Pluto’s moon Charon sports a striking reddish “cap” of organic material at its north pole. Planetary researchers have now found out more about how these tholin deposits formed. Accordingly, they are the result of a complex chain of reactions, at the beginning of which there is methane blown over from Pluto. This freezes out in the polar winter of Charon and is converted into ethane by cosmic UV radiation. In the polar summer, the remaining methane gass out again, but the ethane remains and reacts under the influence of the solar wind to form increasingly complex organic molecules. The result is the reddish tholin deposits.
The dwarf planet Pluto and its largest moon Charon form an exceptional duo in the solar system. Because they are so similar in size and behavior that they almost resemble a double planet. Both celestial bodies are also only 17 plutorarades apart and always face each other on the same side. There is one important difference, however: while Pluto has an extensive atmosphere rich in nitrogen and methane, Charon’s gas envelope is extremely thin and barely detectable. Planetary researchers were all the more surprised when the first images taken by NASA’s New Horizons spacecraft in the summer of 2015 revealed the existence of a strikingly red polar cap on Pluto’s moon Charon. “It was just a fuzzy patch of reflected light visible in front of New Horizons,” explains Randall Gladstone of the Southwest Research Institute in Texas. He is the co-author of two specialist articles that deal with the formation of Charon’s red polar cap.
Charon’s chemistry brought to Earth
The reddish-hued polar cap covers the area north of the 70th parallel of Charon and has been informally named Mordor Macula. The reddish color of the polar deposits led researchers to suspect that they might be tholins, organic molecules formed when hydrocarbons such as methane and ethane are photodegraded by ultraviolet radiation. Such tholins give the dense atmosphere of Saturn’s moon Titan its orange-red hue, among other things. In 2016, planetary researchers determined using data from the New Horizons space probe and model simulations that the Pluto moon Charon could get the raw material for the tholin from its neighbor Pluto: The dwarf planet constantly loses methane to space through outgassing, parts of which then get into Charon’s gravitational field and freezes there on its night side. According to the calculations, around 27 billion methane molecules arrive at the Pluto moon per second and square meter.
Using laboratory experiments and model simulations, Gladstone and his colleagues have now used laboratory experiments and model simulations to take a closer look at how the methane will continue to develop on Charon. To do this, they simulated the surface conditions on the Pluto moon in a special vacuum chamber and observed what happens in the interplanetary space under the influence of the seasons and the UV radiation of excited hydrogen molecules, the so-called Lyman alpha radiation. “Our photolysis experiments provided indications of limitations for the synthesis of Charon’s red material,” reports Gladstone’s colleague Ujjwal Raut. Because, as has been shown, there is a kind of hemisphere change of methane, especially during the short transitions between summer and winter: The methane ice, which was previously mainly deposited at the South Pole, sublimes under the increasing solar radiation there and creates a for a short transition period of little more than a year temporary, thin methane atmosphere around Pluto’s moon. However, the methane quickly freezes out again over the now dark northern polar region.
Two-step conversion
“These drastic seasonal upheavals in Charon’s thin atmosphere are key to understanding the formation of Charon’s red polar zone,” says Raut. Because, as the investigations have shown, a layer of methane ice several dozen micrometers thick is depleted at the North Pole as a result of the freezing of the methane in the autumn. Additional micrometers are added during the roughly 100-year long winter. The problem with this is that this layer of methane is too thick to be converted into tholin by the diffuse UV radiation from space. Because even before the multi-step chemical process can take place, the top layer is covered by new methane and thus removed from the influence of radiation, as the team explains. However, their experiments and models indicate that there is sufficient time during the northern winter on Charon for the methane ice to convert to colorless ethane.
Contrary to previous assumptions, the decisive step in the formation of the red tholins only takes place when the polar region of Charon is illuminated by the sun again: The remaining methane then evaporates again, but not the ethane: “Ethane is less volatile than methane and remains therefore also frozen on the Charon surface long after the spring sunrise,” explains Raut. As a result, this ethane now comes under the influence of the charged particles of the solar wind – and these set the further chemical transformations in motion: “During the roughly 30 years of the polar summer, the solar wind can cause the ethane ice previously produced by the Lyman-alpha radiation to react and produce the complex molecular structures that produce the red color,” the researchers explain. The red polar cap of the Pluto moon is therefore the product of two transformation steps that take place at different times.
Source: Kurt Retherford (Southwest Research Institute, San Antonio) et al., Geophysical Research Letters, doi: 10.1029/2021GL097580; science advances, doi: 10.1126/sciadv.abq5701