mass. Beginning in the spring of 2015, New Horizons started its detailed studies of Pluto including searches for additional moons and for rings. Various studies continued through its close approach on July 14 at a distance of 8507 miles (13,691 kilometers) and after. The return of New Horizons' best data began shortly after its close approach and will continue for more than a year due to the large transmission distance. Because of the speed of the flyby and Pluto's slow rotation rate only one hemisphere of this dwarf planet has been photographed and measured at high resolution, limiting generalizations about all of Pluto's surface. Still, Pluto's diameter could be measured and had to be revised upward to 1475 miles (2,374 kilometers) based on New Horizons' imagery.Results from New Horizons, besides forcing the revision of textbooks, left planetary scientists struggling to explain this cold, distant world and its system of moons. Pluto's surface exhibits craters as large as 162 miles (260 kilometers) in diameter on the dayside, near encounter hemisphere. Craters are widely distributed there and show degradation or infill. This was a surprise to find on a dwarf planet that was anticipated to be heavily cratered and lacking activity that might affect craters. Dense cratering is seen in some areas, but so are tectonic features including scarps and troughs as long as about 370 miles (600 kilometers).Mountains are also seen rising 6500 to 9800 feet (2 to 3 kilometers) above their surroundings. The likely materials that can hold up the mountains and maintain their shapes over millions of years in Pluto's cold is limited. The science team concludes that the mountains are made of "water ice-based 'bedrock'." Frozen gases on Pluto's surface (the temperature there is about -391 F = -235 C = 38 K) include nitrogen (N2), carbon monoxide (CO),and methane (CH4). These were detected by ground-based telescopes and are now thought to be thin layers on top of the 'bedrock' water ice.Dark surface coloring appears to be due carbon residues called tholins. These are created by solar ultraviolet rays or charged particles falling on mixtures of nitrogen and methane.Detailed views of a plain on this dwarf planet show no confirmed craters but it has large (tens of kilometers) polygonal or egg-shaped features defined and separated by troughs between neighbors. Features similar in look to glaciers on Earth are seen in this region. This is consistent with the strength and flow properties of the frozen gases mentioned above.Radio transmission measurements from New Horizons measured Pluto's atmosphere having a pressure of 10 microbars (millionths of a bar). For comparison, Earth's atmospheric pressure at sea level is about 1 bar, 100,000 times greater than Pluto's surface pressure. Those measurements also showed that Pluto has a shallow tropospheric boundary layer (Earth has a troposphere too). Imagery from the cameras shows a haze layer and even some structure in it.It isn't known whether Pluto has a magnetic field, but its small size and slow rotation suggest little or none. Data from two of New Horizons instruments may give an indirect answer to this question.Charon was