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Jupiter
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There has been much speculation that any life on Jupiter, or on other gas giants, might be ammonia-based life. The possibility of "abundant biota" in the upper regions of Jupiter's atmosphere was considered in a 1976 paper by Carl Sagan and Edwin E. Salpeter1 prior to the arrival of the first Jupiter probe, Pioneer 10. Sagan and Salpeter compared the ecology of the Jovian atmosphere with that of terrestrial seas which have simple photosynthetic plankton at the top level, fish at lower levels feeding on these creatures, and marine predators which hunt the fish. The three hypothetical Jovian equivalents of these organisms, Sagan and Salpeter termed "sinkers", "floaters", and "hunters". They envisaged creatures like giant gas-bags (see bubble life) that move by pumping out helium and calculated that the "hunter" variety might grow to be many kilometers across (and therefore visible from space). Jovian aerial life-forms like these are portrayed in Arthur C. Clarke's short story "A Meeting with Medusa" (in The Wind From the Sun).
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The planet Jupiter is primarily composed of hydrogen with a small proportion of helium; it may ... have a rocky core of heavier elements. Because of its rapid rotation the planet is an oblate spheroid (it possesses a slight but noticeable bulge around the equator). The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the seventeenth century. Surrounding the planet is a faint planetary ring system and a powerful magnetosphere. There are also at least 63 moons, including the four large moons called the Galilean moons that were first discovered by Galileo Galilei in 1610.
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Jupiter has a huge magnetic field, much stronger than Earth's. Its magnetosphere extends more than 650 million km (past the orbit of Saturn!). (Note that Jupiter's magnetosphere is far from spherical -- it extends "only" a few million kilometers in the direction toward the Sun.) Jupiter's moons therefore lie within its magnetosphere, a fact which may partially explain some of the activity on Io. Unfortunately for future space travelers and of real concern to the designers of the Voyager and Galileo spacecraft, the environment near Jupiter contains high levels of energetic particles trapped by Jupiter's magnetic field. This "radiation" is similar to, but much more intense than, that found within Earth's Van Allen belts. It would be immediately fatal to an unprotected human being.
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Jupiter has a faint planetary ring system composed of smoke-like dust particles knocked from its moons by meteor impacts. The main ring is made of dust from the satellites Adrastea and Metis. Two wide gossamer rings encircle the main ring, originating from Thebe and Amalthea. There is ... an extremely tenuous and distant outer ring that circles Jupiter backwards. Its origin is uncertain, but this outer ring might be made of captured interplanetary dust.
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Known as a "gas giant," Jupiter's deep gaseous atmosphere merges imperceptibly into a layer of liquid hydrogen. Within the planet pressure and temperature are so high that there is no clear boundary between the gas and liquid. The atmosphere is composed primarily of hydrogen and helium in nearly the same abundances found in the Sun and other stars. This enormous planet rotates faster than any other in the solar system, so fast that its shape is noticeably flattened out of a sphere. The banded and colorful atmosphere contains many gigantic oval-shaped storms. One of these, the Great Red Spot, is larger than two Earths and has persisted in Jupiter's atmosphere at least since the 1700s.
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Jupiter rotates faster than any other planet. It takes 9 hours 56 minutes to spin around once on its axis, compared with 24 hours for Earth. Scientists cannot measure the rotation of the interior of the giant planet directly, so they have calculated the speed from indirect measurements. They first calculated the speed using an average of the speeds of the visible clouds that move with interior currents, except for a more rapid zone near the equator.
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