The interplanetary space probe NASA New Horizons was launched in 2006. Thanks to it, researchers were able to get an unobstructed view of the dwarf planetary system Pluto and Charon, at a distance of more than 5 billion kilometers from the Sun. In 2015, she photographed a “cap” red on Charon, largest of Pluto’s moons.
Intrigued by the planetary processes responsible for such a result, the researchers initially thought that the red spot, dubbed Macula Mordor, was methane captured on the surface of Pluto. This red color would result from a slow cooking in the ultraviolet light (UV) of the Sun.
Where does this red cap come from?
This red color is common for iron-rich planets such as Earth or Mars. On the other hand, deep in the solar system, this color could indicate the presence of a diverse group of tar-like compounds called tholins. To transform into tholins, methane would have to absorb a very specific color of UV light filtered by orbiting hydrogen clouds, called Lyman-alpha.
The methane released by Pluto could drift to its orbiting moon, researchers say. Nevertheless, the time required for the gas to settle and freeze in this red spot is still unknown. There is also the spring dawn, even a very faint one, which would be enough to melt the methane frost, driving it off the surface again.
While modeling the movement of the planetary system, the researchers discovered that melting ice could be the cause of the arrival of spring. The sudden warming of the North Pole would occur over several years. During this period, a thin layer of methane frost would evaporate at one pole, while the other pole would begin to freeze.
Surprising new information
The modeling results suggest that this movement would be too fast to allow much of the frozen methane to absorb sufficient amounts of Lyman-alpha to transform into a tholin. Therefore, researchers have oriented their study towards ethane.
The results of laboratory experiments by planetary scientist Ujjwal Raut and his team have shown that it is possible to transform methane into ethane at the poles. However, Lyman-alpha radiation lacked the ability to turn ethane into a reddish sludge.
Charged particles from the Sun over a long period could generate increasingly long chains of hydrocarbons. This would give Charon his characteristic red cap.