X Marks the Spot

Explanation: "X" marks the spot of some intriguing surface activity in the latest picture of Pluto returned from NASA's New Horizons spacecraft. "This part of Pluto is acting like a lava lamp," said William McKinnon, deputy lead of the New Horizons Geology, Geophysics and Imaging team, from Washington University in St. Louis, "if you can imagine a lava lamp as wide as, and even deeper than, Hudson Bay." Computer models by the New Horizons team show that these blobs of overturning solid nitrogen can slowly evolve and merge over millions of years. The ridged margins, which mark where cooled nitrogen ice sinks back down, can be pinched off and abandoned. The "X" feature is likely one of these — a former quadruple junction where four convection cells meet. Numerous, active triple junctions can be seen elsewhere in the LORRI mosaic.

Pluto

Image credit: NASA/JHUAPL/SwRI