topPLUTO
ATMOSPHERE
PICTURES
Pluto is the farthest planet from the Sun (usually) and by far the smallest. Pluto is
smaller than seven of the
solar system's moons. In Roman mythology, Pluto (Greek: Hades) is the god of the
underworld. The planet received this name
(after many other suggestions) perhaps because it's so far from the Sun that it is in
perpetual darkness and
perhaps because "PL" are the initials of Percival Lowell.
Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out
to be in error had predicted a
planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the
error, Clyde W. Tombaugh at
Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.
After the discovery of Pluto, it was quickly determined that Pluto was too small to
account for the discrepancies in the orbits
of the other planets. The search for Planet X continued but nothing was found. Nor is it
likely that it ever will be: the
discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with
Neptune is used. There is no tenth
planet.
Pluto is the only planet that has not been visited by a spacecraft. Even the Hubble Space
Telescope can
resolve only the largest features on its surface (left and above).
Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was discovered (in
1978) just before
its orbital plane moved edge-on toward the inner solar system. It was therefore possible
to observe many
transits of Pluto over Charon and vice versa. By carefully calculating which portions of
which body would be covered at what
times, and watching brightness curves, astronomers were able to construct a rough map of
light and dark areas on both bodies.
Pluto's radius is not well known. JPL's value of 1137 is given with an error of +/-8,
almost one percent.
Though the sum of the masses of Pluto and Charon is known pretty well (it can be
determined from careful measurements of
the period and radius of Charon's orbit and basic physics) the individual masses of Pluto
and Charon are difficult to determine
because that requires determining their mutual motions around the center of mass of the
system which requires much finer
measurements -- they're so small and far away that even HST has difficulty. The ratio of
their masses is probably somewhere
between 0.084 and 0.157; more observations are underway but we won't get really accurate
data until a spacecraft is sent.
Pluto is the second most contrasty body in the Solar System (after Iapetus). Exploring the
origin of that contrast is one of the
high-priority goals for the proposed Pluto Express mission.
There are some who think Pluto would be better classified as a large asteroid or comet
rather than as a planet. Some
consider it to be the largest of the Kuiper Belt objects (also known as Trans-Neptunian
Objects). There is considerable merit
to the later position, but historically Pluto has been classified as a planet and it is
likely to remain so.
Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it
was from January 1979 thru February 11
1999). Pluto rotates in the opposite direction from most of the other planets.
Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is exactly
1.5 times longer than Neptune's. Its
orbital inclination is also much higher than the other planets'. Thus though it appears
that Pluto's orbit crosses Neptune's, it really
doesn't and they will never collide. (Here is a more detailed explanation.) Like Uranus,
the plane of Pluto's equator is at almost right angles to the plane of its orbit.
The surface temperature on Pluto is not well known but is probably between 35 and 45
Kelvins (-228 to
-238 C).
Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it is
probably a mixture of 70% rock and
30% water ice much like Triton. The bright areas of the surface seem to be covered with
ices of nitrogen with smaller amounts
of (solid) methane, ethane and carbon monoxide. The composition of the darker areas of
Pluto's surface is unknown but may
be due to primordial organic material or photochemical reactions driven by cosmic rays.
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. The Pluto Express mission
planners want to arrive at Pluto while the atmosphere is unfrozen.
The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk
properties between Pluto and Triton suggest
some historical connection between them. It was once thought that Pluto may have once been
a satellite of Neptune's, but this
now seems unlikely. A more popular idea is that Triton, like Pluto, once moved in an
independent orbit around the Sun and was
later captured by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members
of a large class of similar objects
the rest of which were ejected into the Oort cloud. Like the Earth's Moon, Charon may be
the result of a collision between
Pluto and another body.
Pluto can be seen with an amateur telescope but it is not easy.
