CHAPTER 24 | URANUS, NEPTUNE, AND THE DWARF PLANETS 545
Th e discovery of Pluto’s moons is important for a number of
reasons. Charon, the largest and easiest to track, orbits Pluto in
a nearly circular orbit in the plane of Pluto’s equator. Observations
show that the moon and Pluto are tidally locked to each other
and that Pluto’s axis of rotation is highly inclined to its orbit
around the sun (■ Figure 24-19). Furthermore, tracking the
orbital motion of Charon allowed calculation of the mass of
Pluto. Charon orbits 19,640 km from Pluto with an orbital
period of 6.387 days. Kepler’s third law reveals that the mass of
the system is 6.5 10 −9 solar masses or only about 0.002 Earth
mass. Most of that mass is Pluto, which seems to be about
12 times more massive than Charon.
You know that the mass of a body is important in astronomy
because mass divided by volume is density. Th e density of Pluto
is about 2 g/cm^3 , and the density of Charon is just a bit less.
Th ose densities indicate that Pluto and Charon must contain
about 35 percent ice and 65 percent rock. Spectra of Charon
show that the small moon has a surface that is mostly water ice
with not much evidence of other volatiles that are detected in
spectra of Pluto. Perhaps Charon has lost its more volatile com-
pounds because of its lower escape velocity. Water ice at Charon’s
surface temperature is no more volatile than is a piece of rock on
Earth, so a water ice surface could last a long time.
Another reason Pluto’s moon Charon has proved important
is that, as Charon and Pluto orbit the sun, occasionally their
mutual orbit is seen edge-on from Earth. During those times,
astronomers can watch Pluto and Charon eclipse each other;
and, by carefully measuring the combined light from the two
objects, they can produce crude maps (■ Figure 24-20). Th oseastronomers have used a clever trick to make low-resolution maps
of Pluto and its moon Charon. If all goes well the New Horizons
probe, due to arrive in 2015, will send the fi rst close-up images of
Pluto and its moons.
Most planetary orbits in our solar system are nearly circular,
but Pluto’s is signifi cantly eccentric. In fact, from 1979 to 1999,
Pluto was closer to the sun than Neptune. Th e two worlds will
never collide, however, because Pluto’s orbit is inclined 17° to the
ecliptic and because, as you will learn later in this chapter, Pluto
and Neptune orbit in resonance with each other so they never
come close together.
If you land on the surface of Pluto, your spacesuit will have
to work hard to keep you warm. Orbiting so far from the sun,
Pluto is cold enough to freeze most compounds that you think
of as gases, and spectroscopic observations have found evidence
of solid nitrogen ice on its surface with traces of frozen methane
and carbon monoxide. Th e maximum daytime temperature of
about 55 K (360°F) is enough to vaporize some of the nitro-
gen and carbon monoxide and a little of the methane to form a
thin atmosphere around Pluto. Th is atmosphere was detected in
1988 when Pluto occulted a distant star, and the starlight was
observed to fade gradually rather than winking out suddenly.
Pluto’s largest moon was discovered on photographs in
- It is very faint and about half the diameter of Pluto
(■ Figure 24-18). Th e moon was named Charon after the mytho-
logical ferryman who transports souls across the river Styx into
the underworld. Two smaller moons, named Nix and Hydra,
were found in 2005 and confi rmed in 2006 by astronomers
using the Hubble Space Telescope.
■ Figure 24-18
(a) A high-quality, ground-based photo shows Pluto and its moon, Charon, badly blurred by seeing. (NASA) (b) The Hubble Space Telescope
image clearly separates the planet and its moon and allows more accurate measurements of the position of the moon. (R. Albrecht, ESA/ESO
Space Telescope European Coordinating Facility, NASA) (c) A long-exposure photograph made in 2006 with the Hubble Space Telescope confi rmed
discovery of two more moons of Pluto that were named Nix and Hydra. (NASA, ESA, H.Weaver/JHU/APL, A. Stern/SwRI)ab cVisual-wavelength image Visual-wavelength image Visual-wavelength image