Pluto’s Sputnik Planitia is a bright, roughly circular feature that resembles a polar ice cap. It is approximately 1,000 kilometres across and is centred on a latitude of 25 degrees north and a longitude of 175 degrees, almost directly opposite the side of Pluto that always faces Charon as a result of tidal locking1. One explanation for its location includes the formation of a basin in a giant impact, with subsequent upwelling of a dense interior ocean2. Once the basin was established, ice would naturally have accumulated there3. Then, provided that the basin was a positive gravity anomaly (with or without the ocean), true polar wander could have moved the feature towards the Pluto–Charon tidal axis, on the far side of Pluto from Charon2,4. Here we report modelling that shows that ice quickly accumulates on Pluto near latitudes of 30 degrees north and south, even in the absence of a basin, because, averaged over its orbital period, those are Pluto’s coldest regions. Within a million years of Charon’s formation, ice deposits on Pluto concentrate into a single cap centred near a latitude of 30 degrees, owing to the runaway albedo effect. This accumulation of ice causes a positive gravity signature that locks, as Pluto’s rotation slows, to a longitude directly opposite Charon. Once locked, Charon raises a permanent tidal bulge on Pluto, which greatly enhances the gravity signature of the ice cap. Meanwhile, the weight of the ice in Sputnik Planitia causes the crust under it to slump, creating its own basin (as has happened on Earth in Greenland5). Even if the feature is now a modest negative gravity anomaly, it remains locked in place because of the permanent tidal bulge raised by Charon. Any movement of the feature away from 30 degrees latitude is countered by the preferential recondensation of ices near the coldest extremities of the cap. Therefore, our modelling suggests that Sputnik Planitia formed shortly after Charon did and has been stable, albeit gradually losing volume, over the age of the Solar System.
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We thank NASA for its support of the New Horizons mission and the New Horizons mission team for making the July 2015 flyby possible. We thank V. Bray, B. Carcich, J. Hofgartner and F. Nimmo for helpful comments. This research was supported by a grant from NASA Origins (to D.P.H.).
The authors declare no competing financial interests.
Extended data figures and tables
Sputnik Planitia, the informally named western lobe of the white heart-shaped feature, is approximately 1,000 km across and is centred on a latitude of 25° N and longitude of 175°, with the zero of longitude defined to run directly underneath Pluto’s moon Charon. Owing to Pluto’s slow rotation and the approach vector of New Horizons, the highest-resolution images were of Sputnik Planitia, and most other regions were imaged at much lower resolution from farther away. Consequently, this map is a mosaic of multiple images of differing resolution. Faint grid lines of latitude and longitude are spaced by 30°. Regions south of about 30° S were not sunlit and, hence, were not imaged, because they are currently experiencing polar night.
The large circle represents Pluto, with our view centred on the intersection of the equator and the noon meridian. At the location of the red star, the Sun is directly overhead, and it tracks along the latitude ls over the course of a full rotation of Pluto. The thick horizontal red line segment shows the regions along latitude ly that are currently illuminated by sunlight. The angle γ is the angular distance between the Sun and the point of interest on Pluto’s surface (black dot labelled ‘P’ at latitude ly), as measured from the centre of Pluto. The Sun is on the horizon when γ = 90°, which we define to occur at the meridional angle of β = βmax. The spherical triangle formed by γ and the two meridians connecting the north pole to the above-mentioned points defines γ in terms of the other variables and simplifies derivations of average energy fluxes.
Here we zoom in on the region surrounding Sputnik Planitia and show (solid curves) the solar energy flux with Pluto at its minimum (e = 0.222), current (e = 0.25) and maximum (e = 0.266) eccentricity, assuming a present-day obliquity of 120°. Black arrows show the future changes that are expected as a result of these eccentricity variations. As we show in Methods, Pluto’s eccentricity affects insolation equally at all latitudes. Incident radiation from Charon, a 0.1% effect, moves the solid e = 0.266 curve slightly to the right, most noticeably at the equator, as indicated by the dashed curve and discussed in Methods. Eccentricity effects are at the 1% level, 20 times weaker than the obliquity effects highlighted in Fig. 1. The effect of Pluto’s eccentricity varies with a 3.95-million-year period8. The maximum insolation to the entire planet occurred about 0.8 million years ago and the minimum will next occur in about 1.2 million years.
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Hamilton, D., Stern, S., Moore, J. et al. The rapid formation of Sputnik Planitia early in Pluto’s history. Nature 540, 97–99 (2016). https://doi.org/10.1038/nature20586
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