Was Charon’s red cap formed by cryovolcanoes?
The mystery of Charon’s red cap
Pluto’s largest moon Charon is similar to many other small rocky moons, with valleys and many craters dotting its gray surface. It does, however, have one unique feature that stands out. There is a large reddish patch at its North Pole that almost looks like dried paint, or even blood. NASA’s New Horizons spacecraft discovered the intriguing feature when it passed by Pluto and its moons in 2015. Scientists know the patch consists of organic compounds called tholins, but there is still debate as to how it got there. In early August, researchers at Purdue University published a new theory that suggests that ancient cryovolcanoes – ice volcanoes – on Charon are the cause.
Kiona Smith wrote about the findings in Inverse on September 6, 2022.
Scientists named the reddish region Mordor Macula, after the black land that J.R.R. Tolkien called Mordor in The Lord of the Rings. Although in this case, it’s red, not black. It kind of looks like a red ice cap at the moon’s North Pole, but it’s not ice. Rather, it is composed of organic tholins. Tholins are formed by solar ultraviolet or cosmic ray irradiation of simple carbon-containing molecules such as carbon dioxide, methane or ethane. They are quite common on icy bodies in the outer solar system. They can also be found as aerosols in the atmospheres of outer solar system planets and moons. In this case, the molecules would be methane.
As the paper notes, there may also be a similar feature at the Charon’s South Pole.
Last June, EarthSky reported on the leading hypothesis that the methane molecules originate from Pluto and freeze on Charon’s surface at the North Pole. Seasonal surges in Charon’s very thin methane atmosphere also play a role.
Menten and her colleagues, however, now propose a different scenario. Charon has a vast smooth ice field in its Southern Hemisphere called Vulcan Planitia (or Vulcan Planum). Ancient cryovolcanic eruptions are thought to have created that ice field. The researchers wanted to know how much methane those eruptions might have released. As Inverse quoted Menten:
The gas particles will end up hopping across Charon’s surface, and there’s a set of equations that you can use to track, randomly, what will happen to these methane particles.
The paper states:
Here, we hypothesize an endogenic source for the volatiles on these objects, including Charon, and instead propose that Charon’s widespread cryovolcanic resurfacing erupted large amounts of methane, some of which migrated to the poles and were processed into the tholins observed by New Horizons.
Calculating movement of methane particles
The researchers modeled the probable dispersal of 1,000 individual methane molecules. Their calculations showed that indeed, enough methane could have been released during that time to form the reddish cap. The methane molecules found their way to cold traps at Charon’s poles to leave a layer of frozen methane. The scientists estimate that layer could be nine meters deep in the region now known as Mordor Macula.
Just as in the tholins from Pluto scenario, solar radiation and cosmic rays then break down the top layer of methane. That process converts them into darker-colored tholins. The only real difference is that the methane molecules originated from Charon’s cryovolcanoes, not Pluto.
How did eruptions create Charon’s red cap?
So, if there were once cryovolcanic eruptions on Charon, how did they happen and how did they create the red cap? As the researchers say, it had to do with Charon’s former ocean. Scientists think that the moon once had a subsurface ocean, just like some of the other icy moons in the outer solar system (and Pluto may still have one). However, scientists think the ocean froze about 4 billion years ago.
As we know, water expands when it freezes. This means that the subsurface ice would likely have pushed its way through Charon’s crust. The eruptions occurred when ice broke through the crust and onto the surface. As Inverse quoted Menten:
The ocean that was left, which is probably like a lot less than what was there to begin with, was pretty pressurized [by the expanding ice]. So, because it was pressurized, there were fractures all the way up to the surface. You can have an eruption that happens.
Indeed, that expanding ice may have created large fractures on Charon’s surface that we see today.
Eruptions of ice, not lava
These eruptions aren’t like volcanic eruptions as we think of them on Earth, however. They are eruptions of ice, not hot lava. On Charon, the “lava” would have been a thick, slow-moving, icy fluid. The researchers compare them to Icelandic eruptions on Earth. They estimate that this period of cryovolcanism lasted about one million years. Much of the outpouring ice covered Charon’s Southern Hemisphere in a kilometer-thick ice sheet (Vulcan Planitia).
It still isn’t known for certain that the venting ice actually contained methane. But if it did, the researchers estimate that the eruptions would have released about 100 billion metric tons of the gas. According to the calculations, by far most of the methane should have reached the poles of Charon, about 97 percent.
It is quite likely that Charon did have methane in its interior, though, since Pluto does. And scientists think that Pluto and Charon formed out of the same cloud of primordial material.
Cryovolcanism common in the outer solar system?
The findings also support the possibility that cryovolcanism is common on small icy bodies in the outer solar system. Menten said:
We can tell from telescopic observations from Earth that a lot of them have methane or methane products detected on their surfaces. But not a lot has been proposed for exactly where the methane comes from. It could be that cryovolcanism is a pretty common process on these bodies, and it could be the thing that’s actually providing this methane that we’re detecting from Earth-based observations. It could be that cryovolcanism is a pretty common process on these bodies, and it could be the thing that’s actually providing this methane that we’re detecting from Earth-based observations.
So, just as “hot” volcanism has been common on rocky planets closer to the sun like Mars, Earth and Venus, “cold” volcanism may also be a frequent occurrence on colder worlds farther out.
More observations needed of Charon’s red cap
More observations of Charon’s red cap will be needed to fully understood how it formed. As noted in the Inverse article, tholins darken over time. If Mordor Macula was about 2 billion years old, then it should appear black by now, just like in The Lord of the Rings. But it’s not, it’s still red. One possible answer is that meteorites gradually erode the topmost layer of the tholins. If so, they could expose redder terrain beneath. Or, the methane gas coming from Pluto may replenish the methane particles at Charon’s North Pole. In that case, both the previous hypothesis and this new one may be correct.
For the next step, Menten and her team will study Charon’s geological past for more clues. Menten said:
This modeling and geological analysis project was about the last step of what happened [with cryovolcanism] on Charon. But I think next, what we’re going to be focusing on is what happens internally for Charon, to see exactly how you can generate enough pressure to start actually cracking the surface.
Tholins elsewhere in the solar system
Learning about tholins on Charon will also help scientists better understand other small bodies in the outer solar system that also have them. These include Makemake, Sedna and others in the Kuiper Belt. These kinds of bodies are also often red in color, just like Charon’s North Pole. Sedna in particular appears to be completely dark red in color.
Some of these small worlds may still have internal oceans while other may show evidence of cryovolcanism, as the paper mentioned:
Future spacecraft visiting bodies like Makemake or Sedna may observe evidence of widespread cryovolcanism across their surfaces in the form of smooth, lobate plains similar to the smooth plains of Vulcan Planitia observed on Charon, whereas objects like Eris may be found to be more Pluto-like with the possible existence of present-day oceans and no extensive volcanic plains.
Bottom line: Why does Pluto’s moon Charon look red on top? A new study from researchers at Purdue University suggests that cryovolcanic eruptions created Charon’s red cap.