Although the detection of methane ice on Pluto’s surface in the 1970s (see below The surface and interior) gave scientists confidence that the body had an atmosphere, direct observation of it had to wait until the next decade. Discovery of its atmosphere was made in 1988 when Pluto passed in front of (occulted) a star as observed from Earth. The star’s light gradually dimmed just before it disappeared behind Pluto, demonstrating the presence of a thin, greatly distended atmosphere. Because Pluto’s atmosphere must consist of vapours in equilibrium with their ices, small changes in temperature should have a large effect on the amount of gas in the atmosphere. During the years surrounding Pluto’s perihelion in 1989, when Pluto was slightly less cold than average, more of its frozen gases vaporized; the atmosphere was then at or near its thickest, making it a favourable time to study the body. Astronomers in the year 2000 estimated a surface pressure in the range of a few to several tens of microbars (one microbar is one-millionth of sea-level pressure on Earth). At aphelion, when Pluto is receiving the least sunlight, its atmosphere may not be detectable at all.

Observations made during occultations showed that nitrogen was the primary gas in the Plutonian atmosphere, which also contained small amounts of methane, carbon monoxide, and hydrogen cyanide. (Nitrogen is also the main constituent of the atmospheres of both Triton and Saturn’s largest satellite, Titan, as well as of Earth.) During its flyby, New Horizons determined the surface pressure to be 10 microbars and found acetylene, ethylene, and ethane in the atmosphere. The temperature near the surface is 45 K (−228 °C, or −379 °F). Haze layers are seen up to an altitude of 200 km (120 miles). The upper atmosphere is quite extended, going up to 1,800 km (1,100 miles) from the surface, and quite cold, which keeps nitrogen from escaping into space.

The surface and interior

New Horizons observed only one hemisphere of Pluto. That hemisphere is dominated by Tombaugh Regio, a white heart-shaped plain. The western half of Tombaugh Regio is Sputnik Planitia, a smooth plain of nitrogen ice without impact craters. The lack of craters shows that Sputnik Planitia is a very young feature and thus that Pluto likely has some geologic activity. Tombaugh Regio is surrounded by less smooth regions that contain some mountain ranges. These mountains are made of water ice, which are likely floating in the surrounding nitrogen ice. Higher northern latitudes are covered in darker plains. To the west of Tombaugh Regio is the darkest region on Pluto. Originally nicknamed “the whale” for its shape and later dubbed Cthulhu Regio, this region has a varied topography with plains, scarps, mountains, and craters. This region’s dark colour arises from organic compounds called tholins.

Pluto’s average reflectivity, or albedo, is 0.72 (i.e., it returns 55 percent of the light that strikes it), compared with 0.1 for the Moon and 0.8 for Triton. However, this average albedo encompasses a wide range of reflectivities, with Cthulhu Regio having a reflectivity of 0.1 to 0.2 and Tombaugh Regio having a reflectivity from 0.8 to 1.

The first crude infrared spectroscopic measurements (see spectroscopy), made in 1976, revealed the presence of solid methane on Pluto’s surface. Using new ground-based instrumentation available in the early 1990s, observers discovered ices of water, carbon monoxide, and molecular nitrogen. Although nitrogen’s spectral signature is intrinsically very weak, it is now clear that this substance must be the dominant surface constituent. The methane is present both as patches of pure methane ice and as a frozen “solution” of methane in the nitrogen ice.

Pluto has a density of 1.85 grams per cubic cm, and Charon has a density of 1.7 grams per cubic cm. These values suggest that both bodies are composed of a significant fraction of materials such as silicate rock and organic compounds denser than water ice (which has a density of 1 gram per cubic cm). Charon’s lower density may have arisen from its being more porous or having a lower fraction of rock. Pluto, like the icy moons of Jupiter and Saturn, likely has an inner rocky core surrounded by a thick mantle of water ice. The frozen nitrogen, carbon monoxide, and methane observed on its surface are in the form of a relatively thin layer, similar to the layer of water on Earth’s surface. Sputnik Planitia is a deep basin that may have formed as a result of an impact. It is located on Pluto’s tidal axis; that is, it is on the opposite side of the dwarf planet from Charon. Sputnik Planitia’s location requires that there be extra mass underneath it, and this extra mass may be from a subsurface ocean above the rocky core and below the water ice mantle.