Charon Surface – Evolution of a Pale Grey World

Charon: Old Chap With a Red Cap

Charon Surface, explained!

There are quite interesting planets and moons in Solar System. Thousands of rocky objects, hundreds of kilometers wide.. And Beyond the Neptune orbit is like a catalog of similar rocks.
Since we have few knowledge about them, they are complete new worlds to explore for scientists.

That is valid for Charon too. There is a big world there: Another story to tell!

Charon Surface – Before and After New Horizons

We use those terms quite often.. Maybe we should abbreviate as BNH and ANH!

Charon Moon
Best view of Moon Charon, captured by New Horizons Spacecraft. Source: NASA

From discovery of Charon (1978) till closest approach of New Horizons Spacecraft to Pluto System (2015, BNH), Charon observations resulted with basic characteristic info like orbit, size, atmospheric status, surface composition and albedo measurements with some margin of error. Due to the huge distance from Earth based observatories, BNH noone knew past story of Charon and how she looked like.

While signing its name to reveal many Pluto facts, New Horizons Spacecraft did not hold back its cameras from Charon surface. Several different researches performed and articles published based on New Horizons images and calculations.

Beginning with second half of 2015 (ANH), we gained extremely detailed knowledge about Charon surface: Detailed enough to know its formation, past regeneration and surface features of the encountered hemisphere.

Charon Surface Features

As the lifetime partner of Pluto, moon Charon has always been a good company and a comrade. Big gravitational influence of Pluto, Charon’s own evolution and the chaotic environment in Kuiper Belt has been shaping Charon surface through billions of years..

Charon Map
In 2018, IAU announced a set of names for Charon’s surface features. Source: IAU

Charon Surface – Basic Surface Features

Thanks to New Horizons Mission, we have detailed maps: A map for Pluto and another map for Charon. Well, at least clear details for the encountered hemispheres.. And researchers investigated all interesting features on each surface.

Very similar to surface features of Pluto, on Charon surface there are big crater zones, plain terrains, rugged areas, mountanious fields, scarped and eroded areas mapped. In addition, signs of tectonism, cryovolcanism are clearly observed as well.

Mountains on Charon surface can go up to 3-4 kilometers while channels and chasms have depths in similar scales.

A close-up view from Charon surface. Credit: USRA

There is a strap of canyon clearly visible on the middle area, reclining all the way from east to west. This line can grow up to 50 kilometers in width and includes pits as deep as 7 kilometers.

We can easily divide Charon into two from this equatorial area: North and South. The reason researchers do such differentiation is not this natural line itself. It’s a consequence of basic form of surface features.

Mostly the northern parts include bumpy regions with lots of craters where south side is comparatively smooth. However the south side stands out with its geologic complexity, which will be described below in details.

Counting the amount of craters on Charon surface, New Horizons Team estimates more than 2.200 impact based craters are visible. The amount is less than half of the amount of craters on Pluto surface, which makes sense considering gravitational influences and size.

Craters on Charon Surface
Close-up view of several craters on Charon surface. This image reflects surface details as small as 800 meters. Source: NASA

Scientists believe those craters on both Charon and Pluto were formed during early stages of Solar System and Kuiper Belt. And the calculated size of early Kuiper Belt Objects crushing to those surfaces is between 300 meters to 40 kilometers in diameter.

Charon Surface – Temperature, Surface Composition and Color

We are talking about cold worlds.. Average temperature on Charon surface is around -220°C and polar regions (especially the north part) are even colder.

Which materials can stand out despite this cold weather?

Charon surface mostly consists of composite icy rocks dominated by water ice among with ammonia compunds (ammonia hydrate and ammonia salt) diffused within. The surface is basically made of cristalline water in powder form. However it will not feel like tough, icy but collapsable snow that you have stepped on Earth before. Because this one has more like a rocky form.

While Charon surface is mostly uniform in terms of color and albedo with a grey view in common, there is a visible red-brown cap with significantly less albedo on the northern side of the satellite.

If researchers are right, this part has an interesting story.

Charon North Pole
This is what you’d see if you flip Charon 90 degrees. The area on the right has nothing to be ashamed of: It’s just the non-encountered hemisphere during New Horizon Spacecraft’s closest approach. Source: NASA.

Charon’s Red Cap

There are complex organic molecules called tholins existing on Pluto surface. In order tholins to be formed and layered on the surface, a radiative process via solar winds should interact with the materials like methane. Analyzing and comparing the surface features between Pluto and Charon, researchers believe that same case should apply to Charon’s north pole as well. However, the story includes an appealing way of transportation.

Since Charon does not seem to have fountains made of methane to initiate such process, easiest conclusion is that some of the gaseous methane molecules escapes from Pluto atmosphere in time, Charon restrains them and those molecules are frozen on cold north pole area. When methane is there, it’s just a matter of time for the tholin formation.

What is more interesting, similar reddish formations are observed on smaller Pluto moons Hydra and Nix too. And their formation are expected to occur in the same manner.

Pluto Moons: Nix and Hydra. Source: NASA

OK, sounds strained.. However the researchers have already eliminated other possibilities and this is the best scenario to explain those formations.

One more thing to note down here is the distribution of tholins on Charon surface. Although north pole has high intensity in terms of tholins, researchers found that those tholins are distributed all over the surface! But the denseness on other areas is too less to form a visible and colorful layer.

Charon Surface – Activity and Regeneration

How can “tholins of the north” spread all over Charon surface? This is where time and surface activities take place.

Based on evidences, Charon surface is estimated to be extended by tectonism; especially during the early stages of formation. However, scientists believe “most” of the tectonic activities ended long time ago.

A close-by view of Charon surface in north-south section. Source: NASA

And one of the reasons could be an ocean: Frozen, melted and frozen again 2.5 to 0.5 billion years ago. Resaerchers think that the cracked strip in the equatorial area is a direct result of this oceanic cycle.

The surface on South of Charon, which is respectively smoother than the north part, is considered to be flattened by cryovolcanism. Cryovolcanic activity probably started after the ocean of Charon vanished. Researchers believe the powdery state of surface material is also supportive for smoothing the southern surface.

Click on following link for details of cryovolcanism on Pluto.

Some of the Charon mountains have interesting features: They are covered with trenches! Scientists believe this is a direct result of erosion caused by the flowing material from the tops.

Charon Mountain
In interesting fault plane: A big mountanin sitting in a trench. Source: NASA

According to researches among the craters, it’s calculated that majority of Charon surface is older than 4 billion years while there are rare newly shaped surface areas, which are estimated to be couple of hundred million years old.

As most of the crater surfaces are older than 4 billion years, that supports scientists’ theories about early Kuiper Belt War: Smaller rocks raining on bigger ones.

Beneath Charon Surface

Researchers believe the icy rocks in Kuiper Belt region retain the possibility to hide oceans beneath their icy crusts. While Pluto saves her chance to include one, simulations show that Charon had an underworld ocean some time ago. However it was frozen and unified with the crust. We have to settle for the Pluto ocean instead..

Charon hides a rocky core which is 55% of the complete body. Rest of it is the icy surface made of water. We are not quite sure what the rocky core is made of, however scientists believe the KBO cores include forms of silicates, sulfides and oxides.

Pluto and Charon
Adorable view of Charon and Pluto. Source: NASA

Charon Surface – Further Understanding

ANH, scientists are able to unterstand and estimate Charon’s formation and shaping process throughout years. And this is directly based on surface analysis.

For gathering detailed information in future, we should probably wait for a pioneer technology. We need spacecrafts which can travel billions of kilometers and can also be equipped with powerful drilling and analyzing machines. When that future comes, a new way of understanding for those icy objects will mirror the secrets behind our Solar System.

References

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    https://en.wikipedia.org/wiki/Charon_(moon)
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