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SYFY WIRE Bad Astronomy

Pluto’s Moon Charon Has an Ammonia Leak

By Phil Plait

It turns out, Luke and Leia really were different, despite being fraternal twins. Not so much in Star Wars, but in real life.

Well, on Charon at least. OK, let me explain.

Pluto’s big moon Charon is a mess. I mean, literally; in the New Horizons spacecraft’s photos it looks like someone took a bunch of moon puzzle pieces and forced them together whether they fit or not. Despite that, a lot of features on it do make some sense. For example, a lot of the smaller (say, 5-kilometer size) craters give every indication that the surface and subsurface at the impact site is mostly water ice.

But astronomers got a surprise when they looked at maps of Charon made in the infrared. Ammonia is very efficient at absorbing infrared light at a wavelength of 2.2 microns (about three times the wavelength of the reddest light your eye can see). By looking at Charon at that wavelength, ammonia on the surface shows up as dark spots. We’ve known there’s ammonia on the surface of the moon for more than a decade, but these maps pinpoint it.

The surprise came when they looked at an area near the north pole of Charon: Two small similarly sized craters had been previously mapped in high resolution by the optical camera on New Horizons. Informally, features on the moon are named after people and places in science fiction, and these twin craters were named Organa (after Princess Leia) and Skywalker (after Luke; by the way, a nearby crater is named Vader).

Skywalker looks like a normal enough crater, but Organa is very rich in ammonia! That’s interesting; they’re only about 100 kilometers apart, and visually look very much alike. So why is the ammonia so strong with Organa?

Visible-light image with the infrared ammonia map (shown in green) superposed. Clearly, Organa is loaded with ammonia while Skywalker is not. Credit: NASA/JHUAPL/SwRI

There are three possibilities. One is that whatever impacted to create Organa crater was itself rich in ammonia. Another is that there’s ammonia under the surface near Organa that got excavated by the impact. A third is that the two craters are vastly different ages; over time ammonia degrades on the surface, so perhaps Organa is much younger than Skywalker (though that seems unlikely; both are relatively young, showing bright rays of material splashed around them that tend to fade with time).

I wonder, though: If you take a step back, you see that both craters are near the weird red region at Charon’s north pole (nicknamed Mordor). But note that while Skywalker is well outside that region, Organa is right on the edge and has a reddish tint itself. When you see two odd things going on at the same time, it’s very tempting to link them. You have to be careful, but it’s worth sniffing out.

The red pole of Charon is probably due to the presence of tholins, complex molecules created when ultraviolet light from the Sun breaks down simple molecules like methane, ethane, and ammonia, and they recombine into bigger molecules (read more about this in a blog post by my pal Sarah Hörst). There’s an idea that somehow Pluto’s thin atmosphere is (or was) escaping and collecting on Charon. The tholins are created out of that raw material.

So maybe this is all tied together, and Organa is just revealing an ammonia-rich subsurface region, while Skywalker is farther away and over different material.

But here’s an added bonus if Organa and the red pole are related: Tholins are carbon--based, so technically they are termed organic molecules. Seriously. The force is strong with this idea.

Or, don’t forget, this may be a total coincidence and something else entirely is happening here. We just don’t know enough about what’s going on with Pluto and Charon to be sure of much yet. There are always new ideas, and if you think one particular idea is our last hope, you can rest assured there is another.

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