Dwarf Planet Facts: Pluto

Dwarf Planet Facts: Pluto

The image above shows a near true color picture of the dwarf planet designated 134340 Pluto, the first object to be discovered in the Kuiper Belt after Clyde Tombaugh identified it in 1930. Pluto is the biggest known dwarf planet, and second most massive in the solar system after Eris, which is 27% more massive. It is also the the ninth biggest and tenth most massive object in the entire solar system that directly orbits the Sun.

Pluto was not named by its discoverer, but by an 11-year-old, British school girl named Venetia Burney, who suggested the name Pluto (god of the underworld) to her grandfather, a librarian working at Bodleian Library at the time. The grandfather, Herbert Hall Turner, forwarded the girls’ suggestion to colleagues in the US, and the name Pluto was officially adopted on May 1, 1930. The two other names suggested on the shortlist included Minerva and Cronus.

Quick Facts

• Aphelion: 49.305 AU
• Perihelion: 29.658 AU
• Eccentricity: 0.2488
• Orbital period: 248 years (90 560 days)
• Equatorial rotation velocity: 47.18 kilometres/hour
• Average orbital speed: 4.67 km/sec
• Dimensions: 2,376.6 ±3.2 km (Mean Diameter)
• Mean temperature : 44K ( -229C / -380F)
• Volume: 7.057 ± 0.004 × 109 cubic kilometres (0.00651 Earths)
• Mass: 1.303 ± 0.003 × 1022 kilograms (0.00218 Earths / 0.177 Moons)
• Surface area: 1.779×107 square kilometres ( 0.035 Earths)
• Mean density: 1.860±0.013 gram/cubic centimetre
• Escape velocity: 1.212 km/sec
• Apparent magnitude: Variable from 13.65 to 16.3
• Satellites: Charon, Hydra, Kerberos, Nix, Styx

Visibility

While Pluto is not readily visible with modest amateur equipment, observers that have access to a large telescope of at least  8-inches may be able to see the dwarf planet on a dark, clear, country night.

Physical Properties

Due to its vast distance from Earth, not much was known about Pluto’s size and mass until the discovery of Charon, Pluto’s closest satellite, in 1978. Occultations of Pluto by Charon allowed investigators to measure the dwarf planet’s diameter accurately, while the use of adaptive optics on ground-based telescopes facilitated a more precise determination of Pluto’s shape to be made.

However, it was only during the New Horizons spacecrafts’ flyby in 2015 that detailed information about the surface, internal structure, and atmosphere of the dwarf planet became available, some key findings of which are summarized below:

Surface

New Horizons showed that Pluto’s surface is at least as varied in terms of color and brightness as Saturn’s moon Iapetus, with areas that range in color from charcoal black to dark orange and even almost pure white. For the most part, the plains on Pluto consist primarily of nitrogen ice that hold trace amounts of methane and carbon monoxide. It is worth noting that Pluto and Charon are tidally locked, and that nitrogen and carbon monoxide predominate on Pluto’s hemisphere that faces away from Charon, while most methane is concentrated in one location at about 300 degrees east.

Pluto’s surface hosts many notable geological features, such as mountains that consist mostly of various ices. Some particularly noteworthy features include Tombaugh Regio (Tombaugh’s Heart), an enormous bright area on the side of Pluto facing away from Charon; Cthulhu Regio, a large dark area on the trailing hemisphere that faintly resembles a whale; and a series of dark areas on Pluto’s equator that rejoices in the name “The Brass Knuckles”, after their resemblance to this offensive weapon.

One other notable feature is named Sputnik Planitia (the western lobe of Tombaugh’s Heart), a 1,000 km- wide basin of nitrogen and carbon and monoxide ices that is divided into huge polygonal cells. While the origin of these structures is not clear, most investigators believe that they are convection cells that carry blocks of frozen water ice towards the margins of the basin, since there is clear evidence of glacial flows both in and out of the basin.

The absence of impact cratering on Pluto’s surface suggests that the surface is less than 10 million years old. In fact, recent studies suggest that Pluto’s surface is only about 180,000 years old. However, there is not enough information available to explain all of the surface features on Pluto at this time, which prompted the New Horizons science team to summarize the available data thus- “Pluto displays a surprisingly wide variety of geological landforms, including those resulting from glaciological and surface–atmosphere interactions as well as impact, tectonic, possible cryovolcanic, and mass-wasting processes.”

Atmosphere

New Horizons also discovered that Pluto’s tenuous atmosphere consists primarily of nitrogen (N2), methane (CH4), and carbon monoxide (CO), all of which are in equilibrium with their ices on Pluto’s surface. Available data shows that the surface atmospheric pressure on Pluto is about 1 Pa (10 µbar), which is between 100,000 and 1 million times more than the sea-level atmospheric pressure on Earth.

Somewhat surprisingly, New Horizons also found that instead of decreasing in density as Pluto moved away from the Sun, causing the atmosphere to freeze and fall back to the surface, the atmospheric density actually increases, and that the atmosphere therefore likely remains in a gaseous phase throughout Pluto’s orbit around the Sun.

While the exact mechanisms that regulate Pluto’s atmosphere are complex and not yet fully understood, it is thought that the presence of large quantities of methane in the atmosphere acts like green house gas on Pluto just as it does on Earth. In practice, the methane in Pluto’s atmosphere causes a temperature inversion layer that in turn, causes Pluto’s atmosphere to be several tens of degrees warmer than the surface, but it must be noted that Pluto’s upper atmosphere is about 30K (-243C/-405F) colder than expected.

Interestingly, Pluto’s atmosphere extends to an altitude of 1,670 km above the surface, and while it does not have a sharply defined upper limit, it is divided into about 20 or so layers that are each up to 150 km thick and thought to be the result of pressure waves that are created when atmospheric winds blow across Pluto’s ice mountains.

Pluto Internal Structure
Image Credit: John Hopkins Applied Physics Laboratory

Internal structure

The image above represents the best estimates on what constitutes the internal structure of Pluto. Investigators think that Pluto’s’ structure is differentiated due to the decay of radioactive elements, which would have caused the rocky material to aggregate into a rocky core estimated to be about 1,700 km in diameter. This in turn accounts for about 70% of the dwarf planet’s overall diameter. Note, however, that Pluto has no magnetic field.

The core is likely overlaid by a mantle of water ice, with a 100-180 km deep ocean of liquid water trapped between the core and the mantle. While the existence of a sub-surface ocean is not certain, computer modelling/simulations of the impact that had created the Sputnik Planitia suggest that the basin might have been created by the upwelling of liquid water after the impact.

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