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PLANETARY SCIENCE. Scientists ponder an improbably active Pluto.

On 14 July, NASA's New Horizons spacecraft flew past Pluto, the first reconnaissance of a body in the Kuiper belt, the zone of icy objects beyond the orbit of Neptune. With the flyby complete and the data trickling home, mission scientists focused on a new challenge: making sense of an unexpectedly complex and dynamic world. Pluto contains ice mountains and smooth, crater-free plains—features suggestive of active geological processes. But mission scientists are debating whether these are the result of an atmosphere that shapes the landscape from above, or residual heat in Pluto's interior that could be driving fresh flows of ice onto the surface.