Jupiter

Jupiter named after the Roman king of the gods is the fifth planet from the Sun orbiting at a distance of 770,330,000 kilometers or 5.15 AU and with a diameter of 142,800 kilometers is by far the largest planet in the Solar System, one thousand times the size of Earth.

Jupiter like Saturn is a true gas giant with an extremely deep atmospheric blanket as befits a planet with more matter than all the other planets combined. In the southern hemisphere a large reddish oval called the "Great Red Spot" located high in the atmosphere's outer layer moves majestically westward through clouds that have a mean temperature of -121 °C and are composed of ammonia, hydrogen sulphide and water. The atmosphere is mainly hydrogen and helium; a downward direction toward the planet's center produces a steady increase in pressure along with a corresponding increase in temperature until the gas finally reaches a blistering 5,700 °C and begins to resemble molten metal; below this we reach a region of incredibly superheated water, ammonia and methane in a liquid mix surrounding a comparatively small rocky core with a mass of 10 Earths or more.

Although the likelihood of life on Jupiter is highly improbable, an interesting hypothesis was advanced some years ago by the late American astronomer, author and popularizer of science Carl Sagan concerning the possibility of ammonia based life existing in Jupiter’s upper atmosphere.

A Jovian ecology was envisioned based loosely on that of Earth’s oceans. The principal life forms were labeled “sinkers, floaters and hunters.” The “sinkers” were short lived plankton type organisms, “floaters“ giant sentient gas bags and “hunters” gas propelled squid like predators that attacked and fed on the “floaters.”

So far there has been no evidence from the many probes that have explored the Jovian system that Sagan’s hypothesis has any validity (though in all fairness Galileo is the only spacecraft to have actually orbited the giant planet witnessing the impact of Comet Shoemaker-Levy 9 upon approach in 1994 and parachuting an atmospheric probe deep into its atmosphere in 1995).

Juno the second mission in NASA's New Frontiers programme, launched on August 5, 2011, is destined to enter Jupiter's orbit early July 2016. Its instruments will amongst other things measure thermal radiation emanating from deep in the atmosphere, the observations complementing earlier findings by asssessing the abundance and distribution of water and oxygen, the overall goal to provide a broader insight into the planets composition and origin.

Jupiter has a faint ring system [1] composed of three main segments: Innermost is a halo ring which is actually a 10,000 km thick torus of dust and debris, then a main ring and then a gossamer ring, itself divided into inner and outer bands. In addition a very faint and tenuous outer ring circles at great distance from and in an opposite direction to the other rings. The main ring is composed of material from the moons Adrastea and Metis. The inner and outer gossamer rings are composed of material from the shepherd moons Thebe and Amalthea. [2] The origin of the faint and tenuous outer ring is unknown.

To date Jupiter has 63 known moons: Metis, Adrastea, Amalthea, Thebe, Io, Europa, Ganymede, Callisto, Themisto, Leda, Himalia, Lysithia, Elara, Carpo, Euporie, Thelxinoe, Euanthe, Helike, Orthosie, Iocaste, Praxidike, Harpalyke, Mneme, Hermippe, Thyone, Ananke, Aitne, Kale, Taygete, Chaldene, Erinome, Aoede, Kallichore, Kalyke, Carme, Callirrhoe, Eurydome, Pasithee, Cyllene, Eukelade, Pasiphae, Hegemone, Arche, Isonoe, Sinope, Sponde, Autonoe, Megaclite, Kore, Herse and thirteen as yet unnamed S/2000 J 11, S/2003 J 12, S/2003 J 3, S/2003 J 18, S/2003 J 16, S/2003 J 19, S/2003 J 15, S/2003 J 16, S/2003 J 23, S/2003 J 4, S/2003 J 9, S/2003 J 5, and S/2003 J 2.

[1] A planetary ring is a flat disk shaped band composed of rock or ice dust, larger rocks, boulders and ice chunks which circle in a planet's equatorial plane.

[2] “Shepherd” moons are small satellites orbiting within gaps or at the periphery of planetary rings their gravitational presence producing a distinct edge to the matter comprising the ring.

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