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Jupiter

Jupiter is the largest planet in the Solar system, a huge giant made of gasses, the fifth planet from the Sun. Its mass is approximately one-thousandth of the solar mass (~318 earth masses). It is the dominant planet as it exerts the strongest gravity after the Sun and it has almost all the angular momentum of the solar system, as the Sun rotates relatively slow. Its radius at the equator is 71,490 km (11.2 earth radii) and 66,850 km (10.5 earth radii) at the poles. It is composed by 71% hydrogen, 24% helium and 5% other elements and molecules, methane, ammonia, hydrogen deuteride (HD), ethane, water, ammonium hydrosulfide (NH4SH). Jupiter is a very large planet as its orbit is sufficiently far from the Sun and its gravitational attraction collected all the hydrogen, helium and other volatiles that were orbiting the protosun during the formation of the solar system. The low temperature of the planet combined with its large gravity kept all light elements forming a huge planet made of gasses. If the same planet was formed near the Sun, at Earth’s orbit it was going to lose all volatile material, like the Earth.

It could not have become a star unless the mass was much larger, as a celestial body with composition similar to Jupiter must have a mass 50 to 100 times larger than Jupiter to become a small star, i.e. to start energy production via hydrogen fusion.

Jupiter from NASA/ESA Hubble Space Telescope (credit ESA/NASA)

Jupiter is composed of gasses, like all four giant planets or jovian planets (Jupiter, Saturn, Uranus, and Neptune). It does not have a solid surface. As surface is defined the region that the atmospheric pressure is equal to 10 bars (10 Earth atmospheres). On Jupiter’s surface, the gravity at the equator is more than double of Earth’s gravitational acceleration, equal to 2.52g  and the escape velocity is 59.5km/s, much higher that at Earth. At the level of 1 bar, the mean temperature is 165 K (-108°C). Jupiter's prominent features are the belts and zones and the red spot that are visible even with a small telescope. The belts and zones are dark and light bands  that circle the planet, parallel to the equator.

Notice the symmetric dark and light belts and zones formed by upwelling hot air currents that circulate the planet caused by heat that comes from Jupiter. The great red spot is always prominent in the southern hemisphere between a belt and a zone. Europa‘s shadow is seen on Jupiter (December 2000).

Image of Jupiter by Cassini spacecraft

The darker belts have higher temperatures and are descending parts of the atmosphere, while the lighter coloured zones are regions of rising warmer gas forming huge cells, currents of air, symmetric with respect to the equator. Like at the Earth they are thermally driven zonally symmetric circulations initially proposed by George Hadley to explain trade winds at the Earth at latitudes around 30°. These streams of gasses circling the planet running with differential rotational speed. A circular seemingly permanent storm, the Great Red Spot, which is a huge high pressure anti-cyclonic storm similar to hurricane, which has been active for four centuries.

Jupiter radiates ~1.6 times more energy than it receives from the Sun. This extra energy comes from its internal heat source that comes from the collapse of the nebula that created the planet and it is believed that it creates the complex circulation of the atmosphere with zones and belts.

Jupiter orbits the Sun in 11.86 years with a speed of 13 km/s, in a nearly circular low eccentricity (0.048) orbit. Its perihelion is 5.458104 AU and its aphelion 4.950429 AU.

Jupiter is the planet with the largest number of known satellites of the Solar System. At the time, 64 satellites are confirmed. The first satellites that were discovered are the Galilean satellites Ganymede, Callisto, Io and Europa observed by Galileo in 1609-10, although these names have been given by astronomer Simon Marius who discovered them too at the same time. The discovery of these satellites, that orbit another planet and not the Earth, lead to the establishment of the heliocentric system of Aristarchus that put forward Copernicus in 1543. Jovian satellites are classed in the Regular Satellites, which are close to Jupiter, very large (the Galilean) and relatively large satellites that have almost circular orbits and the Irregular Satellites that are smaller satellites with eccentric orbits and have eccentric orbits and large semi-major axis. The Galilean satellites have almost all the mass of satellites as the other 60 have only 0.003 percent of the mass orbiting Jupiter in the form of satellites or rings. They are much larger than any of the dwarf planets (Pluto, Ceres) and asteroids (Vesta).

The periods of Ganymede, Europa and Io are in a 4:2:1 Laplacian orbital resonances and the orbits of Ganymede and Callisto have a 7:3 resonance, like a large cosmic very well synchronized clock of satellites dancing under the music of the spheres. These resonance phenomena not well understood. The average moon density decreases with jovicentric distance. Io, the closest to Jupiter Galilean satellite is the densest moon, with density between rock and iron, while Callisto consists of ice and rock and probably has one of the oldest surfaces in the solar system. Ganymede is the largest satellite in the Solar System, it is bigger than planet Mercury and it even has a magnetosphere. Galileo spacecraft measured the magnetic fields of the Galilean satellites. The equatorial magnetic fields are for Io ~25nT, for Europa 120nT, Ganymede 720nT and Callisto 14 nT. These data combined with the average density revealed that Europa and Callisto, perhaps have spherical layers of liquid salty water that can permit the existence of magnetic fields in these small bodies that can be explained in terms of induced magnetic fields that need a near-surface electrically conducting shell of salty water. Ganymede's magnetic field is probably the result of electric currents perhaps due to convection, possibly in a liquid iron core, with a dynamo mechanism, similar to the mechanism that produces the magnetic field of the Earth. Io's magnetic field cannot be explained in terms of a dynamo, or a purely induced field. Io is the first satellite with active volcanoes discovered by Voyager. Many of the small satellites of Jupiter have been captured by the giant planet, because the planet is the second after the Sun great attractor of bodies in the solar system, and have become satellites. Studying their trajectories reveals their possible origin.

Jupiter has a system of small rings discovered initially by Pioneer 11 (1974) magnetometer data and confirmed by Voyager images (1979). The Halo Ring is a diffuse cloud-like ring followed by the narrow Main Ring, and the almost invisible double ring, the Amalthea and Thebe Gossamer Rings. Metis, Adrastea, Amalthea and Thebe play an important role with their gravity and trajectory in maintaining the rings of Jupiter. Some of the small satellites have retrograde orbits, turning the opposite way to Jupiter and prograde satellites.  Many satellites have similar properties, like rertograe orbit, semimajor axis, eccentricity, inclination etc, and probably common origin, perhaps are the debris of a larger body that has been destroyed and its pieces are the satellites of the group. The Himalia satellite group consists of Himalia, Leda, Lysithea, Elara and S/2000 J 11,  as they have almost the same inclination ~27.5 and eccentricities 0.11 to 0.25 perhaps reminiscent of their common origin, presumably an asteroid. Another is thw Carme group, that includes Taygete, Eukelade, S/2003 J 5, Chaldene, Isonoe, Kalyke, Erinome, Aitne, Kale, Pasithee, S/2003 J 9, S/2003 J 10 that are retrograde orbit satellites too as the Ananke group with Praxidike, Iocaste, Harpalyke, Thyone, Euanthe, Euporie and the group of Pasiphae that includes Sinope, Callirrhoe, Megaclite, Autonoe, Eurydome and  Sponde are all retrograde satellites.

Jupiter interior is liquid, very hot, between 13,000 to 35,000 K, made of hydrogen 90%, ~10% helium and small amounts of heavier elements. They are in liquid form due to the extreme pressure of some 100 million Atmospheres (at the center of the Sun the pressure is around 340 billion). Hydrogen under these extreme pressures becomes Liquid Metallic Hydrogen , called like this, because it conducts heat and electricity like metal because the electrons of hydrogen are released from the molecules and are free to move, making hydrogen to behave like a metal, and its fast rotation with the planet leads to the generation of electrical currents that produce the strong magnetic field. It is believed that a small rocky core with a mass few times the one of the Earth is in the center of Jupiter.

The magnetic field of Jupiter is almost the stronger magnetic field of all planets. It is dipolar close to the planet. The dipole is tilted ~10° off the axis of rotation (almost like the Earth's tilt, ~11.3o). The field at the equator is approximately 428 ?T . The dipole magnetic moment is 1.53 1020 Tm3, thousand time more than the Earth's. The field originates from currents in the interior and it is constant since it has been first measured by Pioneer 10 in 1972. The magnetosphere of Jupiter is the largest object in the solar system, much larger than the Sun and it changes in size and shape enormously with time as the solar wind pressure and magnetic field change.  The magnetic field is generated from electric currents in the outer metallic hydrogen core of Jupiter.

The magnetosphere of Jupiter, as all magnetospheres of planets with intrinsic magnetic field, is the result of the interaction of the solar wind with the magnetic field of the planet, which is almost a dipole. At the heliospheric distance of Jupiter the density of solar wind is almost 30 times smaller than at the Earth and as the magnetic moment of Jupiter is 18000 larger than the Earth’s the resulting magnetosphere of Jupiter is enormous. Near the planet, as it always happens, it is almost dipolar. It has energetic particles trapped in Van Allen radiation belts and Io’s volcanoes every second contribute with 1000 kg of new material (mainly S+, O+, S2+ and O2+ and NaCl dust) that add up to the magnetosphere forming the Io plasma torus that is a ring of plasma as in a few hours the particles diffuse forward backward around the trajectory of Io and form a banana-shaped initially cloud. A huge electric current (10 million amps) produced by Io makes the magnetosphere more complex. The circuit uses the flux tube that passes through Io, its ionosphere and the ionosphere of Jupiter. The aurora of Jupiter is very bright and it is influenced by the flux tubes of Io, Ganymede and Europa as particles from these satellites guided by the magnetic field lines hit the planet’s ionosphere.

The dipolar filed is tilted 10o with respect to the axis of the planet that rotates in 9h 55.5m and hence the magnetosphere that is charged with plasma particles wobbles and it goes up and down continuously, and this combined with the variations of solar wind pressure makes it extremely variable. 

The magnetosphere of Jupiter is an important source of radio wave bursts with frequencies between 0.6 to 30 MHz produced by electrons in the plasma population.

LINKS:

http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jupiter
http://www.nasa.gov/topics/solarsystem/features/20090218.html

PICTURES:

The Galilean satellites Io, Europa, Ganymede and Callisto, first observed by Galileo in 1610.

http://pds.nasa.gov/planets/special/jupiter.htm

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