Pluto

Pluto was discovered in 1930 as the “ninth planet” of the solar system. It was discovered by Clyde Tombaugh (1906 – 1997). Named after the ruler of the underworld in classical mythology, its existence was predicted in 1905 by American astronomer Percival Lowell (1855 – 1916).

Pluto taken on 2015
A high-resolution enhanced colour image captured 14th July 2015. Nasa/JHUAPL/SwRI

He discovered that the orbits of Neptune and Uranus had “odd deviations”, which suggested to him that the gravity of another large body was tugging at these two planets. It was a remarkable piece of astronomical observation and deduction and he was rightly honoured in the naming of the Lowell Observatory.

Pluto was reclassified as a dwarf planet in a controversial decision by the International Astronomical Union in 2006. One important definition of a planet is a body that has cleared its orbital path. Pluto has not achieved this. And at a diameter of 2 370 kilometres it is indeed a dwarf, being smaller even than our very own moon (which has a diameter of 3 476 kilometres). Even so, Pluto at least has the title of being the largest body travelling in the Kuiper Belt. Eris is the second largest with a diameter of 2 336 kilometres, although it is heavier than Pluto.

Structure

Pluto is a tiny world of rock (accounting for two-thirds of its mass) covered with a mantle rich in water ice and frozen gas. On top is a thin icy crust. Compared to Earth it is 174 times smaller and only 0.22% of its mass. No surprise this is reflected in a miniscule gravity where a 68kg person on Earth would weigh just 4.5 kg on Pluto.

Pluto resembles some of the moons of Neptune and Saturn but has big differences too. Its surface is made of three types of ice: carbon dioxide, methane, and nitrogen; with solid silicates. The surface temperature is -233°C. Nitrogen glaciers flow down into plains of frozen methane.

The most primordial part of Pluto’s surface may be a hemisphere-spanning splash of red called Cthulhu Regio, a region so thoroughly pulverized by craters it is thought to be some four billion years old. Curiously, it is directly adjacent to the western edge of what could be Pluto’s youngest landform, the fresh, cream-coloured ices of Sputnik Planum. But even this feature, as it turns out, has surprisingly ancient roots: its youthful ice fills a deep basin that may be the oldest, largest impact crater on Pluto. Northeast of Sputnik Planum, past stretches of pitted plains, the surface is wrinkled with closely spaced ridges that rise sharp and knife-like half a kilometre into the air. This “bladed terrain” may be vestiges of an old, once-buried layer of highly durable material exhumed and weathered by some combination of scouring ice, swirling winds and glaring sunlight. Or it may be newer, formed from airborne methane frost glazing rigid crests of water ice.

To the south, Nasa scientists have identified two young cryovolcanoes, Wright Mons and Piccard Mons. These are relatively unblemished kilometres-high mounds surrounding central pits at least as deep. Taken together, these features show that more than four billion years after its formation, Pluto still holds enough internal heat to maintain an active geology and, here and there, a very youthful surface replenished by cryovolcanism and the seasonal sublimation and deposition of volatile ices. Deep within the world, Nasa says Pluto’s heat could be sufficient to sustain an ocean of ammonia-rich water beneath a thick roof of water-ice bedrock. Long linear striations upon parts of Pluto’s surface hint that any subsurface ocean may be slowly freezing, deforming the ground and releasing additional latent heat as it turns to ice.

Most of the rest of Pluto’s exterior is far craggier and ancient, altered extensively across hundreds of millions or billions of years. Varying mixtures and combinations of nitrogen, water, carbon monoxide and methane that make up Pluto’s crust create different varieties of ice and terrain. This is similar to how rocks on Earth can form cliffs of soft chalk or mountains of hard granite. These varying substrates can then be textured with pits, grooves and channels produced by subliming ice, eroding glaciers and precipitating frost—effects driven by Pluto’s weather, which fluctuates in decades-long seasons.

There are some regions on the surface of Pluto near the equator that scientists say are as dark as anything so far discovered in the solar system. But as to why Pluto’s surface is so dark, and what the various splotches could be, remains a mystery. The light-coloured polar regions are made of methane and nitrogen ice.

Atmosphere

Pluto has a thin atmosphere of nitrogen, methane, and other gases that might sometimes freeze entirely onto the surface. Nasa’s New Horizons also revealed that the atmosphere is layered with hazes of soot-like hydrocarbon particles produced by ultraviolet light and cosmic rays. The particles are reddish, but at sunrise and sunset, when sunlight passes through the thickest hazes, they scatter the light to give Pluto’s sky a blue tint. The particles are also “sticky” and grow like snowflakes over tens of thousands of years, until at last they become heavy enough to fall, accumulating as crimson sludge on the surface.

Orbit

Like Uranus Pluto is tipped on its side, in this case with an axial tilt of 122°, which means it spins in the opposite direction to Earth. Pluto has the greatest orbital eccentricity of any of the eight planets: aphelion is 7.38 billion kilometres distant; perihelion is 4.44 billion kilometres distant. Pluto has an orbital inclination of 17° to the major planets and has an elongated orbit (though not as stretched out as a comet’s). Pluto is usually farther from the Sun than any of the planets; however, due to the eccentricity of its orbit, it is closer than Neptune for 20 years out of its 248-year orbit. Pluto crossed Neptune’s orbit on the 21st January 1979 and made its closest approach on the 5th September1989. It returned to its outermost spot around February 1999 and will not venture across Neptune’s path again until September 2226.

As Pluto approaches perihelion it reaches its maximum distance from the ecliptic due to its 17-degree orbital inclination. Thus, it is far above or below the plane of Neptune’s orbit. Under these conditions, Pluto and Neptune will never collide.

A day on Pluto is equivalent to 6 days and 9 hours on Earth. So its axial spin is very slow with a speed of just 48 kilometres an hour at the equator.

The moons of Pluto

Pluto has one large moon, Charon —the largest of any moon in the solar system in proportion to the size of its planet— and four tiny moons: Nix, Hydra, Kerberos, and Styx.

Charon was discovered telescopically on 22nd June 1978, by James W. Christy and Robert S. Harrington at the U.S. Naval Observatory station in Flagstaff, Arizona. The moon was named after the ferryman conveying the dead souls to the realm of Hades (the Greek counterpart of the Roman god Pluto).

Charon is so large and massive with respect to Pluto that some scientists prefer to think of the two bodies as a double system. Its diameter is 1 208 kilometres, so a little more than half that of Pluto. Its mass is more than one-tenth of Pluto’s mass. Its lack of substantial atmosphere made it easy to determine the moon’s diameter even using ground-based telescopes.

Scientists think that Charon was once joined to Pluto and split away when an asteroid smashed into it.

Charon also seems to have a subsurface ocean—or at least it used to. Nasa’s New Horizons discovered a deep gash snaking across some 1 800 kilometres of its surface—a furrow four times longer than the Grand Canyon in the US. The gash seems to be from a time when Charon’s inner ocean froze, bulging as it turned to ice and rupturing the moon’s crust from within.

As for Pluto’s tiny moons Nasa’s New Horizons found them to be much brighter and smaller than most researchers expected. They all spin rapidly and have extreme axial tilts that are so off-kilter to those of Pluto and Charon that they are not easily explained. These smaller moons were probably also produced by the great Charon-forming impact. They also possess crater-battered surfaces of four-billion-year-old water ice. But they are oblong rather than spherical, as if they are all less moons and more barely-held-together piles of coalesced rubble. At least one of them, Kerberos, is shaped like a dumbbell, signalling its formation from two smaller bodies merging after a cataclysmic impact

Nix and Hydra are estimated at 35 kilometres and 45 kilometres in diameter respectively. Hydra must be dizzy. It wobbles like a spinning top, and spins 89 times for each orbit of Pluto.

Kerberos and Styx are even smaller and not nearly as bright, making them difficult to measure—and to spot. They were discovered using the Hubble Space Telescope in 2011 and 2012 respectively.

Exploration

Nasa’s New Horizons craft left Earth on 19th January 2006 and reached Pluto on 9th July 2015. On its journey it achieved the fastest recorded speed of any spacecraft reaching a speed of 72 420 kilometres an hour (20 kilometres a second) on its 4.8-billion-kilometre journey to Pluto. The craft flew by multiple planets through the course of its nine-year trip, snapping a photo of Neptune and its large moon Triton, for example.

It was the size of a baby grand piano and flew about 12 550 kilometres above Pluto’s surface, closer than any human-made spacecraft has ever been to the cosmic body. It carried some of the ashes of its discoverer.

By Nigel Benetton, science fiction author of Red Moon and The Sands of Rotar.
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Last updated: Tuesday, 24th March 2020