Pluto's atmosphere consists of a thin envelope of nitrogen, methane, and carbon monoxide, derived from the ices on its surface.[62] As Pluto moves away from the Sun, its atmosphere gradually freezes and falls to the ground. As it edges closer to the Sun, the temperature of Pluto's solid surface increases, causing the ices to sublimate into gas. This creates an anti-greenhouse effect; much like sweat cools the body as it evaporates from the surface of the skin, this sublimation has a cooling effect on the surface of Pluto. Scientists using the Submillimeter Array have recently discovered that Pluto's temperature is about 43 K (−230 °C), 10 K colder than expected.[63]
Pluto was found to have an atmosphere from an occultation observation in 1985; the finding was confirmed and significantly strengthened by extensive observations of another occultation in 1988. When an object with no atmosphere occults a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually.[64] From the rate of dimming, the atmospheric pressure was determined to be 0.15 pascal, roughly 1/700,000 that of Earth.[65]
In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the Paris Observatory,[66] James L. Elliot of MIT,[67] and Jay Pasachoff of Williams College.[68] The atmospheric pressure was estimated to be 0.3 pascal, even though Pluto was farther from the Sun than in 1988 and thus should have been colder and had a more rarefied atmosphere. One explanation for the discrepancy is that in 1987 the south pole of Pluto came out of shadow for the first time in 120 years, causing extra nitrogen to sublimate from the polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere.[69] Another stellar occultation was observed by the MIT-Williams College team of James Elliot, Jay Pasachoff, and a Southwest Research Institute team led by Leslie Young on June 12, 2006 from sites in Australia.[70]
In October 2006, Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and his colleagues announced the spectroscopic discovery of ethane on Pluto's surface. This ethane is produced from the photolysis or radiolysis (i.e., the chemical conversion driven by sunlight and charged particles) of frozen methane on Pluto's surface and suspended in its atmosphere.[71]
Orbit
Pluto's orbit is different from those of the planets. The planets all orbit the Sun close to a flat reference plane called the ecliptic and have nearly circular orbits. In contrast, Pluto's orbit is highly inclined relative to the ecliptic (over 17°) and highly eccentric (elliptical). This high eccentricity leads to a small region of Pluto's orbit lying closer to the Sun than Neptune's. Pluto was last interior to Neptune's orbit between February 7, 1979 and February 11, 1999. Detailed calculations indicate that the previous such occurrence lasted only fourteen years, from July 11, 1735 to September 15, 1749, whereas between April 30, 1483 and July 23, 1503, it had also lasted 20 years.
Although this repeating pattern may suggest a regular structure, in the long term Pluto's orbit is in fact chaotic. While computer simulations can be used to predict its position for several million years (both forward and backward in time), after intervals longer than the Lyapunov time of 10–20 million years, it is impossible to determine exactly where Pluto will be because its position becomes too sensitive to unmeasurably small details of the present state of the solar system.[72][73] For example, at any specific time many millions of years from now, Pluto may be at aphelion or perihelion (or anywhere in between), with no way for us to predict which. This does not mean that the orbit of Pluto itself is unstable, however, only that its position along that orbit is impossible to determine far into the future. In fact, several resonances and other dynamical effects keep Pluto's orbit stable, safe from planetary collision or scattering.
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