Introduction
Pluto
is the last planet in our solar system. It has a mass of 1.5 x 10 22
kg or about 0.0025 the mass of the Earth. Its diameter is 2250 km, while its
density is 2.3 g/cm3. Its density is lower than that
for a terrestrial planet, but similar to that of the icy moons of a Jovian
planet. The rotation of Pluto is retrograde.Atmosphere
Little is known about Pluto's atmosphere, but it probably consists primarily
of nitrogen with some carbon monoxide and methane. It is extremely tenuous, the
surface pressure being only a few microbars. Pluto's atmosphere may exist as a
gas only when Pluto is near its perihelion;
for the majority of Pluto's long year, the atmospheric gases are frozen into
ice.
Near perihelion, it is likely that some of
the atmosphere escapes to space, perhaps even interacting with Charon, one of
its moon (see below). The surface temperature on Pluto varies between about 38 K
to 63 K. The "warmer" regions roughly correspond to the regions that
appear darker in optical wavelengths.
Terrain
Pluto
is made up mostly of water ice. Frozen methane is also present in the surface as
revealed by spectroscopy. Its surface
temperature has to be less than 50 K or else methane cannot exist in solid form.
Moons
Pluto
has a moon named Charon, after the mythical boatman who
ferried the dead across the river Styx into Hades, the domain of Pluto. Charon's
diameter is 1270 km, and appears bluer than Pluto, implying that they have
different surface composition and structure.
Special Features
Pluto and Charon are also unique in that not only does Charon rotate synchronously, but Pluto does too: they both keep the same face toward one another.