Recently, NASA released a new image from last summer’s close approach of the Pluto system by New Horizons. It isn’t a jaw-dropping image we are accustomed to, but it does give scientists a new look at Pluto’s largest moon Charon. The dark side.

Charon dark side

A small sliver on Charon is lit by the sun while the rest basks in light reflecting off Pluto. We see a similar phenomenon on Earth as a new moon approaches. For us, it’s called ‘Earthshine.’


Charon’s dark side might not look like much, but the New Horizons team is using this image (and others like it) to map portions of the far side of Charon. Scientists hope to learn more about Charon’s south pole, seen at the top of this image.

Unfortunately, New Horizons was about 26 years too late to see Charon’s south pole clearly. In 1989, Charon’s south pole entered polar night. And, it’s going to be a long one. The next time sunlight will hit that part of the surface again? 2107. The temperatures during this 100+ year span hover near absolute zero (-459 degrees Fahrenheit, -273 degrees Celsius).

You may not have known about Charon’s south pole being in polar winter, but you saw it in earlier images of Charon. Check it out.

charon red patch

The south pole (bottom of this image) is completely engulfed in darkness while the north pole dominates with a large reddish patch.

Five Facts about Charon

1. Charon is the largest moon relative to its planet in the entire solar system. With a diameter of 1,212 kilometers, Charon is just half the diameter of Pluto.

Pluto’s Sky May Have Clouds and What It Means

2. Parts of Charon’s surface appear to be much younger than other regions. There are fewer large craters in the plains south of Charon’s canyon.

Charon canyons and plains

3. It could be cryovolcanism. That’s one of the possibilities New Horizons scientists are entertaining. “The team is discussing the possibility that an internal water ocean could have frozen long ago, and the resulting volume change could have led to Charon cracking open, allowing water-based lavas to reach the surface at that time,” said Paul Schenk, a New Horizons team member.

4. Charon’s canyon system is huge. It stretches more than 1,000 miles across Charon and scienitsts believe it likely continues on its far side. It dwarfs the Grand Canyon at more than four times long and twice as deep in spots.

5. Charon has a reddish patch at its north pole. I’ll let Carly Howett from the Southwest Research Institute explain: “One theory is that small amounts of Pluto’s atmosphere can escape and eventually reach Charon, where it would be temporarily trapped by Charon’s gravity before escaping to space.” Howett adds, “any gases that arrive on the winter pole would freeze solid instead of escaping, a process scientists refer to as “cold trapping.”

The key to this theory is what happens when sunlight hits the ice. Solar radiation modifies the ice and produces a new substance called tholins (which have a higher sublimation temperature and can’t escape from Charon). Depending on the radiation, molecules and conditions it forms in tholins can range in color from yellow to red to black. In Charon’s case, it’s red.


New Horizons continues to downlink data from its close encounter with Pluto and its moons. It’s just a shame that New Horizons couldn’t enter orbit and study Pluto and all of its moons even closer. I would have loved to see more of Styx, Nix, Hydra and Kerberos.

Charon’s Glaring Red Feature Explained

Today, New Horizons speeds towards a new target. A Kuiper belt object called 2014 MU69. If NASA gives approval, and everything goes well with New Horizons – we’ll see another incredible close encounter in January 2019.

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