This detailed image of Pluto from NASA’s New Horizons mission shows a strip some 80km wide running from the informally named Al-Idrisi mountains in the northwest into Sputnik Planum, the icy plains that form the “heart” seen on the fly-by images. The terrain is surprisingly varied, with cratering, mountains up to 3500m high and signs of glaciation that suggest geological activity. Modelling of ice properties suggests that an icy ammonia–water slurry could give rise to Pluto’s surprisingly active appearance.
Pluto is made mainly of water ice, which would be solid at the temperatures expected at Pluto. But if ammonia were mixed with the water ice, it would give Pluto a mushy, mobile mantle able to convect. Ammonia is a plausible component of these icy bodies and has been found on the surface of Charon. It would reduce the viscosity of water ice by a factor of 100,000, enough to give rise to mantle convection under the conditions at Pluto. Alex Trowbridge and Jay Melosh of Purdue University described their modelling work at the meeting of the Division for Planetary Sciences of the American Astronomical Society in November 2015.
New Horizons’ Long Range Reconnaissance Imager (LORRI) provided the highest resolution data here, at ~80m per pixel; the colour data added from the Ralph/Multispectral Visible Imaging Camera is less detailed.