Free Astronomy Magazine January-February 2016

11 PLANETOLOGY pressure of the solar radia- tion at a rate a thousand times slower than previously esti- mated. Furthermore, the fact that the atmosphere is also cooler than originally antici- pated makes its consistency even more enigmatic, since it was expected that already at higher temperatures the at- mospheric gases should quick- ly fall to the ground. Entirely unpredicted was also the bluish hue of Pluto’s atmo- sphere, clearly visible in the previous page image, taken when New Horizons was behind the planet with re- spect to the Sun. According to the astron- omers, that particular colour is due to the scattering of sunlight by the aforesaid tholins, through a process similar to that previously observed in the atmosphere of Titan, Saturn’s largest satellite. Remarkable are also the news about Char- on, which like Pluto appears to have been geologically active until very recently. Above, an animation allowing us to fly over part of the satellite, while below we have an image that intrigues geologists and which portrays Organa and Skywalk- er craters. The picture was taken in the in- P revious page, high-resolu- tion view (0.8 km/pixel) of a large area of Charon’s sur- face, centred downwards on Serenity Chasma and Vulcan Pla- num. Right, a virtual over- flight of Char- on’s surface. Below, the enig- matic Organa crater and sur- rounding areas. [NASA/Johns Hopkins Univer- sity Applied Physics Labora- tory/South- west Research Institute]