Scientists on NASA’s New Horizons mission have learned that reddish material in the north polar region of Pluto’s biggest moon, Charon, is chemically processed methane that escaped from the atmosphere of the dwarf planet onto the moon.
This impressive view of Charon was captured on July 14, 2015. Charon’s color palette is not as diverse as Pluto’s; most striking is the reddish north (top) polar region, informally named Mordor Macula. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.
NASA’s New Horizons robotic spacecraft first spotted the dark red polar region on Pluto’s largest moon in June 2015. Over the past year, after analyzing the images and other data from the spacecraft, New Horizons mission scientists think they’ve solved the mystery.
Charon’s polar coloring comes from Pluto itself – as methane gas that escapes from the dwarf planet’s atmosphere and becomes ‘trapped’ by the moon’s gravity and freezes to the cold, icy surface at Charon’s north pole.
This is followed by chemical processing by solar UV light that transforms the methane into heavier hydrocarbons and eventually into reddish organic materials called tholins.
“Who would have thought that Pluto is a graffiti artist, spray-painting its companion with a reddish stain that covers an area the size of New Mexico? Every time we explore, we find surprises. Nature is amazingly inventive in using the basic laws of physics and chemistry to create spectacular landscapes,” said New Horizons project scientist Dr. Will Grundy, from Lowell Observatory.
Dr. Grundy and his colleagues from the United States, Germany and France combined analyses from detailed images of Charon with computer models of how ice evolves on the moon’s poles.
They had previously speculated that methane from Pluto’s atmosphere was trapped in Charon’s north pole and slowly converted into the reddish material, but had no models to support that theory.
They dug into the data to determine whether conditions on Charon could allow the capture and processing of methane gas.
The models using Pluto and Charon’s 248-year orbit around the Sun show some extreme weather at Charon’s poles, where 100 years of continuous sunlight alternate with another century of continuous darkness.
Surface temperatures during these long winters dip to minus 430 degrees Fahrenheit (minus 257 degrees Celsius), cold enough to freeze methane gas into a solid.
“The methane molecules bounce around on Charon’s surface until they either escape back into space or land on the cold pole, where they freeze solid, forming a thin coating of methane ice that lasts until sunlight comes back in the spring,” Dr. Grundy said.
“But while the methane ice quickly sublimates away, the heavier hydrocarbons created from it remain on the surface.”
The models also suggested that in Charon’s springtime the returning sunlight triggers conversion of the frozen methane back into gas. But while the methane ice quickly sublimates away, the heavier hydrocarbons created from this evaporative process remain on the surface.
Sunlight further irradiates those leftovers into reddish tholins that have slowly accumulated on Charon’s poles over millions of years.
New Horizons’ observations of Charon’s other pole, currently in winter darkness – and seen by New Horizons only by light reflecting from Pluto– confirmed that the same activity was occurring at both poles.
The team’s findings were published online this week in the journal Nature.
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W.M. Grundy et al. The formation of Charon’s red poles from seasonally cold-trapped volatiles. Nature, published online September 14, 2016; doi: 10.1038/nature19340
This article is based on a press-release issued by NASA.
