The "Giant" Planet:
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
Note that, due to gravity, the heavier elements sink to the core of proto-Jupiter, thus, we expect the core region to be rocky. While the lighter elements (H and He) remain in the atmosphere.
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.
The zones and belts are zonal jet streams moving with velocities up to 400 miles/hr. Wind direction alternates between adjacent zones and belts. The light colored zones are regions of upward moving convective currents. The darker belts are made of downward sinking material. The two are therefore always found next to each other. The boundaries of the zones and belts (called bands) display complex turbulence and vortex phenomenon.
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.
On the Earth, the energy to power our storm systems comes from sunlight. Jupiter is too far from the Sun and receives very little energy. The energy needed to power all the turbulence in Jupiter's atmosphere comes from heat released from the planet's core.
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.
Jupiter's interior consists mostly of hydrogen and helium. These elements are gaseous at the top of Jupiter's atmosphere down to several thousand kilometers. At this point, the pressures and temperatures compress these gases into a liquid state.
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.
A planet absorbs energy from the Sun in the form of light and converts the energy into heat. The heat is then reradiated back into space (mostly from the nightside of the planet). Based on how much energy Jupiter absorbs from the Sun, then its mean temperature should be 105 K (about -280 F). However, IR and radio measurements of Jupiter show that it has a mean temperature of 125 K, or 20 degrees too warm. In other words, Jupiter radiates about twice as much energy as it receives. Conservation of energy requires that this heat come from someplace and the only reservoir is the core of Jupiter. Thus, this extra heat is leftover energy from the time of Jupiter's formation.
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.
A thermal component is associated with the leftover heat of formation (see above). The non-thermal component is associated with radiation that does not follow a Planck curve but follows what is know as a power-law spectrum. A spectrum that associated with synchrotron radiation.
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.
A visible result of this interaction is aurora or northern lights on Jupiter.
The interaction of Jupiter's strong magnetic field and nearby space produces a region known as Jupiter's magnetosphere. The magnetosphere has several features:
- North and South magnetic poles
- current sheet along the magnetic equator
- plasma torus associated with Io's orbit
- magnetopause - the boundary where the magnetosphere encounters the solar wind
The rapid rotation of Jupiter spews charged particles into a current sheet around the magnetic equator of Jupiter. Inside this current sheet orbits the moon Io. The current sheet sweeps up ejected ions from Io's geysers to make a plasma torus. The region around this plasma torus and the inner moon system is intensely radioactive with levels around 1000 times the radioactive levels of the Earth's surface. This region of space is inhabitable by man or machine without heavy shielding.
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.
The principal Jupiter's characteristcs are:
- Is the fourth brightest object in the sky (after the Sun, the Moon and Venus).
- As a gas planet it does not have a solid surface, it's gaseous material simply gets denser with depth.
- It is about 90% hydrogen and 10% helium (by numbers of atoms, 75/25% by mass) with traces of methane, water, ammonia and "rock".
- It 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.
- 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.
- Have high velocity winds which are confined in wide bands of latitude.
- The vivid colors seen in Jupiter's clouds are probably the result of subtle chemical reactions. Those 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.
- 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. A star can be larger only because of its internal (nuclear) heat source. 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!).
- Jupiter has rings like Saturn's, but much fainter and smaller.
- It has 66 confirmed satellites. The four large Galilean moons plus many more small ones.
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 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.