The largest of the planets in our Solar System, the name Jupiter was an accident since the ancient astronomers did not know Jupiter's real size. Its radius is 11.3 Earth radii, its mass is 317 Earth masses. It is composed mostly of hydrogen (90%) and helium (9%) and traces of everything else. Jupiter's mean density is 1.3 gm/cc, close to that of water.
As we discussed before, the mean density of the Jovian worlds is near the value for water, 1 gm/cc, versus the terrestrial worlds which have average densities near the value of rocks, 3 to 5 gm/cc. This was due to the fact that temperatures in the outer Solar System are low because of the large distance from the warm Sun. So volatile compounds, such as ices like H2O, CO2, NH3, CH4, which tended to evaporate in the inner Solar System (although not all of them since there was plenty leftover to form secondary atmospheres) are abundent in the outer Solar System and make-up most of the comets, moons and rings around the Jovian worlds.
Note that H2O (water), CO2 (carbon dioxide), NH3 (ammonia) and CH4 (methane) are the simplest molecules you can make with hydrogen (H), carbon (C), oxygen (O) and nitrogen (N) = often called the HCNO compounds. Jupiter is also rich in NH4SH = ammonium hydrosulfide.
The formation of Jupiter (and the other Jovian worlds) starts with the accretion (build-up) of ice-covered dust in the outer, cold solar nebula
Jupiter's Atmospheric Features:
The exterior of Jupiter is noted by its brightly colored latitudinal zones, dark belts and thin bands dotted with numerous storms and eddies. Due to differential rotation, the equatorial zones and belts rotate faster than the higher latitudes and poles as seen in this Jupiter movie.
Note: Upward moving gases in Jupiter's atmosphere bring white clouds of ammonia/water ice from lower layers. Downward moving gases sink and allow us to view the lower, darker layers.
The most obvious feature on Jupiter is the Great Red Spot which is actually the top of a large cyclone twice the size of the Earth.
Gas planets do not have solid surfaces, but rather build-up in pressure and density as one goes deeper towards the core. Different colors represent different depths into Jupiter's atmosphere. The colors (reds, browns, yellows, oranges) are due to subtle chemical reactions involving sulfur. Whites and blues are due to CO2 and H2O ices.
The detailed structure in Jupiter's atmosphere is dominated by physics known as fluid mechanics. Note that the atmosphere of Jupiter is so dense and cold that it behaves as a fluid rather than a gas. At the point were we see atmospheric features the pressure is 5 to 10 times that of the Earth's atmospheric pressure at sea level.
The simplest theories in fluid mechanics predict two types of patterns. One pattern occurs when a fluid slips by a second fluid of a different density. Such an event is known as a viscous flow and produces wave-like features at the boundary of the two fluids. A second pattern is produced by a stream of fluid in a constant medium, called turbulent flow. The stream breaks up into individual elements, called eddies. These eddies can develop into cyclones.
Cyclones develop due to the Coriolis effect where the lower latitudes travel faster than the higher latitudes producing a net spin on a pressure zone. The cyclones on Jupiter are regions of local high or low pressure spun in such a fashion. Note that the direction of the spin differs in the two hemispheres where clockwise spin is in the North and counter-clockwise spin is in the South.
Brown ovals are low pressure cyclones/storms in the North. White ovals are high pressure cyclones/storms in the South. Both can last on the order of tens of years. The Great Red Spot is a large high pressure storm that has lasted over 600 years.
Jupiter is highly oblate (flattened). Plus, Jupiter has a very high rotation rate (once every 9.8 hours). These two facts combine to tell us that Jupiter has a very small solid core.
The liquid hydrogen, in molecular form at these levels (H2), continues to be compressed further reaching a metallic state. This occurs in a transition zone located 20,000 km below the atmosphere. Notice that at no time is there any real ``surface'' as one drops into Jupiter's interior.
At the very center of Jupiter is a small (15 Earth masses) rocky core, leftover from the icy dust particles that originally collected in the early solar nebula.
Many textbooks refer to Jupiter as a ``failed star''. This is due to the fact that if Jupiter were slightly more massive the temperatures in its core would have reached the ignition point for thermonuclear fusion. This is the process where stars turn hydrogen into helium and release energy (i.e. the star shines). If Jupiter were 100 times more massive, our Solar System would have had two stars.
Jupiter's radiation output:
IR and radio measurements revealed two components to Jupiter's radiation output; a thermal and non-thermal component.
Jupiter's magnetic field:
The magnetic field of Jupiter is 19,000 times stronger than the Earth's magnetic field. Even with a large rocky core and high rotation rate, the magnetic field is too strong. The origin of Jupiter (and other Jovian planets) strong magnetic field is the metallic hydrogen shell that surrounds Jupiter's rocky core. Metal is an excellent conductor of electric current and supplies the energy for the generation of an intense and large magnetic field.
A strong magnetic field can capture charged particles from the solar wind (i.e. high speed protons and electrons) and particles ejected from the inner moon, Io. These particles are trapped in the inner magnetic belts and are reflected back and forth between the north and south magnetic poles.
The interaction of Jupiter's strong magnetic field and nearby space produces a region known as Jupiter's magnetosphere. The magnetosphere has several features:
The magnetosphere of Jupiter encounters the solar wind at about a million kilometers from the planet. The bow shock from this boundary reaches beyond the orbit of Saturn.