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Abstract: Peak-ring basins are important in understanding the formation of large impact basins on planetary bodies; however, debate still exists as to how peak rings form. Using altimetry and image data from the MESSENGER and LRO spacecraft in orbit around Mercury and the Moon, respectively, we measured the morphometric properties of impact structures in the transition from complex craters with central peaks to peak-ring basins. This work provides a comprehensive morphometric framework for craters and basins in this morphological transition that may be used to further develop and refine various models for peak-ring formation. First, we updated catalogs of craters and basins ≥50km in diameter possessing interior peaks on Mercury and the Moon. Crater degradation states were assessed and morphometric measurements were made on the freshest examples, including depths to the crater floor, areas contained within the outlines of the rim crest and floor, crater volumes, and rim-crest and floor circularity. There is an abrupt decrease in crater depth in the crater to basin transition on both Mercury and the Moon. Peak-ring basins have larger floor area/interior area ratios than complex craters; this ratio is larger in craters on Mercury than on the Moon. The dimensions of central peaks (heights, areas, and volumes exposed above the surface) increase continuously up to the transition to basins. Compared with central peaks, peak rings have reduced heights; however, all interior peaks are typically >1km below the rim-crest elevations. Topographic profiles of peak-ring basins on Mercury and the Moon are distinct from complex craters and exhibit interior cavities or depressions that are bounded by the peak ring with outer annuli that are at higher elevations. We interpret the trends in floor and interior area to be largely due to differences in impact melt production and retention, although variations in types and thicknesses of impactites, including proximal ejecta, could also contribute to the observed trends. Our trends illustrate that the transition from craters to basins is characterized by an abrupt change in morphology, implying a change in process for the formation of peak rings. Refinement of models for peak-ring formation through improved quantitative predictions of crater morphometry and cross-validation with our morphometric framework are needed to better constrain the processes that form peak rings on the terrestrial planets. [Copyright &y& Elsevier]

Abstract: The study of peak-ring basins and other impact crater morphologies transitional between complex craters and multi-ring basins is important to our understanding of the mechanisms for basin formation on the terrestrial planets. Mercury has the largest population, and the largest population per area, of peak-ring basins and protobasins in the inner solar system and thus provides important data for examining questions surrounding peak-ring basin formation. New flyby images from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft have more than doubled the area of Mercury viewed at close range, providing nearly complete global coverage of the planet''s surface when combined with flyby data from Mariner 10. We use this new near-global dataset to compile a catalog of peak-ring basins and protobasins on Mercury, including measurements of the diameters of the basin rim crest, interior ring, and central peak (if present). Our catalog increases the population of peak-ring basins by ∼150% and protobasins by ∼100% over previous catalogs, including 44 newly identified peak-ring basins (total=74) and 17 newly identified protobasins (total=32). A newly defined transitional basin type, the ringed peak-cluster basin (total=9), is also described. The new basin catalog confirms that Mercury has the largest population of peak-ring basins of the terrestrial planets and also places the onset rim-crest diameter for peak-ring basins at , which is intermediate between the onset diameter for peak-ring basins on the Moon and those for the other terrestrial planets. The ratios of ring diameter to rim-crest diameter further emphasize that protobasins and peak-ring basins are parts of a continuum of basin morphologies relating to their processes of formation, in contrast to previous views that these forms are distinct. Comparisons of the predictions of peak-ring basin-formation models with the characteristics of the basin catalog for Mercury suggest that formation and modification of an interior melt cavity and nonlinear scaling of impact melt volume with crater diameter provide important controls on the development of peak rings. The relationship between impact-melt production and peak-ring formation is strengthened further by agreement between power laws fit to ratios of ring diameter to rim-crest diameter for peak-ring basins and protobasins and the power-law relation between the dimension of a melt cavity and the crater diameter. More detailed examination of Mercury''s peak-ring basins awaits the planned insertion of the MESSENGER spacecraft into orbit about Mercury in 2011. [Copyright &y& Elsevier]

Abstract: Impact craters on planetary bodies transition with increasing size from simple, to complex, to peak-ring basins and finally to multi-ring basins. Important to understanding the relationship between complex craters with central peaks and multi-ring basins is the analysis of protobasins (exhibiting a rim crest and interior ring plus a central peak) and peak-ring basins (exhibiting a rim crest and an interior ring). New data have permitted improved portrayal and classification of these transitional features on the Moon. We used new 128pixel/degree gridded topographic data from the Lunar Orbiter Laser Altimeter (LOLA) instrument onboard the Lunar Reconnaissance Orbiter, combined with image mosaics, to conduct a survey of craters>50km in diameter on the Moon and to update the existing catalogs of lunar peak-ring basins and protobasins. Our updated catalog includes 17 peak-ring basins (rim-crest diameters range from 207km to 582km, geometric mean=343km) and 3 protobasins (137–170km, geometric mean=157km). Several basins inferred to be multi-ring basins in prior studies (Apollo, Moscoviense, Grimaldi, Freundlich–Sharonov, Coulomb–Sarton, and Korolev) are now classified as peak-ring basins due to their similarities with lunar peak-ring basin morphologies and absence of definitive topographic ring structures greater than two in number. We also include in our catalog 23 craters exhibiting small ring-like clusters of peaks (50–205km, geometric mean=81km); one (Humboldt) exhibits a rim-crest diameter and an interior morphology that may be uniquely transitional to the process of forming peak rings. A power-law fit to ring diameters (D ring) and rim-crest diameters (D r) of peak-ring basins on the Moon [D ring =0.14±0.10(D r)1.21±0.13] reveals a trend that is very similar to a power-law fit to peak-ring basin diameters on Mercury [D ring =0.25±0.14(D rim)1.13±0.10] [Baker, D.M.H. et al. [2011]. Planet. Space Sci., in press]. Plots of ring/rim-crest ratios versus rim-crest diameters for peak-ring basins and protobasins on the Moon also reveal a continuous, nonlinear trend that is similar to trends observed for Mercury and Venus and suggest that protobasins and peak-ring basins are parts of a continuum of basin morphologies. The surface density of peak-ring basins on the Moon (4.5×10−7 perkm2) is a factor of two less than Mercury (9.9×10−7 perkm2), which may be a function of their widely different mean impact velocities (19.4km/s and 42.5km/s, respectively) and differences in peak-ring basin onset diameters. New calculations of the onset diameter for peak-ring basins on the Moon and the terrestrial planets re-affirm previous analyses that the Moon has the largest onset diameter for peak-ring basins in the inner Solar System. Comparisons of the predictions of models for the formation of peak-ring basins with the characteristics of the new basin catalog for the Moon suggest that formation and modification of an interior melt cavity and nonlinear scaling of impact melt volume with crater diameter provide important controls on the development of peak rings. In particular, a power-law model of growth of an interior melt cavity with increasing crater diameter is consistent with power-law fits to the peak-ring basin data for the Moon and Mercury. We suggest that the relationship between the depth of melting and depth of the transient cavity offers a plausible control on the onset diameter and subsequent development of peak-ring basins and also multi-ring basins, which is consistent with both planetary gravitational acceleration and mean impact velocity being important in determining the onset of basin morphological forms on the terrestrial planets. [Copyright &y& Elsevier]

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 (∼125km) 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. [Copyright &y& Elsevier]