Pluto, a dwarf planet located in the distant Kuiper Belt, possesses a thin, transient atmosphere. Unlike the thick gaseous envelopes of larger planets, Pluto’s atmosphere is incredibly tenuous and fragile. It exists in a state of perpetual flux, driven by the dwarf planet’s unique orbital dynamics and the extreme cold of its environment. This wispy layer of gas profoundly affects Pluto’s surface, creating a dynamic world where volatile ices constantly interact with the space around them.
Composition and Structure
The bulk of Pluto’s thin atmosphere is composed of molecular nitrogen gas (\(N_2\)), along with trace amounts of methane (\(CH_4\)) and carbon monoxide (\(CO\)). These gases are vaporized directly from the surface ices covering large areas of the dwarf planet. The surface pressure is exceptionally low, measured by New Horizons to be about 10 microbars, roughly 1/100,000th the pressure of Earth’s atmosphere at sea level.
Above the surface, the atmosphere features multiple layers of photochemical haze, or smog, extending to altitudes of more than 200 kilometers. This haze is created when ultraviolet light breaks apart atmospheric methane and nitrogen molecules. The resulting fragments recombine to form heavier, non-volatile hydrocarbon particles called tholins.
These submicron-sized particles scatter sunlight, giving the atmosphere a soft, blue tint in images taken from behind the dwarf planet. The haze is structured into as many as 20 discernible layers that are not strictly parallel to the surface. This layered structure suggests the influence of atmospheric gravity waves or other meteorological phenomena that stir the upper atmosphere.
The Dramatic Cycle of Freezing and Sublimation
Pluto’s atmosphere is governed by a dramatic cycle tied to its 248-year orbit around the Sun. The dwarf planet follows a highly eccentric path, meaning its distance from the Sun changes significantly. Since the atmosphere is formed by volatile ices in equilibrium with their gas phase, tiny changes in surface temperature lead to massive shifts in atmospheric density.
When Pluto is relatively closer to the Sun, a period known as perihelion, the slight increase in solar energy causes the surface ices to warm. This warming triggers a process called sublimation, where solid nitrogen, methane, and carbon monoxide turn directly into gas, bypassing the liquid state. This rapid gas release effectively inflates the atmosphere, making it temporarily thicker and more extensive.
As Pluto moves farther away from the Sun toward aphelion, the surface temperature drops, and the process reverses. The atmospheric gases cool dramatically, causing them to freeze and condense back onto the surface as ice. This cycle suggests that as Pluto continues to recede from the Sun, its atmosphere will eventually collapse, freezing almost entirely onto the ground.
The current state of the atmosphere is one of slow dissipation, though it was relatively thick when observed by New Horizons shortly after the dwarf planet passed perihelion in 1989. The gases are continually freezing onto the surface while also escaping into space. This constant volatile transport reshapes the surface features, forming vast ice plains and contributing to the formation of methane ice caps on mountain peaks.
Confirmation through the New Horizons Mission
The existence of Pluto’s atmosphere was first hinted at in 1988 through Earth-based observations of a stellar occultation. As Pluto passed in front of a distant star, the star’s light dimmed gradually rather than disappearing abruptly, a clear sign that a gaseous layer was diffusing the starlight. This early evidence, however, could not provide detailed atmospheric composition or structure.
The definitive confirmation and detailed study of the atmosphere came with the New Horizons flyby in July 2015. The spacecraft carried a suite of instruments designed to analyze the distant world, including the Alice ultraviolet imaging spectrometer. Alice was used to measure the atmospheric composition by analyzing how the gases absorbed ultraviolet light from the Sun.
The Radio Science Experiment (REX) confirmed the atmospheric structure and temperature profile. REX worked by having NASA’s Deep Space Network beam radio signals to the spacecraft as it passed behind Pluto. The attenuation of the signal as it traveled through the atmosphere was used to calculate density and temperature, confirming the incredibly low surface pressure and the high-altitude haze layers.
Instruments like the Solar Wind Around Pluto (SWAP) and the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) measured the rate at which the atmosphere is being lost to space. These observations revealed that Pluto’s weak gravity allows its atmosphere to escape at a high rate, with the solar wind sweeping away atmospheric particles. The New Horizons data provided a clear, close-up picture of an active, yet highly fragile, atmosphere in the outer solar system.