12.4 Pluto and Charon

Pluto and Charon

Pluto and Charon form one of the most unusual pairings in the Solar System. Studying them helps us understand the diverse icy worlds that populate the region beyond Neptune, and NASA's New Horizons flyby in 2015 transformed them from blurry dots into richly detailed worlds.

Pluto's orbit vs planetary orbits

Pluto's orbit stands out from the eight planets in two major ways: it's far more elliptical (stretched out) and more inclined (tilted about 17° relative to the plane most planets orbit in). Its orbital period is 248 Earth years, and its average distance from the Sun is about 39.5 AU (roughly 5.9 billion km).

Because Pluto's elliptical orbit actually crosses inside Neptune's orbit for part of its journey, you might wonder why they never collide. The answer is orbital resonance: Pluto completes exactly 2 orbits for every 3 orbits Neptune completes (a 2:3 resonance). This timing ensures the two bodies are never in the same part of their orbits at the same time.

Pluto was reclassified as a dwarf planet in 2006 by the International Astronomical Union. To qualify as a full planet, an object must meet three criteria:

1. It orbits the Sun
2. It has enough mass for gravity to pull it into a roughly round shape (hydrostatic equilibrium)
3. It has "cleared the neighborhood" around its orbit

Pluto meets the first two but fails the third. Its orbit is full of other Kuiper Belt Objects, so it hasn't gravitationally dominated its region the way Earth or Jupiter have. Pluto is also classified as a trans-Neptunian object because its orbit lies beyond Neptune.

Surface features from New Horizons data

The New Horizons spacecraft flew past Pluto in July 2015, revealing a world far more geologically complex than anyone expected.

• Sputnik Planitia is a vast, heart-shaped plain of nitrogen ice roughly 1,000 km across. Its surface shows polygon-shaped convection cells where warmer nitrogen ice slowly rises, spreads, and sinks again, similar to water boiling in a pot but happening over millions of years.
• Mountainous regions rise several kilometers high and are likely made of water ice, which at Pluto's temperatures (around $-230°C$) behaves like rock.
• Cthulhu Macula is a large, dark, heavily cratered region, suggesting it's an ancient surface that hasn't been resurfaced.

Some areas of Pluto have surprisingly few craters, which tells scientists those surfaces must be geologically young. Cryovolcanism (volcanism involving ices like water or ammonia instead of molten rock) may be responsible for resurfacing parts of the terrain.

Pluto also has a thin atmosphere, composed mainly of nitrogen with traces of methane and carbon monoxide. New Horizons photographed distinct haze layers extending high above the surface. Ultraviolet light breaks down methane in this atmosphere, producing complex organic molecules called tholins, which give some of Pluto's surface its reddish-brown color.

Charon's properties and Pluto relationship

Charon is unusually large compared to Pluto. Its diameter is about 1,212 km, roughly half of Pluto's 2,377 km. No other known moon in the Solar System comes close to that size ratio relative to its parent body.

This size relationship has a major consequence: the barycenter (the center of mass that both bodies orbit) lies in the space between Pluto and Charon, not inside Pluto. Because of this, some scientists describe the pair as a binary system or double dwarf planet, since both bodies visibly orbit a shared point.

Pluto and Charon are also mutually tidally locked. Each one always shows the same face to the other, much like how our Moon always shows the same face to Earth, except here it works both ways.

Charon has its own interesting geology:

• Serenity Chasma, a system of canyons and fractures stretching across Charon's surface, points to past tectonic activity, possibly caused by an ancient internal ocean freezing and expanding.
• Kubrick Mons is a mountain sitting inside a depression, which may be evidence of past cryovolcanism.
• Mordor Macula, the dark reddish cap on Charon's north pole, is thought to be made of tholins. These organic compounds likely escaped from Pluto's atmosphere, drifted to Charon, and became trapped at the cold pole.

Surface composition and reflectivity

Pluto's surface is dominated by nitrogen ice, which forms the smooth plains of the region informally called Tombaugh Regio (which includes Sputnik Planitia). Methane and carbon monoxide ices are also present in various locations.

The mix of surface ices gives Pluto a wide range of albedo (reflectivity) values. Brighter areas with high albedo tend to be fresher, cleaner ices like nitrogen. Darker areas with low albedo, such as Cthulhu Macula, contain more tholins and other complex organic compounds that absorb sunlight rather than reflecting it. This contrast is what gives Pluto its striking patchwork appearance in New Horizons images.