Since its discovery in 1930, Pluto has not failed to incite wonder over its bizarre and mysterious features. For one, it exhibits radically different terrain over its surface, with a mix of ancient and newly formed features. Its largest moon, Charon, was deemed geologically active until about 2 billion years ago when it “ran out of juice.”
The atmosphere of Pluto is an enigma in itself.
Before NASA’s New Horizons spacecraft’s flyby in July 2015, it was thought that the dwarf planet’s nitrogen-rich atmosphere was fizzing out into space at a 1027 molecules-per-second rate. However, data revealed that the actual atmospheric escape rate is four orders of magnitude lower due to a cooling effect in the atmosphere.
“The observed nitrogen opacity at high altitudes was lower than expected, which is consistent with a cold upper atmosphere. Such low temperatures imply an additional, but as yet unidentified, cooling agent,” wrote the authors in the journal Science.
On Earth, nitrogen as well as methane and carbon monoxide exist as in the form of gases, while on Pluto the temperatures drop so low that the molecules freeze into solid form. A separate study in Science, for instance, reported up to 3-mile-high water ice mountains and proof of nitrogen-made active glaciers carving expansive troughs across the planet surface.
What could possibly explain this mystery?
One explanation: a thick layer of haze particles resembling smog serve as coolants, then absorbing and emitting energy from the sun that would otherwise heat up nitrogen in the atmosphere.
Another is the presence of hydrogen cyanide, which acts as an efficient coolant. It was recently found in the dwarf planet's atmosphere by the Atacama Large Millimeter/submillimeter Array.
But regardless of the operating mechanism, this cold atmosphere could help explain why Pluto kept having features such as the nitrogen-rich pool of ice Sputnik Planum at its heart. In addition, the cold, dense atmosphere also lends insight into why a shockingly small amount of it is colliding with solar wind.
The New Horizons spacecraft helped scientists make these puzzling discoveries as it made its closest approach to Pluto back on July 14, taking high-res images of its surface and observing its different satellites, atmosphere, and solar-wind interaction.
At present, the spacecraft has beamed a mere 40 percent of the total data back to Earth, but for scientists it’s more than enough to see how Pluto defies a number of scientific explanations and expectations.