Terrestrial Planets: The Cratering Process
New Insights From High-Resolution Observations of Linne Crater, Moon
The young, 2.2-km Linné crater formed on the volcanic plains in W. Mare Serenitatis. Fifty-cm/px image pairs and a 2-m/px DTM have been developed by the LRO camera team led by Mark Robinson.
Topography and regional slope over Linné constrain the proportions of structural uplift and ejecta that constitute the crater rim. The regional slope map reveals that much of the upper crater walls is considerably steeper than the angle of repose for fragmental debris (30°–35°). High-resolution images confirm that these steep slopes are coherent, layered outcrops of mare basalt.
Linné's rim extends ~122 m above the pre-impact surface, estimated to be at elevation –2630 m, as shown with shading in the perspective image. Uplifted target outcrops comprise ~85% of the rim relief, leaving only 15% (or ~20 m) of ballistically emplaced ejecta. This is considerably more uplift and less ejecta than previously estimated based on laboratory-scale impact experiments.
Rim Formation at Dawes Crater, Moon
The 18-km-wide Dawes crater is an example of an enigmatic class of lunar craters that are irregular in planform and intermediate between simple, bowl-shaped craters and ones with a more complex morphology characterized by shallow flat floors, central peaks, and terraced rims.
Recently acquired imagery and elevation data from the Lunar Reconnaissance Orbiter (LRO) have revealed details of Dawes that are important for advancing knowledge of the ways craters form and distribute ejecta. For instance, these 3D models show impressive variations in rim height along its crest.
The southern rim of Dawes is marked by a significant gap. Contorted, uplifted target blocks comprise the majority of rim relief. Ballistically emplaced ejecta makes up less than 15% of the rim — considerably less than currently accepted models predict. This result suggests that the amount of primary ejecta distributed at great distances from ancient basin-forming events may be less than previously thought.
The western rim of Dawes reaches a height of nearly 800 m above the pre-impact mare surface. Outcrops of uplifted target, apparently mantled by impact melt rock, are clearly shown in this 3D model. Here the maximum thickness of ejecta on the rim does not exceed 35% of the rim elevation.
Characterizing Crater ejecta to Understand Lunar Mare Stratigraphy
Datasets. The four images on the right cover the region between the Serenitatis and Tranquillitatis basins (10°E; 10°N to 35°E; 30°N). Albedo and elevation data were derived from Lunar Reconnaissance Orbiter; iron and titanium data are from the Clementine mission. All data have a nominal resolution of 100 m/pixel.
Principal Component Analysis. The first three principal components were calculated from the albedo, FeO, and TiO2 data to minimize redundancy and accentuate distinguishing characteristics that may reveal geologically relevant information.
This image is a perspective view of the first principal component draped on elevation data with a 15× vertical exaggeration. This image clearly differentiates highlands (areas in violet-magenta) from high-TiO2 basalts (HTB) (areas in orange-yellow) and low-TiO2 basalts (LTB) (areas in green-cyan). Results. The 42-km Plinius crater and the 18-km Dawes crater both appear to have excavated through the HTB layer and ejected LTB. Crater scaling relations, therefore, suggest that the HTB surface unit is less than ~1 km thick.
Haughton Impact Structure, Canada: A Peak-Ring Crater
Location: The ~20-km Haughton structure is located at 89.7°W, 75.4°N on Devon Island in the Arctic Archipelago of Canada.
Geology: The target rocks consist of a nearly flat-lying sequence of platform sediments. The lake that filled the Haughton crater deposited sediments (TH) on top of the original crater floor (including TIB). The base of the remaining lake deposits, therefore, preserves the original morphology of the Haughton crater.
Topography reveals that the uplifted, shatter-coned Eleanor River outcrops (OE) near the basin center extend above the lake beds flanking them. Therefore, the OE outcrops represent a true central peak or peak ring.
The TH breccias contain crystalline rock fragments indicating excavation through the 2-km-thick platform sequence. Preservation of OE outcrops, however, constrains the areal extent of this zone of deep excavation. The OE exposures, therefore, appear to mark the boundary between deep and shallow excavation, suggesting that they represent a poorly formed central peak ring.
Morphological Reconstruction of the Terny Impact Structure, Central Ukraine
The 280-million-year-old Terny impact structure formed in Precambrian rocks of the Ukrainian Shield. The structure has been modified by erosion and subsequently buried by recent sediments. (Fig 1: Location of Terny Impact Structure in central Ukraine.
(Fig. 2: Photograph of iron mine near Terny ca. 1900. Steeply dipping Precambrian crystalline rocks (tan) are overlain by ~25 m of recent sediments.)
Although there are no natural outcrops of the deformed basement rocks within the area, mining exploration has provided surface and subsurface access to the structure, exposing impact melt rocks, shocked parautochthonous target rocks, and allogenic impact breccias, including suevites. ( Fig. 3: Color satellite image covering our study area annotated to show sample sites, target stratigraphy, and structural interpretations. P1 and P2 indicate the Pervomaysk-1 and -2 mine shafts, respectively.)
These rocks display a variety of diagnostic indicators of shock metamorphism, including shatter cones, planar deformation features in quartz, diaplectic glass, selective melting of minerals, and whole-rock melting. We have collected and studied samples from surface and subsurface exposures to a depth of ~750 m below the surface. (Fig. 4: Typical shatter cone from the Annovsk open pit.)
This analysis indicates the Terny crater is centered on geographic coordinates 48.13°N, 33.52°E. The center location and the distribution of shock pressures constrain the transient crater diameter to be no less than ~8.4 km. (Fig.5: Radial distribution of shock pressures observed within the Terny structure. Open circles denote the maximum observed distance from the estimated crater center for each of the four shock pressure indicators shown. The power law relating peak pressure P to radial distance R was derived by fitting the outward extents of diaplectic glasses and shatter cones.)
Using widely accepted morphometric scaling relations, we estimate the pre-erosional rim diameter of Terny crater to be ~16–19 km, making it close in original size to the well-preserved El'gygytgyn crater in Siberia. (Fig. 6: Aerial view of El'gygytgyn crater.)
Comparisons with El'gygytgyn allow the original morphometric properties of Terny crater to be reconstructed as shown in the cross section. (Fig. 7: Cross-sectional model of the pristine Terny impact crater based on a comparison with the well-preserved El'gygytgyn crater. Rtc denotes the transient crater radius; Ra, the apparent crater radius; and Rr, the radial distance to the rim crest.)
Dr. Virgil L. Sharpton
Senior Staff Scientist
Dr. Sharpton's research focuses on deriving a better understanding of the cratering process and how impact events have shaped the geological evolution of terrestrial planets and the biological evolution of planet Earth. He has taken a multidisciplinary approach over his three-decade-long career. The methods he has used include field validation studies, remote sensing/image processing, synthetic aperture radar (processing and interpretation), geophysical exploration techniques, petrographic microscopy (including universal-stage and refractive index measurements), and instrumental geochemical techniques.
He is particularly interested in the new high-resolution data gathered from recent lunar orbital missions. He believes that these resources hold considerable potential for advancing our knowledge of the process and planetary consequences of hypervelocity impact.
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