Scientists at the Southwest Research Institute used data from NASA's New Horizons mission, unique laboratory experiments and exospheric modeling to figure out what the red cap on Pluto's moon Charon is made of and how it arose.
Charon or Pluto I is the largest of the five natural satellites of the dwarf planet Pluto. It has a radius of 606 kilometers.
For the first time, new experimental data has been used to depict Charon's dynamic methane atmosphere, providing a fascinating peek into the origins of the moon's red spot, as reported in two recent articles.
"Prior to New Horizons, the best Hubble images of Pluto revealed only a fuzzy blob of reflected light," said SwRI's Randy Gladstone, a member of the New Horizons science team, Science Daily reported.
"In addition to all the fascinating features discovered on Pluto's surface, the flyby revealed an unusual feature on Charon, a surprising red cap centered on its north pole."
The flyby revealed an unexpected feature on Charon, a startling crimson cap centered on its north pole, in addition to all the remarkable features observed on Pluto's surface.
In this handout provided by NASA, the dwarf planet Pluto and Charon are shown July 11, 2015. NASA's New Horizons spacecraft was nearing its July 14 flyby when it would close to a distance of about 7,800 miles.
Charon or Pluto I's Red Polar Cap Origin Explained
SwRI's Dr. Ujjwal Raut, lead author of a paper titled "Charon's Refractory Factory" said their findings were pointing to the "drastic seasonal surges" in Charon's thin atmosphere and the light that broke down the frozen methane. He noted that it was one of the most illustrative surface-atmospheric interactions they observed at a planetary body.
Every equinox, polar methane frozen during the centuries-long winter darkness may be reintroduced into Charon's atmosphere. SciNews said this factor caused the entire atmosphere to a spike in pressure by a factor of approximately 1,000.
During these extremely brief episodes, which occur only a few years after the Pluto-Charon system completes its 248-year orbit around the Sun, polar caps of tens of microns thick methane frost may be abruptly swapped between north and south, evaporated, and then re-frozen from the spring to the autumnal polar zones.
The material synthesized from frozen methane by backscattered solar ultraviolet (UV) light, as seen by NASA's New Horizons spacecraft, was suspected to be material synthesized from Charon's polar red spot.
The authors discovered, however, that Charon's polar caps are frozen too quickly and thickly for the synthesis of much more complex material than ethane.
Nonetheless, because ethane is less volatile than methane, it remains frozen to Charon's surface for decades after spring dawn and may be changed to permanent red-colored surface deposits when exposed to solar wind, which may contribute to the origins of Charon's red spot.
Soon after the 2015 flyby, New Horizons scientists speculated that UV radiation breaking down methane molecules may produce a reddish tholin-like substance at Charon's pole.
After fleeing Pluto, they are trapped and frozen in the moon's polar regions during their long winter nights.
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How Researchers Uncovered The Mystery
The researchers used a realistic simulation of Charon's surface conditions to determine the composition and color of hydrocarbons formed as methane freezes beneath the Lyman-alpha light on the planet's winter hemisphere.
The scientists created a novel computer simulation to mimic Charon's tiny methane atmosphere. They used the data to create a new Charon atmospheric model, which showed methane breaking down into residue on the planet's north polar region.
The researchers used the findings of ultra-realistic experiments to estimate the distribution of complex hydrocarbons produced by methane breakdown under the impact of UV radiation in an atmospheric model.
Polar zones in the model essentially produce ethane, a colorless gas that does not contribute to the reddish hue.
The researchers think that ionizing radiation from the Sun decomposes the Lyman-alpha-cooked polar frost, resulting in the formation of progressively complex, redder compounds, which are responsible for the moon's mysterious albedo.
Ethane is less volatile than methane, according to specialists, and remains frozen to Charon's surface well after spring daylight.
Ethane may be converted to permanent reddish surface deposits by exposure to the solar wind, leading to Charon's red cap.
The findings were reported in two studies in the journals Geophysical Research Letters and Science Advances.
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