Pluto

An image of Pluto taken in 1979 showed that Pluto has a moon. Named Charon, this moon orbits its parent planet in close proximity--at only 19,640 km from Pluto. This proximity to Pluto is the main reason why Charon was not discovered earlier; the moon is often hidden behind the planet. Charon has a radius of 600 km, while Pluto has a radius of 1,150 km. Pluto's moon has the same density as itself, which means that the two bodies have a mass ratio of seven. Such a small ratio is quite unusual for a planet and its moon, and indicates an unusual formation process.

The radii of Pluto and Charon are known with such accuracy because we observed the two bodies during a brief five year period 1985-1990, when Charon's orbit was seen edge-on from Earth. Because of the orientation of the orbit Pluto and Charon periodically eclipsed each other, and from the light variations during the eclipses we could accurately calculate their radii. From prior information about mass and radius, scientists determined their density. Pluto and Charon have the same density, about 2 g/cm³.

All the best and most recent data about Pluto comes from observations made using the Hubble Space Telescope (HST). This orbiting telescope has the best resolution available today, because its orbit is above Earth's atmosphere. Our atmosphere is quite turbulent, even at the top of the mountains and in the desert regions where Earth-based telescopes reside, and can result in distorted images. The HST, by contrast, is so accurate that it can distinguish details as small as a penny at a 50-km distance.

Using the HST, scientists have discovered that Pluto does not reflect light homogeneously, and that the planet has an average albedo of 30%. There are brighter and darker regions, on the scale of a big fraction of the planet's radius. One of the bright spots corresponds to Pluto's south pole. By observing the motion of the pattern of dark and bright spots, scientists know that the planet as a whole rotates around an axis oriented sideways, along Pluto's orbit, with a tilt of 118 degrees. The rotation is retrograde, i.e., east to west, and 6.39 days long.

Observation of Charon's spots has yielded much knowledge as well. These spots' motion reveals that Charon's rotation period is the same as Pluto's, and equal to its orbital period. Pluto and Charon always show the same side to each other. This is not a coincidence: Pluto and Charon may have had quite different periods of rotation and revolution when they first became a planet-moon system, but tides between the two bodies probably eventually equalized the periods to the value we measure today.

Similarly, tides are responsible for the fact that our own Moon always shows the same side to Earth; scientists were only able to map the other side by sending probes. Charon is so close to Pluto that the force of gravity exerted by these two bodies on each other is quite uneven, with the closest side attracted more than the farthest side. Consequently, Pluto and Charon are both slightly deformed; they have taken on a prolate shape.

If their period of rotation did not match their orbital period, different parts of Pluto and Charon would periodically rise and then fall to a lower level, compared to their center. The stress generated on the rocks in their interior by these periodic motions would be transformed into heat, until the rotation period matched the orbital period and the stresses disappeared.

The phenomenon we have just described is happening in the Earth-Moon system as well. A little known fact is that Earth's rotation period is slowing down, albeit at a very miniscule rate: 300 million years ago, one day on Earth used to last only 19 hours; far in the future, the Earth will rotate with the same rate as the orbital period of the Moon.

Pluto also has a very tenuous atmosphere, made of nitrogen (N₂) and carbon monoxide (CO). This is probably the result of the sublimation of some gas when exposed to the dim sunlight. Charon is not massive enough to retain any atmosphere, since any gas produced just escapes its low surface gravity.

The analysis that the HST made of Pluto's surface revealed that the planet is covered in nitrogen (N₂), carbon monoxide (CO) and methane (CH₄) ices. Its temperature is only 50 K, although this value is not known with great accuracy. Observation of Charon yielded slightly different results: there the ice appears to be made of water, and there is no clear evidence of any methane ice. The darker spots on both Pluto and Charon might be the result of a phenomenon known as "radiation darkening."

Similarly, radiation darkening may explain the darker color and low albedo of many of the moons and the rings of Uranus and Neptune. When methane ice is exposed to some form of radiation, e.g., ultraviolet rays, it can form more complex hydrocarbons. These compounds confer a darker tint to the external ice layer (the layer exposed to the radiation).

The similarities between Pluto and Charon, and the fact that Charon is quite a large moon, suggest that the formation of this moon might be the consequence of an unusual mechanism. Perhaps the moon originated from a collision between Pluto and another planet of similar size. Alternatively, Charon and Pluto may have formed separately, but under similar conditions; Charon may have then been captured by Pluto's gravity at a later time.