In 2015, NASA's New Horizons spacecraft snapped the first close-up photos of Pluto and its surrounding moons during a historic fly-by. The images revealed Pluto's vivid, eye-catching terrain and a large, maroon patch sweeping across nearly half of the dwarf planet's equator, the Cthulhu Macula. The bright, red regions were thought to be caused by molecules known as tholins, which are organic compounds that rain down onto the surface after cosmic rays or ultraviolet light interact with the methane in Pluto's surface and atmosphere.
A new study published in the journal Icarus suggests tholins alone may not be the only cause of the red patches, reports Michelle Starr for Science Alert. More research is needed to identify the exact combination of factors influencing Pluto's coloration.
The dwarf planet's atmosphere comprises a hazy, thin layer of nitrogen, methane, and carbon monoxide. One of Pluto's prominent features is the heart-shaped glacier towards the right side of the dwarf planet, also known as the Sputnik Planitia. The region is vaporized during the day, freezes over each night, and may play an essential role in Pluto's weather patterns. To the left of Pluto's heart-shaped plain is the mud red Cthulhu Macula.
To further understand what gives the Cthulhu Macula it's signature red color, scientists at the Delft University in the Netherlands created tholins in their laboratory. The team recreated Pluto's atmosphere in a chamber and then blasted it with plasma to mimic radiation in space, reports Victor Tangermann for Futurism. The low-density cloud of atmospheric gases reacted with the plasma by condensing into dust-like particles, reports Leah Crane for New Scientist.
The researchers then shined a light on the artificial tholins and compared them on how Pluto's surface reflects and absorbs light based on readings the New Horizons spacecraft took, reports Science Alert. The lab-created tholins did not match the observed conditions on Pluto. The artificial tholins absorbed some light that the red regions on the dwarf planet did not. However, this discrepancy does not mean that Pluto's red patches do not contain tholin, but instead suggests another factor contributes to the differences in light absorption in addition to tholin, reports Science Alert.
The study authors have hypothesized several potential causes to investigate in future research. First, they suggest space radiation may be darkening the Cthulhu Macula and changing the way it absorbs light, per Science Alert.
Another reason for the mismatch may be that the texture of Pluto's surface is more porous than expected. The New Horizons spacecraft did not detect methane ice near the red regions, but seasonal methane frosts may occur that the spacecraft did not catch, per Science Alert. Ice in these regions could explain changes in color. When a substance sits on top of an icy surface and some of the ice turns into vapor, the porous structure left behind may affect how light is absorbed or reflected on the surface, which in turn affects the substance's light spectrum, per New Scientist.
Pluto's weak gravity may also create a thin layer of tholins, resulting in its porosity, Futurism reports.
"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," study author Marie Fayolle, an aerospace engineer at the Delft University of Technology, tells New Scientist. "It might be more like a fluffy, porous snow that isn't packed down."
More research is needed to pin down the exact cause of Pluto's red plains. The team is planning future experiments using the tholins to explain further what is causing the Cthulhu region's signature red deposits and could help explain how the Pluto’s surface interacts with its atmosphere, per Science Alert.