pluto Pluto gives up its icy secrets as New Horizons data pours in

[Reefa]

 

It’s all connected. Eight months after NASA’s New Horizons spacecraft’s historic fly-by of Pluto, the tiny world and its moons are still offering up surprises. A series of startling links between Pluto, Charon, their four smaller satellites – and the space environment that surrounds them – have been revealed by a new series of papers.

 

New Horizons spent less than an hour in the Pluto system, but it has been beaming the data it gathered back to Earth ever since. The mission team is analysing it, and has already released some of the most fantastic images of the dwarf planet’s surface we’re likely to see.

 

“What you’ve seen before is the individual blind men coming out and describing their part of the elephant,” says Jeffrey Moore at NASA Ames Research Center in Moffett Field, California. This is all the blind men getting together and describing the elephant in totality. It’s an attempt to provide an integrated, global view.”

 

Collision chunks

Pluto and its moons are a tight-knit family: they were all formed from the same cataclysmic collision about 4 billion years ago. Scientists already suspected that Pluto and Charon formed when a larger body smacked into proto-Pluto, similar to how Earth and its moon formed.

 

But before the fly-by, it was unclear if the four smaller moons – Styx, Nix, Kerberos and Hydra – were formed in that same crash, or if they were captured later. New Horizons revealed that they have bright surfaces, suggesting they’re mostly composed of water-ice. That is similar to Pluto and Charon, but unlike many other Kuiper belt bodies, which favours the idea that the small moons are chips left over from the Charon-forming collision.

 

“They’re all chunks from the same event,” Moore says.

 

That seemed to be where the resemblance stops: the four small moons spin rapidly and asynchronously, while Pluto and Charon are locked in a face-to-face dance. Pluto is also currently cryovolcanically active, while Charon appears to have gone dormant billions of years ago after a brief period of cryovolcanic eruptions.

 

Long winter

But Pluto and Charon are still in close communication. One of the biggest surprises of the mission was how closely Pluto holds its atmosphere.

 

“Pluto’s really tiny, so it doesn’t have the mass to hold on to an atmosphere over the age of the solar system – at least we wouldn’t have thought that,” says Randy Gladstone of the Southwest Research Institute in San Antonio, Texas.

 

Remote observations showed that the atmosphere was dominated by nitrogen, and astronomers expected to find a tail of nitrogen streaming off the dwarf planet like a comet. But in reality, the chief material Pluto is losing to space is methane, and much less of it than predicted.

 

G.R. Gladstone et al. /Science (2016)

 

“Instead of having this big comet, blasting out lots of gas, Pluto is a lot more like Earth and Mars,” says Fran Bagenal at the University of Colorado, Boulder.

 

Once that methane escapes, some of it smacks into Charon. Most of the time, it will just bounce around on the surface until it gains enough energy to fly off into space again. But during the system’s long winter, when Charon’s polar regions receive no sunlight for decades at a time, it can get as cold as 20 degrees Kelvin – cold enough to freeze a bouncing methane molecule in its tracks.

 

That idea gave half of an explanation for the mysterious reddish cap at Charon’s north pole, which scientists are calling Mordor Macula. But they didn’t have a way to make it stick: when the sun returned in the spring, the frozen methane should just evaporate out again.

 

On Pluto, though, sunlight striking atmospheric methane can split a hydrogen atom off methane molecules, leaving them free to join up into larger particles called tholins. These particles make up the layers of haze in Pluto’s atmosphere, and also settle onto the surface in dark reddish splotches.

Paint the Charon red

There’s no sunlight shining on Charon’s winter poles, but there are energetic photons called Lyman-alpha particles scattering around space in all directions. These photons could help split up the methane on Charon and encourage tholins to form there too. Pluto is essentially painting Charon red.

 

“You lay down a layer every Pluto year, and that layer builds up like a varnish over time,” Gladstone says.

 

The material from Pluto’s atmosphere that doesn’t end up on Charon becomes part of the chaotic jumble of particles streaming around the dwarf planet – and also exert some control over how they move.

 

The solar wind – a constant stream of charged particles emanating from the sun – wreaks havoc on every body with an atmosphere by stripping away its outer layers bit by bit. But in the inner solar system, where the solar wind’s magnetic field is strong, the wind hitting the atmosphere acts like a fluid splashing into another fluid.

 

H.A. Weaver et al. / Science (2016)

 

At Pluto’s distance from the sun, that magnetic field is much weaker, making the particles in the wind smack into the atmosphere more like individual billiard balls. This interaction sends the stolen atmospheric particles spiraling in a wild, gyrating motion as they are carried off into space. These gyrations in turn rob the solar wind of some momentum, slowing it down, Bagenal says.

 

“We simply haven’t had time to sit down and just step away from data reduction, looking at things and making basic measurements,” Moore says. With so much data now in hand, they can now begin building models or running laboratory experiments to figure out how the Pluto system works.

 

“We have hypotheses,” Moore says. “But can we make landscapes that look the way Pluto does using physics and chemistry? That’s what’s next.”

 

Journal references: Science, DOI: 10.1126/science.aad9189, 10.1126/science.aad7055, 10.1126/science.aad8866, 10.1126/science.aad9045, 10.1126/science.aae0030

 

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[freeforever03]

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[Jordan]

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