinity of Eminescu is thisspectrally intermediate terrain.

A bright crater-floor deposit (BCFD) unit, of much more limitedextent, was also defined by Robinson et al. (2008) on the basis ofspectral characteristics. BCFDs are spectrally distinct from freshcrater ejecta and were identified at isolated, geographically sepa-rated locations such as Sander, a 50-km-diameter complex craterwithin the Caloris basin, and the peak ring of the Raditladi basin.The annular and peak-ring units mapped in Eminescu correspondto the BCFD material of Robinson et al. (2008). Analyses of Mariner10 data also identified similar bright crater interior deposits(Dzurisin, 1977; Schultz, 1977; Denevi and Robinson, 2008). How-ever, hypotheses for the origin of these deposits invoking severelyshock-altered crustal material (Schultz, 1977) and localized phy-siochemical alteration associated with impact-related fractures(Dzurisin, 1977) have remained speculative due to the limited dataavailable. Blewett et al. (2010) concluded that BCFDs constitute adistinct composition, genetically unrelated to lunar swirls or spaceweathering phenomena. Ernst et al. (2010) analyzed impact craterexcavation of buried stratigraphic units and found that the BCFDsin the cases they examined represented excavation of a lithology at

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).

S.C. Schon et al. / Planetary and Space Science 59 (2011) 1949–1959 1953

depth that had been exposed by impact cratering events. Ourinvestigation of Eminescu provides context and new geologicconstraints on the occurrence of this potential unit and suggeststhat here it represents impact exposure of a buried lithology in theEminescu region.

4. Relative ages

Mercury’s geologic timescale (Fig. 7) is divided into fiveperiods (Spudis, 1985; Spudis and Guest, 1988) that are compar-able to the five divisions of the lunar timescale (e.g., Wilhelms,1987) and similarly based on the formation of large basins andprominent younger craters. The Mercury timescale was developedwith Mariner 10 data, and many questions have remained sincethat mission regarding the early geologic history and planetaryevolution of Mercury, in particular regarding the prevalence andlifetime of surface volcanism (e.g., Head et al., 2000, 2007). TheMESSENGER flybys have provided images from which volcanicplains were conclusively identified (Head et al., 2008; Head et al.,2009a), and that identification permitted evaluation of plainschronology with crater size-frequency distributions (Strom et al.,2008, this issue). After the first flyby, volcanic plains weredefinitively identified within and exterior to the 1500-km-diameter 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.

Moon

Age (

Ga)

1.0

2.0

3.0

4.0

Mercury

COPERNICAN

ERATOSTHENIAN

IMBRIAN

NECTARIAN

PRE-NECTARIAN

KUIPERIAN

MANSURIAN

CALORIAN

TOLSTOJAN

PRE-TOLSTOJAN

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.

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 km2; Raditladi:

number of craters counted¼178, area¼53,500 km2). These data are displayed

using a ‘‘relative’’ or R plot, which is referenced to crater-size frequency distribu-

tion defined by a power law of slope �3. Vertical position measures crater density,

with cratered surfaces reaching an empirical saturation density at R�0.2–0.25

(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.

S.C. Schon et al. / Planetary and Space Science 59 (2011) 1949–19591954

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-km-diameter peak-ring basin (Rachmaninoff) with unambiguous evi-dence of post-impact volcanic plains on the interior floor (Prockteret al., 2010). Spectrally distinct smooth plains, within the basinand embaying the ejecta deposit, are chronologically separablefrom the age of the basin and are associated with a likely ventstructure (Prockter et al., 2010). This basin is substantially youngerthan 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 plainsare older than Raditladi (Fig. 6), a 250-km-diameter peak-ringbasin with a prominent peak ring of BCFD material that is alsolikely Mansurian in age (Prockter et al., 2009, 2010). The peak ringof Raditladi (Fig. 6) lacks extensive bright annuli of BCFD material

similar to the annular deposits surrounding the peak-ring massifsin Eminescu (Fig. 1A).

Crater-size frequency distributions from the continuous ejectaand the interior of Eminescu show that Eminescu is unambigu-ously younger than Raditladi (Fig. 8). Crater size-frequency dis-tributions observed on the dark smooth plains unit in the centralportion of the basin are not distinguishable from crater size-frequency distributions found on the other interior units andcontinuous ejecta deposit. Therefore, the dark smooth plains arenot chronologically separable from basin formation on the basisof their crater-retention age. Although Eminescu is younger thancomparable basins, we cannot constrain well its absolute age.

The bright-rayed crater Kuiper defines the youngest strati-graphic system on Mercury, the Kuiperian (Spudis, 1985; Spudisand Guest, 1988) (Fig. 7). Kuiperian-age deposits are restricted torayed craters because analysis of Mariner 10 data showed noevidence of volcanic or tectonic activity during this period (Spudisand Guest, 1988; Neukum et al., 2001; Head et al., 2007). Sincethe time of this definition of the Kuiperian, analyses of lunardeposits have shown that crater rays are not homogenousfeatures. Rather, lunar crater rays have been divided into threetypes: immaturity rays, compositional rays, or a combinationof immaturity and compositional rays (Hawke et al., 2004).

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).

S.C. Schon et al. / Planetary and Space Science 59 (2011) 1949–1959 1955

Immaturity rays are visually apparent due to the presence of freshhigh-albedo material that is compositionally similar to its sur-roundings but has not been exposed to space weathering pro-cesses (e.g., Hapke, 2001) for a sufficient time to have itsreflectance reduced to background levels. Compositional raysare visible because of the contrast between high-albedo raymaterial of highlands mineralogy, for example, and backgroundmaterial of lower reflectance, typically maria (Hawke et al., 2004).Whereas immaturity rays are sensitive to degradation by spaceweathering and are generally young, compositional rays tend tobe more persistent and can therefore be older features (Hawkeet al., 2004; Werner and Medvedev, 2010).

We interpret the ray segments of Eminescu (Fig. 5) as con-sistent with the preservation of compositional rays. In Fig. 5, tworays appear distinctly on regions of smooth plains where they areassociated with clusters and chains of secondary craters fromEminescu. Rays from Eminescu are difficult to distinguish, how-ever, and are generally not observed on cratered terrain closer tothe crater. Bright rays nearly perpendicular to Eminescu second-ary crater chains (Fig. 5) are from Xiao Zhao crater. We interpretthese rays as immaturity rays given the brightness of the XiaoZhao rays, superposition relationships, and their preservation onthe cratered terrain.

Therefore, although Eminescu meets the strict definition forinclusion in the Kuiperian period, any immaturity rays fromEminescu have apparently been space weathered compared tothose from Xiao Zhao crater. These relationships suggest that theage of Eminescu is intermediate between the characteristicdegradation times of immaturity rays and compositional rayson Mercury (Blewett et al., 2007). Investigations of rayed cratersystems and ray degradation during the orbital phase of theMESSENGER mission should help to resolve uncertainties in theMansurian–Kuiperian boundary. Crater retention-age criteriashould be established for period boundaries, and ray preservationshould be investigated with high-resolution spectra obtainedwith the Mercury Atmospheric and Surface Composition Spectro-meter (McClintock and Lankton, 2007) to quantify optical matur-ity differences and compositional variations between rays andtheir 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 interiorunits, which could result from either distributed impact melt orpost-impact volcanic activity. The bright material surrounding theEminescu peak ring extends outward from the peak massifs bygreater distances (1–2 km) than talus and scree associated withlunar central peaks, none of which have a similar annulus (Haleand Head, 1979; Pieters et al., 1994). This geometry suggests thatboth the peak ring and annular material were embayed by thedark smooth plains. However, no evidence of volcanic vents hasbeen observed in Eminescu. For comparison, the lunar protobasinAntoniadi (Fig. 9) has a peak ring and diminutive central peak thatwere 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 ofthe crater (Wilhelms et al., 1979).

Eminescu is demonstrably younger than Raditladi (Fig. 8) andtherefore postdates the extensive Calorian-age plains volcanismthat has been documented elsewhere on Mercury (Robinson andLucey, 1997; Head et al., 2008; Prockter et al., 2010). In contrast tothe mare deposits of Antoniadi, the dark smooth plains unit ofEminescu is not chronologically separable from the formation ofthe basin on the basis of crater retention ages. Although the peakring and annuli appear embayed by the dark smooth plains unit,there is no clear morphological or crater chronological evidencethat those plains are volcanic deposits. As described in greater

detail in the next section, we suggest that these plains, as well asthe brighter plains, are both impact melt. However, a volcanicorigin for the dark smooth plains cannot be excluded if theinterval between basin formation and volcanism was sufficientlyshort as to lie within the uncertainty in relative age determinationfrom crater size-frequency distributions.

5. Conceptual model for the formation of Eminescu

Geologic mapping of Eminescu (Fig. 1) provides the basis topropose a conceptual scenario for the basin-forming eventderived from general impact cratering principles (e.g., Melosh,1989). Geological observations (e.g., Roddy et al., 1977), numer-ical modeling (e.g., Ahrens and O’Keefe, 1977), and laboratoryimpact experiments (Gault et al., 1974; Schultz, 1987) haveelucidated the primary mechanics of impact cratering, and thereis general consensus on the sequential crater-formation stages ofcontact and compression, excavation, and collapse and modifica-tion (see Melosh, 1989; Spudis, 1993). On the basis of theseprinciples, in the case of Eminescu, as with other craters of thissize, hypervelocity impact of a projectile into the substrateresulted in the formation of a displaced zone and an excavationzone (Gault et al., 1968), which together formed a transient cavity(Fig. 10). Peak levels of shock stress and impact heating in thecentral portion of the transient cavity caused melting of the targetrock and a central concentration of impact melt. Evolution of thegrowing transient cavity caused streaming of a portion of the meltout along the growing melt-rock interface, with pure impact meltfrom the interior cavity mixing with rock debris dislodged andejected from the growing cavity. The boundary between comple-tely melted material and highly shocked but unmelted material isrepresented by a zone of peak heating just below the liquidus ofthe target material. A variety of evidence (e.g., Melosh, 1989;Grieve and Cintala, 1992; Pierazzo et al., 1997) indicates that the

MELT

VAPOR

BOLIDETransient Cavity Configuration

(During Impact)

Final Configuration

(Post-Impact)

A1

A2

B1

B2

TRANSIENT CAVITY

DISPLACED ZONE

EXCAVATED ZONE

melt expulsion

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).

S.C. Schon et al. / Planetary and Space Science 59 (2011) 1949–19591956

volume of melt in the interior of the cavity increases with craterdimensions faster than crater volume.

The projectile that formed Eminescu impacted a substratecomposed of spectrally intermediate terrain (Robinson et al., 2008)and formed a transient cavity that grew laterally out to a diametersomewhat less than the current diameter of 125 km. Structuraluplift of the rim occurred by the downward, lateral, and radialmovement of material in the displaced zone (Fig. 10). The ejectaexpelled from the transient cavity thinned with radial distance fromthe transient cavity rim crest; proximal ejecta was continuous onthe crater rim and discontinuous farther out, forming secondarycrater chains, clusters, and rays in more distal regions.

Within the transient crater, the melt cavity continued to growto greater depths, penetrating into and exposing the BCFDspectral unit. In our preferred interpretation, the BCFD spectralunit is a specific lithology, occurring at depth, which was exposedduring formation of the Eminescu peak ring in a fashion similar tothe exposure of buried lithologies by impact craters documentedby Ernst et al. (2010). In the later stages of the cratering event, bythe scenario developed here, collapse of the edge of the meltcavity caused uplift of the peak ring to form the peaks andassociated annuli of BCFD (Fig. 10). The kipukas of BCFD aredistributed away from the ringed peak cluster on the southern,eastern, and western part of the crater floor; their distance fromthe peak ring suggests that the melt cavity penetrated to asubstantial depth into the BCFD material. In this same stage ofthe cratering event, downward and inward rotation of thetransient cavity rim crest occurred along listric faults, forming astep-like series of terraces (e.g., Gault et al., 1968; Melosh, 1989)currently comprising the crater wall (Fig. 1). This collapse of thetransient cavity rim crest enlarged the diameter to its currentvalue of 125 km. Cavity wall debris mixed with the impact meltand the toes of slump blocks to produce the primary floorroughness of the rough interior facies (Fig. 1).

The dark smooth plains and the bright smooth plains thatmake up a major portion of the crater floor are interpreted to bedifferent facies of impact melt (e.g., Hawke and Cintala, 1977).The outer bright smooth plains are interpreted to correspond toimpact melt deposits in similar positions within terrestrial craters

(e.g., Grieve and Cintala, 1992) where the melt was mixed withsubstantial amounts of fragmental debris and cooled and solidi-fied rapidly. The dark smooth plains are interpreted to representthe more pure impact melt deposits characteristic of terrestrialcrater interiors; often these deposits are sufficiently thick toundergo differentiation (e.g., Grieve et al., 1991; Stoffler et al.,1994; Therriault et al., 2002; Zieg and Marsh, 2005; Spray andThompson, 2008).

What is the relationship of the peak ring and the dark smoothplains, interpreted to be impact melt? Dark smooth plains line thefloor of the interior of the ringed peak cluster, embay the BCFDannuli, and extend through gaps in the peaks out to the surround-ing crater floor, where they embay other kipukas of BCFD smallerthan peaks of the ringed peak cluster. On the basis of the positionof Eminescu (yellow star in Fig. 3B) along the extension of thecentral-peak diameter relationship for complex craters, and at theuppermost end of the trend of the ringed peak-cluster basins(Fig. 3B), we interpret Eminescu to represent a type example ofthe transition from complex craters to peak-ring basins. On thebasis of cratering theory (e.g., Melosh, 1989; Grieve and Cintala,1992; Pierazzo et al., 1997; Cintala and Grieve, 1998), impact meltshould be concentrated in the central part of the cavity and thenbe redistributed during cavity collapse. In oblique impacts, themelt region may be shifted downrange, but the melt volume willremain comparable for impact angles down to �301 from thehorizontal (Pierazzo and Melosh, 2000). We interpret the dis-tribution of the dark smooth facies to be the result of the uplift ofthe innermost crater floor to form the ringed peak cluster and theconcurrent expulsion of melt from the innermost part of thecavity onto the adjacent crater floor, draining through gaps in theringed peak cluster. The present volume of the interior ofthe Eminescu peak ring (from the average height of the peak-ring massifs to the level of the inner floor), which shouldapproximately correspond to the volume of the melt cavity inthis scenario (Fig. 10), is equivalent to a thickness of plainssurrounding the peak ring on the Eminescu floor (for an area of1670 km2) of about 100 m. This result is consistent with thedistribution of the dark plains unit and its embayment relation-ships by the scenario described here (Fig. 1).

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6. Discussion of peak-ring basin formation processes

Although the primary mechanics of impact cratering are known,debate remains regarding the formation process responsible forpeak-ring basins. Early work, informed by lunar exploration, sug-gested scaling relations based on gravitational acceleration(e.g., Hartmann, 1972) or kinetic energy (e.g., Head, 1978) thatmight govern the onset of peak-ring basins. More recent work hasfocused on theories of acoustic fluidization and hydrodynamiccollapse of central-uplift materials (Melosh, 1979) and so-called‘‘differential melt scaling,’’ a power-law description for the increasein melt volume as a fraction of crater volume with increasing cratersize (e.g., Grieve and Cintala, 1992). Here we focus on assessingpredictions of impact-melt scaling for the transition to peak-ringbasins as manifested in the geology of Eminescu and our inter-pretive scenario developed in the previous section.

A continuous transition from complex craters with centralpeaks to peak-ring basins has been suggested on the basis ofobservations of craters on Earth (Grieve and Cintala, 1992), Venus(Alexopoulos and McKinnon, 1994), and the Moon (Wood andHead, 1976). A continuum of central uplift features (Fig. 3) marksthe transition from complex crater to basin, ranging from singlecentral peaks, to multiple central peaks, and finally to peak rings(sometimes with central peaks). Such a continuum, at face valueinconsistent with the discrete classification scheme of Pike (1988),calls for a more detailed approach to the onset and formationof peak-ring basins with increasing crater size. Many workershave explored relationships between central peak area andcrater 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-ringbasins’’ from ‘‘protobasins’’ and, on the basis of differing ratios ofinner-ring and rim-crest diameters, declared them to be differenttypes of two-ring structures that should not be commingled instatistical analyses. In contrast, Alexopoulos and McKinnon (1994)rejected this distinction and considered ring-diameter ratios asconsistent with a morphological continuum between central-peakcraters and peak-ring basins. They presented data showing thatratios of the diameters of crater rim to peak ring decrease withincreasing 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 tocrater-rim diameter (0.21) of Eminescu. This low ratio is belowPike’s average ratios (with 95% confidence intervals) for two-ringbasins (0.4970.01) and protobasins (0.3970.03). Comparison tothe data presented by Alexopoulos and McKinnon (1994) alsoindicates that Eminescu’s ring/rim ratio is an extreme value.However, Baker et al. (this issue) have shown that Eminescu isnot 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 toinner rings, none of these rings are as completely developed as thepeak ring of Eminescu, and none are markedly smaller in diameter.How can the onset diameter (for peak-ring basins, Fig. 3) anddevelopment of Eminescu’s peak ring be used in conjunction withthe mapped interior units to assess formation models for peak-ring basins?

Eminescu is below the onset diameter observed for peak-ringbasins on the Moon (Wood and Head, 1976) but above thediameter range for peak-ring basins on Venus (Alexopoulos andMcKinnon, 1994), consistent with Mercury’s intermediate valueof surface gravitational acceleration (3.7 m/s2). Although gravityis the major factor in determining crater dimensions, it is only aminor influence on the volume of melt generated during ahypervelocity impact (Cintala and Grieve, 1994). A body of workdeveloped by Cintala and Grieve has focused on characterizing

the non-linear scaling between impact melt volume and cratervolume (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 experi-mental studies (Grieve and Cintala, 1992), as well as the work ofothers (e.g., Pierazzo et al., 1997), shows that larger impact eventslead to the generation of disproportionately more impact meltthan smaller events because melt generation is a function ofkinetic energy, whereas crater dimensions are strongly influencedby gravity. The impact of a given size object on Mercury or theMoon would generate dimensionally similar craters because ofthe counterbalancing effects of higher gravity and higher impactvelocity at Mercury, but the impact would generate approxi-mately twice as much melt on Mercury (Cintala, 1992).

There are several other geologic effects arising from theincreasing fractional melt volume with increasing crater size.The composition and thermal evolution of impact melt depositswill differ, because larger volumes of impact melt have lesscontact with crater walls, incorporate fewer lithic clasts, andtherefore cool more slowly (Cintala and Grieve, 1998). The depthof melting also influences the depth of origin of central upliftfeatures (Fig. 10). Grieve and Cintala (1992) suggested the grow-ing fractional volume of melt as a possible mechanism for thetransition from central-peak craters to peak-ring basins (seealso Cintala and Grieve, 1991). They also noted that beneath thezone of complete melting, the target is weakened and undergoespartial melting, physical changes that may be manifested incentral uplift features (Grieve and Cintala, 1992, 1997). Thenon-linear relation between melt volume and crater volume formthe basis for the peak-ring basin formation scenario of Head(2010) called the nested melt-cavity model. In that scenario, thedepth of greatest melting exceeds the depth of the transientcavity for impact features of sufficient size, resulting in thedevelopment of a so-called ‘‘nested melt cavity’’ within thedisplaced zone underlying the transient cavity. Deepening andexpansion of the melt cavity during the cratering event preventsthe formation of a single central peak. Collapse of the transientcrater results in the formation of a peak ring surrounding theformer melt cavity (Head, 2010).

In our interpretation of Eminescu, interior smooth plains unitsare impact melt sheets, and the rough interior facies is partiallycomposed of impact melt breccia. Our analysis is not a completetest of the nested melt-cavity model. However, our mappingof the ringed peak cluster and distribution of impact melt isconsistent with retardation of central peak development withincreased depth of melting as shown in Fig. 10 (Cintala, 1992;Grieve and Cintala, 1992, 1997; Head, 2010). Therefore, wesuggest that Eminescu and other ringed peak-cluster basinsfollow the trend on depth-diameter plots of central-peak craters(Fig. 3) because uplift in these craters and basins is similar, butgreater depths of melting at the sub-impact point than forcentral-peak craters lead to small melt cavities (Fig. 10). Thesesmall melt cavities are contained within the central uplift regionand are responsible for the formation of ringed peak clustersrather than traditional central peaks in these features (Baker et al.,this issue). In this conceptual model, the dark smooth plainsfound inside the peak ring (Fig. 1) extend peripherally around thebright deposits due to an expulsion and drainage of impact meltfrom a central pooling as a result of inward and upward move-ment of underlying material to form the peak ring during collapseand modification of the transient crater. This scenario is consis-tent with peak-ring emplacement as envisioned by Grieve andCintala (1992) and Head (2010) (Fig. 10).

This study is consistent with predictions of impact melt scalingand the nested melt-cavity model for basin evolution on Mercury(Grieve and Cintala, 1992, 1997; Cintala and Grieve, 1994, 1998;

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Head, 2010; Baker et al., this issue). However, rigorous testing ofthis scenario must await improved topographic, compositional,and imaging observations from the MESSENGER orbital missionphase. Additional geological effects predicted by melt scalingwill provide fruitful lines of inquiry during MESSENGER’sorbital mission phase at which time the provenance of the BCFDshould be further investigated and the potential occurrence ofvolcanic plains in Eminescu should be reevaluated, especiallywith high-resolution topography data (Cavanaugh et al., 2007;Zuber et al., 2008).

7. Conclusions

Eminescu is a 125-km-diameter ringed peak-cluster basinnewly observed in flyby data from the MESSENGER mission. Itssize and nature provide insight into the transition betweencomplex craters and peak-ring basins. The peak ring and sur-rounding bright material are composed of bright crater floordeposits (BCFDs), a unit seen elsewhere on Mercury that weinterpret as a buried lithology that was exposed by the Eminescuimpact event. Dark smooth plains found on the floor of Eminescuhave an impact crater size-frequency distribution similar todeposits coeval with the Eminescu impact and thus are inter-preted to be impact melt, rather than products of post-impactvolcanic flooding. However, volcanic emplacement at a time afterbasin formation less than the relative time uncertainty in cratersize-frequency distributions cannot be excluded. On the basis ofthe size and morphology of the ringed peak cluster, and compar-isons to other crater transitional forms (Fig. 3), we interpret theformation of Eminescu as being consistent with an impact meltscaling (nested melt cavity) model of basin formation. The interiorof the ringed peak cluster represents the formation of a centralmelt cavity at a crater size just below that at which distinctivepeak rings develop in larger impact structures (Fig. 3). Composi-tional rays from Eminescu are preserved on adjacent smoothplains, but immaturity rays are not preserved. Eminescu exhibitsgood morphological preservation, and crater counts support theinference that it is Kuiperian in age and likely the youngest basinon Mercury. New data from MESSENGER orbital observations willhelp to provide further tests of this and other models for thetransition from complex crater to peak-ring basin on Mercury.

Acknowledgments

We thank Jay Dickson, Caleb Fassett, Seth Kadish, and LauraKerber for help with data preparation. Thanks also to Time ScaleCreator (Huang and Ogg, 2008), www.tscreator.com. The authorsgratefully acknowledge the MESSENGER operations teams forspacecraft operations and the MESSENGER Geology DisciplineGroup for helpful reviews. Mark Cintala and an anonymousreviewer provided detailed and constructive appraisals that haveimproved the manuscript. This work was partly supported bythe NASA Earth and Space Fellowship Program under grantNNX09AQ93H. The MESSENGER project is supported by the NASADiscovery Program under contracts NASW-00002 to the CarnegieInstitution of Washington and NAS5-97271 to the Johns HopkinsUniversity Applied Physics Laboratory.

References

Ahrens, T.J., O’Keefe, J.D., 1977. Equations of state and impact-induced shock-waveattenuation on the Moon. In: Roddy, D.J., Pepin, R.O., Merrill, R.B. (Eds.), Impactand Explosion Cratering. Pergamon Press, New York, pp. 639–656.

Alexopoulos, J.S., McKinnon, W.B., 1994. 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 PlanetaryEvolution. Geological Society of America, Boulder, Colo., pp. 29–50 (SpecialPaper 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-ringbasin on Mercury: new observations from MESSENGER flyby data andconstraints 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., 2009. Assessing the relationship between crater depth and diameter onMercury with topographic measurements by MESSENGER. Lunar Planet. Sci.40 (abstract 1638).

Blewett, D.T., Hawke, B.R., Lucey, P.G., Robinson, M.S., 2007. A Mariner 10 colorstudy of Mercurian craters. J. Geophys. Res. 112, E02005. doi:10.1029/2006JE002713.

Blewett, D.T., Denevi, B.W., Robinson, M.S., Ernst, C.M., Purucker, M.E., Solomon,S.C., 2010. The apparent lack of lunar-like swirls on Mercury: implications forthe formation of lunar swirls and for the agent of space weathering. Icarus209, 239–246. doi:10.1016/j.icarus.2010.03.008.

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., 2007. The Mercury Laser Altimeter instrument for theMESSENGER mission. Space Sci. Rev. 131, 451–479. doi:10.1007/s11214-007-9273-4.

Cintala, M.J., 1992. Impact-induced thermal effects in the lunar and Mercurianregoliths. J. Geophys. Res. 97, 947–973. doi:10.1029/91JE02207.

Cintala, M.J., Grieve, R.A.F., 1991. Impact melting and peak-ring basins: inter-planetary comparisons. Lunar Planet. Sci. 22, 215–216.

Cintala, M.J., Grieve, R.A.F., 1994. The effects of differential scaling of impactmelt and crater dimensions on lunar and terrestrial craters: some briefexamples. In: Dressler, B.O., Grieve, R.A.F., Sharpton, V.L. (Eds.), Large Meteor-ite Impacts and Planetary Evolution. Geological Society of America, Boulder,Colo., pp. 51–59 (Special Paper 293).

Cintala, M.J., Grieve, R.A.F., 1998. Scaling impact melting and crater dimensions:implications for the lunar cratering record. Meteorit. Planet. Sci. 33, 889–912.

Cintala, M.J., Wood, C.A., Head, J.W., 1977. The effects of target characteristics onfresh crater morphology: preliminary results for the Moon and Mercury. Proc.Lunar Sci. Conf. 8, 3409–3425.

Denevi, B.W., Robinson, M.S., 2008. Mercury’s albedo from Mariner 10: implica-tions for the presence of ferrous iron. Icarus 197, 239–246. doi:10.1016/j.icarus.2008.04.021.

Dzurisin, D., 1977. Mercurian bright patches: evidence for physio-chemicalalteration of surface material? Geophys. Res. Lett. 4, 383–386.

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., 2010. Exposure ofspectrally distinct material by impact craters on Mercury: implications forglobal stratigraphy. Icarus 209, 210–223. doi:10.1016/j.icarus.2010.05.022.

Fischer, E.M., Pieters, C.M., 1994. Remote determination of exposure degree andiron concentration of lunar soils using VIS–NIR spectroscopic methods. Icarus111, 475–488.

Gault, D.E., Guest, J.E., Murray, J.B., Dzurisin, D., Malin, M.C., 1975. Somecomparisons of impact craters on Mercury and the Moon. J. Geophys. Res.80, 2444–2460.

Gault, D.E., Quaide, W.L., Oberbeck, V.R., 1968. Impact cratering mechanics andstructures. In: French, B.M., Short, N.M. (Eds.), Shock Metamorphism of NaturalMaterials. Mono Book Corporation, Baltimore, Md., pp. 87–99.

Gault, D.E., Quaide, W.L., Oberbeck, V.R., 1974. Impact cratering mechanics andstructures. In: Greeley, R., Schultz, P.H. (Eds.), A Primer in Lunar Geology.NASA Ames Research Center, Moffett Field, Calif., pp. 177–189.

Grieve, R.A.F., Cintala, M.J., 1992. An analysis of differential impact melt-craterscaling and implications for the terrestrial cratering record. Meteoritics 27,526–538.

Grieve, R.A.F., Cintala, M.J., 1997. Planetary differences in impact melting. Adv.Space Res. 20, 1551–1560. doi:10.1016/S0273-1177(97)00877-6.

Grieve, R.A.F., Stoffler, D., Deutsch, A., 1991. The Sudbury structure: controversialor misunderstood? J. Geophys. Res. 96, 22,753–22,764. doi:10.1029/91JE02513.

Hale, W., Head, J.W., 1979. Central peaks in lunar craters: morphology andmorphometry. Proc. Planet. Lunar Sci. Conf. 10, 2623–2633.

Hapke, B., 2001. Space weathering from Mercury to the asteroid belt. J. Geophys.Res. 106, 10,039–10,073. doi:10.1029/2000JE001338.

Harmon, J.K., Slade, M.A., Butler, B.J., Head, J.W., Rice, M.S., Campbell, D.B., 2007.Mercury: radar images of the equatorial and midlatitude zones. Icarus 187,374–405. doi:10.1016/j.icarus.2006.09.026.

Hartmann, W.K., 1972. Interplanet variations in scale of crater morphology—Earth,Mars, Moon. Icarus 17, 707–713. doi:10.1016/0019-1035(72)90036-X.

Hartmann, W.K., Wood, C.A., 1971. Moon: origin and evolution of multi-ringbasins. Earth Moon Planets 3, 3–78. doi:10.1007/BF00620390.

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.,2009. Long-lived volcanism on the lunar farside revealed by SELENE terraincamera. Science 323, 905–908. doi:10.1126/science.1163382.

Hawke, B.R., Cintala, M.J., 1977. Impact melts on Mercury and the Moon. Bull. Am.Astron. Soc. 9, 531 (abstract 9.13).

S.C. Schon et al. / Planetary and Space Science 59 (2011) 1949–1959 1959

Hawke, B.R., Head, J.W., 1977. Impact melt on lunar crater rims. In: Roddy, D.J.,Pepin, R.O., Merrill, R.B. (Eds.), Impact and Explosion Cratering. PergamonPress, New York, pp. 815–841.

Hawke, B.R., Blewett, D.T., Lucey, P.G., Smith, G.A., Bell, J.F., Campbell, B.A.,Robinson, M.S., 2004. The origin of lunar crater rays. Icarus 170, 1–16.doi:10.1016/j.icarus.2004.02.013.

Hawkins, S.E., Boldt, J.D., Darlington, E.H., Espiritu, R., Gold, R.E., Gotwols, B., Grey,M.P., Hash, C.D., Hayes, J.R., Jaskulek, S.E., Kardian, C.J., Keller, M.R., Malaret,E.R., Murchie, S.L., Murphy, P.K., Peacock, K., Prockter, L.M., Reiter, R.A.,Robinson, M.S., Schaefer, E.D., Shelton, R.G., Sterner, R.E., Taylor, H.W., Watters,T.R., Williams, B.D., 2007. The Mercury Dual Imaging System on theMESSENGER spacecraft. Space Sci. Rev. 131, 247–338. doi:10.1007/s11214-007-9266-3.

Head, J.W., 1975. Processes of lunar crater degradation: changes in style withgeologic time. Moon 12, 299–329.

Head, J.W., 1978. Origin of central peaks and peak rings: evidence from peak-ringbasins on Moon, Mars, and Mercury. Proc. Planet. Lunar Sci. Conf. 9, 485–487.

Head, J.W., 1982. Lava flooding of ancient planetary crusts: geometry, thickness,and volumes of flooded lunar impact basins. Earth Moon Planets 26, 61–88.doi:10.1007/BF00941369.

Head, J.W., 2010. Transition from complex craters to multi-ringed basins onterrestrial planetary bodies: scale-dependent role of the expanding melt cavityand progressive interaction with the displaced zone. Geophys. Res. Lett. 37,L02203. doi:10.1029/2009GL041790.

Head, J.W., Wilson, L., Robinson, M., Hiesinger, H., Weitz, C., Yingst, A., 2000. Moonand Mercury: volcanism in early planetary history. In: Gregg, T.K.P., Zimbel-man, J.R. (Eds.), Environmental Effects on Volcanic Eruptions: From DeepOceans to Deep Space. Plenum, New York, pp. 143–178.

Head, J.W., Chapman, C.R., Domingue, D.L., Hawkins, S.E., McClintock, W.E.,Murchie, S.L., Prockter, L.M., Robinson, M.S., Strom, R.G., Watters, T.R., 2007.The geology of Mercury: the view prior to the MESSENGER mission. Space Sci.Rev. 131, 41–84. doi:10.1007/s11214-007-9263-6.

Head, J.W., Murchie, S.L., Prockter, L.M., Robinson, M.S., Solomon, S.C., Strom, R.G.,Chapman, C.R., Watters, T.R., McClintock, W.E., Blewett, D.T., Gillis-Davis, J.J.,2008. Volcanism on Mercury: evidence from the first MESSENGER flyby.Science 321, 69–72. doi:10.1126/science.1159256.

Head, J.W., Murchie, S.L., Prockter, L.M., Solomon, S.C., Chapman, C.R., Strom, R.G.,Watters, T.R., Blewett, D.T., Gillis-Davis, J.J., Fassett, C.I., Dickson, J.L., Morgan,G.A., Kerber, L., 2009a. Volcanism on Mercury: evidence from the firstMESSENGER flyby for extrusive and explosive activity and the volcanic originof plains. Earth Planet. Sci. Lett. 285, 227–242. doi:10.1016/j.epsl.2009.03.007.

Head, J.W., Murchie, S.L., Prockter, L.M., Solomon, S.C., Strom, R.G., Chapman, C.R.,Watters, T.R., Blewett, D.T., Gillis-Davis, J.J., Fassett, C.I., Dickson, J.L., Hurwitz,D.M., Ostrach, L.R., 2009b. Evidence for intrusive activity on Mercury from thefirst MESSENGER flyby. Earth Planet. Sci. Lett. 285, 251–262. doi:10.1016/j.epsl.2009.03.008.

Huang, A., Ogg, J., 2008. Time scale creator: a visualization and database tool forEarth history. Eos Trans. Am. Geophys. Union 89 (53) (Fall Meeting suppl.,abstract IN23B-1088).

Lucey, P.G., Blewett, D.T., Taylor, G.J., Hawke, B.K., 2000. Imaging of lunar surfacematurity. J. Geophys. Res. 105, 20,377–20,386.

Malin, M.C., Dzurisin, D., 1978. Modification of fresh crater landforms: evidencefrom the Moon and Mercury. J. Geophys. Res. 83, 233–243.

McClintock, W.E., Lankton, M.R., 2007. The Mercury Atmospheric and SurfaceComposition Spectrometer for the MESSENGER mission. Space Sci. Rev. 131,481–521. doi:10.1007/s11214-007-9264-5.

Melosh, H.J., 1979. Acoustic fluidization–a new geologic process. J. Geophys. Res.84, 7513–7520.

Melosh, H.J., 1989. Impact Cratering: A Geologic Process. Oxford University Press,London, 253 pp.

Murchie, S.L., Watters, T.R., Robinson, M.S., Head, J.W., Strom, R.G., Chapman, C.R.,Solomon, S.C., McClintock, W.E., Prockter, L.M., Domingue, D.L., Blewett, D.T.,2008. Geology of the Caloris basin, Mercury: a view from MESSENGER. Science321, 73–76. doi:10.1126/science.1159261.

Murray, B.C., 1975. The Mariner 10 pictures of Mercury—an overview. J. Geophys.Res. 80, 2342–2344. doi:10.1029/JB080i017p02342.

Murray, B.C., Belton, M.J.S., Danielson, G.E., Davies, M.E., Gault, D.E., Hapke, B.,O’Leary, B., Strom, R.G., Suomi, V., Trask, N., 1974. Mercury’s surface:preliminary description and interpretation from Mariner 10 pictures. Science185, 169–179. doi:10.1126/science.185.4146.169.

Neukum, G., Oberst, J., Hoffmann, H., Wagner, R., Ivanov, B., 2001. Geologicevolution and cratering history of Mercury. Planet. Space Sci. 49, 1507–1521.doi:10.1016/S0032-0633(01)00089-7.

Noble, S.K., Pieters, C.M., 2003. Space weathering on Mercury: implications forremote sensing. Sol. Syst. Res. 37, 31–35. doi:10.1023/A:1022395605024.

Oberst, J., Preusker, F., Phillips, R.J., Watters, T.R., Head, J.W., Zuber, M.T., Solomon,S.C., 2010. The morphology of Mercury’s Caloris basin as seen in MESSENGERstereo topographic models. Icarus 209, 230–238. doi:10.1016/j.icarus.2010.03.009.

Pierazzo, E., Melosh, H.J., 2000. Melt production in oblique impacts. Icarus 145,252–261. doi:10.1006/icar.1999.6332.

Pierazzo, E., Vickery, A.M., Melosh, H.J., 1997. A reevaluation of impact meltproduction. Icarus 127, 408–423. doi:10.1006/icar.1997.5713.

Pieters, C.M., Staid, M.I., Fischer, E.M., Tompkins, S., He, G., 1994. A sharper view ofimpact craters from Clementine data. Science 266, 1844–1848. doi:10.1126/science.266.5192.1844.

Pike, R.J., 1988. Geomorphology of impact craters on Mercury. In: Vilas, F.,Chapman, C.R., Matthews, M.S. (Eds.), Mercury. Univ. Ariz. Press, Tucson,Ariz., pp. 165–273.

Preusker, F., Oberst, J. Head, J.W., Watters, T.R., Robinson, M.S., Zuber, M.T.,Solomon, S.C. Stereo topographic models of Mercury after three MESSENGERflybys. Planet Space Sci., this issue.

Prockter, L.M., Watters, T.R., Chapman, C.R., Denevi, B.W., Head III, J.W., Solomon,S.C., Murchie, S.L., Barnouin-Jha, O.S., Robinson, M.S., Blewett, D.T., Gillis-Davis,J., Gaskell, R.W., 2009. The curious case of Raditladi basin. Lunar Planet. Sci. 40(abstract 1758).

Prockter, L.M., Ernst, C.M., Denevi, B.W., Chapman, C.R., Head, J.W., Fassett, C.I.,Merline, W.J., Solomon, S.C., Watters, T.R., Strom, R.G., Cremonese, G., Marchi,S., Massironi, M., 2010. Evidence for young volcanism on Mercury from thethird MESSENGER flyby. Science 329, 668–671. doi:10.1126/science.1188186.

Robinson, M.S., Lucey, P.G., 1997. Recalibrated Mariner 10 color mosaics: implica-tions for Mercurian volcanism. Science 275, 197–200. doi:10.1126/science.275.5297.197.

Robinson, M.S., Murchie, S.L., Blewett, D.T., Domingue, D.L., Hawkins, S.E., Head,J.W., Holsclaw, G.M., McClintock, W.E., McCoy, T.J., McNutt, R.L., Prockter, L.M.,Solomon, S.C., Watters, T.R., 2008. Reflectance and color variations onMercury: regolith processes and compositional heterogeneity. Science 321,66–69. doi:10.1126/science.1160080.

Roddy, D.J., Pepin, R.O., Merrill, R.B. (Eds.), 1977. Impact and Explosion Cratering.Pergamon Press, New York, 1315 pp.

Schultz, P.H., 1977. Endogenic modification of impact craters on Mercury. Phys.Earth Planet. Inter. 15, 202–219. doi:10.1016/0031-9201(77)90032-2.

Schultz, P.H., 1987. Experimental planetary impact research. Int. J. Impact Eng. 5,569–576. doi:10.1016/0734-743X(87)90071-6.

Smith, E.I., Hartnell, J.A., 1978. Crater size-shape profiles for the Moon andMercury: terrain effects and interplanetary comparisons. Earth Moon Planets19, 479–511. doi:10.1007/BF00901976.

Solomon, S.C., 2003. Mercury: the enigmatic innermost planet. Earth Planet. Sci.Lett. 216, 441–455. doi:10.1016/S0012-821X(03)00546-6.

Solomon, S.C., McNutt, R.L., Gold, R.E., Acuna, M.H., Baker, D.N., Boynton, W.V.,Chapman, C.R., Cheng, A.F., Gloeckler, G., Head, J.W., Krimigis, S.M., McClintock,W.E., Murchie, S.L., Peale, S.J., Phillips, R.J., Robinson, M.S., Slavin, J.A., Smith,D.E., Strom, R.G., Trombka, J.I., Zuber, M.T., 2001. The MESSENGER mission toMercury: scientific objectives and implementation. Planet. Space Sci. 49,1445–1465. doi:10.1016/S0032-0633(01)00085-X.

Solomon, S.C., McNutt, R.L., Watters, T.R., Lawrence, D.J., Feldman, W.C., Head, J.W.,Krimigis, S.M., Murchie, S.L., Phillips, R.J., Slavin, J.A., Zuber, M.T., 2008. Returnto Mercury: a global perspective on MESSENGER’s first Mercury flyby. Science321, 59–62. doi:10.1126/science.1159706.

Spray, J.G., Thompson, L.M., 2008. Constraints on central uplift structure from theManicouagan impact crater. Meteorit. Planet. Sci. 43, 2049–2057.

Spudis, P.D., 1985. A Mercurian chronostratigraphic classification. In: Reports ofPlanetary Geology and Geophysics Program—1984. TM 87563, NationalAeronautics and Space Administration, Washington, DC, pp. 595–597.

Spudis, P.D., 1993. The Geology of Multi-Ring Impact Basins. Cambridge Univ.Press, Cambridge, 177 pp.

Spudis, P.D., Guest, J.E., 1988. Stratigraphy and geologic history of Mercury. In:Vilas, F., Chapman, C.R., Matthews, M.S. (Eds.), Mercury. Univ. Ariz. Press,Tucson, Ariz., pp. 118–164.

Stoffler, D., Deutsch, A., Avermann, M., Bischoff, L., Brockmeyer, P., Buhl, D.,Lakomy, R., Muller-Mohr, V., 1994. The formation of the Sudbury structure,Canada: toward a unified impact model. 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. 303–318 (SpecialPaper 293).

Strom, R.G., Chapman, C.R., Merline, W.J., Solomon, S.C., Head, J.W., 2008. Mercurycratering record viewed from MESSENGER’s first flyby. Science 321, 79–81.doi:10.1126/science.1159317.

Strom, R.G., Banks, M.E., Chapman, C.R., Fassett, C.I., Forde, J.A., Head, J.W., III,Merline, W.J., Prockter, L.M., Solomon, S.C. Mercury crater statistics fromMESSENGER flybys: implications for stratigraphy and resurfacing history.Planet Space Sci., this issue.

Therriault, A.M., Fowler, A.D., Grieve, R.A.F., 2002. The Sudbury igneous complex:a differentiated impact melt sheet. Economic Geology 97, 1521–1540.doi:10.2113/gsecongeo.97.7.1521.

Werner, S.C., Medvedev, S., 2010. The lunar rayed-crater population—

characteristics of the spatial distribution and ray retention. Earth Planet. Sci.Lett. 295, 147–158. doi:10.1016/j.epsl.2010.03.036.

Wilhelms, D.E., 1987. The geologic history of the Moon. Professional Paper 1348,US Geological Survey, Denver, Colo., 302 pp.

Wilhelms, D.E., Howard, K.A., Wilshire, W.G., 1979. Geologic map of the south sideof the Moon. Map I-1192, Misc. Investigations Ser., US Geological Survey,Denver, Colo.

Wood, C.A., Head, J.W., 1976. Comparisons of impact basins on Mercury, Mars andthe Moon. Proc. Lunar Sci. Conf. 7, 3629–3651.

Zieg, M.J., Marsh, B.D., 2005. The Sudbury igneous complex: viscous emulsiondifferentiation of a superheated impact melt sheet. Geol. Soc. Am. Bull. 117,1427–1450. doi:10.1130/B25579.1.

Zuber, M.T., Smith, D.E., Solomon, S.C., Phillips, R.J., Peale, S.J., Head, J.W., Hauck,S.A., McNutt, R.L., Oberst, J., Neumann, G.A., Lemoine, F.G., Sun, X., Barnouin-Jha, O., Harmon, J.K., 2008. Laser altimeter observations from MESSENGER’sfirst Mercury flyby. Science 321, 77–79. doi:10.1126/science.1159086.