Another study suggests there’s more to the dwarf planet’s rust-colored deposits than meets the eye
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| Pluto as seen by New Horizons (Credit: NASA/JHUAPL/SwRI) |
When NASA’s New Horizons spacecraft flew past the dwarf planet Pluto in 2015, it found that huge swathes of the surface are covered in a strange red material. Planetary scientists thought they knew the identity of this material, but it turns out their best guess isn’t a good match, leaving it a mystery.
Many assumed that these red patches were made of tholins, organic substances that form in a world’s atmosphere and then drift down to the surface. Pluto’s atmosphere, while extremely tenuous, has the ingredients to produce this brown or red gunk, so it was a reasonable assumption.
Now Marie Fayolle at the Delft University of Technology in the Netherlands and her colleagues have made artificial Pluto tholins, using a laboratory setup designed to study the tholins that are abundant in the atmosphere of Saturn’s moon Titan.
The experiment works by producing a low-density cloud of molecules and then blasting them with radiation similar to what would hit a world’s atmosphere in space, causing the molecules to react and condense into dust-like particles. The researchers used a mixture of carbon monoxide, nitrogen and methane to match the composition of Pluto’s atmosphere as measured by New Horizons.
They then compared these artificial tholins to measurements of the red material on Pluto’s surface. Surprisingly, the two didn’t match. “If you were to look at these two materials side by side, they might look the same – but they are not,” says Fayolle. “It’s quite a big problem, but at the same time quite interesting.”
They tested the match by bouncing light off the artificial samples to see which parts of the spectrum they reflected or absorbed – New Horizons made similar measurements of Pluto’s surface. While some parts of the resulting spectrum matched, the artificial tholins absorbed some light that the red material on Pluto didn’t.
“Tholins just basically means mud, it’s just a bunch of goo, so it could be that they have made something similar to what’s there, but there might be a bunch of effects contributing to this colour,” says Mark Loeffler at Northern Arizona University.
One possible reason for the mismatch could be the texture of Pluto’s surface. Previous laboratory experiments have shown that when a material sits atop an icy surface and some of the ice sublimates, turning into gas and floating away, the porous structure left behind can affect the light spectrum of the material by limiting light absorption.
While we don’t see much sublimation in Pluto’s red areas, it is plausible that they could be porous. “Given that Pluto is pretty small and has weak gravity, it might be that if you’re depositing very small particles in very weak gravity, you might end up with a porous surface,” says Fayolle. “It might be more like a fluffy, porous snow that isn’t packed down.”
That is a subject for future work. For now, though, we still don’t know what sort of material is painting Pluto red. “It’s definitely still a mystery,” says Fayolle.
Many assumed that these red patches were made of tholins, organic substances that form in a world’s atmosphere and then drift down to the surface. Pluto’s atmosphere, while extremely tenuous, has the ingredients to produce this brown or red gunk, so it was a reasonable assumption.
Now Marie Fayolle at the Delft University of Technology in the Netherlands and her colleagues have made artificial Pluto tholins, using a laboratory setup designed to study the tholins that are abundant in the atmosphere of Saturn’s moon Titan.
The experiment works by producing a low-density cloud of molecules and then blasting them with radiation similar to what would hit a world’s atmosphere in space, causing the molecules to react and condense into dust-like particles. The researchers used a mixture of carbon monoxide, nitrogen and methane to match the composition of Pluto’s atmosphere as measured by New Horizons.
They then compared these artificial tholins to measurements of the red material on Pluto’s surface. Surprisingly, the two didn’t match. “If you were to look at these two materials side by side, they might look the same – but they are not,” says Fayolle. “It’s quite a big problem, but at the same time quite interesting.”
They tested the match by bouncing light off the artificial samples to see which parts of the spectrum they reflected or absorbed – New Horizons made similar measurements of Pluto’s surface. While some parts of the resulting spectrum matched, the artificial tholins absorbed some light that the red material on Pluto didn’t.
“Tholins just basically means mud, it’s just a bunch of goo, so it could be that they have made something similar to what’s there, but there might be a bunch of effects contributing to this colour,” says Mark Loeffler at Northern Arizona University.
One possible reason for the mismatch could be the texture of Pluto’s surface. Previous laboratory experiments have shown that when a material sits atop an icy surface and some of the ice sublimates, turning into gas and floating away, the porous structure left behind can affect the light spectrum of the material by limiting light absorption.
While we don’t see much sublimation in Pluto’s red areas, it is plausible that they could be porous. “Given that Pluto is pretty small and has weak gravity, it might be that if you’re depositing very small particles in very weak gravity, you might end up with a porous surface,” says Fayolle. “It might be more like a fluffy, porous snow that isn’t packed down.”
That is a subject for future work. For now, though, we still don’t know what sort of material is painting Pluto red. “It’s definitely still a mystery,” says Fayolle.