Charon is the largest of Pluto’s five moons. NASA’s New Horizons spacecraft first saw this fascinating world up close when it flew past Pluto in 2015. The moon has craters, deep valleys and a strange, isolated mountain that appears to lie at the inside a ditch. And there’s another strange feature on Charon: a large red “cap” at the moon’s North Pole. It’s reminiscent of an ice cap, except it’s, well, red. On June 21, 2022, scientists at the Southwest Research Institute (SwRI) in San Antonio, Texas said they believe they know how it formed.
Basically, they explain how a combination of seasonal “atmospheric surges” in Charon’s very thin atmosphere, ultraviolet radiation, and methane form the distinctive red mark.
The researchers have detailed their findings in two new papers. They published the first article in the peer-reviewed journal Geophysical Research Letters (GRL) on April 15, 2022. And they published the second on June 17, 2022 in the peer-reviewed journal Scientists progress.
Charon’s red cap
Images from New Horizons have revealed the confusing ceiling for the first time. It looked rather unusual on the otherwise mostly gray surface. Randy Gladstone, New Horizons Science Team Member, said:
Prior to New Horizons, the best Hubble images of Pluto revealed only a fuzzy blob of reflected light. In addition to all the fascinating features discovered on Pluto’s surface, the flyby revealed an unusual feature on Charon, a startling red cap centered on its North Pole.
So what caused it? New Horizons data showed it was likely composed of reddish tholins. Tholins are sticky organic residues formed by chemical reactions fueled by light. Scientists speculated that it was the result of methane molecules being broken down by ultraviolet light from the sun. The molecules originate from Pluto and freeze to the surface of Charon at the moon’s North Pole. In the case of Charon, the light is the ultraviolet glow of the Lyman-alpha line scattered by interplanetary hydrogen atoms.
Seasonal Surges in the Atmosphere Create Charon’s Red Cap
Another key factor in the formation of the ceiling is Charon’s atmosphere. It is extremely thin, but still experiences seasonal changes. As described in the GRL article:
Charon’s exosphere can exhibit extreme seasonal dynamics, with centuries of quiescence punctuated by short-lived (about four Earth years) exospheric surges near the equinoxes, as the spring sunrise twice a year flushes frozen methane polar night zones.
Tholins freeze to the surface of Charon during the long winter nights. Ujjwal Raut, lead author of the Scientists progress paper, says:
Our findings indicate that drastic seasonal surges in Charon’s thin atmosphere as well as light breaking down the condensed methane gel are key to understanding the origins of Charon’s red polar zone. It is one of the most illustrative and striking examples of surface-atmosphere interactions observed so far on a planetary body.
Replicating conditions on Charon
To determine how the red cap likely formed, the researchers replicated the conditions found on Charon’s surface in a lab. They conducted the experiments at SwRI’s new Center for Astrophysics and Space Science Experiments (CLASSE). The main objective of the researchers was to measure the composition and color of hydrocarbons produced on Charon’s winter hemisphere when methane freezes under the Lyman-alpha glow. The researchers used a new atmospheric model of Charon to analyze the measurements.
Indeed, they found evidence that methane was turning into a tholin-like residue at the North Pole of Charon. As Raut explained:
Our team’s new “dynamic photolysis” experiments have provided new limits to the contribution of interplanetary Lyman-alpha to the synthesis of Charon’s red material. Our experiment condensed methane in an ultra-high vacuum chamber under exposure to Lyman-alpha photons to reproduce with high fidelity the conditions at the poles of Charon.
Charon’s thin methane atmosphere
In another model, the researchers simulated the moon’s thin methane atmosphere. This is how seasonal surges were discovered. According to Ben Teolis, lead author of the GRL article:
The model indicates “explosive” seasonal pulsations in Charon’s atmosphere due to extreme changes in conditions during Pluto’s long journey around the sun.
Methane and ethane
As methane deposits break down at Charon’s North Pole, they release ethane as a byproduct. Ethane itself is colorless, but scientists say that on Charon, ionizing radiation from the solar wind turns it reddish. Ethane also stays frozen longer than methane. Rault said:
We believe that ionizing radiation from the solar wind breaks down Lyman-alpha baked polar frost to synthesize increasingly complex and redder materials responsible for the unique albedo on this enigmatic moon. Ethane is less volatile than methane and remains frozen on Charon’s surface long after spring sunrise. Exposure to solar wind can convert ethane into persistent reddish surface deposits contributing to Charon’s red cap.
Excerpt from the GRL article:
Ethane, the main photoproduct of methane under these conditions, can, unlike methane, remain frozen decades after polar sunrise under exposure to the solar wind. Solar wind radiolysis of polar ethane frost synthesizes higher-order refractories that may contribute to the coloration of Charon’s polar zones.
Scientists have, it seems, solved the mystery of Charon’s brilliant polar cap. This weird mountain in a ditch still needs an explanation, though.
Bottom line: A strange reddish polar cap on Pluto’s moon Charon has puzzled scientists since it was first seen in 2015. Now researchers may have solved the mystery of Pluto’s red cap. Charon.
Source: Extreme exospheric dynamics at Charon: implications for the red spot
Source: Charon Refractory Factory