New Horizons’s New View of Pluto

summary by Laurie Averill

As observations of Pluto made during New Horizons’s July 2015 flyby trickle back to Earth at a rate of 3 kilobytes per second, theories about Pluto’s geology, surface composition, atmosphere, and interaction with the solar wind are being upended. Instead of consisting of half rock and half ice, New Horizons’s density observations of 1.86 grahams per cubic centimeter indicate the Pluto is made up of a third ice and two-thirds rock. Around a rocky core, the outer ice layer is about 180 miles thick and is made up of water, nitrogen, carbon monoxide, and methane.

Water ice acts like rock being solid, rigid, and forms mountains the size of the Rockies on the surface on the older more crater-marked surface of the planet. Methane snow covers high altitude areas like crater rim and mountains, but also the plains. Nitrogen ice forms polygonal features that flow glacially and subvectively possibly completely turning over in about half a million years exposing newer surfaces.

Other usual surface features include ice volcanoes, scaly areas, and pits the size of small towns and big buildings.

Observed during occultation with the Sun, the atmosphere contains hazes that the Sun turns blue. The hazes are made up of complex hydrocarbons that Sagan named tholins that are large carbon-rich molecules formed by the interaction of the Sun’s ultraviolet rays with methane, ethylene, and acetylene. The tholins form layers of haze in the atmosphere as they settle out. Hydrogen cyanide radiates out to the high atmosphere, cools, and shrinks the atmosphere. There seems to be little interaction between the atmosphere and the solar wind.

To keep up to date with new observations, visit New Horizons: NASA’s Mission to Pluto at http://pluto.jhuapl.edu/index.php . New articles about analyses of the New Horizons observations are forthcoming in Science. All images are courtesy of NASA and the information was drawn from Fran Bragenal’s presentation to Solar System Ambassdors “New Horizon’s Mission to Pluto,” February 26, 2016.