The dwarf planet Pluto orbits in the distant Kuiper Belt, roughly 40 times farther from the Sun than Earth. This extreme distance results in incredibly frigid surface temperatures, yet Pluto maintains a thin, tenuous atmosphere. The New Horizons mission, which performed a rapid flyby in 2015, revolutionized our understanding of this distant world’s structure and weather. The spacecraft’s observations offered the first detailed look at the complex atmospheric layers surrounding Pluto. Scientists used this data to investigate whether the dwarf planet’s atmosphere supports the formation of clouds.
Haze Versus Discrete Clouds
Pluto’s atmosphere is dominated by a pervasive, layered blue haze that extends hundreds of kilometers above the surface. This haze is created when ultraviolet sunlight breaks down atmospheric gases through photochemistry. The resulting complex hydrocarbon molecules, such as acetylene and ethylene, coagulate into tiny solid particles that scatter sunlight, giving the atmosphere its distinct blue appearance. These particles are global, constantly forming, and slowly settling down onto the surface.
A traditional cloud is a discrete, localized feature where a gas condenses into liquid droplets or ice crystals. Clouds are transient and typically require specific conditions like localized cooling or uplift to form. Scientists define a true cloud as a feature that is optically thick enough to block the surface below it. While the haze is confirmed, evidence for these distinct, traditional clouds remains suggestive rather than definitive.
Pluto’s Atmospheric Composition
Pluto’s thin atmosphere is primarily composed of molecular nitrogen gas, which makes up more than 99% of its volume. Trace amounts of methane and carbon monoxide are also present, having sublimated—turned directly into gas—from the ices on Pluto’s surface. The surface pressure of this atmosphere is extremely low, approximately 100,000 times less than that of Earth.
The extremely cold temperatures on the dwarf planet, which range from about 34 K to 60 K on the surface, govern the behavior of these atmospheric components. Nitrogen, carbon monoxide, and methane are volatile ices that are constantly sublimating and refreezing in a cycle driven by Pluto’s orbit and seasonal changes. The atmosphere is thickest when Pluto is closer to the Sun, causing more surface ice to sublimate.
Methane, despite its small percentage, plays a significant role because it absorbs solar radiation, creating a temperature inversion in the atmosphere. This causes temperatures to increase with altitude up to around 110 K at an altitude of about 30 kilometers, before cooling again. This higher-altitude warmth and the presence of methane suggest that low-lying, localized methane ice clouds are the most likely type of cloud to form on Pluto.
Observational Evidence from New Horizons
During the 2015 flyby, the New Horizons spacecraft captured images that provided intriguing suggestions of discrete atmospheric features. Scientists identified several low-lying, bright features in images taken near Pluto’s dawn and dusk terminator, where temperatures would be low enough for condensation to occur. These features appeared to be isolated and were located close to the surface, consistent with models for cloud formation in Pluto’s atmosphere.
One specific image taken just after closest approach showed an intriguing bright wisp, tens of miles across, that may be a localized, low-lying cloud. These potential cloud features are visible because they are illuminated by sunlight grazing Pluto’s surface at a low angle. Furthermore, some images showed subtle parallel streaks in the haze that could be crepuscular rays, which are shadows cast by surface topography like mountains, suggesting the presence of something discrete in the atmosphere.
The New Horizons team identified seven specific candidates for these condensation clouds, but they have not been definitively confirmed due to limitations in the data. The spacecraft’s rapid flyby did not allow for the continuous observation necessary to track the features over time, which would be required to confirm their transient nature. Therefore, while the data suggests that Pluto has the potential for rare, localized clouds, a definitive confirmation awaits further study.