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Jupiter is the fifth planet from the Sun and is the largest one in the solar system. If Jupiter were hollow, more than one thousand Earths could fit inside. It also contains more matter than all of the other planets combined. It has a mass of 1.9 x 1027 kg and is 142,800 kilometers (88,736 miles) across the equator. Jupiter possesses 16 satellites, four of which - Callisto, Europa, Ganymede and Io - were observed by Galileo as long ago as 1610. There is a ring system, but it is very faint and is totally invisible from the Earth. (The rings were discovered in 1979 by Voyager 1.) The atmosphere is very deep, perhaps comprising the whole planet, and is somewhat like the Sun. It is composed mainly of hydrogen and helium, with small amounts of methane, ammonia, water vapor and other compounds. At great depths within Jupiter, the pressure is so great that the hydrogen atoms are broken up and the electrons are freed so that the resulting atoms consist of bare protons. This produces a state in which the hydrogen becomes metallic.
Colorful latitudinal bands, atmospheric clouds and storms illustrate Jupiter's dynamic weather systems. The cloud patterns change within hours or days. The Great Red Spot is a complex storm moving in a counter-clockwise direction. At the outer edge, material appears to rotate in four to six days; near the center, motions are small and nearly random in direction. An array of other smaller storms and eddies can be found through out the banded clouds.
Auroral emissions, similar to Earth's northern lights, were observed in the polar regions of Jupiter. The auroral emissions appear to be related to material from Io that spirals along magnetic field lines to fall into Jupiter's atmosphere. Cloud-top lightning bolts, similar to superbolts in Earth's high atmosphere, were also observed.

      MEAN RADIUS:                      69911 km 
      MASS:                             317.8 (Earth=1)
      DENSITY:                          1.31 (gm/cm)
      GRAVITY:                          2.34 (Earth=1)
      ORBIT PERIOD:                     11.86 (Earth years)
      ROTATION PERIOD:                  0.414 (Earth days)
      SEMIMAJOR AXIS OF ORBIT:          5.2 au (Earth=1)
      ECCENTRICITY OF ORBIT:            0.004 (Earth=0.017)
      EQUATORIAL INCLINATION (deg):     3.12 (Earth=23.44) 
      SURFACE PRESSURE:                 >>100 bars  
      AVERAGE TEMPERATURE:              ~129 K
      DENSITY AT 1 BAR:                 ~0.16 kg/m3
      WIND SPEEDS:                      Up to ~150 m/s (<30 degrees latitude)
                                        Up to ~40 m/s (>30 degrees latitude)
      Atmospheric composition
      Major:       Molecular hydrogen (H2) - 89%; Helium (He) - 11%
      Minor (ppm): Methane (CH4) - ~2000; Ammonia (NH3) - ~200;
                   Hydrogen Deuteride (HD) - 20; Ethane (C2H6) - ~5;
                   Water (H2O) - 1
      Aerosols:    Ammonia ice, water ice, ammonia hydrosulfide

Jupiter's Moon Summary

Sixteen moons have been discovered orbiting around Jupiter. Most of them are relatively small and seem to have been more
likely captured than to have been formed in orbit around Jupiter. Four of the largest moons, Io, Europa, Ganymede and
Callisto, are believed to have accreted as part of the process by which Jupiter itself formed. The following table summarizes
the radius, mass, distance from the planet center, discoverer and the date of discovery of each of the moons of Jupiter: 

              Radius      Mass     Distance
Moon     #    (km)        (kg)     (km)       Discoverer       Date
Metis    XVI  20          9.56e+16 127,969    S. Synnott       1979
Adrastea XV   12.5x10x7.5 1.91e+16 128,971    Jewitt-Danielson 1979
Amalthea V    135x84x75   7.17e+18 181,300    E. Barnard       1892
Thebe    XIV  55x45       7.77e+17 221,895    S. Synnott       1979
Io       I    1,815       8.94e+22 421,600    Marius-Galileo   1610
Europa   II   1,569       4.80e+22 670,900    Marius-Galileo   1610
Ganymede III  2,631       1.48e+23 1,070,000  Marius-Galileo   1610
Callisto IV   2,400       1.08e+23 1,883,000  Marius-Galileo   1610
Leda     XIII 8           5.68e+15 11,094,000 C. Kowal         1974
Himalia  VI   93          9.56e+18 11,480,000 C. Perrine       1904
Lysithea X    18          7.77e+16 11,720,000 S. Nicholson     1938
Elara    VII  38          7.77e+17 11,737,000 C. Perrine       1905
Ananke   XII  15          3.82e+16 21,200,000 S. Nicholson     1951
Carme    XI   20          9.56e+16 22,600,000 S. Nicholson     1938
Pasiphae VIII 25          1.91e+17 23,500,000 P. Melotte       1908
Sinope   IX   18          7.77e+16 23,700,000 S. Nicholson     1914