Formation

The formation of the outer planets began with the accretion of ice-covered dust in the outer, cold solar nebula. As ice-covered dust particles began to clump together, they began to collect more volatiles, creating increasingly massive bodies. This process of dust particles gradually adhering to one another eventually led to the formation of spherical bodies of asteroid size, called planetesimals. The collisions between these planetesimals led to the destruction of some; however, it also led to the creation of even larger planetesimals. Those that survived were able to trap the remains of those that did not, further increasing their masses.

Jupiter formed at such a distance from the Sun that the temperatures were low enough to keep water ice frozen. As a result, Jupiter had much more material at its disposal than any of the inner planets. In addition, the ice acted as a good “glue” to hold the other raw materials together. These two factors allowed Jupiter to become very massive, increasing the strength of its gravitational attraction to a level appropriate for capturing hydrogen and helium. Since these gasses are not able to escape Jupiter’s atmosphere, the planet was able to grow very massive indeed.

Composition

Due to the extreme atmospheric pressures of Jupiter making probing impossible, the internal structures or compositions of the planet are deduced through indirect clues. Such clues include the planets shape, which can be used to determine the size of its rocky core. The gas giants are not spherical, but rather slightly flattened or oblate. The equatorial diameter of Jupiter is roughly 142,980 km which is 6.5% larger than the diameter from pole to pole. This discrepancy in diameters is referred to as the planets oblateness, therefore giving Jupiter an oblateness of 6.5% or 0.065.

The oblateness of the gas giants is also a direct result of their rotations. If the planets did not rotate, they would settle into perfect spheres assuring that each atom on the surface experiences the same gravitational pull toward the planets center. However, due to their rotations, the mass of each planet swings outward away from its axis of rotation giving them their oblate shape.

The shape of Jupiter also depends on the distribution of mass per volume of the planet. Planetary scientists aim to design accurate models of the planets mass distribution which accounts for the correct value of oblateness. It is suggested that Jupiter has a dense concentrated rocky core which contains roughly 2.6% of its mass. The extreme weight of the remainder of Jupiter forces the core into roughly 11,000 km in diameter. It is also estimated that the pressure and temperature of the rocky core are roughly 70 million atmospheres and 22,000 K respectively. This temperature is severe when compared to Jupiter’s cloud top temperature of 165 K.

It is also believed that material from icy planetesimals sunk deep within Jupiter adding to the rocky core and forming a layer 3,000 km thick surrounding it. These ices consist mainly of water, methane, ammonia and other substances produced due to the chemical reactions of these three molecules. The smaller densities of these ‘ices’ allows them to float on top of the rocky core.

Atmosphere

Jupiter has four major atmospheric layers, the troposphere, stratosphere, mesosphere and thermosphere. Jupiter’s atmosphere is composed of mostly hydrogen and helium. The ratio of hydrogen to helium in Jupiter’s atmosphere is 3:1 where hydrogen is 88% of the atmosphere and helium is 11% . This hydrogen-helium ratio is very similar to that of the Sun which suggests that Jupiter still has its original atmosphere.

The remaining 1% of the atmosphere is composed of other trace elements. Elements such as ethane, acetylene, phosphine, ammonia, methane, carbon monoxide and hydrogen cyanide can be found in Jupiter’s upper atmosphere. Although scientist are unsure of some of the elements and their abundance in the atmosphere they can infer as to their presence. For example it is believed that ethane and acetylene are produced as incoming solar ultraviolet radiation disassociates molecular hydrogen and organic molecules such as methane and ammonia.

The temperature of Jupiter’s atmosphere increases from the outer levels in. The temperature at the cloud tops is 125 K and it increases 2 K for every kilometer dropped. Because the temperature fall off with altitude, the temperature below the clouds is close to the adiabatic rate which suggests convection is an important form of energy transport in Jupiter’s atmosphere.

Jupiter has three major cloud layers. The highest cloud layer is composed of ammonia and ice at 120 km, the middle cloud layer is composed of ammonium and hydrosulfide at about 90 km and the lowest layer is composed of water in a mixture of liquid and ice at 50 km. Different cloud colors come from different temperatures and different heights. Blues correspond to the warmest regions and are closest to the surface. These clouds are only seen through holes in the higher levels. Brown, white and red clouds are from progressively lower temperatures and progressively higher levels. Other factors also affect the varying chemical compounds such as lightening, which could act as a catalyst for specific reactions.

Jupiter has many distinct atmospheric features. Zones are where gas is rising and they appear as brightly colored bands in photographs. Belts are where gas is sinking and they appear as darkly colored bands in photographs. The tops of Jupiter’s belts are 20 km lower than the tops of the zones. The locations of bands are consistent across the surface even though their coloration often changes.

Eddies are another atmospheric feature which can be seen in close up pictures and appear as small circular regions in the clouds. Because eddies are associated with a dissipation of energy scientists would expect the cloud patterns to wash out. However, since the bands are steady and consistent across the surface, they indicate that the east-west flow must continue deeper into the atmosphere.

Jupiter has substantial east-west winds which flow at 100 m/s at the equator and 25 m/s at higher latitudes. These winds flow in alternating east-west and west-east bands which correspond to the alternating color bands as seen from photographs. There are five or six of these alternating bands in each hemisphere.

Magnetic Field

The oblateness of the gas giants reveals information about their rocky cores, but the remainder of their internal structures is determined through observation with radio telescopes. Radio waves from these telescopes have proven that there are electric currents flowing through the hydrogen-rich interior of Jupiter.

A small portion of the radiation emitted is thermal radiation, typical of a planetesimal due to their temperatures. Conversely, most of the radiation from Jupiter is nonthermal which is unlike radiation from a heated body and has two separate wavelength ranges. The first of which is referred to as decametric radiation and are sporadic bursts of radio waves believed to be caused by electrical discharges related to powerful electric currents within Jupiter’s ionosphere. Furthermore, it is believed that these discharges are a result of electromagnetic interactions with Io. Jupiter also emits a constant stream of decimetric radiation at shorter wavelengths of a few tenths of a meter. This radiation is associated with electrons moving through strong magnetic fields at speeds close to the speed of light. Due to deflection by the magnetic field, electrons are sent into spiraling trajectories. This form of energy is known as synchrotron radiation.

The presence of synchrotron radiation supports the existence of Jupiter’s magnetic field. This was further supported by the Pioneer and Voyager missions which discovered that the field at Jupiter’s equator was 14 times greater than the magnetic field of Earth. The field is thought to be generated by motions of an electrically conducting fluid in the planets interior believed to be liquid metallic hydrogen. Hydrogen becomes a liquid metal when subjected to 1.4 million atmospheres which occurs almost 7,000 km below Jupiter’s cloudtops. With the bulk of Jupiter composed of liquid metallic hydrogen, the planets rapid rotation sets the liquid into motion generating an intense magnetic field 20,000 times greater than Earth’s. It is therefore assumed that Jupiter consists of a dense rocky core, a 3,000 km thick layer of ‘ices’, a layer of helium and liquid metallic hydrogen 56,000 km thick and a layer of helium and ordinary hydrogen roughly 7,000 km thick.

Jupiter’s strong magnetic field results in a magnetosphere so large that several of the planet’s moons are enveloped within it. The shock wave which surrounds the magnetosphere is 30 million km across. The Pioneer and Voyager reported that the outer boundary of the magnetosphere ranged between 3 and 7 million kilometers above the planet, while the downstream side of the magnetosphere extends more than a billion kilometers from Jupiter.

The inner region of Jupiter’s magnetosphere entraps enormous numbers of charged particles in belts surrounding the planet. Jupiter’s rapid rotation discharges these charge particles outward into a huge ‘current sheet’. The current sheet lies close to the plane of Jupiter’s magnetic equator, while the magnetic axis is 11o from the planet’s axis of rotation. However, Jupiter’s magnetic field is the reverse of the Earth’s magnetic field orientation.

Radio emissions from charged particles at the denser regions of Jupiter’s magnetosphere vary slightly, with a period of 9 hours 55 minutes and 30 seconds. This variation is a result of Jupiter’s rotation changing the angle at which we view its magnetic field. This variation further reveals Jupiter’s internal rotation period, because the magnetic field is anchored deep within the planet. The internal rotation is slightly slower than the atmospheric rotation of 9 hours, 55 minutes and 28 seconds, but nearly the same as the rotation at the poles of 9 hours 55 minutes and 41 seconds. The faster rotation of the atmosphere relative to the bulk of planet’s mass is driven by Jupiter’s internal heat.

The Voyager also discovered that within the inner regions of Jupiter’s magnetosphere, there is a hot gas like mixture of charged particles known as plasma. Plasma is an extremely hot gas that is composed of free-floating positively charged ions and free negatively charged electrons. A plasma behaves much differently than a neutral gas, and is considered the fourth state of matter, capable of conducting electrical currents. Jupiter’s plasma consists mainly of electrons, protons and a small amount of helium, sulfur and oxygen ions. Io is a major source of these ions, with its volcanic activity ejecting material from its surface and volcanic plumes each second into the magnetosphere. The plasma particles are caught up in the planet’s rapidly rotating magnetic field and accelerated to high speeds, which exert a considerable pressure that holds off the solar wind. Data suggests that the pressure equilibrium between the solar wind and hot plasma inside the Jovian magnetosphere is unsteady. Solar wind can blow away plasma shrinking the magnetosphere to nearly half its original size. Jupiter’s rotating magnetic field soon replenishes the magnetosphere with additional electrons and ions to its original size.

Unique Features

One of the very first things that most people notice about Jupiter is the Great Red Spot. This swirling area in the cloud bands of Jupiter is actually a hurricane that has lasted for at least three hundred years. Because the atmosphere of Jupiter is always changing, the storm is always moving. Other hurricanes are visible on the surfaces of Jupiter and the other outer planets, but none are so large nor as old as the Great Red Spot.

The Numbers

Discovered By	Known by the Ancients
Discovery Date	Unknown
Average Distance from the Sun	778,412,020 km (5.20336 AU)
Perihelion	740,742,600 km (4.952 AU)
Aphelion	816,081,400 km (5.455 AU)
Equitorial Radius	71,492 km
Equitorial Circumference	449,197 km
Volume	1.4255 * 1015 km³
Mass	1.8987 * 1024 kg
Density	1.33 g/cm³
Surface Area	6.21796 * 1010 km²
Equitorial Surface Gravity	20.87 m/s²
Escape Velocity	214,300 km/h
Sidereal Day	0.41354 Earth days
Sidereal Year	11.8565 Earth years
Mean Orbital Velocity	47,051 km/h
Orbital Eccentricity	.04839
Orbital Inclination to Ecliptic	1.305 degrees
Equitorial Inclination to Orbit	3.12 degrees
Orbital Circumference	4.774 * 109 km
Effective Temperature	-148 °C
Namesake	King of the Roman gods

Source: http://sse.jpl.nasa.gov/planets