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Reorientation of Sputnik Planitia implies a subsurface ocean on Pluto

Abstract

The deep nitrogen-covered basin on Pluto, informally named Sputnik Planitia, is located very close to the longitude of Pluto’s tidal axis1 and may be an impact feature2, by analogy with other large basins in the Solar System3,4. Reorientation5,6,7 of Sputnik Planitia arising from tidal and rotational torques can explain the basin’s present-day location, but requires the feature to be a positive gravity anomaly7, despite its negative topography. Here we argue that if Sputnik Planitia did indeed form as a result of an impact and if Pluto possesses a subsurface ocean, the required positive gravity anomaly would naturally result because of shell thinning and ocean uplift, followed by later modest nitrogen deposition. Without a subsurface ocean, a positive gravity anomaly requires an implausibly thick nitrogen layer (exceeding 40 kilometres). To prolong the lifetime of such a subsurface ocean to the present day8 and to maintain ocean uplift, a rigid, conductive water-ice shell is required. Because nitrogen deposition is latitude-dependent9, nitrogen loading and reorientation may have exhibited complex feedbacks7.

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Figure 1: Sputnik Planitia topography and reorientation.
Figure 2: Load thicknesses L and resulting gravity anomalies Δg for present-day Sputnik Planitia topography.
Figure 3: Basal shell temperature required to maintain a thinned shell for 4 billion years.

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Acknowledgements

New Horizons was built and operated by the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, USA, for NASA. We thank the many engineers, flight controllers and others who have contributed to the success of the New Horizons mission and NASA’s Deep Space Network for a decade of excellent support to New Horizons. We thank B. Johnson for discussions on impact physics and J. Conrad for cryovolcanism calculations.

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Contributions

D.P.H. originated the reorientation hypothesis. F.N. developed the subsurface ocean scenario and carried out the bulk of the calculations. C.J.B. calculated the effect of realistic basin geometries and ejecta blanket. P.M.S. and R.A.B. provided the stereo topography. All authors read or commented on the manuscript.

Corresponding author

Correspondence to F. Nimmo.

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The authors declare no competing financial interests.

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Nature thanks G. Collins and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Schematic of the way in which the gravity anomaly is affected by an uplifted ocean and the thickness of the nitrogen layer.

ac, Either a nitrogen layer more than 40 km thick (b) or an uplifted ocean (c) could result in the present-day positive gravity anomaly at Sputnik Planitia; if neither is present, then a negative gravity anomaly results (a). The peak gravity anomaly is calculated using the flat-plate formula 2πGΔρh for each layer, where h represents the thickness, Δρ is the lateral density contrast and the densities of H2O ice, water and N2 ice are 0.92 g cm−3, 1.0 g cm−3 and 1.0 g cm−3 (ref. 23), respectively. In c, the gravitational contribution of the ocean is reduced as a result of upwards attenuation assuming a shell thickness of 150 km (see Methods). The structure in c is similar to the inferred structure of lunar mascon basins, which also show positive gravity anomalies (refs 15, 16).

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Nimmo, F., Hamilton, D., McKinnon, W. et al. Reorientation of Sputnik Planitia implies a subsurface ocean on Pluto. Nature 540, 94–96 (2016). https://doi.org/10.1038/nature20148

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