Pluto’s life partner, Charon, wears a disarming red “cap”. Ever since New Horizons snapped the moons rust-toned North Pole on its bypass in 2015, scientists have pondered the planetary processes responsible for leaving such a bold landmark.
Researchers initially suspected the iron-colored batter (nicknamed Mordor Macula) was methane captured from Pluto’s surface, its red color was the result of a slow baking in the sun’s ultraviolet light. It was a good idea to just beg to be tested.
Now a mix of modeling and laboratory experiments has found that these early assumptions were not too far from the mark, with a slight twist. The research adds surprising new details to our understanding of Pluto and Charon’s intimate engagement, suggesting that there is more to do the moons staining than first meets the eye.
NASA’s interplanetary space probe New Horizons, launched in 2006, gave scientists an unparalleled view of the dwarf planetary system Pluto and Charon at a distance of more than 5 billion kilometers (3.1 billion miles) from the sun.
“Before New Horizons, the best Hubble images of Pluto revealed only a faint ink of reflected light.” says Randy Gladstone, a planetary scientist from the Southwest Research Institute (SwRI) in the United States.
“In addition to all the fascinating features discovered on Pluto’s surface, the bypass revealed an unusual feature on Charon; a surprising red cap centered at its north pole.”
Red may not be an unusual color to look at in iron-rich worlds like ours, either March for that part. But all the way out into the frozen suburbs of the solar system, red is much more likely to indicate the presence of a diverse group of tar-like compounds called tolins.
If it helps, just replace the word tholin with “gunk”. The brownish-red mixture of chemicals is like the residue left in the oven, if the oven used UV light to bake brownies made from simple gases such as carbon dioxide or ammonia.
On Pluto, methane would be a likely starting point. To grow into a tolin, these small hydrocarbons would simply need to absorb one very specific UV color light filtered by orbiting hydrogen clouds, called Lyman-alpha.
Pluto’s pink glow has been the subject of study for decades. New Horizons simply revealed the exact pattern of tolins on its surface in brilliant high resolution. To find a rusty shade thrown over the hat at his companion, however, was an exciting surprise.
It was assumed that methane from Pluto could drift over to its orbiting moon. But the exact time needed for the gas to settle and freeze to such a distinct diffuse smear was always a problem.
Part of the problem is the competition between Charon’s weak gravity and the cold light from the distant sun that warmed its surface. As weak as it was, the spring dawn could be enough to melt the methane frost and drive it from the surface again.
To determine what would really happen, SwRI scientists modeled the rocking motion of the largely inclined planetary system. The secret behind the batter, they found, may be the explosive nature of the arrival of spring.
The relatively sudden warming of the North Pole would take place over several years – just a flash in the moon’s 248-year orbit around the sun. During this short period, a thick methane frost would evaporate only tens of micrometers thick at one pole when it began to freeze over at the other.
Unfortunately, the modeling found that this rapid motion would be far too fast for much of the frozen methane to absorb sufficient amounts of Lyman alpha to become a toluene.
But ethane – methane’s slightly longer hydrocarbon cousin – would be a completely different story.
“Ethane is less volatile than methane and remains frozen to the surface of Charon long after the spring sunrise.” says planetary scientist Ujjwal Raut, lead author of a second study as modeled changes in the density of methane that evaporates and freezes.
“Exposure to the solar wind can convert ethane into persistent reddish surface deposits that contribute to Charon’s red cap.”
Together with the results of laboratory experiments, Raut and his team’s study showed a feasible way to convert methane to ethane at the poles.
It was just a problem. Lyman alpha radiation will not turn ethane into a reddish sludge.
It does not exclude the hydrocarbon. Charged particles that flow from the sun for an extended period of time can still generate longer and longer chains of hydrocarbons that would give Charon its characteristic red cap.
“We believe that ionizing radiation from the solar wind breaks down the Lyman-alpha-cooked polar frost to synthesize increasingly complex, redder materials that are responsible for the unique albedon on this enigmatic moon.” says Raut.
Additional laboratory tests and modeling can help confirm the hypothesis that Charon’s blush staining is much more complex than we ever realized.
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