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Pluto looks as if it’s haunted. That ghostly blue ring around it could conjure up visions of disembodied spirits floating in space, but the reality is possibly even spookier.
Many planets and moons, including Earth, are shrouded by hazes, but Pluto’s is by far the most bizarre one yet. Even NASA calls it a “photochemical smog”. New research suggests the haze is made up of ice crystals with cyanide lurking inside. You really can’t make this stuff up. The researchers believe sunlight starts chemical reactions in Pluto’s upper atmosphere, forming molecules like hydrogen cyanide—which is, by the way, extremely poisonous—along with acetylene and ethylene. This is not something anyone would want to breathe in.
“Solar radiation can break these molecules and reactions among the fragments are the starting point of a complex organic chemistry,” planetary scientist Panayotis Lavvas, who recently led a study published in Nature Astronomy, told SYFY WIRE. “One typical product of this photochemistry is hydrogen cyanide.”
The newly formed molecules in Pluto’s atmosphere then freeze into tiny ice particles that scatter sunlight to make it appear an otherworldly blue. As gravity sinks them, other gases condense around them and form the haze.
The haze on Saturn’s moon Titan, which was observed by the Cassini-Huygens mission, was often compared that of Pluto and Neptune’s moon Triton (below). What the new research also suggests that there are drastic differences in how Titan’s haze emerged. Pluto and Triton were investigated by New Horizons. Using Cassini and New Horizons data, computer models revealed that if Titan experienced similar chemical reactions to those that happened on Pluto, it would only end up with about half of Pluto’s haze.
“This difference is due to the atmospheric temperature of Pluto being significantly colder than that of Titan,” Lavvas said. “Titan’s atmosphere particles are formed from the organic chemistry through the formation of large molecules. In Pluto’s atmosphere, there is also an active organic chemistry, but the organic molecules formed can condense before reaching to the large molecular size observed on Titan.”
Another difference between Pluto and Titan is that the haze of Pluto is made up of organic ices, while Titan’s is full of polycyclic aromatic hydrocarbons, or PAHs. These are the large molecules Lavvas mentioned, which are carbon-based and form from smaller hydrocarbons that cluster together.
It doesn't stop there. PAHs can go on and aggregate into even larger molecules that become too heavy to stay buoyant in the upper atmosphere, so they sink into the lower layers, where they can grow even more because of the higher density there. This results in carbon-based aerosols in Titan’s lower atmosphere.
The organic particles in Pluto’s haze also interact less with solar energy that streams in from a distance. Titan’s haze has an easier time of interacting with energy from the Sun. What Pluto, Triton and Titan do have in common are their atmospheric chemical compositions. They all have atmospheres made of mostly nitrogen dioxide, methane and carbon monoxide (another infamously toxic gas).
Because Pluto is brutally cold, anything that can freeze, or at least condense, will. Its average temperature is -269 degrees Fahrenheit when it orbits closest to the sun and -387 degrees Fahrenheit when it strays the furthest. This explains why the hydrogen cyanide in its atmosphere condenses. Its saturated upper atmosphere just means it will take forever for all the hydrogen cyanide molecules to condense. The molecules have a much easier time condensing in the lower atmosphere, where other organic gases like diacetylene (C4H2) also take less time to freeze.
While Pluto’s haze is not meant for human consumption, it could tell us more about what kind of haze could be surrounding Triton.
“Applying the theory developed for Pluto's haze to Triton’s case, we found that organic ices can also explain the Triton haze observations from the Voyager II mission,” Lavvas asid. “The dominant organic ice component in this case is ethylene (C2H4).”
At least this toxic no-man’s land is the last place anyone is thinking of sending astronauts. Mars is dangerous enough.