The atmosphere of Pluto is the layer of gases that surround the dwarf planet Pluto. It consists mainly of nitrogen (N2), with minor amounts of methane (CH4) and carbon monoxide (CO), all of which are produced by the sublimation of surface ices from Pluto's surface. It contains layered haze, probably consisting of heavier compounds which form from these gases due to high-energy radiation. The atmosphere of Pluto is notable for its strong and not completely understood seasonal changes caused by peculiarities of the orbital and axial rotation of Pluto.
The surface pressure of the atmosphere of Pluto, measured by New Horizons in 2015, is about (), roughly 1/100,000 of Earth's atmospheric pressure. The temperature on the surface is , but it quickly rises with altitude due to a methane-generated greenhouse effect. Near the altitude of it reaches , but then slowly decreases with altitude.
Pluto is the first trans-Neptunian object with a known atmosphere. Its closest analog is the atmosphere of Triton, although in some aspects it resembles even the atmosphere of Mars.
The atmosphere of Pluto has been studied since the 1980s by way of Earth-based observation of occultations of stars by Pluto and spectroscopy. In 2015, it was studied from a close distance by the spacecraft New Horizons.
Composition
The main component of the atmosphere of Pluto is nitrogen. The methane content, according to measurements by New Horizons, is 0.25%. For carbon monoxide, estimates are around 0.0515%. Under the influence of high-energy cosmic radiation, these gases react to form more complex compounds (not volatile at Pluto's surface temperatures), including ethane (C2H6), ethylene (C2H4, about 0.0001%), acetylene (C2H2, about 0.0003%), heavier hydrocarbons and nitriles and hydrogen cyanide (HCN) , These compounds slowly precipitate on the surface. They probably also include tholins, which are responsible for the brown color of Pluto (like some other bodies in the outer solar system).
The most volatile component of the atmosphere of Pluto is nitrogen, the second is carbon monoxide and the third is methane (CH4. The indicator of volatility is saturated vapor pressure (sublimation pressure). At temperature (close to minimum value for surface of Pluto) it is about for nitrogen, for carbon monoxide and for methane. It quickly increases with temperature, and at (close to the maximum value) approaches , and respectively. For heavier-than-methane hydrocarbons, water, ammonia, carbon dioxide and hydrogen cyanide, this pressure remains negligibly low (about or still lower), which indicates absence of volatility at Pluto's conditions (at least in cold lower atmosphere).
Methane and carbon monoxide, due to their lower abundance and volatility, could be expected to demonstrate stronger deviations from pressure equilibrium with surface ices and bigger temporal and spatial variations of concentration. But actually the concentration of methane, at least, does not vary noticeably with height (at least, in the lower 20–30 km), longitude or time. But temperature dependences of the volatilities of methane and nitrogen suggest that the concentration of methane will decrease as Pluto moves further from the Sun. It is notable that the observed concentration of methane is 2 orders of magnitude higher than expected from Raoult's law on the basis of its concentration in surface ice and the ratio of the sublimation pressures of methane and nitrogen. The reasons for this discrepancy are unknown. It could be due to the existence of separate patches of relatively clean methane ice, or due to an increased methane content in the uppermost layer of usual mixed ice.
Seasonal and orbital changes of insolation result in migration of surface ices: they sublimate in some places and condensate in others. According to some estimates, this causes meter-sized changes of their thickness. This, alongside changes in viewing geometry, results in appreciable changes in the brightness and color of Pluto.
Methane and carbon monoxide, despite their low abundance, are significant for the thermal structure of the atmosphere: methane is a strong heating agent and carbon monoxide is a cooling one (although the degree of cooling contributed by carbon monoxide is not completely clear).
Haze
New Horizons discovered in the atmosphere of Pluto a multi-layered haze, which covers the entirety of the dwarf planet and reaches altitudes over 200 km. The best images show about 20 layers of the haze. Horizontal extent of the layers is no less than 1000 km. The thickness of the layers varies from 1 to >10 km, and vertical distance between them is about 10 km. In northern regions the haze is 2-3 times denser than near the equator.
Despite the very low density of the atmosphere, the haze is rather appreciable: it even scatters enough light to allow photographing some details of Pluto's night side. Long shadows from mountains are seen on the haze. Its normal optical depth is estimated as 0.004 or 0.013 (thus, it diminishes the intensity of a vertical beam of light by or ; for grazing light the effect is much stronger). Scale height of the haze is ; it approximately coincides with scale height of pressure in the middle atmosphere. At the heights of it diminishes to 30 km.
Size of the haze particles is unclear. Its blue color points to a particle radius near 10 nm, but the ratio of brightnesses at different phase angles indicates a radius exceeding 100 nm. This can be explained by aggregation of small (tens of nm) particles into larger (hundreds of nm) clusters.
The haze probably consists of particles of non-volatile compounds, which are synthesized from atmospheric gases under influence of cosmic high-energy radiation. The layers show the presence of atmospheric waves (presence of which is also suggested by observations of occultations), and such waves can be created by wind blowing over Pluto's rough surface.
The haze is the most probable reason for a kink in the curve of light intensity vs. time obtained by New Horizons during the flight through Pluto's shadow (see image on right) – below altitude the atmosphere attenuates light much more strongly than above. A similar kink was observed during stellar occultation in 1988. It was initially interpreted as weakening of light by haze, but now it is thought to be mainly a result of a strong inverse temperature gradient in the lower atmosphere. During later occultations (when the atmosphere of Pluto was already denser) this kink was absent.
More evidence of the haze was obtained in 2002 due to a new occultation. The stellar light which reached Earth during the occultation (due to refraction in Pluto's atmosphere), demonstrated an increase in intensity with wavelength. This was interpreted as reliable evidence of light scattering by aerosols (similar to the reddening of the rising sun). However, this feature was absent during later occultations (including 29 June 2015), and on 14 July 2015, New Horizons found the haze to be blue.
In the final batch of images received from New Horizons, a number of potential clouds were observed.
Structure Pluto has no or almost no troposphere; observations by New Horizons suggest only a thin tropospheric boundary layer. Its thickness in the place of measurement was 4 km, and the temperature was 37±3 K. The layer is not continuous.
Above it is a layer with rapid increase of temperature with height, the stratosphere. The temperature gradient is estimated by various sources to be 2.2, or 5.5 degrees per km. This is a result of a greenhouse effect, caused by methane. The mean temperature of the surface is (measured in 2005), and the mean value for the whole atmosphere is (2008).
At height the temperature reaches its maximum (; stratopause) and then slowly decreases (about ; mesosphere). Causes of this decrease are unclear; it could be related to the cooling effect of carbon monoxide, or hydrogen cyanide, or other reasons. Above 200 km the temperature reaches approximately and then remains constant.
The temperature of the upper layers of the atmosphere does not show noticeable temporal changes. In 1988, 2002 and 2006 it was approximately constant and equal to (with uncertainty about ), despite a twofold increase in pressure. Dependence on latitude or morning/evening conditions is also absent: temperature is the same above every part of the surface. It is consistent with theoretical data, which predict fast mixing of the atmosphere. But there is evidence for small vertical heterogeneities in temperature. They reveal themselves in sharp and brief spikes of brightness during stellar occultations. Amplitude of these heterogeneities is estimated to be on the scale of a few km. They can be caused by atmospheric gravity waves or turbulence, which can be related to convection or wind.
Interaction with the atmosphere significantly influences the temperature of the surface. Calculations show that the atmosphere, despite its very low pressure, can significantly diminish diurnal variations in temperature. But there still remain temperature variations of about – partly because of cooling of the surface due to sublimation of ices.
Pressure The pressure of the atmosphere of Pluto is very low and strongly time-dependent. Observations of stellar occultations by Pluto show that it increased about 3 times between 1988 and 2015, even though Pluto has been moving away from the Sun since 1989. This is probably caused by Pluto's north pole coming into sunlight in 1987, which intensified evaporation of nitrogen from the northern hemisphere, whereas its southern pole is still too warm for condensation of nitrogen. Absolute values of surface pressure are difficult to obtain from occultation data, because this data does not usually reach the lowest layers of the atmosphere. So, surface pressure has to be extrapolated, and this is somewhat ambiguous due to the height dependence of temperature and, consequently, pressure not being completely clear. The radius of Pluto must also be known, but it was poorly constrained before 2015. So, precise values of Pluto's surface pressure were impossible to calculate in previous times. For some occultations since 1988, pressure was calculated for a reference level of from the center of Pluto (which later turned out to be 88±4 km from the surface).
Curves of pressure vs. distance from the center, obtained from occultations in 1988 and 2002, in combination with the now known radius of Pluto () give values of about for 1988 and for 2002. Spectral data provided values in 2008 and in 2012 for the distance from the center (1±4 km from the surface). An occultation on 4 May 2013 gave data almost precisely for the surface level (1190 km from the center, or 3±4 km from the surface): . An occultation on 29/30 June 2015, just 2 weeks before New Horizons encounter, provided a surface pressure of .
The first direct and reliable data about the lowermost layers of the atmosphere of Pluto were obtained by New Horizons on 14 July 2015 from radio-occultation measurements. The surface pressure was estimated to be ( at entry of the spacecraft behind Pluto and at the exit). This is consistent with occultation data from previous years, although some of the previous calculations based on this data gave about 2 times higher results.
The stellar occultation on 17 July 2019 has shown Pluto's atmospheric pressure have dropped about 30% from maximal values in 2015, reaching 0.967 Pa. 6 June 2020 further pressure decline to 0.91 Pa was measured.
The scale height of pressure in Pluto's atmosphere varies significantly with height (in other words, the height dependence of the pressure deviates from exponential). This is caused by strong height variations of temperature. For the lowermost layer of the atmosphere the scale height is about , and for heights — .
Seasonal changes