Announcing Charon’s Dark Pole

What’s Causing THAT?

As the New Horizons spacecraft gets closer to Pluto, we are seeing more detailed images of the planet and its companion, Charon. The latest ones, taken with the LORRI instrument onboard the spacecraft, show what looks like a darkened pole on Charon, a somewhat lighter region below it, and some bright regions along the limb. “Fascinating” as Mr. Spock would say. But, what is happening at Charon to make it look like that? The final answer is a couple of weeks away, so let’s talk about how we figure out what’s happening at a planet from images of it.

As in all other aspects of planetary science, you have to look for processes on the world you’re studying to understand how they affect the surface of that place. For example, if you were approaching Earth and were still quite a ways away as you came in to assume a standard orbit (Mr. Sulu), you’d likely notice the poles, the bluish color, and the darker areas that indicate land masses. The existence of ice at the poles tells you something about the climate and temperature in those regions. The bluish water in a liquid state tells you that conditions are good enough to permit liquid water. And, the land masses have many messages of their own, from the signatures of volcanoes to the ongoing (and long-term) deformation of the surface due to plate tectonics. What you see on Earth, even at the most cursory level — and at Pluto and Charon — are all caused by complex interactions comprising chemical reactions, atmospheric mixing, and actions going on below the surface.

So, with that in mind, what’s going on at Charon? I wish I could tell you for sure. But, it looks really, really interesting! Now that we’re seeing a great variety of surface features (or, as the scientists call it, “terrain types”) it’s a hint that Charon is not just a frozen dead world. A dark terrain could indicate some sort of chemical interaction as sunlight hits specific ices on the surface. That normally happens with methane-rich ice, which Charon doesn’t appear to have much (if any) of. Instead, it has been measured to be mostly water and nitrogen ice.

However, I suspect there’s more going on at Charon than meets the eye.

A few years ago, we created a graphic “approximation” of Charon for a project with Gemini Observatory. We had to guess at what the surface looked like. You can see that, even in 2007, we had an idea that there’d be darker areas on an already darkish object. The real interesting bit was that astronomers using Gemini telescope had spotted what looked like evidence for geyser-like activity on Charon. I will be really interested to see if New Horizons finds that same evidence and confirms such activity. If it does, then we’re looking at a dynamic world with an interior that is forcing mixtures of ammonia hydrates (ammonia mixed with water) and water crystals onto the surface.  And, THAT’s cool.

I don’t know why Charon has a dark pole, yet. I suspect that the New Horizons mission team members don’t YET know for sure. They’re likely going through all the ideas en masse, and once they have more data, we’ll all know what’s ticking inside this little world.