|
|
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 (318 times Earth).
orbit: 778,330,000 km (5.20 AU) from Sun
diameter: 142,984 km (equatorial)
mass: 1.900e27 kg
Jupiter (also known as 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; at some times Mars is also brighter). It has been known since prehistoric times. Galileo discovered, in 1610, Jupiter's four large moons Io, Europa, Ganymede and Callisto.
Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11, Voyager 1, Voyager 2 and Ulysses. The spacecraft Galileo is currently in orbit around Jupiter and will be sending back data for at least the next two years.
The
gas
planets do not have solid surfaces. Their
gaseous material simply gets denser with depth . What we see when
looking at these planets is the tops of clouds high in their
atmospheres.
Jupiter is about 90% hydrogen and 10% helium 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 times that of the Earth.
Above the core lies the main bulk of the planet in the form of liquid metallic hydrogen.
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 .
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. 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.
The GRS is a high-pressure region whose cloud tops
are significantly higher and colder than the surrounding
regions.
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.
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.)
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.
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.
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). Galileo found clear evidence that the rings are continuously resupplied by dust formed by small meteor impacts on the four inner moons, which are very energetic because of Jupiter's large gravitational 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.
Jupiter has 16 known satellites, the four large Galilean
moons and 12 small ones.
Distance Radius Mass Satellite (000 km) (km) (kg) Discoverer Date --------- -------- ------ ------- ---------- ----- Metis 128 20 9.56e16 Synnott 1979 Adrastea 129 10 1.91e16 Jewitt 1979 Amalthea 181 98 7.17e18 Barnard 1892 Thebe 222 50 7.77e17 Synnott 1979 Io 422 1815 8.94e22 Galileo 1610 Europa 671 1569 4.80e22 Galileo 1610 Ganymede 1070 2631 1.48e23 Galileo 1610 Callisto 1883 2400 1.08e23 Galileo 1610 Leda 11094 8 5.68e15 Kowal 1974 Himalia 11480 93 9.56e18 Perrine 1904 Lysithea 11720 18 7.77e16 Nicholson 1938 Elara 11737 38 7.77e17 Perrine 1905 Ananke 21200 15 3.82e16 Nicholson 1951 Carme 22600 20 9.56e16 Nicholson 1938 Pasiphae 23500 25 1.91e17 Melotte 1908 Sinope 23700 18 7.77e16 Nicholson 1914
Values for the smaller moons are approximate.
Distance Width Mass Ring (km) (km) (kg) ---- -------- ----- ------ Halo 92000 30500 ? Main 122500 6440 1e13 Gossamer 128940 100000 ?
(distance is from Jupiter's center to the ring's inner edge)
... Sun ... Mars ...
Deimos ... Jupiter ... Metis
... Saturn ... 
