Antipodal Terrains on Pluto

Post contributed by C. Adeene Denton, Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, USA.

Antipodal terrains are unusual regions of hilly, lineated, or otherwise disrupted terrain that are on the direct opposite side of planetary bodies to large impact basins. These mysterious terrains have been observed at the antipodes to the Caloris basin on Mercury and the Imbrium basin on the Moon, where their formation is considered to be indicative both of the impact’s size and the specificities of the planetary body’s interior structure. Recent revisiting of data from the New Horizons spacecraft revealed an unusual region of disrupted and lineated terrain on Pluto’s far side that is roughly antipodal to the massive Sputnik Planitia basin, the feature sometimes referred to as “Pluto’s Heart” (Image 1). If the lineated terrain is indeed connected to the large impact believed to have formed Sputnik Planitia, then the two geologic features offer a new and unusual way to probe Pluto’s interior: seismology through giant impact.

Image 1: Comparison of Pluto’s nearside (left) and farside (right) with Sputnik Planitia and its proposed antipodal terrain indicated. The location of Image 3 is also indicated. Images modified from full-scale planetary images taken by the New Horizons spacecraft, via NASA/JHUAPL/SWRI.

Antipodal terrains to giant impacts are believed to be produced as a result of focusing of the seismic waves released from the impact point: as these seismic waves expand hemispherically through the planetary body, planetary geometry tends to focus them such that they converge at the antipode (i.e., Image 2), where the concentration of energy can produce the surficial deformation that we observe. However, the magnitude and extent of the deformation that occurs at the antipode to a large impact is known to be highly dependent on the internal structure of the target body – in the case of Pluto, that includes whether it might possess an ocean and whether the core has experienced any hydrothermal alteration. Our recent numerical models simulated the Sputnik Planitia-forming giant impact into Pluto and found that antipodal focusing is indeed a consistent result; the moment of wave arrival from the impact point to the antipode is shown in Image 2.

More detailed analysis also revealed that the extent of the lineations is most consistent with the presence of a thick subsurface ocean and a serpentinized core in Pluto’s interior, a structure which maximizes wave transmission over a broad area. The serpentinized core is of particular interest, as it indicates extensive water-rock interaction in Pluto’s interior and is associated with potentially habitable ocean world environs.

Image 2: Snapshot taken from a best-fit impact model of the Sputnik Planitia-forming impact showing arrival of impact waves at the antipode, ~1110 seconds after impact. The formation of Sputnik Planitia is ongoing at the top of the image, and the arrival of waves at the antipode can be observed at the bottom. Material is colored according to magnitude of material velocity in the z-direction (i.e., downward) so that wave arrival at the antipode is visible. This best-fit case includes a 150-km-thick ocean (178-km-thick ice shell) and a serpentine core (labeled). Impact model was produced by the author using the iSALE shock physics code.

If these lineations are indeed antipodal features, what sorts of tectonic expression might they be? Topographic data for Pluto’s farside cannot resolve these features in detail, but combining impact model results with surface topography facilitate some broad-scale inferences. The strains experienced at the antipode seem to be largely extensional, facilitating the formation of large-scale graben. Basic estimates of graben depth can be obtained assuming the strain in the region is uniformly distributed across the lineations. In this scenario, resulting graben are between 2-4 km deep, similar to the massive graben observed on Pluto’s nearside (i.e., Virgil Fossae, Image 3).

Image 3: Virgil Fossae, one of the large graben observed on Pluto’s nearside, which has much higher (20-50 times!) image resolution relative to the proposed antipodal graben on Pluto’s farside. The scale of the features is believed to be similar, however (see text). Image via NASA/JHUAPL/SWRI.

While these lineations do indeed seem consistent with large-scale graben formed as a result of antipodal focusing from Sputnik Planitia, confirming that hypothesis with improved imaging will have to wait until the next time a spacecraft makes it to Pluto. The additional global implications of the presence of a thick ocean and an at least partially hydrated core are also intriguing, as it implies that the interior as well as the exterior of Pluto are both more geologically active than anticipated. The unusual geology of the dwarf planet remains mysterious!

Further Reading

Bowling, T. J., Johnson, B. C., Melosh, H. J., Ivanov, B. A., O’Brien, D. P., Gaskell, R., & Marchi, S. (2013). Antipodal terrains created by the Rheasilvia basin forming impact on asteroid 4 Vesta. Journal of Geophysical Research: Planets, 118(9), 1821–1834.

Denton, C. A., Johnson, B. C., Wakita, S., Freed, A. M., Melosh, H. J., & Stern, S. A. (2021). Pluto’s antipodal terrains imply a thick subsurface ocean and hydrated core. Geophysical Research Letters, 48, e2020GL091596.

Schultz, P. H., & Gault, D. E. (1975). Seismic effects from major basin formations on the moon and mercury. The Moon, 12(2), 159–177.

Stern, S. A., Grundy, W. M., Mckinnon, W. B., Weaver, H. A., & Young, L. A. (2018). The Pluto system after New Horizons. Annual Review of Astronomy and Astrophysics, 56, 357–392.

Stern, S. A., White, O. L., McGovern, P. J., Keane, J. T., Conrad, J. W., Bierson, C. J., et al. (2020). Pluto’s far side. Icarus, 356, 113805.

Watts, A. W., Greeley, R., & Melosh, H. J. (1991). The formation of terrains antipodal to major impacts. Icarus, 93(1), 159–168.

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