Saturn, however, does have seasons, since its spin axis is inclined at 27° to the perpendicular to its orbit. What are the seasons like for the giant planets? The spin axis of Jupiter is tilted by only 3°, so there are no seasons to speak of. Due to the tilt of Uranus, we see the rings completely surround the planet, giving the appearance of a The left-hand panel shows Uranus, with its rings, moons and some surface features. Uranus and Neptune have slightly longer rotation periods of about 17 hours, also determined from the rotation of their magnetic fields.įigure 1: Uranus in the Infrared. In the same way, we can measure that the underlying rotation period of Saturn is 10 hours 40 minutes. The rotation period we measure in this way is 9 hours 56 minutes, which gives Jupiter the shortest “day” of any planet. Since the magnetic field (which we will discuss below) originates deep inside the planet, it shares the rotation of the interior. Much more fundamental is the rotation of the mantle and core these can be determined by periodic variations in radio waves coming from Jupiter, which are controlled by its magnetic field. Distinct details in its cloud patterns allow us to determine the rotation rate of its atmosphere at the cloud level, although such atmosphere rotation may have little to do with the spin of the underlying planet. Seen through a telescope, Jupiter is a colorful and dynamic planet. On Uranus, we see no obvious cloud layer at all, but only a deep and featureless haze. On Neptune, the upper clouds are made of methane. When we look at the planets, we see only their atmospheres, composed primarily of hydrogen and helium gas. The uppermost clouds of Jupiter and Saturn, the part we see when looking down at these planets from above, are composed of ammonia crystals. When astronomers began to discover other planetary systems (exoplanets), we found that planets the size of Uranus and Neptune are common, and that there are even more exoplanets intermediate in size between Earth and these ice giants, a type of planet not found in our solar system. This is one piece of evidence that tells us that their composition must differ fundamentally from the gas giants. Their densities of 1.3 g/cm 3 and 1.6 g/cm 3, respectively, are much higher than that of Saturn. Uranus and Neptune each have a mass about 15 times that of Earth and, hence, are only 5% as massive as Jupiter. Since this is less than the density of water, Saturn would be light enough to float. Saturn’s mass is 95 times that of Earth, and its average density is only 0.7 g/cm 3-the lowest of any planet. (Recall that water has a density of 1 g/cm 3.) Jupiter’s material is spread out over a volume so large that more than 1400 Earths could fit within it. Jupiter’s average density is 1.3 g/cm 3, much lower than that of any of the terrestrial planets. Its diameter is about 11 times that of Earth (and about one tenth that of the Sun). Jupiter, the giant among giants, has enough mass to make 318 Earths. Some of the main properties of these four planets are summarized in Table 1. Uranus and Neptune are smaller and differ in composition and internal structure from their large siblings. Jupiter and Saturn have many similarities in composition and internal structure, although Jupiter is nearly four times more massive. These long timescales make it difficult for us short-lived humans to study seasonal change on the outer planets. Uranus orbits at 19 AU with a period of 84 years, while Neptune, at 30 AU, requires 165 years for each circuit of the Sun. Saturn is about twice as far away as Jupiter (almost 10 AU) and takes nearly 30 years to complete one orbit. Jupiter is more than five times farther from the Sun than Earth’s distance (5 AU), and takes just under 12 years to circle the Sun. The giant planets are very far from the Sun. Our approach is not just to catalog their characteristics, but to compare them with each other, noting their similarities and differences and attempting to relate their properties to their differing masses and distances from the Sun. Let us now examine the four giant (or jovian ) planets in more detail. Describe the discovery and characteristics of the giant planets’ magnetic fields.Compare and contrast the internal heat sources of the giant planets.Describe the composition and structure of Jupiter, Saturn, Uranus, and Neptune.Describe the basic physical characteristics, general appearance, and rotation of the giant planets.By the end of this section, you will be able to:
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