Image: Mosiac of New Horizons images of Charon. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

When NASA's New Horizons spacecraft flew through the Pluto system last year, scientists were surprised to find that Charon, Pluto's largest moon, has a dark red polar cap. 

A new study may have figured out the reason why: trapped gas.

RELATED: Pluto Moon Charon 'Bursting' With Frozen Ocean?

Lowell Observatory astronomer Will Grundy and colleagues combined analysis of New Horizons imagery with computer models to show that Charon's north pole grew cold enough during its century-long winter to trap methane escaping from Pluto. As sunlight returned to the pole, the methane was then converted into red-colored chemicals, known as tholins, the study shows.

"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?" Grundy said in a press release . "Nature is amazingly inventive in using the basic laws of physics and chemistry to create spectacular landscapes."

WATCH VIDEO: Where Is New Horizons Now?

Surface temperatures during Charon's long winters dip to -430 Fahrenheit, 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," Grundy said.

At point, the methane ice quickly vaporizes, leaving heavier hydrocarbons that were created from it on the surface.

Sunlight further irradiates the hydrocarbons and turns them red. 

"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," Johns Hopkins University Applied Physics Lab wrote in a press release about the study.

"The distribution of dark, reddish material around Charon's northern pole is notable for its generally symmetric distribution across longitudes and its gradual increase with latitude, although there are local irregularities associated with craters, topographic features and perhaps subsurface variations in thermal properties," Grundy and colleagues write in this week's Nature.

"These characteristics … are consistent with our hypothesis that the combination of Pluto's escaping atmosphere and Charon's long, cold winters enables methane to be seasonally cold-trapped at high latitudes, where some is photolytically processed into heavier molecules that are subsequently converted to reddish tholin-like materials," the study shows.

That left the team wondering if the process could be happening elsewhere. Nix, one of Pluto's four small moons, has a reddish spot, but it orbits farther away form Pluto and is much smaller, which would make the process less efficient, the scientists note.

Quelle: Seeker


Update: 20.09.2016


How Pluto got its heart of ice

New climate simulations help explain how Pluto’s heart-shaped Sputnik Planum (lower right) got its thick glaciers of nitrogen and carbon monoxide. Some of the glaciers, first seen by the New Horizons spacecraft in July, hold ice that circulates like the material in a lava lamp. Now, to find out how the glaciers formed in the first place, scientists created models that simulated atmospheric circulation on the dwarf planet for the last 50,000 years (a mere 200 orbits around the sun for Pluto). At the beginning of the simulations, the researchers gave Pluto a planet-wide veneer of nitrogen, carbon monoxide, and methane ices a few millimeters thick; then, the planet’s surface and atmosphere evolved as the icy orb passed through orbit after orbit. If Pluto were a completely smooth sphere, it would have either a permanent swath of nitrogen ice at the equator or seasonal snow caps at its poles. But that’s not what the planet looks like today. When researchers added realistic topography to the model, including the 4-kilometer-deep Sputnik Planum and two other large craters, the basin gradually trapped Pluto’s nitrogen, carbon monoxide, and much of its methane, the researchers report online today in Nature. That’s because the dwarf planet’s sparse atmosphere is thickest at lowest elevation, making condensation of the ices most effective there. Besides helping explain the current pattern of ices on Pluto’s surface, the new simulations also shed light on substantial changes in Pluto’s atmospheric pressure observed in recent years. The team’s models suggest that in the coming decade, the dwarf planet’s atmospheric pressure will decrease and the frosts now seen in Pluto’s northern hemisphere will disappear. If it happens, this will be a crucial verification of their model, the researchers say. 

Quelle. AAAS


How Pluto's big frozen heart grew and evolved

Numerical simulations of the dwarf planet's chemistry and atmosphere show its famous glacier growing, as well as seasonal frosts expanding and shrinking.

This high-resolution enhanced colour view of Pluto, snapped by New Horizons in July 2015, shows the dwarf planet's heart of hearts – the relatively flat, icy expanse of a massive glacier of nitrogen ice covering the basin of Sputnik Planum.

The vast glacier in Pluto's heart could have grown in just 10,000 Earth years as nitrogen ice froze out of its atmosphere and was sequestered in the deep basin, according to new modelling.

Tanguy Bertrand and François Forget from Sorbonne University in France simulated the climate and topography of the dwarf planet and found nitrogen ice, along with some carbon monoxide and methane, naturally congregated in the lower elevations.

They published their work in Nature.

Since New Horizons beamed back images of Pluto's big, pale heart – called Tombaugh Regio after Pluto's discovered Clyde Tombaugh – planetary scientists have been trying to get to the bottom of the vast, craterless region within.

Informally named Sputnik Planum, the glacier is mostly nitrogen ice. But how did the basin, which is more than 1,000 kilometres wide and thought to be four kilometres deep, fill up?

Computer models, like those used by climate scientists to recreate out planet's ancient climate history, have tried to track how the Plutonian atmosphere changed and mixed, but have been too slow to run over multiple seasons.

The depths of Hellas Basin on Mars, snapped here by the Mars Reconnaissance Orbiter, accumulates ice and frost – like Sputnik Planum. Click to enlarge.

So Bertrand and Forget developed a new model – one that could run through Earth 50,000 years and see what happened to nitrogen (and other volatiles) on the icy little dwarf planet.

They found, when they added a basin and a couple of craters to their modelled Pluto, all the nitrogen ice was sequestered in the centre of the basin – just as it does on Sputnik Planum – after just 10,000 Earth years.

This is because the basin floor has higher surface pressure than the surrounds. Higher surface pressure means nitrogen is more likely to condense and freeze. This "cold trap" sets the ball rolling for more nitrogen ice to pile on.

They note this phenomenon is also seen on Mars, where carbon dioxide frost preferentially forms at low elevations, such as at the Hellas Basin.

Their model also ended up with carbon monoxide and methane frozen into the glacier ice. 

Indeed, data from New Horizons indicated carbon monoxide in Sputnik Planum, while methane tended to spread all the way around Pluto's equatorial region.

Finally, the model accounted for seasonal methane frosts that have been seen on Pluto's north pole in the 1980s, 1994 and 2002.

They predict these frosts should, for the most part, disappear in the next decade.

Quelle: COSMOS