The validity of the saying “All cats are gray at night” can be confirmed by astronomers, especially when studying celestial bodies in the outer solar system. They are all happier when they discover that the places they crave are more colorful than they thought.
When American astronomer James W. Christy accidentally discovered Charon, the largest moon on the dwarf planet Pluto, in 1978, all he saw in the image was a gray bump on the edge of a dim light. Not until the NASA spacecraft New Horizons When it flew next to Charon in 2015 at a distance of almost 30,000 kilometers, astronomers discovered a clear red drop in the north polar region, a little fainter but visually comparable to a red spot that Jupiter also has. However, Charon’s cute sign has a completely different cause.
Scientists quickly speculated that the stain may be a tholine-like substance. After that, a brown-red mixture of hydrocarbons would cover a plain nearly 500 kilometers wide at the North Pole – informally called “Mordor Macula” by astronomers, from the dark realm to JRR Tolkien’s “Lord of the Rings”.
It is true that this hypothesis has not yet been conclusively proven. Charon and Pluto are currently about five billion kilometers away from us and New Horizons was the only visitor so far. However, in two related studies, scientists have now experimentally reconstructed and theoretically modeled atmospheric conditions on Charon. “Until now, no one has been able to directly detect the composition of such halos,” said Ujjwal Raut of the Southwest Research Institute in Texas, who led and co-authored both studies. “However, our experiments strongly suggest that complex hydrocarbons can be formed from methane ice through energy processes.”
The required methane does not come from Charon, but from neighboring Pluto. The moon, 2,300 kilometers in size, and its main body, which is only twice as large, form a system that orbits each other for less than 20,000 kilometers. So Charon continues to fly through a cloud of methane fired by Pluto. Charon’s gravity traps many gas molecules, which then bounce off its surface until they are caught in the extreme cold of the polar plains, the study authors said. Since the Charon-Pluto system takes 248 years to complete an orbit around the sun and also leans sharply out of the orbital plane, Charon, like the Earth, has separate eras. Temperatures in the deep cold traps can drop to less than minus 260 degrees Celsius during Charon’s 124-year winter. Therefore, even the leaking methane settles there as ice until it melts in early Charon’s spring and continues its journey to the surface the following winter.
However, a small portion of methane ice is decomposed photolytically by ultraviolet radiation and converted to other substances, mainly slightly larger and longer-lasting ethane molecules. However – even ethane ice is transparent. “Photolysis of methane alone can not explain the color red,” says Raut. “Therefore, the solar wind will also have to process ethane to produce larger, reddish hydrocarbons. But we have not yet found out exactly what these molecules are.”
Such planetary gas transports are unique to the solar system, Raut says, because no other planetary celestial body is so close to Pluto and Charon. The freezing processes at the poles cannot be compared to those on Earth or Mars. “We estimate that about 80 individual layers of photoproducts accumulate at the poles in a year of Pluto,” says Raut. “Over millions of years, they are likely to grow to many microns in thickness.” For comparison: the foil is about 15 microns thick.