Free Astronomy Magazine September-October 2015

SEPTEMBER-OCTOBER 2015 previously estimated, making it the big- gest known body among the transneptu- nian objects. Given its mass, a larger di- ameter means a lower average density, and thus a fraction of ice in the planetary mass greater than that estimated so far. A larger diameter reduces also the thickness of the troposphere, whose outer bound- ary was previously known, and that re- mains the same. In the case of Charon, which in not having an atmosphere has in the past permitted to calculate its diameter without uncertainties, the images taken by LORRI have confirmed the previous estimat- ed value of 1,208 km. On 13 July, the picture shot from a distance of 768,000 km will be probably considered the iconic image of the entire mission, a spectacular overview of Pluto’s globe, in which stands out the most noticeable of all surface formations, a heart-shaped, whitish region about 1,600 km across, close to the equator and already spotted in earlier images taken, though, at much greater distances. The image resolution is such that for the first time can be clearly observed low reflec- tance regions dotted with impact craters, and hence rather ancient (with an age of 14 July, photo- graphed, from a distance of 77,000 km, the first mountain range on Pluto, Norgay Montes, as high as 3,500 metres. The video below shows its location. [NASA/ Johns Hopkins University Ap- plied Physics Lab- oratory/South- west Research Institute]