It contains layered haze, probably consisting of heavier compounds which form from these gases due to high-energy radiation.

[5] Under the influence of high-energy cosmic radiation, these gases react to form more complex compounds (not volatile at Pluto's surface temperatures[14]), including ethane (C2H6), ethylene (C2H4), acetylene (C2H2), heavier hydrocarbons and nitriles[15][16][17] and hydrogen cyanide (HCN)[18] (the amount of ethylene is about 0.0001%, and the amount of acetylene is about 0.0003%).

They probably also include tholins, which are responsible for the brown color of Pluto (like some other bodies in the outer solar system).

[14][20] 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.

[8][22] Seasonal and orbital changes of insolation result in migration of surface ices: they sublimate in some places and condensate in others.

[7][24] 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.

[2][26] 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.

[2] The haze probably consists of particles of non-volatile compounds, which are synthesized from atmospheric gases under influence of cosmic high-energy radiation.

Firstly it was also interpreted as weakening of light by haze,[30] but now it is thought to be mainly a result of strong inverse temperature gradient in lower atmosphere.

The stellar light which managed to reach Earth during the occultation (due to refraction in Pluto's atmosphere), demonstrated an increase of intensity with wavelength.

[7][8][10] Causes of this decrease are unclear; it could be related to the cooling effect of carbon monoxide,[24] or hydrogen cyanide, or other reasons.

However, the situation is complicated by its big axial tilt (122.5°[44]), which results in long polar days and nights on large parts of its surface.

Shortly before the perihelion, on 16 December 1987, Pluto underwent equinox, and its north pole[f] came out of the polar night, which had lasted 124 Earth years.

Data, existing as of 2014, allowed the scientists to build a model of seasonal changes in Pluto's atmosphere.

Nonetheless, its surface was already substantially heated, and its big thermal inertia (provided by non-volatile water ice) greatly slowed down its cooling.

That is why gases, which now intensively evaporate from northern hemisphere, cannot quickly condense in the southern, and keep accumulating in the atmosphere, increasing its pressure.

[45] In 2021, astronomers at the Southwest Research Institute confirmed the result using data from an occultation in 2018, which showed that light was appearing less gradually from behind Pluto's disc, indicating a thinning atmosphere.

[46] Early data suggested that Pluto's atmosphere loses 1027–1028 molecules (50–500 kg) of nitrogen per second, an amount corresponding to the loss of a surface layer of volatile ices several hundred meters or several kilometers thick during the lifetime of the Solar System.

The Solar Wind around Pluto (SWAP) instrument on the New Horizons spacecraft made the first measurements of this region of low-energy atmospheric ions shortly after its closest approach on 14 July 2015.

[50] The reddish-brown cap of the north pole of Charon, the largest of Pluto's moons (Mordor Macula), may be composed of tholins, organic macromolecules produced from methane, nitrogen and other gases released from the atmosphere of Pluto and transferred over about 19,000 km (12,000 mi) distance to the orbiting moon.

[12] In the 1970s, some astronomers forwarded the hypothesis of a thick atmosphere and even oceans of neon: according to some views of those times, all other gases that are abundant in the Solar System would either freeze or escape.

Infrared photometry by the 4-meter Nicholas U. Mayall Telescope revealed methane ice[55] on Pluto's surface, which must sublimate significantly at Plutonian temperatures.

[6][32] The first such observations were made on 19 August 1985 by Noah Brosch and Haim Mendelson of the Wise Observatory in Israel.

[31][56] But quality of the data was rather low due to unfavorable observational conditions (in addition, the detailed description[57] was published only 10 years later).

[12] On 9 June 1988 the existence of the atmosphere was convincingly proven[6] by occultation observations from eight sites (the best data were obtained by the Kuiper Airborne Observatory).

Scale height of the atmosphere was measured, making it possible to calculate the ratio of the temperature to the mean molecular mass.

The temperature and pressure themselves were impossible to calculate at the time due to an absence of data on the chemical composition of the atmosphere and a large uncertainty in the radius and mass of Pluto.

[11][24][60] The same year observations by the 3.0-meter NASA Infrared Telescope Facility revealed the first conclusive evidence of gaseous methane.

The first stellar occultations after 1988 were on 20 July and 21 August 2002 by teams led by Bruno Sicardy of the Paris Observatory[31] and James L. Elliot of MIT.

[7][11] An occultation of an exceptionally bright star, about 10 times brighter than the Sun itself, was observed on 29/30 June 2015 – only 2 weeks before the New Horizons encounter.