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Physics LibreTexts

7.34 Pluto

Pluto and its largest satellite, Charon (pronounced care-on) are frontier outposts of the Solar System. From the cold distance of Pluto’s orbit the Sun would appear only as bright as a streetlight seen at night from several hundred yards away. Even as imaged by the Hubble Space Telescope, it is only a tiny, fuzzy disk. Even Pluto’s size is not known precisely – the best values still have errors of about a percent. Astronomers have done some studies of how the brightness changes as Pluto rotates and Charon eclipses it. These data reveal patchy bright and dark markings that may be deposits of different kinds of frosts. While our understanding is currently without detail, this will all change when the NASA space mission New Horizons reaches Pluto in July, 2015.


A photomap of Dithering image based on pictures of Pluto from 2002 to 2003 taken by the Advance Camera for Survey of the Hubble telescope. Click here for original source URL

Charon is half the size of Pluto itself – the largest ratio between two co-orbiting bodies in the Solar System, as far as we know. Pluto rotates in 6.4 days and Charon moves around Pluto once in the same time period 6.4 days. The two are locked in a dance, always facing each other. This unique configuration is probably the result of tidal forces between the two bodies.

Pluto has a density close to 2000 kg m-3, indicating that it’s about 70% rock and 30% ice, similar in composition to Neptune’s large satellite Triton. Charon’s density may be a bit lower – meaning it may be more ice-rich. The daytime surface temperature on both Pluto and Charon is about the same as that on Triton, about 38 K (-391 °F). In the past decade, four additional small satellites have been found orbiting Pluto: Nix and Hydra were discovered in 2005, Kerberos was discovered in 2011, and Styx was discovered in 2012. Some of the small moons may have been formed by collisions.

Curiously,spectra show that the ice and frost on Charon have a very different composition than on Pluto. Pluto has mostly nitrogen ice, with a few percent of frozen methane and carbon monoxide mixed in. It’s a bright mixture, reflecting 60% of incoming sunlight. Charon’s surface is darker, and it’s mostly frozen water. Why would two such intimately linked worlds have different surface materials? Researchers think that lighter molecules such as methane escaped from Charon because of its lower gravity. Once these molecules escaped into space within the Pluto/Charon system, some of them collided with the surface of Pluto itself. So over time, Charon has lost methane molecules, and Pluto has gained some of them. This would explain the difference in their surfaces.
The challenge of exploration is one of the principal reasons for considering a mission to study Pluto and Charon. Recent telescopic observations have revealed some basic physical characteristics of these two bodies, but until New Horizons visits this system, Pluto and Charon will remain as tantalizing reminders of unfinished business at the edge of the Solar System. Meanwhile, after initial dismay and some consternation, the general public is getting used to the fact that Pluto was demoted to the status of a dwarf (or minor) planet in 2006. The rationale was the fact that at least one, and possibly more, outer Solar System objects are larger than Pluto, and Pluto has low mass and so did not clear out the material in its orbit when it formed.


This is a close up look at Pluto and Charon taken January 2015. Click here for original source URL.