Impact Crater Degradation on Mercury

Post by Mallory Kinczyk, PhD candidate, Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University

The formation of impact craters may be the most ubiquitous exogenic surface process in the Solar System. These craters take on many shapes and sizes and can hint at underlying rock types, tell us about the nature of the impactor, and can shed light on the body’s geological history. Even on bodies without atmospheres, erosive forces are at play, changing the crater shape through time via processes such as seismic shaking and disruption from debris thrown outward by subsequent, nearby impacts. Because Mercury is the only terrestrial planet without an atmosphere, it maintains a unique snapshot of the inner Solar System’s impactor population (Image 1) and, in turn, can shed light onto Earth’s own geological history.

converted PNM file

Image 1: View of Mercury from the MESSENGER spacecraft, which orbited Mercury between 2011 and 2015 (Image PIA17280). A variety of impact crater sizes and shapes are evident from very fresh craters to subdued to almost completely obliterated crater forms. Bach crater (arrow) hosts a well-defined central peak ring, but its subdued form indicates that it has been disrupted by subsequent craters. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

One of the first elements of a freshly made impact crater to degrade is its rays (Image 2). Rays are bright, surficial deposits of fine-grained material that can extend many crater radii from the impact site. Beyond rays, morphological changes that Mercury’s craters undergo include overprinting by subsequent craters as well as by secondary craters and debris from nearby impacts. Additionally, seismic shaking from later, nearby impacts causes rims and wall terraces to become more subdued, and craters can become infilled with later volcanic or impact-related melts.


Image 2:  The progression of central peak craters from ‘very fresh’ (left) to ‘very degraded’ (right). The crater on the left is Kuiper crater, which has distinct wall terraces, hummocky floor material, and bright rays (not shown here). At the right is an unnamed crater that has been overprinted by the ejecta deposits from the crater in the upper right-hand corner of the image. The older crater has also likely been filled in by volcanic material or impact melt, as indicated by its shallow nature.

Classifying craters on Mercury based on their degree of degradation provides a window into the planet’s past, where massive basin-forming impacts were common and large swaths of the planet were resurfaced by impact-ejected material. We carried out a global survey of crater degradation state on Mercury and found a dearth of craters of the oldest and most degraded morphological class in a region where two large, ancient impact basins were thought to have formed (Image 3). While the exact timing of these ancient events is unknown, our findings offer us a better understanding of the extent of resurfacing of Mercury by impact events compared with widespread effusive volcanism, and even what the earliest days of Earth might have been like.


Image 3: MESSENGER MDIS image PIA14339 of Bach (bottom) and Alencar (top center) craters. Alencar is a very fresh, young crater with a hummocky floor, crisp wall terraces, and textured radial ejecta deposits outside the rim. Note the small crater to the left that has been almost completely obscured by Alencar’s ejecta. On the other hand, Bach crater has undergone substantial degradation. There are many smaller impact craters on top of Bach, and the larger crater’s shallow nature indicates that it has been infilled with material almost to the rim.

Further Reading

Kinczyk, M.J., et al. (2020), A morphological evaluation of crater degradation on Mercury: Revisiting crater classification with MESSENGER data, Icarus, 341, 1-11.

Baker, D.M.H., et al. (2011), The transition from complex crater to peak-ring basin on Mercury: New observations from MESSENGER flyby data and constraints on basin formation models, Planetary and Space Science, 59, 1932–1948.

Denevi, B.W., et al. (2018), The Geologic History of Mercury, in Mercury: The View after MESSENGER, Cambridge University Press, pp. 144–175.

Fassett, C.I., et al. (2012), Large impact basins on Mercury: Global distribution, characteristics, and modification history from MESSENGER orbital data. Journal of Geophysical Research Planets, 117, 1–15.

Fassett, C.I., et al. (2017), Evidence for rapid topographic evolution and crater degradation on Mercury from simple crater morphometry. Geophysical Research Letters 44, 5326–5335.

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