Ice Dwarfs - Pluto and Charon

Pluto-like planet, Pov-Ray model

Above: a Pov-Ray simulation of an orange ice-dwarf like Pluto.

Below: The Ice Mountains of a Pluto-like planet. A Sun-like star is visible in the Plutonian sky at a an equivalent distance of Sol from Pluto to illustrate the size of Sol as seen from Pluto.

Pluto, Pov-Ray model

Pluto, an orange ice dwarf is aptly named after a mythical god of the Underworld as it is far from the Sun. It is
so far from the Sun that the Sun appears as a bright star when Pluto is furthest away (at aphelion) and the disc
of the Sun is just about discernible when Pluto is at its closest (at perihelion). Light from the Sun is about one to
two thousand times as dim here, but since the eye detects the log of light intensity the perceived light levels are
only about 3 times lower (about one-third) than on Earth and looking at the Sun will still hurt the eyes.

Planet type: carbohydrate and water (?) ices, cold desert planet. The largest known object in the Kuiper Belt.
The Kuiper Belt is a ring or band of objects orbiting Sol beyond Neptune from about 30 to 50 AU (astronomical
units). I consider Pluto to be a planet of the dwarf category (rather than a 'dwarf planet' per se).

Equatorial Diameter: 2372 km (radius 1186 km).

Orbit: Pluto orbits Sol in a very eccentric (elliptical) orbit ranging from 29.657 to 48.871 AU from Sol, with a year
lasting 247.68 Earth years and a day length of 6.387 Earth days.

Atmosphere: Very thin (about 1 pa or one hundred thousandth of Earth's atmospheric pressure), consists of
nitrogen, methane and carbon monoxide, a visible hydrocarbon haze extends about 130 km (80 miles) above
the planet's surface; it possible snows hydrocarbon ice. The atmospheric gases are in equilibrium with surface
ices of nitrogen, methane and carbon monoxide. A cycle of ice sublimation and ice snow probably maintains
this equilibrium.

Surface Temperature: 33 to 55 K (-240 to -218 degrees C).

Magnetic Field: no data.

Life: none as yet discovered.

Some key tourist attractions

The surface of Pluto is relatively crater-free, suggesting that it is relatively young and geologically active. Large uncratered plains occur, such as Sputnik Planum.

Color - Pluto's surface varies in hue and brightness, but has an orange color due to organic compounds such as tholins (polymers formed by the action of the Sun's UV light on hydrocarbons such as methane).

Ice Mountains - a layer of water ice probably exists beneath Pluto's surface layers of methane, nitrogen and hydrocarbon ices. This is thought to account for the existence of mountain ranges on Pluto, with the mountains consisting of a water ice core (water ice is stronger than hydrocarbon ice). The tallest mountain range on Pluto is 6.2 km (Tenzing Montes).

Ice Plains - vast plains with a network of troughs containing darker material (some of which forms hills) possibly consist of frozen 'mud', the 'mud' consisting of particles of methane ices.

Dune Fields - large plains rippled with regular dune systems thought to be made chiefly of grains of methane ice acting like silica sand. this implies that winds on Pluto can cause significant geological activity, at least during certain times of the year. These transverse dunes are regularly spaced, with a wavelength of 0.4 to 1 km and are thought to have been deposited by winds between 1 m/s and 10 m/s in speed. (Pluto's cold and thin atmosphere is generally thought not to support higher wind speeds).

Nitrogen Glaciers - evidence indicates that Pluto possibly has glaciers of frozen nitrogen (and other ices such as CO and methane). Water ice is too brittle at these low temperatures to flow, behaving more like silica rock on Earth. Despite the low temperatures frozen nitrogen may remain sufficiently soft and fluid to slowly flow across Pluto's surface. Nitrogen freezes at about -210 degrees C (63 K) and boils at about -196 degrees C (77 k).

Giant Blades of Ice - jagged ridges of methane ice near Pluto's equator several hundred feet tall. These are thought to form when methane ice deposits are eroded by sublimation (evaporation directly into methane gas) due to relatively warm periods in Pluto's climate. Material can be seen that has flowed around hills and through breeches in crater rims.

Impact craters - though not especially frequent do occur and "many appear to be substantially degraded or infilled, and some are highlighted by bright ice-rich deposits on their rims and/or floors" (Stern et al. 2015) such as those in the dark equatorial region called Cthulhu Regio (CR).

Features are generally named as follows:

Planum - large plain

Below - artistic renditions of Pluto's ice mountains produced by a computer simulation. Can you see Sol in the sky on one of these views?

Pluto, Pov-Ray model

Pluto, Pov-Ray model

Pluto, Pov-Ray model

Pluto, Pov-Ray model

Below: the frozen mud-planes of Pluto. Charon is visible in the sky. Charon is Pluto's largest moon and although much smaller than Earth's Moon it is also much closer to its parent planet and so appears about 6 or 7 times as large in Pluto's sky as the Moon does on Earth.

Pluto and Charon pov-Ray model

Pluto is tiny as far as planets go. It is a dwarf planet, but in my classification a dwarf planet is not a distinct class from a planet. Rather, planets can be divided into dwarf planets, like Pluto, medium planets, like Earth, and giant planets, like Jupiter. In this scheme, a 'dwarf planet' is a subtype of 'planet'. Anything below 1000 km in radius I would not classify as a planet but rather as a planetoid. Let us compare Pluto, its largest moon Charon, The Earth and Earth's Moon. Also included for comparison is Eris, probably the most massive Kuiper Belt object and all the moons in the Solar system with a radius above 1000 km.

Values listed are: radius, mass relative to the Earth and surface gravity relative to the Earth

Pluto:           1186 km, 0.00218, 0.063g
Charon:        603.5 km, 0.000254, 0.028g
Earth:           63741 km, 1, 1g
Moon:          1737 km, 0.0123, 0.1654g
Eris:             1163 km, 0.0028, 0.084g
Triton:          1353 km, 0.00359, 0.0794g
Titan:            2576 km, 0.0225, 0.14g
IO:                1822 km, 0.015, 0.183g
Ganymede:  2634 km, 0.025, 0.146g
Callisto:        2410 km, 0.018, 0.126g
Europa:        1561 km, 0.008, 0.134g

In our classification the Moon would be a planet of the dwarf type, however, since it orbits the Earth which is much more massive the Moon can be considered a secondary planet. Objects like Charon and Ceres, a more-or-less spherical asteroid with a mean radius of 473 km would not be classes as a dwarf planet, but rather as a planetoid, since it has enough gravity to form a roughly spherical object. Objects with a radius of about 1000 km or greater usually have a richer and more varied geology. The four largest moons of Jupiter: IO, Ganymede, Callisto and Europa are secondary planets of the dwarf type, along with Titan and Triton. This would give the Solar System a total of 17 planets, of which 9 are dwarf planets. Furthermore, 7 of the nine dwarf planets are secondary planets (major moons), leaving ten primary planets in the Solar System. This seems the most natural and convenient classification to me. Eris is currently about three times as far from Sol as Pluto, though its highly eccentric orbit means that it is sometimes closer (perihelion of 37.9 AU, aphelion of 97.65 AU).

Pluto - NASA Horizons

Above:Pluto itself as imaged by NASA's New Horizons space probe:

The bright heart-shaped region in the south is Tombaugh Regio. To the left of Tombaugh Regio is the dark region: Cthulhu Regio. The upper left lobe of Tomaugn Regio is Sputnik Planitia (Sputnik Planum). The region of bright and dark streaks near the equator on the right is Tartarus Dorsa - an area of large ridges.

The Moons of Pluto

Pluto has one large moon, Charon. Indeed, Charon is more than half of Pluto's diameter, though much less massive, and both Pluto and Charon orbit a common center of mass which is above Pluto's surface (making this a binary system of two primary bodies, of which one is a planet and one a planetoid). Charon orbits about 19 570 km from Pluto's center or 17 540 km from the common center of mass (barycenter). The Earth-Moon system is sometimes considered a double or binary planet, however, the center of mass lies beneath the surface of the Earth and so the Moon is a secondary planet in our classification.

Pluto has four other moons, which are tiny asteroids captured from the Kuiper Belt. These are Styx, Nix, Kerberos and Hydra. Of these, Hydra is the largest at 55 by 40 km across (and hence not even a planetoid in our classification as it is not even roughly spherical). These orbit around the central Pluto-Charon system.

Charon's surface is apparently dominated by water ice rather than by methane or nitrogen ices and is less orange and more brownish in color. One theory is that an impact with Pluto vaporised part of its icy mantle which condensed into Charon. There is some evidence that Charon may have cryogeysers. The surface of Charon is also relatively crater-free, suggesting that it's surface is relatively young and geologically active.

Is Pluto a Planet?

Scientists disagree on whether Pluto is a planet or not. The IAU (International Astronomical Union) decided to reclassify Pluto as a 'dwarf planet', which is somehow not a 'planet'. Yes, Pluto is a dwarf planet as in a planet that's small but it can still be considered a planet. Why? Because science does not work by diktat: unions and 'official' bodies can define their own definitions and classification systems but they are not absolutely official as the Media have portrayed them to be. Many scientists still regard Pluto as a planet and they have the official capacity to do so. Geologists and interplanetary scientists are among those who study planets, apart from 'astronomers' per se and they have just as much say in the matter. The Media have been disingenuous in explicitly stating the IAU definition as set in stone and official, as if in some legal capacity: they are wrong! Science is not politics!

Cronodon agrees with many planetary scientists in defining planets on their own merits, as objects in their own right, rather than on orbital properties. Other scientists may agree or disagree. A planet can be considered as an object with sufficient gravity to pull itself into an approximate sphere. Large moons can be considered secondary planets. Pluto is clearly sufficiently spherical and has interesting geological features showing clear signs of current geological activity. Cronodon considers Pluto to be a true planet of the dwarf class.

Pluto and Charon are almost Perfect Spheres

In terms of sphericity, measurements suggest that the difference between the equatorial and polar diameters of Pluto are less than 12 km different, so the polar diameter is at least 99% of the equatorial diameter. Generally, planets with liquid interiors or extensive gaseous envelopes extend along their equator due to the centrifugal force of the planet's rotation. Tidal bulges due to the gravitational pull of other nearby bodies can also deform a planet. In Pluto's case no oblateness (polar flattening) has been detected. Similarly, Charon has no detectable oblateness, placing a maximum limit of a 1% difference in its polar and equatorial radii.

Pluto is tidally locked with Charon: both bodies present the same face to one-another (their rate of spin on their own axes is equal to the average orbital period of the two bodies about their common center of mass or barycenter which is outside the surface of Pluto). The orbits of the two bodies about the barycenter are circular. Furthermore, the equators and orbital plane are coplanar. This situation could have arisen in two probable ways: either two bodies collided, forming Pluto and Charon from the impact debris in circular orbits, or Charon was captured by Pluto, perhaps after a grazing impact, in which case the orbit of Charon may have been elliptical or eccentric to begin with. Two such bodies orbiting in close proximity exert gravitational pulls on one-another, resulting in tidal bulges on each body: the bulges move as the bodies spin, but because of friction within the deforming bodies, the bulges lag behind the gravitational pull. This results in tugs acting on the two bodies to slow their spins and transfer angular momentum to their orbits. The total angular momentum must be conserved in an isolated system, so as angular momentum is transferred, the spins slow as the orbits speed up, until the two become equal and the bodies become tidally locked. The orbits also become circular as the tidal forces resist eccentric orbits in which resistive tidal forces increase sharply when the bodies are closer together: circular orbits offer less resistance.

The fact that very little or no polar flattening / equatorial extension has occurred in Pluto suggests that Pluto remained warm, soft and deformable during the early evolution of the Pluto-Charon binary system either during or after the tidal locking. It may also suggest that Pluto was not spinning very fast to begin with.

References / Further Reading

Stern, S.A., Bagenal, F., Ennico, K., et al. 2015. The Pluto system: Initial results from its exploration by New Horizons. Science 350(6258): aad1815. DOI: 10.1126/science.aad1815

Telfer, M.W., Parteli, E.J.R., Radebaugh, J., et al. 2018. Dunes on Pluto. Science 360(6392): 992-997.
DOI: 10.1126/science.aao2975

Article updated:
26 July 2015
30 June 2020