Pluto’s Cthulhu Macula’s Red Plains May Not Be What We Thought
When New Horizons approached Pluto in 2015, it provided us with a spectacular sight: the clearest glimpse of the faraway, small dwarf planet we would ever seen.
Amazing geography was revealed in sharp photographs, including a huge expanse of red circling Pluto’s equator: a non-frigid landscape on an otherwise incredibly icy body.
The reddish plain was likely created by molecules called tholins, which are organic chemicals that originate in the atmosphere when UV or cosmic radiation cooks carbon-containing compounds such as methane or carbon dioxide, causing them to rain down onto the surface.
Now, new research indicates that we may not have the complete picture.
A team of researchers lead by aerospace engineer Marie Fayolle of the Delft University of Technology in the Netherlands generated tholins in the lab and compared their spectral signatures to those observed on Pluto.
Although tholins are not the only possible explanation for Pluto’s red patches – the largest of which is the Cthulhu Macula – they did appear to be the best fit. New Horizons identified a haze of methane, nitrogen, and carbon dioxide in the dwarf planet’s atmosphere.
When these compounds are irradiated, they should turn russet and fall to the surface, coloring it a muddy red color. Tholins are abundant in the outer Solar System, most notably on ice worlds, thus it is only natural, right?
To validate it, Fayolla and her team decided to synthesize tholins in the laboratory. They blended nitrogen, methane, and carbon dioxide in quantities close to those found in Pluto’s atmosphere, with one containing 1% methane and the other containing 5%. They then blasted them with plasma to simulate space irradiation.
This produced manufactured tholins, samples of submillimeter-sized spherical particles on which the researchers could flash light in order to compare the reflected light to that discovered by New Horizons.
Although the 1% methane was a better match for the New Horizons data, it did not entirely recreate it.
“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 said in their report.
“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 tholins created absorbed somewhat more light than the Cthulhu Macula. This is not to say that tholins are not responsible for the red stain spreading across Pluto’s surface, but it does indicate that something else may be at work.
One possibility is that galactic cosmic rays have darkened the tholins and altered their ability to absorb and reflect light. This may not totally account for the observed spectrum, but additional analysis is necessary to rule it out.
Another option is that the surface of Pluto is more porous than expected in certain locations, maybe due to ice sublimation. These plains are unlikely to hold much nitrogen ice, as they are located near the dwarf planet’s equator. Neither did New Horizons find any methane ice, but the researchers noted that seasonal methane frost may occur during a different season than New Horizons’ visit.
Thirdly, due to Pluto’s light gravity, tholin deposition is mild, resulting in a fluffy, porous crust.
Future trials with synthetic tholins may assist establish the validity of these models, the researchers noted. As a result, we may gain a better understanding of Pluto’s interactions with its atmosphere.
The article was published in Icarus.
With thanks to: New Scientist