Post contributed by Ben Man, The Open University @AstroBenjamin
Mercury is a contracting world as evidenced by the abundance of shortening structures observed across the surface of the planet (Image 1). Global contraction as a result of secular cooling of the planetary interior is most likely responsible for widespread compressional tectonism with shortening structures seen cutting all surface materials at all latitudes and longitudes. Shortening structures are accepted as the surface manifestation of thrust faults and folding. Grabens, such as those observed in the images (Image 1-3), are secondary structures found on top of parent shortening structures. The presence of these grabens provide evidence for recent widespread tectonism on Mercury, confirming that global contraction is ongoing.
Image 1: Aspect view of Alpha Crucis Rūpes with horst and grabens present in the foreground of the image. Alpha Crucis Rūpes is located in H09 Eminescu, an equatorial quadrangle. The image is comprised of the H09 south east high incidence west mosaic tile and five individual narrow-angle camera frames (EN0231136925M, EN0231136927M, EN0231136960M, EN0231136962M, EN0231136998M). The image has not been vertically exaggerated and the scale bar is computed for the centre of the image. Image source: NASA/JHUAPL/CIW made by Benjamin Man. Image frames and mosaic tiles are available from NASA’s Planetary Data System Geosciences Node (https://pds-geosciences.wustl.edu/) and the Cartography and Imaging Sciences Node (https://pds-imaging.jpl.nasa.gov/).
With continued movement of the parent shortening structure, it is possible that bending of the overlying rock occurs. As a result, faults form resulting in a down-dropped fault block, a graben. These grabens are small-scale shallow landforms, 10s to 100s of metres deep, 10s of kilometres in length and commonly less than 1 km wide1 (see for example Image 2). They are found parallel to sub-parallel to the strike of the parent structure and can be singular or, multiple grabens can be found alongside upstanding horsts. Given the size and depth of these secondary landforms, they are not expected to survive for extended periods of time as their signature would quickly be masked by burial and infilling from impactors and their debris in addition to abrasion/erosion from micrometeors1–4. Consequently, the mere existence of these landforms indicates that they are geologically young and in turn, that the parent structure on which they have formed must have been active geologically recently.
Image 2: Aspect view of Zapiola Rupes with grabens present in the centre of the image. Zapiola Rupes is located in H01 Borealis, the north polar quadrangle. The image is comprised of the H01 high incidence west mosaic tile and three individual narrow-angle camera frames (EN1052963743M, EN1052963746M, EN1052963740M). The image has been vertically exaggerated by 2 times and the scale bar is computed for the centre of the image. Image source: NASA/JHUAPL/CIW made by Benjamin Man. Image frames and mosaic tiles are available from NASA’s Planetary Data System Geosciences Node (https://pds-geosciences.wustl.edu/) and the Cartography and Imaging Sciences Node (https://pds-imaging.jpl.nasa.gov/).
Grabens associated with shortening structures on Mercury have recently been discovered to be more common than previously reported1. Hundreds of grabens have been found associated with parent structures that are widespread across Mercury’s surface. Such a discovery indicates that many of Mercury’s shortening structures have continued to be active providing evidence that global contraction on Mercury is ongoing into the present day.
Image 3: Aspect view of Endurance Rupes with a graben present in the centre of the image. Endurance Rupes is located in H09 Eminescu, an equatorial quadrangle. The image is comprised of H09 northeast high incidence west mosaic tile and one narrow-angle camera frame (EN1015627633M). The image has not been vertically exaggerated and the scale bar is computed for the centre of the image. Image source: NASA/JHUAPL/CIW made by Benjamin Man. Image frames and mosaic tiles are available from NASA’s Planetary Data System Geosciences Node (https://pds-geosciences.wustl.edu/) and the Cartography and Imaging Sciences Node (https://pds-imaging.jpl.nasa.gov/).
Further reading:
1. Man, B., Rothery, D. A., Balme, M. R., Conway, S. J. & Wright, J. Widespread small grabens consistent with recent tectonism on Mercury. Nat. Geosci. (2023). doi:10.1038/s41561-023-01281-5
2. Watters, T. R. et al. Recent tectonic activity on Mercury revealed by small thrust fault scarps. Nat. Geosci. 9, 743–747 (2016). doi.org/10.1038/ngeo2814
3. Watters, T. R., Robinson, M. S., Banks, M. E., Tran, T. & Denevi, B. W. Recent extensional tectonics on the Moon revealed by the Lunar Reconnaissance Orbiter Camera. Nat. Geosci. 5, 181–185 (2012). doi.org/10.1038/ngeo1387
4. Fassett, C. I. et al. Evidence for rapid topographic evolution and crater degradation on Mercury from simple crater morphometry. Geophys. Res. Lett. 44, 5326–5335 (2017). doi.org/10.1002/2017GL073769
Planetary Geomorphology Image of the Month 