Free Astronomy Magazine January-February 2016

8 JANUARY-FEBRUARY 2016 PLANETOLOGY these could be cryovolcanoes, or ice volca- noes formed after the eruption of liquid ice masses that solidified while gradually piling on the surface. Provisionally named Piccard Mons (the highest, about 5 km high) and Wright Mons (about 3 km high), the two cryovolcanoes are not particularly ob- vious in indi- vidual photo shoots, but are clearly noticeable in the 3-D topographic maps created by com- bining images of the surface taken from different angles. The two structures could be geologically very recent, as further demonstration that Pluto’s core could still produce heat through the long-term de- cay of radioactive isotopes. But the most interesting findings are not only limited to Pluto’s surface as, in fact, also its at- mosphere produced great surprises. Observations made prior to the New Horizons flyby had convinced astron- omers that the dwarf planet was surrounded by a tenuous shell of gas and that much of its original mass had already dispersed in- to space. The s p a c e c r a f t ’ s ob s e r v a t i on s have instead proved the con- trary: the atmo- sphere is more ex- tended and denser than expected, and the atoms composing it are being dispersed under the A t the top, a virtual over- flight of Pluto’s surface in high definition. Left, the evocative Plu- to’s atmosphere (seen around the night hemi- sphere), that in the lower layers is sufficiently dense to take on a bluish hue rem- iniscent of the Earth's atmo- sphere. Follow- ing page, Pluto and Charon in comparison. The colours have been accentuated and the distance greatly reduced. [NASA/Johns Hopkins Univer- sity Applied Phys- ics Laboratory/ Southwest Re- search Institute]