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5. The Planet Jupiter

        Jupiter is the largest of the nine planets, more than
10 times the diameter of Earth and more than 300 times its mass.
In fact the mass of Jupiter is almost 2.5 times that of all the
other planets combined. Being composed largely of the light
elements hydrogen and helium, its mean density is only 1.314 times
that of water. The mean density of Earth is 5.245 times that of
water. The pull of gravity on Jupiter at the top of the clouds at
the equator is 2.4 times as great as gravity's pull at the surface
of Earth at the equator. The bulk of Jupiter rotates once in 9h
55.5m, although the period determined by watching cloud features
differs by up to five minutes due to intrinsic cloud motions.

The visible "surface" of Jupiter is a deck of clouds of ammonia
crystals, the tops of which occur at a level where the pressure is
about half that at Earth's surface. The bulk of the atmosphere is
made up of 89% molecular hydrogen (H2) and 11% helium (He). There
are small amounts of gaseous ammonia (NH3), methane (CH4), water
(H2O), ethane (C2H6), acetylene (C2H2), carbon monoxide (CO),
hydrogen cyanide (HCN), and even more exotic compounds such as
phosphine (PH3) and germane (GeH4). At levels below the deck of
ammonia clouds there are believed to be ammonium hydro-sulfide
(NH4SH) clouds and water crystal (H2O) clouds, followed by clouds
of liquid water. The visible clouds of Jupiter are very colorful.
The cause of these colors is not yet known. "Contamination" by
various polymers of sulfur (S3, S4, S5, and S8), which are yellow,
red, and brown, has been suggested as a possible cause of the riot
of color, but in fact sulfur has not yet been detected
spectroscopically, and there are many other candidates as the
source of the coloring.

The meteorology of Jupiter is very complex and not well
understood. Even in small telescopes, a series of parallel light
bands called zones and darker bands called belts is quite obvious.
The polar regions of the planet are dark. (See Figure 7.) Also
present are light and dark ovals, the most famous of these being
"the Great Red Spot". The Great Red Spot is larger than Earth, and
although its color has brightened and faded, the spot has
persisted for at least 162.5 years, the earliest definite drawing
of it being Schwabe's of Sept. 5, 1831. (There is less positive
evidence that Hooke observed it as early as 1664.) It is thought
that the brighter zones are cloud-covered regions of upward moving
atmosphere, while the belts are the regions of descending gases,
the circulation driven by interior heat. The spots are thought to
be large-scale vortices, much larger and far more permanent than
any terrestrial weather system.

The interior of Jupiter is totally unlike that of Earth. Earth has
a solid crust "floating" on a denser mantle that is fluid on top
and solid beneath, underlain by a fluid outer core that extends
out to about half of Earth's radius and a solid inner core of
about 1,220-km radius. The core is probably 75% iron, with the
remainder nickel, perhaps silicon, and many different metals in
small amounts. Jupiter on the other hand may well be fluid
throughout, although it could have a "small" solid core (say up to
15 times the mass of Earth!) of heavier elements such as iron and
silicon extending out to perhaps 15% of its radius. The bulk of
Jupiter is fluid hydrogen in two forms or phases, liquid molecular
hydrogen on top and liquid metallic hydrogen below; the latter
phase exists where the pressure is high enough, say 3-4 million
atmospheres. There could be a small layer of liquid helium below
the hydrogen, separated out gravitationally, and there is clearly
some helium mixed in with the hydrogen. The hydrogen is convecting
heat (transporting heat by mass motion) from the interior, and
that heat is easily detected by infrared measurements, since
Jupiter radiates twice as much heat as it receives from the Sun.
The heat is generated largely by gravitational contraction and
perhaps by gravitational separation of helium and other heavier
elements from hydrogen, in other words, by the conversion of
gravitational potential energy to thermal energy. The moving
metallic hydrogen in the interior is believed to be the source of
Jupiter's strong magnetic field.

Jupiter's magnetic field is much stronger than that of Earth. It
is tipped about 11 deg. to Jupiter's axis of rotation, similar to
Earth's, but it is also offset from the center of Jupiter by about
10,000 km (6,200 mi.). The magnetosphere of charged particles
which it affects extends from 3.5 million to 7 million km in the
direction toward the Sun, depending upon solar wind conditions,
and at least 10 times that far in the anti-Sun direction. The
plasma trapped in this rotating, wobbling magnetosphere emits
radio frequency radiation measurable from Earth at wavelengths
from 1 m or less to as much as 30 km. The shorter waves are more
or less continuously emitted, while at longer wavelengths the
radiation is quite sporadic. Scientists will carefully monitor the
Jovian magnetosphere to note the effect of the intrusion of large
amounts of cometary dust into the Jovian magnetosphere.

The two Voyager spacecraft discovered that Jupiter has faint dust
rings extending out to about 53,000 km above the atmosphere. The
brightest ring is the outermost, having only about 800-km width.
Next inside comes a fainter ring about 5,000 km wide, while very
tenuous dust extends down to the atmosphere. Again, the effects of
the intrusion of the dust from Shoemaker-Levy 9 will be
interesting to see, though not easy to study from the ground.

The innermost of the four large satellites of Jupiter, Io, has
numerous large volcanos that emit sulfur and sulfur dioxide. Most
of the material emitted falls back onto the surface, but a small
part of it escapes the satellite. In space this material is
rapidly dissociated (broken into its atomic constituents) and
ionized (stripped of one or more electrons). Once it becomes
charged, the material is trapped by Jupiter's magnetic field and
forms a torus (donut-shape) completely around Jupiter in Io's
orbit. Accompanying the volcanic sulfur and oxygen are many sodium
ions (and perhaps some of the sulfur and oxygen as well) that have
been sputtered (knocked off the surface) from Io by high energy
electrons in Jupiter's magnetosphere. The torus also contains
protons (ionized hydrogen) and electrons. It will be fascinating
to see what the effects are when large amounts of fine
particulates collide with the torus.

Altogether, Jupiter has 16 known satellites. The two innermost,
Metis and Adrastea, are tiny bodies, having radii near 20 and
10 km respectively, that interact strongly with the rings and in
fact may be the source of the rings. That is, the rings may be
debris from impacts on the satellites. Amalthea and Thebe are
still small, having mean radii of 86.2 and about 50 km,
respectively, but they are close to Jupiter and may serve as
useful reflectors of light from some of the impacts. The Galilean
satellites (the four moons discovered by Galileo in 1610), Io,
Europa, Ganymede, and Callisto, range in radius from 1,565 km
(Europa) to 2,634 km (Ganymede). (Earth's Moon has a radius of
1,738 km.) They lie at distances of 421,700 km (Io) to
1,883,000 km (Callisto) from Jupiter. These objects will serve as
the primary reflectors of light from the impacts for those
attempting to indirectly observe the actual impacts. The outer
eight satellites are all tiny (less than 100-km radius) and at
large distances (greater than 11 million km) from Jupiter. They
are expected to play no role in impact studies.