Reorientation of Sputnik Planitia implies a subsurface ocean on Pluto
Nature 540, 7631 (2016). doi:10.1038/nature20148
Authors: F. Nimmo, D. P. Hamilton, W. B. McKinnon, P. M. Schenk, R. P. Binzel, C. J. Bierson, R. A. Beyer, J. M. Moore, S. A. Stern, H. A. Weaver, C. B. Olkin, L. A. Young & K. E. Smith
The deep nitrogen-covered basin on Pluto, informally named Sputnik Planitia, is located very close to the longitude of Pluto’s tidal axis and may be an impact feature, by analogy with other large basins in the Solar System. Reorientation 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 anomaly, 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 day and to maintain ocean uplift, a rigid, conductive water-ice shell is required. Because nitrogen deposition is latitude-dependent, nitrogen loading and reorientation may have exhibited complex feedbacks.
Observed glacier and volatile distribution on Pluto from atmosphere–topography processes
Nature 540, 7631 (2016). doi:10.1038/nature19337
Authors: Tanguy Bertrand & François Forget
Pluto has a variety of surface frosts and landforms as well as a complex atmosphere. There is ongoing geological activity related to the massive Sputnik Planitia glacier, mostly made of nitrogen (N2) ice mixed with solid carbon monoxide and methane, covering the 4-kilometre-deep, 1,000-kilometre-wide basin of Sputnik Planitia near the anti-Charon point. The glacier has been suggested to arise from a source region connected to the deep interior, or from a sink collecting the volatiles released planetwide. Thin deposits of N2 frost, however, were also detected at mid-northern latitudes and methane ice was observed to cover most of Pluto except for the darker, frost-free equatorial regions. Here we report numerical simulations of the evolution of N2, methane and carbon monoxide on Pluto over thousands of years. The model predicts N2 ice accumulation in the deepest low-latitude basin and the threefold increase in atmospheric pressure that has been observed to occur since 1988. This points to atmospheric–topographic processes as the origin of Sputnik Planitia’s N2 glacier. The same simulations also reproduce the observed quantities of volatiles in the atmosphere and show frosts of methane, and sometimes N2, that seasonally cover the mid- and high latitudes, explaining the bright northern polar cap reported in the 1990s and the observed ice distribution in 2015. The model also predicts that most of these seasonal frosts should disappear in the next decade.
Astronomy: A black hole changes its feeding habits
Nature 540, 7631 (2016). doi:10.1038/nature20480
Authors: Stephanie LaMassa
In the 1980s, the gas surrounding a black hole in a nearby galaxy began to emit much more radiation than before. This change has unexpectedly reversed in the past five years, questioning our understanding of these extreme phenomena.
Planetary science: Pluto's telltale heart
Nature 540, 7631 (2016). doi:10.1038/540042a
Authors: Amy C. Barr
Studies of a large frost-filled basin on Pluto show that this feature altered the dwarf planet's spin axis, driving tectonic activity on its surface, and hint at the presence of a subsurface ocean. See Letters p.86, p.90, p.94 & p.97