A Morphological Evaluation of Crater Degradation on Mercury: Revisiting Crater Classification With MESSENGER Data. Mallory J. Kinczyk1, Louise M. Prockter1, Clark R. Chapman2, and Hannah C. M. Susorney3. 1The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723 USA. 2Department of Space Studies, Southwest Research Institute, Boulder, CO 80302 USA. 3Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, MD 21218 USA. SUMMARY ACKNOWLEDGEMENTS The application of crater degradation as a tool for the relative age dating of geological units has been an important component in deriving interplanetary correlations of geologic time. The global image dataset acquired by the Mercury Dual The classification of Mercurian craters was a joint effort with the global geological Imaging System (MDIS) on the MErcury, Surface, Space, ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft allows for rigorous classification of craters previously imaged by Mariner 10, as well as the classification of Mercury mapping project led by Dr. Louise M. Prockter (Abstract #1245). The First craters in newly imaged regions of Mercury. In this study, we attempt to standardize classification for Mercurian craters by developing a consistent scheme based on clear, uniform morphological criteria. We classified all Mercurian Global Geological Map of Mercury is being presented at this poster session at craters ≥40 kilometers in diameter with our system, thus establishing the first global dataset of crater degradation on Mercury and providing a useful tool for understanding the evolution of Mercury’s surface. poster location #401.

METHODOLOGY GLOBAL CLASS DISTRIBUTION

‐180° ‐90° 0° 90° 180° Class 1 (Kuiperian) Class 2 (Mansurian) Class 3 (Calorian) Class 4 (Tolstojan) Class 5 (Pre-Tolstojan) PREVIOUS CLASSIFICATION SYSTEMS

~1.0 Gyr ~3.0—3.5 Gyr ~3.9 Gyr ~3.9—4.0 Gyr Older than ~4.0 Gyr Class 1—Kuiperian (~1.0 Gyr) Unnamed First developed for the Moon [e.g., 2-4], the degradation

classification system was later extrapolated to Mercury [1,5]. 45° 45°

Though these initial studies used a 4- or 5-class system ranging

Simple from fresh to very degraded, there have been several classification

(.225 - 14.4 km) systems used since, and morphological variations exist among Due to the overlapping of rim diameters between simple and modified simple crater morphologies, and 0° 0° 10 km 200km 200km them. These inconsistencies led us to reevaluate the Unnamed the possibility of mistaking secondary craters for simple or modified simple craters, Class 2 through Class 5 craters were not evaluated. MARINER 10 MESSENGER classification criteria. Figure 2. Dostoevskij crater (lat: 45°S, lon: 183° E) was historically the type example of a Calorian aged (Class 3) crater [1] but later was suggested ‐45° ‐45° to be one of the oldest basins on the planet (Class 5)[6]. Our classification scheme places it in the Tolstojan time period (Class 2). This example is one of several craters that had differing historical classifications, revealing that crater classification based on Mariner 10 data needed to be revisited. (4.6 - 12.2 km) Modified Simple a. 10 km

‐180° ‐90° 0° 90° 180° Unnamed Unnamed Unnamed Unnamed Unnamed CHALLENGES WITH PREVIOUS SYSTEMS Class 2—Mansurian (~3.0—3.5 Gyr)  Mariner 10 quadrangle maps, as well as other classification systems, were inconsistent in their interpretations of morphology and how they defined each class 45° 45°

 Classification system based on only half the planet (9.5 - 29.1 km)

Immature-complex Immature-complex  Limited illumination and viewing conditions at each location Kuiper Unnamed Dvorák Unnamed Unnamed 0° 0°

PROXIMITY WEATHERING AND OTHER MECHANISMS

The interpretation of crater morphology can be influenced heavily by geological activity. Though the main ‐45° ‐45° (30 - 160 km)

Mature-complex Mature-complex erosive force on Mercury is space weathering (solar wind and impact gardening), cratering and volcanism in Mercury’s past have caused some craters to appear older and more degraded than most craters formed Amaral Stravinsky Camões Unnamed during the same time period. It is very likely that these processes are at work on the surface in areas that b. ‐180° ‐90° 0° 90° 180° cannot be easily identified.

Class 3—Calorian (~3.9 Gyr)

Basin cfp Figure 3a shows a set of three craters (lat: 38°N, lon: 175°E) superposed on top

(73 - 133 km) of the Caloris interior plains (cfp). Due to their stratigraphic position on top of 45° 45° Ringed Peak-cluster Ringed Peak-cluster No Class 5 ringed peak-cluster basins or Caloris related units, they must be younger than Caloris and therefore Class 3 or Equiano Unnamed Sinan protobasins have been identified. fresher. However, due to the influence of superposed crater ejecta, the crater

referenced by the white arrow displays characteristics of a Class 4, Tolstojan aged, 0° 0°

No Class 1 protobasins have been identified. degraded crater. Though the stratigraphic relationship with Caloris can be used

Protobasin Protobasin (75 - 172 km) to more accurately identify this crater’s age, there are many craters that exhibit Figure 3a these same characteristics that cannot be tied to a specific stratigraphic marker ‐45° ‐45° Ahmad Baba Wang Meng Munkácsy Unnamed ps (e.g. craters that lie within Mercury’s intercrater plains).

No Class 1 peak-ring Figure 3b shows a peak-ring basin (lat: 40°N, lon: 73°E) that has been embayed c. basins have been by the northern smooth plains unit (ps) from the north. The part of the rim that ‐180° ‐90° 0° 90° 180° identified. (84 - 320 km)

Peak-ring Basin Peak-ring has not been embayed is still crisp and a portion of radially textured ejecta is Class 4—Tolstojan (~3.9—4.0 Gyr) visible to the southeast. Though this crater has been largely obliterated Rembrandt Sanai Sobkou pi (characteristic of a Class 4, Tolstojan aged crater), the fresh ejecta suggests that it 45° 45°

may be younger. Figure 3b

No Class 1 or Class 2 multiring basins have been identified.

RESULTS 0° 0° Multiring Basin Multiring (285 - 1600 km)

Figure 1. Crater degradation is broken down by initial crater morphology [10, 11]. Different crater features will be more prominent depending on crater diameter (e. g. plains units are more pronounced in basins than in simple craters). Ages are derived from previous models [6] and indicate the base of each ‐45° ‐45° system. Selected craters are centered in each image with diameters approximately one third the width of the image.

Crater Class 1 Class 2 Class 3 Class 4 Class 5 d. Features ‐180° ‐90° 0° 90° 180° Bright rays extending at No rays No rays No rays No rays Rays least several radii from Class 5—Pre-Tolstojan (> 4.0 Gyr) crater rim Fresh, crisp rims Crisp rim crests Generally continuous, Continuous to Discontinuous, degraded rounded rims with a discontinuous rims with a rims rising only slightly 45° 45° Rim degraded appearance rounded, degraded above surrounding appearance terrain Crisp wall terraces Slightly degraded wall Generally slumped wall No wall terraces, remnant No terraces or wall Terraces* terraces terraces terraces in larger craters structure Figure 4a Figure 4b 0° 0° Floor-wall Distinct contact between Distinct contact between Floor-wall boundary Floor-wall boundary No floor-wall boundary Figure 4a. A cumulative size-frequency distribution using the entirety of Mercury’s surface as the area of interest. Shifts in the cumulative wall and floor wall and floor somewhat indistinct but generally indistinct unless Boundary* still separable clearly embayed numbers of craters in each class may suggest ancient resurfacing events. Error bars have not been included as more analysis will be

Fully or partially covered Fully or partially covered Fully or partially covered, or Filled or partially filled with Filled with plains material; necessary to quantify the error in classification. with plains materials and/or with plains materials and/or partially filled with plains, plains material; no no hummocky deposits Figure 4b. A cumulative plot of the number of craters in each bin where the bin size is 10 kilometers. Though trends are visible, it is clear ‐45° ‐45° Floor* containing hummocky containing modified and/or containing modified hummocky deposits visible visible deposits hummocky deposits hummocky deposits that several outliers skew the cumulative size-frequency plot in Figure 5a. Radially textured continuous Radially textured Continuous to Ejecta blanket No ejecta deposits e. ejecta blanket continuous ejecta blanket discontinuous ejecta blanket discontinuous to ‐180° ‐90° 0° 90° 180° completely absent; largest CONCLUSIONS Ejecta craters may still show Figure 5a-e. Global distribution of craters identified in each class. In 5e, the region of Intercrater Plains between 0° evidence of radial texture  Our classification system is based on morphological criteria broken down by initial crater morphology. and 90°E is devoid of mid-size craters. Unconfirmed basins b30 (1390 km diameter) and b56 (~1000–1500 km in remaining ejecta diameter) [11] have been classified as Pre-Tolstojan in age and are located in this region. These basins are sufficiently Well-defined continuous Well defined field of Some chains/clusters of No secondary craters No secondary craters  Though this system provides a method that can be applied consistently across the globe, proximity Secondary large to have obliterated a large number of mid-size craters, possibly contributing to this pattern. field of relatively crisp secondary craters secondary craters but not a weathering and volcanism can cause craters to look much older, particularly in more heavily cratered Craters secondary craters continuous field Central Crisp central peak/ring Crisp central peak/ring Most have subdued central Rare central peaks/rings, No central peaks/rings regions. This can contribute to uncertainties in classification. REFERENCES peaks/rings those present are heavily Peaks* degraded  The ratio of craters in Classes 4 through 2 remains relatively constant, suggesting that craters spend [1] McCauley, J. F. et al. (1981). Icarus, 47, 184–202. [2] Arthur, D. W. G. et al. (1963) Comm. LPL, 2, #30, 71–78. [3] Trask, N. J. (1967) Icarus, 6, 270–276. [4] Chapman, C. R. (1968) Icarus, 8, 1–22. [5] Wood, C. A. et al. (1977) Superposed None Low density Low to moderate density Moderate density Moderate to high density equal percentages of their lifetimes transitioning through each degradational class LPS, 8, 3503 –3520. [6] Spudis, P. D. and Guest, J. E. (1988). In Mercury, Univ. Arizona Press, pp. 118–164. [7] Craters  There are very few mid-sized Pre-Tolstojan aged craters between 0° and 90°E, possibly suggesting a Fassett, C. I. et al. (2011). Geophys. Res. Lett., 38, 1–6. [8] Chabot, N. L. et. al. (2016) LPS, 47, #1256. [9] Gaskell, R. Table 1. Crater feature matrix segregating aspects of crater morphology into each class of degradation allowing for more consistent classification across the globe. resurfacing event in this area W. et al. (2011). AGU, P41A-1576. [10] Pike, R. J. (1988). In Mercury, Univ. Arizona Press, pp. 165–273. [11] Baker, Feature descriptions were derived from [1], [6], and the Mariner 10 geological quadrangle maps. Bold features are more prominent indicators of class than non- D. M. H. et al. (2011). Planet. Space Sci., 59, 1932–1948. bolded features. *Descriptions only valid for crater morphologies that have these features.