Why Pluto is a frigid prison for nitrogen ice By Eric HandMar. 17, 2016 , 2:00 PM

All planets lose a small portion of their atmospheres to space, if individual gas molecules get hot enough and reach escape velocity. Before NASA’s New Horizons spacecraft swooped past Pluto last July, scientists thought that the dwarf planet’s nitrogen-rich atmosphere was eroding into space at a rate of 1027 molecules per second. But data from the encounter show that the atmospheric escape rate is in fact four orders of magnitude lower, because of a cooling effect high in its atmosphere, scientists report today in Science. One explanation is that a surprisingly thick layer of smoglike haze particles (bands shown above) act as coolants, absorbing and emitting solar energy that would otherwise heat up nitrogen gas molecules in the atmosphere. Another possible explanation is hydrogen cyanide, an efficient coolant that was recently detected in Pluto’s atmosphere by the Atacama Large Millimeter/submillimeter Array in Chile. Regardless of the mechanism, the cold, dense atmosphere could help explain why Pluto has held onto features like the Sputnik Planum, the pool of mostly nitrogen ice at Pluto’s “heart.” If the newly calculated escape rate has held steady over the solar system’s 4.56-billion-year history, Pluto would have lost the equivalent of only 6 centimeters of nitrogen ice—although the rate may have been higher in the past because of variations in Pluto’s orbit and tilt. The cold, compact atmosphere also explains why a surprisingly small amount of it collides with the solar wind, according to another study published today in Science. In addition to the studies of the atmosphere and the near-space environment, three other studies are published today in Science. One focuses on Pluto’s geological features, including the idea that two mountains are actually cryovolcanoes that once spewed ice. Another focuses on the chemical compositions of Pluto and its largest moon, Charon. And a third paper examines Pluto’s four smaller, irregularly shaped moons: Styx, Nix, Kerberos, and Hydra.