9th Ed. Astro Today Ch. 11 RD 2-10 even Ch. 12 RD 1-5 all Ch. 13 RD 1-7 odd Ch. 11 2) What is differential rotation and how is it observed on Jupiter? Differential rotation refers to the way different parts of a rotating fluid body can have different periods of rotation. This occurs on gas planets like Jupiter as well as stars like the Sun. On Jupiter, features like ovals and spots and eddies in the cloud tops can be observed over many rotations of the planet. We see regions near the equator making a complete rotation in the shortest time. Unlike the Sun, which seems to continuously increase in period with latitude, Jupiter has switches from short to long to short period many times as one moves away from the equator. These switches correlate with the change in colorings from belts (dark) to zones (light). 4) What is the Great Red Spot? What is known about the source of its energy? The Great Red Spot is a large, elongated, rotating cloud feature located in the southern hemisphere of Jupiter below its S. equatorial belt. It rotates CCW in the S. hemisphere making it an anticyclone, unlike a hurricane (which rotates in the same sense as the parent planet). So it is more like a high pressure dome than a low pressure hole. It is sandwiched between a eastward moving belt to the north and a westward moving zone to the south. The calm center compared to rapid edges of the spot suggest that it may be sustained and powered by the surrounding zonal flow. However, the powering of the GRS is not considered a solved problem. Juno determined its depth to be about 350 km - quite shallow compared to its ~15,000 km diameter. 6) Why has Jupiter retained most of its original atmosphere? The surface gravity is so great that not even the lightest gas, hydrogen, can escape at the typical cloudtop temperatures of Jupiter (125 K). Jupiter is far from the Sun, which contributes to its low temperature. (It would be around 105 K if not for the transfer of its internal heat outward.) This means that atmospheric molecules have little kinetic energy, and thus are moving at a speed well below Jupiter's substantial escape velocity. 8) What is Jupiter thought to be like beneath its clouds? Why do we think this? First, from observations of different regions we can tell that there are layers of clouds and haze. Some regions only show the topmost haze and ammonia ice layers, but they probably have the ammonium hydrosulfide ice and water ice layers below them. Other regions we can see these layers directly through holes. Below that our predictions come mostly from physics. Hydrostatic equilibrium tells us to expect the gases to get hotter and denser with depth. We also know that, under high enough pressure, molecular hydrogen will be compressed into a liquid at ~2000 km, and eventually a metallic (electrically conductive) liquid state at ~20,000 km. The Galileo probe descended into a hole in the equatorial clouds but only functioned down to a depth of about 150 km. It could confirm the increase in pressure and measure compositions down to that depth. 10) How does the density of the Galilean moons vary with increasing distance from Jupiter? Is there a trend to the variation? If so, why? The density of the Galilean moons decreases with distance from Jupiter. Table 11.1 gives the densities of Io, Europa, Ganymede, and Callisto as 3.5, 3.0, 1.9, and 1.9 g/cm^3, respectively. So Ganymede and Callisto are about the same. The densities are consistent with a progression in core/crust composition from iron/rock to rock/ice (the outermost moons have considerable amounts of water in ice and liquid form.) The reason for this progression is likely something like the condensation sequence for the whole solar system; moons forming close to Jupiter would have formed out of higher melting point materials. [ 5) What is the cause of the colors in Jupiter's atmosphere? Hydrogen and helium are colorless gasses, so the colors of Jupiter's clouds result from trace quantities of certain molecules. The highest coulds are white and composed of ammonia ice. The yellows, reds, and browns are found in lower clouds that contain ammonium hydrosulfide ice. It is possible that sulfur, phosphorus, or their compounds contribute to these clouds. Even organice compounds are a possibility. Below these clouds are bluish clouds of water ice. ] [ 7) Explain the theory that accounts for Jupiter's internal heat source. When jupiter was forming, gravity compressed its gases, causing the temperature to rise. Although the collapse stopped as Jupiter stabilized, the heat remained. The thick atmosphere has allowed this energy to slowly leak out of Jupiter, causing it to continue to emit more energy than it receives from the Sun. It looses most of its heat via infrared radiation. ] Ch. 12 RD 1-5 (all) 1) Why does Saturn have a less varied appearance than Jupiter? What does it's shape tell us about its deep interior? Saturn receives about 1/4 the sunlight that Jupiter receives because it is farther from the Sun. Saturn also produces less internal heat than Jupiter, and so its atmosphere is cooler and much less turbulent and active than Jupiter's. Because of Saturn's weaker gravity, its upper atmosphere is 2.5 times thicker than Jupiter's. The cloud layers are similar to Jupiter's but thicker; the top layers of haze and ammonia ice hide many of the cloud features below it. Saturn's atmosphere is under-abundant in helium because the cold in the upper layers has caused it to precipitate and fall as "rain" into the interior. The highly oblate shape of Saturn is still less oblate than it would be if made only of H and He, so it likely has a core made of higher density materials. The core may contain as much as 15 Earth masses of dense material. 2) As seen from Earth, Saturn's rings sometimes appear broad and brilliant, but at other times seem to disappear. Why? The rings maintain a 27 degree tilt relative to Saturn's orbital plane. But, as Saturn orbits the Sun over 29 years, Earth has a changing view of the rings. Every 15 years the rings go through a ring-plane crossing where they appear edge-on to us. 7.5 years after a ring-plane crossing, they will look the most broad and brilliant. 3) Compare and contrast the atmospheres and weather systems of saturn and Jupiter, and tell how the differences affect each planet's appearance. Saturn's lower mass means that it has a weaker gravity than Jupiter; therefore, the cloud layers are not as compressed as Jupiter's; Saturn's ammonia ice + haze layer covers 100 km instead of 50 km. This gives Saturn a more uniform butterscotch hue. Saturn is thought to have similar, colorful, lower cloud decks to Jupiter but they are generally hidden. Saturn is also similar to Jupiter in that it has zonal winds. In fact, they flow at even higher speeds (up to 1500 km/s) relative to the magnetic field-defined interior. But the belts and zones patterns are difficult to see without computer processing, or IR imaging. Storms occur on both Jupiter and Saturn (mega-lightning has been detected from both). But on Saturn, they are more seasonal (Saturn has a 27 degree tilt compared to Jupiter's 3 degrees). Roughly every 30 years we are likely to see storms appearing in a given hemisphere. The storms originate at great depths but manifest themselves at the surface as whitish ovals and "festoons" which sometimes grow long tails of turbulence indicating a relative motion between the storm source and the upper winds. 4) Compare the thicknesses of Saturn's various layers (clouds, molecular hydrogen, metallic H, and core) with the equivalent layers in Jupiter. Why do the thicknesses differ? Saturn has a cloud layer 2.5 times thicker than Jupiter; its lower gravity allows the cloud layers to cover a larger range in altitudes. Beneath the clouds, its layer of molecular hydrogen is 30,000-km thick compared to Jupiter's 20,000-km thick molecular hydrogen layer. Saturn's weaker gravity leads to less internal compression at a given depth, and so you have to go deaper into Saturn to get enough pressure to create metallic hydrogen. Saturn's core is 15,000-km thick compared to a 10,000 km thick core for Jupiter. Less internal compression means Saturn's core will be less dense. 5) What mechanism is responsible for the relative absence of helium in Saturn's atmosphere, compared with Jupiter's atmosphere? As a whole, Saturn should have approximately as large a percentage of helium as Jupiter. However, the abundance of helium has been severely depleted in the upper atmosphere by condensation. The helium "rain" has sunk beneath the surface, where we cannot detect its presence. Continued helium precipitation is also responsible for internal heating of Saturn. [ ) Saturn's great distance from the Sun means the planet receives very little solar heating. Therefore, after Saturn had lost a great amount of its initial internal heat, the temperature in the upper atmosphere dropped greatly. It got so cold that helium in the atmosphere condensed and fell like rain into the interior. The great internal pressure prevented the helium from evaporating and returning to the surface, and so the upper layers are depleted in helium. ] Ch. 13 RD 1-7 odd 1) Why did astronomers suspect an eigth planet beyond Uranus? They could not find a simple elliptical orbit that would predict Uranus's position. (The best orbit accrued an error of 15 arcseconds in 50 years.) They concluded that Uranus must also be affected gravitationally by an unknown planet. [2) How did Uranus come to be spinning "on its side"? We do not know for sure. It likely involved collisions of smaller bodies with Uranus. If it happened in one collision, it is difficult to explain how the Moon's could get back into the equatorial plane. The moon's could keep up more easily if there were multiple collisions, but large collisions should have been rare in the outer solar system. ] 3) How and why do the overall colors and appearance of Uranus and Neptune differ from those of Jupiter and Saturn? The greater distance from the Sun makes Uranus and Neptune have colder atmospheres than Jupiter and Saturn. This influences which molecules form the visible cloudtops. Ammonia is frozen out of the atmospheres of Uranus and Neptune, so they do not possess the orangish features of Jupiter and Saturn which are largely ammonia hydrosulfide. Methane, on the other hand, is more abundant in Uranus and Neptune. This causes the bluish coloration of these worlds since methane absorbs red preferentially. [4 Why are storms and other atmospheric features more easily seen on Neptune than on Uranus? Neptune has higher level white clouds than Uranus does. Second, Uranus has more photochemical haze in its stratosphere which blocks the view of its uppermost clouds. Third, Neptune has more internal heat which should lead to more convection and building of clouds and storms. ] 5) How are the interiors of Uranus and Neptune thought to differ from those of Jupiter and Saturn? Because they have lower masses than Jupiter and Saturn, Uranus and Neptune cannot generate nearly as much internal pressure. As a result, they are not capable of compressing hydrogen in their interiors to the point where the hydrogen becomes metallic. Instead, they may have thick layers of compressed water with ammonia and other materials dissolved in it. This layer of “ionic slush” could produce the magnetic fields we see around these planets. [6) How do the magnetic field of Uranus and Neptune compare with that on Earth? Internally, both are about 100x stronger than Earth's magnetic field, but at the surface they have comparable field strengths to Earth. They are both highly tilted relative to the spin axes (60 / 46 degrees for Uranus / Neptune compared to 11 degrees for Earth), and offset relative to the centers of Uranus and Neptune. They may still be caused by a dynamo effect, but the conducting liquid would be "slushy" ammonia water in the outer layers rather than metallic H (in Jupiter/Saturn) or a molten liquid core (in Earth).] 7) Describe a day on Titania. [This should have been in the next assignment.] A typical day on Uranus’ moon Titania will depend on the orientation of Uranus and the orbits of its moons to the Sun. This in turn depends on where Uranus is in its orbit. When either pole of Uranus is facing the Sun, either the northern or southern hemisphere of Uranus (and all of its moons) will be in continual sunlight and the other hemisphere will be in total darkness. About 21 years later, the equator of Uranus will be oriented towards the Sun. In this case, as Titania orbits around Uranus in 8.7 days, and with synchronous rotation, it also rotates with the same period. The Sun will rise and set on Titania but Uranus itself will not move in Titania’s sky. [ Why are Uranus and Neptune bluish? The gas methane absorbs red light very efficiently. Uranus and Neptune have a larger abundance of methane compared to Jupiter and Saturn. Therefore, the reflected sunlight we receive from Uranus and Neptune contains less red light and is mostly blue. Because Uranus has less methane than Neptune, it appears more blue-green; Neptune appears quite blue. ]