Enigmatic Normal Faults on Ceres

Post by Kynan Hughson, Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, USA.

Since March of 2015 NASA’s Dawn spacecraft has been actively exploring the main asteroid belt’s largest member and only dwarf planet in the inner solar system: Ceres. Situated around two fifths of the way between the orbits of Mars and Jupiter, Ceres is gargantuan compared to its neighbors. With a mean diameter of ~946 km (approximately the width of the state of Texas) and a bulk density of ~2.16 g/cm3 it comprises around one third of the mass of the entire main belt. Dawn’s continuing examination of this unique object since March 2015 has revealed a geologically diverse world covered with geomorphological features common to both rocky inner solar system planets and icy outer solar system satellites (e.g. Bland et al., 2016; Schmidt et al., 2017; Fu et al., 2017). These observations have exacerbated Ceres’ refusal to be neatly categorized as either a rocky or icy planet.


Image 1: A rotating aerial view of Nar Sulcus (centered at approximately 79.9 °W, 41.9 °S). Note the two nearly perpendicular sets of fractures. In particular, note the imbricated blocks within the longer fracture set. The longer fracture set is approximately 45 km long, and the deepest valleys are ~400 m deep. This scene was created using a stereophotogrammetrically (SPG) derived elevation model (vertical resolution ~15 m) and high resolution (~35 m/pixel) Dawn framing camera mosaics (Roatsch et al., 2016a; Roatsch et al., 2016b), which are available on the Small Bodies Node of NASA’s Planetary Data System.

One of the most striking geomorphological regions on Ceres is Nar Sulcus (Images 1 and 2). These two mutually perpendicular fracture sets are located on Ceres’ southern hemisphere on top of a regional topographic rise within the Yalode impact structure (Ceres atlas with all named features is available here: https://planetarynames.wr.usgs.gov/Page/CERES/target). The east-west trending set is ~10 km wide and extends ~45 km along strike, while the north-south set is ~10 km wide and extends ~15 km along strike. Both sets display multiple valleys with significant vertical displacement, and an inclined blocky appearance (Image 2b). In light of these observations, we interpreted these features to be imbricated listric normal faults (Hughson et al., 2018). Under this paradigm Nar Sulcus represents a region of extensional tectonism. The largest faults in both sets have characteristic vertical displacements of ~400 m, and characteristic horizontal displacements of ~1.5 km.


Image 2: (A) A plan view mosaic of Nar Sulcus overlain with the (SPG) derived elevation model (Roatsch et al., 2016a; Roatsch et al., 2016b). (B) A to scale elevation profile of the D-D’ cross-section. The dashed black lines represent qualitatively inferred fault planes. Note the imbricated appearance of the inferred half-grabens, and how the majority of the vertical displacement is accommodated along one major fault. This last point is representative of the entire Nar Sulcus region.

In order to unravel some of the mystery surrounding the origin of Nar Sulcus and the mechanical properties of the upper layer of Ceres, we (Hughson et al., 2018) applied a flexural-cantilever style model similar to the one developed by Kusznir et al. (1991) to the cross sectional profile of the largest faults in both sets. Initial results from this analysis suggest that the rigidity of the upper layer of Ceres, at least proximal to Nar Sulcus, is very similar to that of Europa’s icy shell, and several orders of magnitude less than what is measured for the Earth’s crust (Nimmo & Schenk, 2006). This finding is further evidence for an exotic water ice rich outer layer on Ceres, and clearly demonstrates the need for continued study into this unusual and unique landform on Ceres.

Further Reading:

Bland, M. T., Raymond, C. A., Schenk, P. M., Fu., R. R., Kneissl, T., Pasckert, J. H., Hiesinger, H., Preusker, F., Park, R. S., Marchi, S., King, S. D., Castillo-Rogez, J. C., Russell, C. T. (2016), Composition and structure of the shallow subsurface of Ceres revealed by crater morphology, Nature Geoscience 9, 538-542.

Fu, R. R., Ermakov, A. I., Marchi, S., Castillo-Rogez, J. C., Raymond, C. A., Hager, B. H., Zuber, M. T., King, S. D., Bland, M. T., De Sanctis, M. C., Preusker, F., Park, R. S., Russell, C. T. (2017), The interior structure of Ceres as revealed by surface topography, Icarus 476, 153-164.

Hughson, K.H.G., Russell, C.T., Sizemore, H.G., Schmidt, B.E., Buczkowski, D.L., Schenk, P., Peltzer, G., Raymond, C.A. (2018), Characterizing the composition and elastic properties of the near-surface of Ceres: Insights from flexural modeling of the Nar Sulcus fractures, LPSC XLIX, Abs. 2348, 2018.

Kusznir, N.J., Marsden, G., Egan, S.S. (1991), A flexural-cantilever simpleshear/pure-shear model of continental lithosphere extension: applications to the Jeanne d’Arc Basin, Grand Banks and Viking graben, North Sea, In: Roberts, A.M., Yielding, G., Freeman, B. (Eds.), The Geometry of Normal Faults, Geological Society, Special Publication 56, 41-60.

Nimmo, F., Schenk, P. (2006), Normal faulting on Europa: implications for ice shell properties, Journal of Structural Geology 28, 2194-2203.

Roatsch,T., Kersten, E., Matz, K.-.D., Preusker, F., Scholten, F., Elgner, S., Schroeder, S.E., Jaumann, R., Raymond, C.A., Russell, C.T. (2016a), Dawn FC2 Derived Ceres HAMO DTM SPG V1. 0, DAWN-A-FC2-5-CERESHAMODTMSPG-V1.0, NASA Planetary Data System.

Roatsch,T., Kersten, E., Matz, K.-D., Preusker, F., Scholten, F., Elgner, S., Schroeder, S.E., Jaumann, R., Raymond, C.A., Russell, C.T. (2016b), Dawn FC2 Derived Ceres Mosaics V1.0, DAWN-A-FC2-5-CERESMOSAIC-V1.0, NASA Planetary Data System.

Schmidt, B. E., Hughson, K. H. G., Chilton, H. T., Scully, J. E. C., Platz, T., Nsthues, A., Sizemore, H., Bland, M. T., Byrne, S., Marchi, S., O’Brian, D. P., Schorghofer, N., Hiesinger, H., Jaumann, R., Pasckert, J. H., Lawrence, J. D., Castillo-Rogez, J. C., Sykes, V., Schenk, P. M., De Sanctis, M. C., Mitri, G., Formisano, M., Li, J. -Y., Reddy, V., LeCorre, L., Russell, C. T., Raymond, C. A. (2017), Geomorphological evidence for ground ice on dwarf planet Ceres, Nature Geoscience 10, 338-343.

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