Pluto''s life partner, Charon, has a disarming red ''cap.'' Ever since New Horizons took the moon''s shiny north pole on its 2015 flyby, scientists have investigated the planetary processes that are causing it to leave a bold landmark.
Scientists initially suspected the iron-colored smear (nicknamed Mordor Macula) was methane recovered from Pluto''s surface, which was the result of a slow baking in the Sun''s ultraviolet light. It was just begging to be tested.
With a brief twist, a mix of modeling and laboratory experiments has discovered that these early assumptions were not too far from the mark. Moreover, the research furthers our understanding of Pluto''s and Charon''s intimate involvement, suggesting that there''s more to the moon''s coloring than it is first.
New Horizons, the space probe used by NASA in 2006, provided researchers with an unprecedented visual of Pluto and Charon, a dwarf planet located at around 5 billion kilometers (3.1 billion miles) from the Sun.
"Prior to New Horizons, the best Hubble pictures of Pluto demonstrated just a fuzzy blob of reflected light," says Randy Gladstone, a planetary scientist at the Southwest Research Institute (SwRI) in the United States.
"Outside of all the fun on Pluto''s surface, the flyby revealed an unusual feature on Charon, with a surprising red cap on its north pole."
On iron-rich planets like ours or Mars for that matter, red might not be an unusual color. However, the red is far more likely to show the presence of a large group of tar-like compounds called tholins.
If it helps, simply replace the word tholin with "gunk." Similar to the brownish-red residue left in the oven, the oven drew inspiration from UV light to make brownies made of simple gases like carbon dioxide or ammonia.
Methane would be a likely starting point for Pluto. These tiny hydrocarbons would simply need to absorb a very specific color of UV light filtered by orbiting hydrogen clouds, called Lyman-alpha.
Pluto''s rosy glow has been subject of research for decades. In a stunning high definition, New Horizons revealed precisely the patterning of tholins on its surface. However, a finding a rusty tint across its companion was an unusual surprise.
The thought that Pluto''s methane would fall across to its orbiting moon. But it was always a sticking point.
The problem is the confrontation between Charon''s weak gravity and the far-off Sun''s cold light. As faint as it was, the spring dawn might be sufficient to melt the methane frost, causing it to fall from the surface again.
Researchers at SwRI investigated the see-sawing motion of the mostly tilted planet system to determine what would happen. They discovered that the greatest danger to the ears is the brightness of the spring.
The relatively slow warming of the north pole would take place over several years a mere blink in the moon''s 248-year orbit of the Sun. During this brief period, a small amount of methane frost would evaporate at one pole as it began to freeze over at the other.
Unfortunately, the modeling found that this rapid progression would be far too quick for much of the frozen methane to absorb sufficient amounts of Lyman-alpha to become a tholin.
The slightly longer hydrocarbon cousin of ethane methane would be a whole different story.
"Ethane is less volatile than methane and is much longer recovered from Charon''s surface after the spring sunrise," says planetary scientist Ujjwal Raut, who is responsible for the second study that investigated changes in the density of methane evaporating and freezing.
"Exposure to the solar wind may convert ethane into persistent reddish surface deposits contributing to Charon''s red cap."
Using laboratory experiments, Raut and his team''s study demonstrated a feasible technique to convert methane into ethane at the poles.
There was just one problem. Lyman-alpha radiation will not transform ethane into a reddish sludge.
The hydrocarbon isn''t rule out, so charged particles that flow from the Sun over a long period may still produce even longer chains of hydrocarbons that would give Charon its characteristic red cap.
"We believe that ionizing radiation from the solar wind decomposes the Lyman-alpha-cooked polar frost to synthesize increasingly complex, redder materials responsible for the unique albedo on this enigmatic moon," says Raut.
Additional laboratory testing and analysis might help strengthen the hypothesis that Charon''s rouge stain is far more complex than previously.
This study was published in Science and Geophysical Research Letters.