![]() For decades, we have assumed that regular variations in radio emissions faithfully track true rotation rates, but alas, by this standard, Saturn's rotation has slowed down by six minutes since 1980 (See Anderson and Schubert, 2007.) The fact that these planets are primarily fluid and lack surfaces frustrates the simplest aims, like determining their rotation rate without solid benchmarks. Extrapolating from its findings, we concluded that ten hours after its entry, it had evaporated but never hit "bottom." down to conditions of 22 bars, 450 K, surviving for 57 minutes. It made many detailed measurements of pressure, temperature, composition, etc. It entered the atmosphere of Jupiter in 1995. Note: By arbitrary convention, the level at which atmospheric pressure is 1 bar is treated as the "surface." This roughly corresponds to the cloud tops.Ĭonsider the fate of the atmospheric probe conveyed to Jupiter by the Galileo spacecraft. (The schematic at right could apply to any major chemical constituent.) Thus, these worlds lack proper surfaces, regardless of how much this strains our imaginations. In all but the upper layers of the jovian planets, their major components are beyond this critical point. This is the state of temperature and pressure beyond which the distinction between liquid and gas becomes meaningless, and one can only refer to materials as fluids. At high temperature and pressure, we see a critical point. The graph at right is a phase diagram that shows, for different combinations of temperature and pressure, whether a given substance exists as a solid, liquid, or gas. Collectively these exotic processes and materials form planets that are fundamentally alien with respect to the solid worlds.hydrogen!) support electrical currents that power magnetic fields greater than that of Earth: Materials that would not conduct electricity elsewhere in the Solar System (E.G.Their great gravitational self-compression causes their materials to take exotic forms unknown on the smaller worlds.Because they can hold onto hydrogen and helium, their bulk compositions are radically different.They lack distinct surfaces, instead becoming gradually thicker as one goes down.This fact cascades into many consequences: The four giant planets together comprise approximately 445 Earth masses: Consider: Mercury + Venus + Earth + Moon + Mars = slightly less than two Earth masses. Being as massive as they are, Jupiter, Saturn, Uranus, and Neptune are qualitatively different from the solid worlds. ![]() That sounds trivial but actually it isn't. The planets to scale from The RASC Calgary Centre The major distinction of the giant planets is their size.
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