杏吧原创

When NASA鈥檚 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鈥檛 a good match, leaving it a mystery.

Many assumed that these red patches were made of tholins, organic substances that form in a world鈥檚 atmosphere and then drift down to the surface. Pluto鈥檚 atmosphere, while extremely tenuous, has the ingredients to produce this brown or red gunk, so it was a reasonable assumption.

Now 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鈥檚 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鈥檚 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鈥檚 atmosphere as measured by New Horizons.

They then compared these artificial tholins to measurements of the red material on Pluto鈥檚 surface. Surprisingly, the two didn鈥檛 match. 鈥淚f you were to look at these two materials side by side, they might look the same 鈥� but they are not,鈥� says Fayolle. 鈥淚t鈥檚 quite a big problem, but at the same time quite interesting.鈥�

They tested the match by bouncing light off the artificial samples to see which parts of the spectrum they reflected or absorbed 鈥� New Horizons made similar measurements of Pluto鈥檚 surface. While some parts of the resulting spectrum matched, the artificial tholins absorbed some light that the red material on Pluto didn鈥檛.

鈥淭holins just basically means mud, it鈥檚 just a bunch of goo, so it could be that they have made something similar to what鈥檚 there, but there might be a bunch of effects contributing to this colour,鈥� says at Northern Arizona University.

One possible reason for the mismatch could be the texture of Pluto鈥檚 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鈥檛 see much sublimation in Pluto鈥檚 red areas, it is plausible that they could be porous. 鈥淕iven that Pluto is pretty small and has weak gravity, it might be that if you鈥檙e depositing very small particles in very weak gravity, you might end up with a porous surface,鈥� says Fayolle. 鈥淚t might be more like a fluffy, porous snow that isn鈥檛 packed down.鈥�

That is a subject for future work. For now, though, we still don鈥檛 know what sort of material is painting Pluto red. 鈥淚t鈥檚 definitely still a mystery,鈥� says Fayolle.

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