When the New Horizons team acquired the latest color images of the planet’s atmosphere from the spacecraft, they were surprised to see that the haze surrounding Pluto is actually blue, which really attracted the team’s attention. The blue tint gives clues as to the size and composition of the particles making up the haze. The particles themselves are believed to be formed high in the atmosphere where ultraviolet radiation splits and ionizes molecules of nitrogen and methane, allowing reactions to take place. These reactions form complex macromolecules, which continue to combine and grow before being covered with condensation from volatile gases and falling to the surface. These soot-like particles are called tholins, first found to form in the upper atmosphere of Saturn’s moon Titan. Tholins themselves are either gray or red in color, but scatter blue wavelengths of light. The fact that they grow and fall to the surface is what explains Pluto’s reddish-brown hue.
Another big finding that the New Horizons team made was noticing small, exposed regions of water ice on Pluto. Usually, the water ice would be masked by other, more volatile ices across most of the planet. One science team member, Jason Cook, of the Southwest Research Institute in Boulder, Colorado, stated that understanding why the water is found where it is and not in other regions of the planet is a mystery that is being explored. Also, color images of the same regions show bright red spots in the areas where the water ice was detected, indicating some relationship between the water and the reddish tholin colorant on the surface. That relationship is not yet understood, and scientists on the team are equally surprised that such a relationship exists on such a faraway planet.
New Horizons is currently 3.1 billion miles from Earth and moving into the Kuiper Belt. All systems are healthy and operating normally, and the team is awaiting NASA approval and funding of the mission extension to perform a flyby of a second, much smaller Kuiper Belt object in early 2019.