Professional Pilot, October 2017

Nitrogen ice flow These plains are geologically young meaning probably no older than 10 million years We know this because they show no evidence of impact craters as we would expect from an older surface In fact impact craters can be seen in many other areas on Plutos older terrains This level of comparatively recent geologic activity is amazing for an object thats so far from the sun Surrounding portions of Tombaugh Regio are mountains composed of water ice that are as tall as the Rocky Mountains There are even mountains that have the equivalent of snow caps on them but in this case those caps are made of methane ice Also there are at least 2 examples of what may be cryovolcanoes volcanoes that erupt what we would think of as cold liquids that then freeze into ices One of those is named Wright Mons for the Wright Brothers There are also other exotic terrains one of which is informally called snakeskin terrain Plutos color and brightness vary considerably These range from bright off white in areas like Tombaugh Regio to dark reddish orange in older and more mountainous terrains Reddish areas are thought to be caused by materials known as tholins falling out of the atmosphere Tholins form when ultraviolet light from the Sun interacts with certain materials in Plutos atmosphere such as methane Pluto is 1 of the highest contrast objects in the solar system Plutos Atmosphere Pluto has a very thin atmosphere with a surface pressure of about 1 100000 the surface pressure on Earth It is composed mostly of nitrogen 108 PROFESSIONAL PILOT October 2017 which also makes up most of Earths atmosphere Plutos atmosphere also has small but significant amounts of methane and carbon monoxide All 3 gases are in equilibrium with the corresponding ices on the surface Even though Pluto has very low atmospheric pressure its atmosphere extends significantly out from the surface because of the low gravity New Horizons found more than 20 haze layers in Plutos atmosphere Methane is a greenhouse gas and as such causes portions of Plutos atmosphere to reach temperatures many tens of Kelvins higher than its surface temperatures but its still really cold relatively Prior to the New Horizons flyby it was thought that Plutos atmosphere may completely freeze onto the surface as Pluto goes farther out in its orbit However data from New Horizons have led some scientists to propose this actually is not be the case Charon Plutos moon Charon is about half the diameter of Pluto and about 1 8th the mass of Pluto a very large body in comparison to its parent body It has no atmosphere and generally is much darker and grayer than Pluto It also has a much older average surface than Pluto does lacking the range of geologically recent activity that Pluto shows Unlike Pluto the majority of Charons surface is covered in water ice which is very stable and less volatile than the nitrogen ice and other ices on Pluto especially at the cold temperatures of this system Plutos surface features vary from mountainous to rolling plains to large canyons one of which is 5 times deeper than the Grand Canyon Charons North Polar Region shows a large dark reddish deposit informally named Mordor The most likely explanation is that during the winter extremely cold temperatures cause gases such as methane and nitrogen that have escaped from Plutos atmosphere to condense in the polar region And UV illumination eventually forms dark reddish tholins Plutos other moons Pluto also has 4 other moons all of which are much smaller than Charon They are Styx Nix Kerberos and Hydra Their rotations around Pluto and Charon are chaotic because of the dynamic gravity field environment caused by these 2 large bodies In other words they are basically tumbling rather than consistently rotating about 1 axis On any given day the Sun could rise in the East and then set in the North Conclusion Whether Pluto is considered a planet or not the New Horizons mission and other data have shown us that Pluto is a complex gem of the solar system with an intriguing set of moons Spacecraft exploration of the Pluto system once again emphasized something I tell my online Introductory Planetary Science classes planetary bodies are almost always more complex and more interesting than we expect them to be before we visit them New Horizons may be done at Pluto but its mission is really not over On January 1 2019 it will do the first flyby of a much smaller trans Neptunian object in this case a 50 km diameter object named 2014 MU69 I look forward to the surprises that await us Bruce Betts PhD is a planetary scientist with degrees from Stanford and Caltech He is Director of Science and Technology at The Planetary Society and has done research focused on infrared studies of planetary surfaces He also managed planetary instrument development programs at NASA Headquarters Sputnik Planum Rugged cratered terrain Polygonal cells Nitrogen ice flow Likely nitrogen ice flows glaciers on the surface of Pluto in Tombaugh Regio Sputnik Planum as discovered by the New Horizons spacecraft