When Nasa's tiny New Horizons probe travelled 3 billion miles to Pluto it spotted something mysterious on Pluto’s moon Charon – a red 'cap'.
Data from the probe, powered by a Star Trek-style ion drive, has been combined this week with data from lab experiments.
Scientists now believe they may understand what makes up the strange red spot.
The area appears to be sticky organic residue created as methane is decomposed by the solar wind into more complex molecules.
Randy Gladstone, of the Southwest Research Institute in San Antonio, Texas, said: "Prior to New Horizons, the best Hubble images of Pluto revealed only a fuzzy blob of reflected light.
"In addition to all the fascinating features discovered on Pluto's surface, the fly-by revealed an unusual feature on Charon, a surprising red cap centered on its north pole."
Soon after the probe's 2015 encounter, scientists proposed that a reddish "tholin-like" material at Charon's pole could be synthesised by ultraviolet light breaking down methane molecules. These are captured after escaping from Pluto and then frozen onto the moon's polar regions during their long winter nights.
Tholins are sticky organic residues formed by chemical reactions powered by light, in this case the Lyman-alpha ultraviolet glow scattered by interplanetary hydrogen atoms.
Dr Ujjwal Raut, of the Southwest Research Institute, said: "Our findings indicate that drastic seasonal surges in Charon’s thin atmosphere as well as light breaking down the condensing methane frost are key to understanding the origins of Charon’s red polar zone.
"This is one of the most illustrative and stark examples of surface-atmospheric interactions so far observed at a planetary body."
The team realistically replicated Charon surface conditions at SwRI's new Center for Laboratory Astrophysics and Space Science Experiments (CLASSE) to measure the composition and colour of hydrocarbons produced on Charon’s winter hemisphere as methane freezes beneath the Lyman-alpha glow.
The researchers fed the measurements into a new atmospheric model of Charon to show methane breaking down into residue on Charon’s north polar spot.
Raut said: "Our team's novel 'dynamic photolysis' experiments provided new limits on the contribution of interplanetary Lyman-alpha to the synthesis of Charon's red material.
"Our experiment condensed methane in an ultra-high vacuum chamber under exposure to Lyman-alpha photons to replicate with high fidelity the conditions at Charon’s poles."
SwRI scientists also developed a new computer simulation to model Charon's thin methane atmosphere.
The team input the results from SwRI’s ultra-realistic experiments into the atmospheric model to estimate the distribution of complex hydrocarbons emerging from methane decomposition under the influence of ultraviolet light.
The model has polar zones primarily generating ethane, a colourless material that does not contribute to a reddish colour.
Raut said: "We think ionising radiation from the solar wind decomposes the Lyman-alpha-cooked polar frost to synthesise increasingly complex, redder materials responsible for the unique albedo on this enigmatic moon.
"Ethane is less volatile than methane and stays frozen to Charon’s surface long after spring sunrise. Exposure to the solar wind may convert ethane into persistent reddish surface deposits contributing to Charon’s red cap."
Watch: Nasa has created flyby videos of Pluto and Charon