When New Horizons approached Pluto in 2015, it gave us something wondrous: the clearest view we'd ever seen of the distant, tiny dwarf planet.

In crisp images, fascinating terrain was revealed - including a broad swath of red sweeping around Pluto's equator: a non-icy landscape on an otherwise remarkably icy body.

Analysis suggested that the reddish plain was produced by molecules known as tholins, organic compounds that form in the atmosphere when ultraviolet or cosmic radiation cooks compounds that contain carbon, such as methane or carbon dioxide, that then rain down onto the surface.

Now, new research suggests that we don't have the whole story.

A team of researchers led by aerospace engineer Marie Fayolle of the Delft University of Technology in the Netherlands has created tholins in the lab to compare the way they reflect light against the observations of Pluto, and found that the spectral signatures don't quite match up.

Tholins aren't the only available explanation for Pluto's red patches - of which the Cthulhu Macula is the largest - but it did seem to be the best fit. New Horizons detected a haze in the dwarf planet's atmosphere, including methane, nitrogen, and carbon dioxide.

When irradiated, these compounds should turn russet and fall down to the surface, staining it a muddy red. Tholins are common in the outer Solar System, particularly on icy bodies, so it only makes sense, right?

To test it out, Fayolla and her team decided to create tholins in a lab. They took nitrogen, methane and carbon dioxide, and mixed them in proportions similar to those seen in Pluto's atmosphere, one at 1 percent methane, and the other at 5 percent. Then, they blasted them with plasma to mimic the irradiation in space.

This produced synthesized tholins, samples of submillimeter-sized spherical particles that the researchers could shine light on to compare the reflections against the light bouncing off Pluto, as detected by New Horizons.

The 1 percent methane was the better match for the New Horizons data, but even it did not entirely reproduce the observation data.

"From reconstructed reflectance spectra and direct comparison with New Horizons data, some of these tholins are shown to reproduce the photometric level (i.e. reflectance continuum) reasonably well in the near-infrared," the researchers wrote in their paper.

"Nevertheless, a misfit of the red visible slope still remains and tholins absorption bands present in the modelled spectra are absent in those collected by the New Horizons instruments."

In other words, the synthesized tholins absorbed a bit more light than the Cthulhu Macula. This doesn't mean that tholins aren't responsible for the red stain creeping across the surface of Pluto, but it does mean something else might be at play.

One hypothesis is irradiation by galactic cosmic rays, which could darken the tholins and change the way they absorb and reflect light. That might not entirely produce the observed spectrum, but further investigation would be required to rule it out.

Another possibility is that the surface of Pluto in these regions is more porous than expected, possibly due to the sublimation of ice. These plains are not expected to have much nitrogen ice, because they are at the equator, where the dwarf planet is warmer. Nor did New Horizons detect much methane ice, but it's possible that seasonal methane frost occurs during a different season than New Horizons' visit, the researchers said.

The third possibility is that, due to Pluto's weak gravity, the deposition of tholins is gentle, producing a fluffy, porous crust.

Future experiments using synthesized tholins could help determine the validity of these models, the researchers said. In turn, that could help us better understand Pluto's interactions with its atmosphere.

The paper has been published in Icarus.

H/t: New Scientist