Pluto Is Turning Red and Nobody Really Knows Why

NASA's New Horizons spacecraft made history in 2015 by passing by Pluto and sending back the first-ever close-up photos of the planet and its moon before transmitting them back to Earth. In the images, it's apparent some of the planet's surface carries a dark red color. At the time, the prevailing theory was the red colored was caused by tholins, molecules formed after cosmic rays interact with the methane in Pluto's atmosphere.

As it turns out, the red may not be caused entirely by tholins after all. According to a new study published in Icarus, tholins are likely not the only cause of the red color. A team led by Delft University of Technology's Marie Fayolle tried to recreate tholins in their lab, and were unable to fully replicate the same process that would produce a the similar red color of Pluto's "Cthulhu Macula" plain.

"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," Fayolle and her 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."

What's this mean exactly?

It means the tholins created by researchers absorbed more light than those on Pluto. While this doesn't rule out the existence of tholins on the surface of Pluto, hence the reddish color, researchers suggest more molecules and matter could be at play.

"Several hypotheses are considered to explain the absence of these absorption features in LEISA data, namely high porosity effects or GCR irradiation," the study adds. "The formation of highly porous structures, which is currently our preferred scenario, could be promoted by either sublimation of ices initially mixed with the aerosols, or gentle deposition under Pluto's weak gravity."

Cover photo by NASA/APL/SwRI via Getty Images