Mallory J. Kinczyk1, Louise M. Prockter1, Clark R. Chapman2, and Hannah ... Figure 3a shows a set of three craters (lat: 38°N, lon: 175°E) superposed on top.
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. The 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.
1
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 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 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 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.
The classification of Mercurian craters was a joint effort with the global geological Mercury mapping project led by Dr. Louise M. Prockter (Abstract #1245). The First Global Geological Map of Mercury is being presented at this poster session at poster location #401.
METHODOLOGY
Class 1 (Kuiperian) ~1.0 Gyr
Class 2 (Mansurian) ~3.0—3.5 Gyr
Class 3 (Calorian) ~3.9 Gyr
Class 4 (Tolstojan) ~3.9—4.0 Gyr
GLOBAL CLASS DISTRIBUTION PREVIOUS CLASSIFICATION SYSTEMS
Class 5 (Pre-Tolstojan) Older than ~4.0 Gyr
Simple (.225 - 14.4 km)
Unnamed
10 km
Modified Simple (4.6 - 12.2 km)
Unnamed
Due to the overlapping of rim diameters between simple and modified simple crater morphologies, and the possibility of mistaking secondary craters for simple or modified simple craters, Class 2 through Class 5 craters were not evaluated.
200km
200km
MARINER 10
MESSENGER
First developed for the Moon [e.g., 2-4], the degradation classification system was later extrapolated to Mercury [1,5]. Though these initial studies used a 4- or 5-class system ranging from fresh to very degraded, there have been several classification systems used since, and morphological variations exist among them. These inconsistencies led us to reevaluate the 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 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.
‐180°
‐90°
Unnamed
Unnamed
Unnamed
CHALLENGES WITH PREVIOUS SYSTEMS
Unnamed
Immature-complex (9.5 - 29.1 km)
Unnamed
Dvorák
Unnamed
Mariner 10 quadrangle maps, as well as other classification systems, were inconsistent in their interpretations of morphology and how they defined each class
Classification system based on only half the planet
Limited illumination and viewing conditions at each location
Unnamed
Mature-complex (30 - 160 km)
Unnamed
PROXIMITY WEATHERING AND OTHER MECHANISMS
Stravinsky
Camões
The interpretation of crater morphology can be influenced heavily by geological activity. Though the main 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 during the same time period. It is very likely that these processes are at work on the surface in areas that cannot be easily identified.
Unnamed
Protobasin (75 - 172 km)
Figure 3a shows a set of three craters (lat: 38°N, lon: 175°E) superposed on top of the Caloris interior plains (cfp). Due to their stratigraphic position on top of Caloris related units, they must be younger than Caloris and therefore Class 3 or 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, degraded crater. Though the stratigraphic relationship with Caloris can be used to more accurately identify this crater’s age, there are many craters that exhibit these same characteristics that cannot be tied to a specific stratigraphic marker (e.g. craters that lie within Mercury’s intercrater plains).
cfp
Equiano
Unnamed
No Class 5 ringed peak-cluster basins or protobasins have been identified.
Sinan
No Class 1 protobasins have been identified. Figure 3a
Peak-ring Basin (84 - 320 km)
Ahmad Baba
Wang Meng
Munkácsy
Multiring Basin (285 - 1600 km)
Unnamed
ps
No Class 1 peak-ring basins have been identified.
Rembrandt
Sanai
Sobkou
pi Figure 3b
No Class 1 or Class 2 multiring basins have been identified.
Figure 3b shows a peak-ring basin (lat: 40°N, lon: 73°E) that has been embayed by the northern smooth plains unit (ps) from the north. The part of the rim that has not been embayed is still crisp and a portion of radially textured ejecta is visible to the southeast. Though this crater has been largely obliterated (characteristic of a Class 4, Tolstojan aged crater), the fresh ejecta suggests that it may be younger.
RESULTS
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 system. Selected craters are centered in each image with diameters approximately one third the width of the image.
Terraces* Floor-wall Boundary* Floor*
Ejecta
Secondary Craters Central Peaks* Superposed Craters
90°
45° 0° ‐45°
180°
45° 0° ‐45°
‐180°
‐90°
Class 1
Class 2
Class 3
Class 4
No rays
No rays
90°
180°
45° 0° ‐45°
‐180°
‐90°
0°
90°
180°
Class 3—Calorian (~3.9 Gyr) 45° 0° ‐45°
45° 0° ‐45°
c. ‐180°
‐90°
0°
90°
180°
Class 4—Tolstojan (~3.9—4.0 Gyr) 45° 0° ‐45°
45° 0° ‐45°
d.
‐90°
No rays
0°
90°
180°
Class 5—Pre-Tolstojan (> 4.0 Gyr)
Generally continuous, rounded rims with a degraded appearance
Table 1. Crater feature matrix segregating aspects of crater morphology into each class of degradation allowing for more consistent classification across the globe. Feature descriptions were derived from [1], [6], and the Mariner 10 geological quadrangle maps. Bold features are more prominent indicators of class than nonbolded features. *Descriptions only valid for crater morphologies that have these features.
0°
b.
Class 5
Continuous to Discontinuous, degraded discontinuous rims with a rims rising only slightly 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 in larger craters structure Distinct contact between Distinct contact between Floor-wall boundary Floor-wall boundary No floor-wall boundary wall and floor wall and floor somewhat indistinct but generally indistinct unless still separable clearly embayed Fully or partially covered Fully or partially covered Fully or partially covered, or Filled or partially filled with Filled with plains material; with plains materials and/or with plains materials and/or partially filled with plains, plains material; no no hummocky deposits containing hummocky containing modified and/or containing modified hummocky deposits visible visible deposits hummocky deposits hummocky deposits Radially textured continuous Radially textured Continuous to Ejecta blanket No ejecta deposits ejecta blanket continuous ejecta blanket discontinuous ejecta blanket discontinuous to completely absent; largest craters may still show evidence of radial texture in remaining ejecta Well-defined continuous Well defined field of Some chains/clusters of No secondary craters No secondary craters field of relatively crisp secondary craters secondary craters but not a secondary craters continuous field Crisp central peak/ring Crisp central peak/ring Most have subdued central Rare central peaks/rings, No central peaks/rings peaks/rings those present are heavily degraded None Low density Low to moderate density Moderate density Moderate to high density
45° 0° ‐45°
‐180°
Bright rays extending at No rays least several radii from crater rim Fresh, crisp rims Crisp rim crests
Class 2—Mansurian (~3.0—3.5 Gyr)
Ringed Peak-cluster Basin (73 - 133 km)
Amaral
Rim
a.
Kuiper
Rays
0°
Class 1—Kuiperian (~1.0 Gyr)
10 km
Crater Features
Figure 4a
Figure 4b
Figure 4a. A cumulative size-frequency distribution using the entirety of Mercury’s surface as the area of interest. Shifts in the cumulative numbers of craters in each class may suggest ancient resurfacing events. Error bars have not been included as more analysis will be necessary to quantify the error in classification. 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 that several outliers skew the cumulative size-frequency plot in Figure 5a.
45° 0° ‐45°
45° 0° ‐45°
e.
CONCLUSIONS
Our classification system is based on morphological criteria broken down by initial crater morphology.
Though this system provides a method that can be applied consistently across the globe, proximity weathering and volcanism can cause craters to look much older, particularly in more heavily cratered regions. This can contribute to uncertainties in classification.
The ratio of craters in Classes 4 through 2 remains relatively constant, suggesting that craters spend equal percentages of their lifetimes transitioning through each degradational class
There are very few mid-sized Pre-Tolstojan aged craters between 0° and 90°E, possibly suggesting a resurfacing event in this area
‐180°
‐90°
0°
90°
180°
Figure 5a-e. Global distribution of craters identified in each class. In 5e, the region of Intercrater Plains between 0° and 90°E is devoid of mid-size craters. Unconfirmed basins b30 (1390 km diameter) and b56 (~1000–1500 km diameter) [11] have been classified as Pre-Tolstojan in age and are located in this region. These basins are sufficiently large to have obliterated a large number of mid-size craters, possibly contributing to this pattern.
REFERENCES [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) LPS, 8, 3503 –3520. [6] Spudis, P. D. and Guest, J. E. (1988). In Mercury, Univ. Arizona Press, pp. 118–164. [7] 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. W. et al. (2011). AGU, P41A-1576. [10] Pike, R. J. (1988). In Mercury, Univ. Arizona Press, pp. 165–273. [11] Baker, D. M. H. et al. (2011). Planet. Space Sci., 59, 1932–1948.
