Since NASA flew a spacecraft by Pluto in July, it has slowly released incredible images of the dwarf planet and its moons.
Some of those new images of Pluto’s largest moon, Charon, have revealed a giant, dark-red blob on its north pole. It’s so ominous-looking, scientists nicknamed the area “Mordor” for the dark, evil land in “Lord of the Rings”:
The question here is: What in the world made “Mordor” on Charon?
Parts of a planet or moon often look different than the rest of its surface, usually because some materials look wildly different as liquids or solids. For example, patches of ice on Earth look nothing like liquid water.
Scientists can rule this out on Charon though — the moon is simply too cold for liquids to hang around. In fact, the temperature at the poles hovers between minus-433 and minus-351 degrees Fahrenheit, planetary scientist Carly Howett wrote in a blog post for NASA. (That’s only “tens of degrees warmer” than the coldest temperature allowed by physics, she wrote.)
The only other explanation is that Charon’s north pole is made of an entirely different material than the rest of the moon.
This implies Charon is a thief: As Pluto slowly loses some of its thin atmosphere to space, Charon is grabbing the gases with its gravity. When the gases get close to Charon’s north pole, they instantly freeze into a solid — completely skipping the liquid state (again, because Charon is so ridiculously cold). Then, once Charon rotates and its pole faces the sun, the frozen gases get blasted with solar radiation.
The radiation might be enough to transform the stuff into entirely new types of materials called “tholins,” planetary scientist Sarah Hörst explains in a blog post for the Planetary Society. Tholins have the consistency of tar and are chemically different than what they’re made from. Many tholin compounds also happen to be dark red when dried out.
So, it’s possible Charon has built up a layer of tholins over millions of years by stealing Pluto’s atmosphere.
The colour of tholin depends on the molecules in the gas, the conditions of the area that it forms in, and the amount of radiation it’s exposed to. We know Pluto’s atmosphere is mostly made of nitrogen with some methane and carbon monoxide, but scientists need to learn more about Charon’s surface before we know for sure tholins are causing the red spot.
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