3. The invisible buffer zone with space - atmospheres, magnetospheres and the solar wind
Atmospheres of the giant planets
Jupiter has very nearly the same composition as the Sun, made up mainly of the light gases hydrogen and helium. Saturn has about the same composition, with a bit more helium, but Uranus and Neptune are depleted of these two gases relative to the heavier hydrogen compounds like methane, ammonia and water.
Definite spectroscopic proof that molecular hydrogen is the most abundant element in Jupiter and Saturnís upper atmosphere did not occur until the 1960s and late 1970s, when high-dispersion infrared spectroscopy, from both the ground and space, showed several weak absorption features due to molecular hydrogen. Precise values for the ingredients of Jupiter's atmosphere were obtained in 1999 from the Galileo probe.
The helium abundance for Uranus and Neptune is consistent with that expected from a solar composition, but helium has been significantly depleted from Saturn's upper atmosphere and somewhat reduced in Jupiter's. Theoretical calculations indicate that helium rain has been falling toward the center of Saturn for the past two billion years, generating heat and producing lower amounts of helium in its outer atmosphere. Helium rain must be similarly settling toward Jupiter's core, but in lesser amounts.
Raging winds on the giant planets
Astronomers have been using telescopes, both small and large, to scrutinize weather patterns on Jupiter and Saturn for more than a century. They observe clouds of various ices, such as ammonia, ammonium hydrogen sulfide and water, that are formed in the cold outer atmospheres. These clouds have been pulled into counter-flowing winds by rapid planetary rotation, moving in opposite eastward or westward directions at constant speed and remaining confined to specific latitudes. These windswept clouds move in alternating light-colored bands, called zones, and dark ones, known as belts. Since Jupiter is all atmosphere, with no solid surface to rub against or continents to disturb the flow, its winds are free to rage unabated in response to the planet's spin, with large-scale configurations that have remained unchanged for as long as they have been observed.
The weather patterns on Uranus and Neptune, where clouds of methane ice are observed, resemble those on Earth, which has low-latitude trade winds that blow westward and a meandering eastward current, the jet stream, in each hemisphere. Nevertheless, the Earth has the weakest winds in the solar system; its fastest jet streams move at speeds of about 40 meters per second. In contrast, Jupiter's winds move at constant speeds of up to 180 meters per second, and Uranus' fastest winds are just a little faster. The high clouds on Venus and Mars also move at faster speeds than those on Earth, both with speeds of up to 100 meters per second. The winds on Neptune and Saturn move at speeds of up to 400 and 450 meters per second, respectively, ten times the fastest winds on Earth. The speed of the Earth's winds are measured with respect to the rapidly rotating surface beneath them; for the giant planets that have no solid surface the internal rotation rate is inferred from their magnetic fields that are generated within their cores.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University