Scientists have recreated conditions on the surface of Pluto's satellite, Charon, and designed the first model of its dynamic atmosphere.
Charon is one of Pluto's five satellites. It's so large that it makes sense to think of it as part of the Pluto-Haron dual system. In relation to its orbital plane, the system lies almost on its side, so the poles are entirely exposed to the sun's rays when the seasons change. But a "year" there lasts 248 terrestrial years, so the "first days of spring" in the hemispheres last for years. A new study has shown that strongest changes take place on Charon twice a "year" during equinoxes influenced by the sun's rays.
Charon's surface is covered by layers of frozen methane, nitrogen and possibly water ice. The sun's heat quickly evaporates the frozen methane to settle at the other pole. For this period - about four Earth years - the density of the atmosphere increases sharply by a factor of nearly a thousand.
The red spot at the pole was thought to be caused by the interaction of solar ultraviolet radiation with the "falling" methane. A new study has shown that the freezing process is too rapid to produce compounds which give off a red colour. The results are published in two papers in Science Advances and Geophysical Research Letters.
To understand what is happening, scientists have reproduced the surface conditions of Charon in the new Center for Laboratory Experiments in Astrophysics and Space Sciences (CLASSE) at the Southwest Research Institute (USA). The experiments in the ultra-high vacuum chamber made it possible to determine the composition and colour of hydrocarbons formed when methane freezes under ultraviolet light.
The research team developed a new model of the body's thin atmosphere to understand what is happening on the scale of Charon. This is how the "explosive" pulsation of the atmosphere due to the changing seasons became known.
By combining the results of ultra-realistic experiments with the new model, the scientists found that mostly ethane - a colourless compound that cannot be explained by the red colour - forms at the poles of Charon. But the authors put forward a new hypothesis:
"We think that, under the influence of ionising radiation from the solar wind, the polar ice frozen [under ultraviolet radiation] turns into the more complex, red compounds that give the mysterious satellite its unique albedo," explains Ujjwal Raut, lead author of the paper in Science Advances. - Ethane is less volatile than methane and remains frozen after the equinox. And over the following decades, it is the one that turns into reddish compounds when exposed to the solar wind. That is, this tint does not appear as a new layer freezes but when the old one is denuded.
Earthlings saw the surface of Charon for the first and so far only time during a flyby of the New Horizons interplanetary station in 2015. It was only then that scientists became aware of the red spot at the pole. Judging by the deep canyons stretching along the equator, Charon has had a turbulent geological past. Scientists are developing possible expeditions to Pluto and Charon, but none have yet been officially planned.
