Jupiter is the fifth planet from the
Sun and by far the largest. Jupiter is more than twice as massive as all the other planets combined (the mass of Jupiter
is 318 times that of Earth).
orbit: 778,330,000 km (5.20 AU) from Sun
diameter: 142,984 km (equatorial)
mass: 1.900e27 kg
Jupiter (a.k.a.
Jove; Greek Zeus) was the
King of the Gods, the ruler of Olympus and the patron of the Roman state. Zeus
was the son of Cronus
(Saturn).
Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus). It has been known since prehistoric times as a bright
"wandering star". But in 1610 when Galileo first
pointed a telescope at the sky he discovered
Jupiter's four large moons Io, Europa, Ganymede and Callisto (now known as the Galilean moons) and recorded
their motions back and forth around Jupiter. This was the first discovery of a
center of motion not apparently centered on the Earth. It was a major point in
favor of Copernicus's heliocentric theory of the motions of the
planets (along with other new evidence from his telescope: the phases of Venus and the mountains on the Moon).
Galileo's outspoken support of the Copernican theory got him in trouble with
the Inquisition. Today anyone
can repeat Galileo's observations (without fear of retribution :-) using
binoculars or an inexpensive telescope.
Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11, Voyager 1, Voyager 2 and Ulysses. The spacecraft Galileo orbited
Jupiter for eight years. It is still regularly observed by the Hubble Space Telescope.
The gas planets do not have solid surfaces, their
gaseous material simply gets denser with depth (the radii and diameters quoted
for the planets are for levels corresponding to a pressure of 1 atmosphere). What we see when looking at these
planets is the tops of clouds high in their atmospheres (slightly above the 1
atmosphere level).
Jupiter is about 90% hydrogen and 10% helium (by
numbers of atoms, 75/25% by mass) with traces of methane, water, ammonia and
"rock". This is very close to the composition of the primordial Solar Nebula from which the entire solar system
was formed. Saturn has a similar composition, but Uranus and Neptune have
much less hydrogen and helium.
Our knowledge of the interior of
Jupiter (and the other gas planets) is highly indirect and likely to remain so
for some time. (The data from Galileo's
atmospheric probe goes
down only about 150 km below the cloud tops.)
Jupiter probably has a core
of rocky material amounting to something like 10 to 15 Earth-masses.
Above the core lies the
main bulk of the planet in the form of liquid metallic hydrogen.
This exotic form of the most common of elements is possible only at pressures
exceeding 4 million bars, as is
the case in the interior of Jupiter (and Saturn). Liquid metallic hydrogen
consists of ionized protons and electrons (like the interior of the Sun but at
a far lower temperature). At the temperature and pressure of Jupiter's interior
hydrogen is a liquid, not a gas. It is an electrical conductor and the source
of Jupiter's magnetic field. This layer probably also contains some helium and
traces of various "ices".
The outermost layer is composed
primarily of ordinary molecular hydrogen and helium which is liquid in the
interior and gaseous further out. The atmosphere we see is just the very top of
this deep layer. Water, carbon dioxide, methane and other simple molecules are
also present in tiny amounts.
Recent experiments have shown that
hydrogen does not change phase suddenly. Therefore the interiors of the jovian
planets probably have indistinct boundaries between their various interior
layers.
Three distinct
layers of clouds are believed to exist consisting of ammonia ice, ammonium
hydrosulfide and a mixture of ice and water. However, the preliminary
results from the Galileo probe show only faint indications of
clouds (one instrument seems to have detected the topmost layer while another
may have seen the second). But the probe's entry point (left) was unusual --
Earth-based telescopic observations and more recent observations by the
Galileo orbiter suggest that the probe entry site may well have been one of the
warmest and least cloudy areas on Jupiter at that time.
Data from the
Galileo atmospheric probe also indicate that there is much less water than
expected. The expectation was that Jupiter's atmosphere would contain about
twice the amount of oxygen (combined with the abundant hydrogen to make water)
as the Sun. But it now appears that the actual concentration much less than the
Sun's. Also surprising was the high temperature and density of the uppermost
parts of the atmosphere.
Jupiter
and the other gas planets have high velocity winds which are confined in wide
bands of latitude. The winds blow in opposite directions in adjacent bands.
Slight chemical and temperature differences between these bands are responsible
for the colored bands that dominate the planet's appearance. The light colored
bands are called zones; the dark ones belts. The bands have been
known for some time on Jupiter, but the complex vortices in the boundary
regions between the bands were first seen by Voyager. The data from the Galileo
probe indicate that the winds are even faster than expected (more than 400 mph)
and extend down into as far as the probe was able to observe; they may extend
down thousands of kilometers into the interior. Jupiter's atmosphere was also
found to be quite turbulent. This indicates that Jupiter's winds are driven in
large part by its internal heat rather than from solar input as on Earth.
The vivid colors seen in Jupiter's
clouds are probably the result of subtle chemical reactions of the trace
elements in Jupiter's atmosphere, perhaps involving sulfur whose compounds take
on a wide variety of colors, but the details are unknown.
The colors correlate with the
cloud's altitude: blue lowest, followed by browns and whites, with reds
highest. Sometimes we see the lower layers through holes in the upper ones.
The Great
Red Spot (GRS) has been seen by Earthly observers for more than 300
years (its discovery is usually attributed to Cassini, or
Robert Hooke in the 17th century). The GRS is an oval about 12,000 by 25,000
km, big enough to hold two Earths. Other smaller but similar spots have been
known for decades. Infrared observations and the direction of its rotation
indicate that the GRS is a high-pressure region whose cloud tops are
significantly higher and colder than the surrounding regions. Similar
structures have been seen on Saturn and Neptune. It is
not known how such structures can persist for so long.
Jupiter radiates more energy into
space than it receives from the Sun. The interior of Jupiter is hot: the core
is probably about 20,000 K. The heat is generated by the Kelvin-Helmholtz
mechanism, the slow gravitational compression of the planet.
(Jupiter does NOT produce energy by nuclear
fusion as in the Sun; it is much too small and hence its
interior is too cool to ignite nuclear reactions.) This interior heat probably
causes convection deep within
Jupiter's liquid layers and is probably responsible for the complex motions we
see in the cloud tops. Saturn and Neptune are similar to Jupiter in this
respect, but oddly, Uranus is not.
Jupiter is just about as large in
diameter as a gas planet can be. If more material were to be added, it would be
compressed by gravity such that the overall radius would increase only
slightly. A star can be larger only because of its internal (nuclear) heat
source. (But Jupiter would have to be at least 80 times more massive to become
a star.)
Jupiter has a huge magnetic field,
much stronger than Earth's. Its magnetosphere extends
more than 650 million km (past the orbit of Saturn!). (Note that Jupiter's magnetosphere
is far from spherical -- it extends "only" a few million kilometers
in the direction toward the Sun.) Jupiter's moons therefore lie within its
magnetosphere, a fact which may partially explain some of the activity on Io. Unfortunately for future space travelers and of real
concern to the designers of the Voyager and Galileo spacecraft, the environment
near Jupiter contains high levels of energetic particles trapped by Jupiter's
magnetic field. This "radiation" is similar to, but much more intense
than, that found within Earth's Van Allen belts.
It would be immediately fatal to an unprotected human being.
The Galileo
atmospheric probe discovered a new intense radiation belt between Jupiter's
ring and the uppermost atmospheric layers. This new belt is approximately 10
times as strong as Earth's Van Allen radiation belts. Surprisingly, this new
belt was also found to contain high energy helium ions of unknown origin.
Jupiter has rings
like Saturn's, but much fainter and smaller (right). They were totally
unexpected and were only discovered when two of the Voyager 1 scientists
insisted that after traveling 1 billion km it was at least worth a quick look
to see if any rings might be present. Everyone else thought that the chance of
finding anything was nil, but there they were. It was a major coup. They have
since been imaged in the
infra-red from ground-based observatories and by Galileo.
Unlike Saturn's, Jupiter's rings are
dark (albedo about .05). They're probably composed of very small
grains of rocky material. Unlike Saturn's rings, they seem to contain no ice.
Particles in Jupiter's rings
probably don't stay there for long (due to atmospheric and magnetic drag). The
Galileo spacecraft found clear evidence that the rings are continuously
resupplied by dust formed by micrometeor impacts on the four inner moons, which are very energetic because
of Jupiter's large gravitational field. The inner halo ring is broadened by
interactions with Jupiter's magnetic field.
In July 1994, Comet Shoemaker-Levy 9 collided with
Jupiter with spectacular results (left). The effects were clearly visible even with amateur telescopes. The debris from the
collision was visible for nearly a year afterward with HST.
When it is in the nighttime sky,
Jupiter is often the brightest "star" in the sky (it is second only
to Venus, which is seldom visible in a dark sky). The four Galilean moons are
easily visible with binoculars; a few bands and the Great Red Spot can be seen
with a small astronomical telescope. There are several Web sites that
show the current position of Jupiter (and the other planets) in the sky. More
detailed and customized charts can be created with a planetarium
program.
Jupiter's Satellites
Jupiter has 63 known satellites (as of Feb 2004): the
four large Galilean moons plus
many more small ones some of which have not yet been named:
- Jupiter is very gradually slowing
down due to the tidal drag produced by the Galilean satellites. Also, the
same tidal forces are changing the orbits of the moons, very slowly
forcing them farther from Jupiter.
- Io, Europa and Ganymede are locked
together in a 1:2:4 orbital resonance and their orbits evolve together. Callisto is
almost part of this as well. In a few hundred million years, Callisto will
be locked in too, orbiting at exactly twice the period of Ganymede (eight
times the period of Io).
- Jupiter's satellites are named for
other figures in the life of Zeus (mostly his numerous lovers).
- Many more small moons have been discovered recently but
have not as yet been officially confirmed or named. The most up to date
info on them can be found at Scott Sheppard's site.
