Pluto moon Charon’s red head tinted by trapped gas

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A mosaic of images snapped by New Horizons as it approached Pluto’s largest moon Charon in July 2016.
NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute

Plutonian moon Charon’s north pole is stained red from compounds spawned by irradiated methane, a new study suggests.

A US and French team took the latest New Horizons data and modelled heat flow on the moon’s surface. They found gas molecules trickling from Pluto, snagged by Charon, could be broken down into reddish molecules to give it its distinctly ruddy cap.

The work, which links the moon and dwarf planet’s atmosphere and is published in Nature, is “elegant”, says Helen Maynard-Casely, a planetary scientist at the Australian Nuclear Science and Technology Organisation in Sydney. “We think of planets as isolated systems, but that’s not so.”

Pluto and its biggest moon Charon are quite a different system to Earth and our moon.

Where our moon is only about a quarter the diameter of Earth and keeps a distance of around 380,000 kilometres, Charon is about half as big as its dwarf planet. They’re separated by only 20,000 kilometres. 

This means volatile molecules from Pluto, such as methane and nitrogen, regularly make their way to Charon and are trapped by its gravitational pull. 

At the moment, Charon’s northern hemisphere is illuminated in summer while its southern is in winter night.

And since New Horizons sent back spectacular images of the moon’s north pole last year, there has been speculation that its red hue was produced as captured methane was converted into compounds called tholins – a molecules produced when simple hydrocarbons are broken down by radiation. 

This probably happens on Saturnian moon Titan, where methane in its upper atmosphere is converted to tholins that “rain down” onto the surface, Maynard-Casely says, and bestowing an orange-red tinge.

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This composite of enhanced colour images of Pluto (lower right) and Charon (upper left), was taken by New Horizons as it passed through on 14 July 2015.

But no one had calculated to see if it was possible on Charon. Being in the far reaches of the solar system, it receives feeble amounts of sunshine – even in summer. And no one knew if it could cling to enough methane in the first place.

So Will Grundy from Lowell Observatory in Arizona, US and his crew modelled Charon’s thermal conditions, incorporating measurements of methane flowing from Pluto by the New Horizons mission and its orbit around the sun.

They found during Charon’s century-long winters, when the pole was plunged into complete darkness, temperatures were frigid enough to freeze methane ice onto pole. This methane ice was thicker closer to the pole. 

Stray photons bouncing around material permeating the solar system, called the interplanetary medium, converted some of that methane into slightly heavier, less volatile, nitrogen-rich molecules.

So when winter finally ended and ultraviolet radiation from the sun reached across Charon’s icy surface, most of that trapped methane sublimated away. But the less volatile products stuck around – long enough that solar radiation could convert them to tholins.

How efficient is this process? The team can only estimate, but if half the frozen methane products were converted, this would produce around 30 centimetres of tholins produced at the poles each billion years.

The pole isn’t completely black with tholins, so – the researchers reason – the compounds must mix with incoming ice dust, or be stirred into the top few metres of underlying water ice by micrometeorite impacts, to dilute the colour.

Tholins are thought to give Pluto its red patches. Another of Pluto’s moons, Nix, has also been snapped with a red spot, the researchers note, so the same methane capture, freeze and conversion process may happen there too.

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