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Today, we will mainly discuss Saturn. Its orbit, and its place in our solar system, are shown on the next two slides. It is a little more than 9.5 times further from the sun than is the earth, and almost twice as distant from the sun as Jupiter. Its orbit, like the earth’s, lies pretty much in the ecliptic plane. Saturn rotates in the same sense as its orbital motion, as does the earth, and its rotational axis is inclined relative to the plane of its orbit, much like the earth’s. Saturn: Diameter, 120,000 km; Mass, 95.2 Earth masses; Density, 0.7 (density of water is 1.0); Rotation Period, 10 hours, 14 minutes; Axis Inclination, 26° 44’; Oblateness, 0.1; Surface Gravity, 1.15 (Earth = 1.0). The inclined rotation axis of Saturn means that Saturn has seasons, like we do on earth, although it is amusing to think how the seasons on Saturn might be affected by the presence of Saturn’s ring system. The following tables, repeated from last time, give comparisons of some of the physical properties of the planets. The most important columns of these tables to see for the moment are those that compare the sizes and material compositions of the planets. These data show that Saturn, like Jupiter, is a giant gaseous planet entirely different from the earth. 1 Sizes of the Giant Gas Planets Saturn seen with Lick Observatory’s 120-inch telescope (using the Coudé focus) Voyager-1 image of Saturn from 76 million km. Moons Mimas & Enceladus are at the bottom, Titan at right, and Dione & Tethys are at the top left. Image obtained 9/17/80. Saturn, like Jupiter, rotates very rapidly. For this reason, it is visibly oblate. Saturn has just about the same radius and rotation period as Jupiter, but it is less than a third as massive. Therefore Saturn exerts much less gravitational force than Jupiter does near its surface. Consequently, Saturn has less gravity than Jupiter to counterbalance the large centrifugal force generated near its q surface byy its rapid p rotation. The result is that Saturn equatorial is visibly much more oblate than Jupiter. Remember that this situation, where Saturn is much less massive, yet nevertheless not very much smaller than Jupiter, arises because both planets are made out of gases rather than rocks or ice. If you were to add more mass to Jupiter, its gases would become more compressed by the increased gravity, and the planet might not become much larger. If you added enough mass, Jupiter might even become smaller! Figure 11.3 (a) Gravity alone makes a planet spherical, but rapid rotation flattens out the spherical shape by flinging material near the equator outward. (b) Saturn is clearly not spherical. 2 The rapid rotation of Saturn, just as on Jupiter, inhibits circulation of any parcel of gas in the atmosphere over a wide range of latitudes. If a parcel of gas in the atmosphere near the equator were to travel very far toward either pole, the principle of conservation of its angular momentum about the planet’s rotation axis would force it to spin up to a tremendous velocity. The gas therefore prefers to organize its circulation into bands, bands so that no part of the atmosphere travels over too wide a range of latitudes. The atmospheric circulation therefore breaks up into a series of “belts” and “zones,” horizontal bands in which prevailing wind velocities alternate. As on Jupiter, these motions are driven by convection. Convection carries heat outward from the interior to the surface, where it is radiated into space. Zonal (east-west) wind velocity for the giant planets as a function of latitude. There are gaps where we lack contrasts suitable for tracking winds or where the atmosphere was in darkness (on the night side or shadowed by Saturn’s rings). From P. Gierasch and B. Conrath (1993), J. Geophys. Res. 98, 54595469. Despite the similarities of Saturn and Jupiter just mentioned, Saturn does not exhibit the garish atmospheric features that are so striking on Jupiter. Because of its overall lower temperatures, Saturn’s clouds lie deeper in its atmosphere than those of Jupiter. As a result, a layer of tan haze overlies the clouds and washes out Saturn’s appearance. Saturn has a powerful equatorial jet (very rapid winds in the direction of the planet’s rotation). (from Encyclopedia of the Solar System) Saturn's atmosphere and its rings are shown here in a false color composite made from Cassini images taken in near infrared light through filters that sense different amounts of methane gas. Portions of the atmosphere with a large abundance of methane above the clouds are red, indicating clouds that are deep in the atmosphere. Grey indicates high clouds, and brown indicates clouds at intermediate altitudes. The rings are bright blue because there is no methane gas between the ring particles and the camera. View of Saturn from the Cassini Spacecrft, 2005. The complex feature with arms and secondary extensions just above and to the right of center is called the Dragon Storm. It lies in a region of the southern hemisphere referred to as "storm alley" by imaging scientists because of the high level of storm activity observed there by Cassini in the last year. Radio bursts from lightning associated with this storm have been observed by the Cassini probe. 3 Saturn's atmosphere and its rings are shown here in a false color composite made from Cassini images taken in near infrared light through filters that sense different amounts of methane gas. Portions of the atmosphere with a large abundance of methane above the clouds are red, indicating clouds that are deep in the atmosphere. Grey indicates high clouds, and brown indicates clouds at intermediate altitudes. The rings are bright blue because there is no methane gas between the ring particles and the camera. Spectra of Saturn’s atmosphere reveal a mysterious depletion of helium to levels well below that in Jupiter’s atmosphere or in the sun (helium is 4 times less abundant relative to hydrogen in Saturn’s visible atmosphere than on the sun). It is believed that the helium originally present in the outer atmospheric layers of Saturn has separated from the hydrogen, much like oil and vinegar separate naturally, and has settled toward the deep interior of the planet. This settling of the helium releases gravitational potential energy as heat. Without the heat input from the settling of helium, model calculations indicate that Saturn should have lost the heat it would have gained in its original accretion and arrived at its presently observed temperature in only 2 billion years, less than half of the age of the solar system. The heating from helium settling seems to resolve this contradiction. Although the structure of Saturn’s interior and its atmospheric circulation patterns are fascinating, by far the most interesting feature of this planet is its magnificent ring system. 4 Artist’s concept of Saturn’s rings and major icy moons. The outer rings, labelled G and E, are diffuse. The E ring, the largest in the solar system, extends from the orbit of Mimas to that of Titan, a distance of 1 million km. The main rings (A,B,C) are less than 100 meters thick. They are perhaps only a few hundred million years old. Saturn’s rings orbit the planet inside what is called the Roche zone. This is the region so close to the planet (within 2 or 3 Saturn radii) that the tidal forces caused by the planet prevent the accumulation of sizable bodies as a result of their selfgravity. Within the Roche zone, the tidal forces tending to pull an object apart are comparable to the gravitational forces tending to hold it together. together Therefore only relatively small bodies, especially bodies held together by non-gravitational forces, as your body is, can survive inside the Roche zone. Saturn’s rings may have formed according to one of the following 2 scenarios: 1. They are remnants of the original nebula from which Saturn formed. They were prevented from accumulating into a single moon by the tidal forces of gravity. 2. They are the result of the tidal disruption of one or more objects that wandered into the Roche zone. If the object did not approach pp Saturn closely y enough g to be completely p y disrupted, its disintegration may have been aided by cometary or meteoroid impacts. One can object to the first scenario by arguing that over a period of perhaps 100 million years (a relatively short period compared to the age of the solar system) the particles now in Saturn’s rings will be ground into dust by collisions or spiral into the planet. However, these particles may be continually resupplied from the progressive disintegration of small moons. Appearance of Saturn’s rings under different lighting conditions An artist’s representation of particles in Saturn’s A ring (from Sky & Telescope Jan., 1981, p. 10) 5 Saturn’s rings look very different when viewed in reflected light or transmitted light. Tiny dust particles scatter light, but mostly do not change its direction very much. (Just think about how dust on your car’s windshield scatters sunlight.) Therefore when the sun is behind Saturn’s rings, we see mostly light scattered by small dust-sized particles. MarbleM bl or boulder-sized b ld i d particles ti l mostly tl scatter tt light li ht back b k in i the th general direction that it came from. Therefore when we view Saturn’s rings lit by sunlight which is reflected back toward us, we see the regions populated by these larger particles as the brightest, and the regions populated mainly by smaller particles appear dark. Here we see light reflected off the large particles in the rings. Here we see light transmitted through ring regions containing primarily small particles. Voyager-1 view of Saturn in its crescent phase Voyager-2 view of Saturn in its crescent phase Saturn’s rings lit from behind. Cassini’s division is bright (small particles), while the B ring is dark. Voyager-1 image of Saturn’s rings from below at 717,000 km distance. Cassini’s division appears bright in this image. 6 Voyager-1 view of Saturn’s rings from 30º above the ring plane and 720,000 km distance. In this image, we see Cassini’s division as a dark band, because we are looking at light reflected by the rings, and only the large particles reflect much light. Instruments aboard the Cassini spacecraft analyze the spectra from Saturn’s rings to determine the size of the ice particles on the surfaces of the boulder-sized snowballs (mainly of water ice) that are thought to make up the rings. Evidence from the visual and infrared mapping spectrometer on the Cassini spacecraft indicates that the grain sizes in Saturn’s rings grade from smaller to larger, related to distance from Saturn. g ) are shown next to a corresponding p g picture p of the Those data ((right) rings taken by Cassini's narrow angle camera. Saturn's rings are thought to be made up of boulder-size snowballs. By looking at the rings with the visual and infrared mapping spectrometer, the size of the ice crystals, or grains, on the surfaces of those boulders can be determined. Saturn’s rings show a wealth of internal structure. Many of the phenomena that cause this structure are complex, and we will not discuss them. However, some of the processes are easy to understand. The rings are exceedingly thin, as can be observed on the occasions when the earth (or a spacecraft) crosses the ring plane. The ring particles are composed mainly of water ice, and when these particles collide with one another, the collisions are not elastic. Such particles tend to chip each other rather than to bounce like superballs (or like the atoms in a gas). Instruments aboard the Cassini spacecraft analyze the spectra from Saturn’s rings to determine the size of the ice particles on the surfaces of the boulder-sized snowballs (mainly of water ice) that are thought to make up the rings. The inelastic collisions of the ring particles have caused the extremely low degree of random velocities compared to the size of the orbital velocities. The small random velocities make the rings so thin. Natural color mosaic of Saturn’s rings at high resolution taken by the Cassini spacecraft in 2004. Gaps, gravitational resonances and wave patterns are all present, and the delicate color variations across the system are clearly visible. This mosaic of six images covers a distance of approximately 62,000 kilometers along the ring plane, from a radius of 74,565 kilometers to 136,780 kilometers (46,333 to 84,991 miles) from the planet's center. This view is from Cassini's vantage point beneath the ring plane. The rings are tilted away from Cassini at an angle of about 4 degrees. Images taken using red, green and blue spectral filters were used to create this natural color mosaic. The images were acquired using the Cassini spacecraft narrow angle camera on Dec. 12, 2004, at a distance of approximately 1.8 million kilometers (1.1 million miles). The image scale is 10.5 kilometers (6.5 miles) per pixel. 7 Here we see Enceladus hovering past the B Ring, with 4 faint bands visible within the Cassini Division below and to the right of this moon. The rings are only tens of meters thick. Here we do not see them precisely edge on, but it is nevertheless quite clear that they are extremely thin and almost perfectly flat. One of the most striking features of the rings are the prominent gaps within them. The most prominent of these is the Cassini Division. The Cassini Division in Saturn’s rings, seen from 13 million km. 8 Voyager-1 image of the Cassini Division in Saturn’s rings from 6 million km. The Cassini Division occurs at 120,000 km from Saturn’s center. At this location, a ring particle orbits Saturn in exactly half the time that it takes the moon Mimas to orbit. Thus every two orbits such a particle receives the same gravitational nudge from Mimas, which tends to make the ring particle’s orbit eccentric. Cassini recently took a far more detailed photo of the outer edge of the Cassini Division, leading into the inner edge of the A ring, at the g The right. structures seen are due to the influence of the co-orbital moons Janus and Epimetheus. A mosaic image of Saturn’s rings, viewed by Voyager-1 from 8 million km. The growing eccentricity of the ring particle’s orbit leads it to collide with other ring particles, and this process clears out a gap in the rings – the Cassini Division. In the following image, the Enke gap in the outer A-ring is clearly visible. It has a different cause than the Cassini Division. Saturn’s complex rings are both an intriguing scientific puzzle and a supreme natural wonder. This view shows, from upper right to lower left, the thin C ring, multi-toned B ring, the dark Cassini Division, the A ring and narrow F ring. At the bottom, Saturn’s moon Mimas (398 kilometers, or 247 miles across) orbits about 45,000 kilometers (28,000 miles) beyond the bright core of the F ring. The little moon is heavily cratered and is thought to be largely composed of water ice. The bright speck just outside of (below) the F ring is the shepherd moon Pandora (84 kilometers, or 52 miles across). The image was taken in visible light with the Cassini spacecraft narrow angle camera on Jan. 19, 2005, at a distance of 1.8 million kilometers (1.1 million miles) from Saturn. The image scale is 11 kilometers (7 miles) per pixel. Pandora was brightened by a factor of seven to aid visibility. M i m a s & P a n d o r a 9 Reflected light from Saturn dimly illuminates the night side of the cratered moon Mimas in this Cassini image. Above, the outer edges of the planet's main rings show some interesting details. Mimas is 398 kilometers (247 miles) across. Several thin ringlets comprising the F ring are nicely visible here, and the bright core of the ring displays a few twisted knots. Perhaps less noticeable are kinks in one of the thin ringlets of material visible within the Encke Gap near the upper left corner. The outer edge of the A ring appears notably brighter than the ring material on the other side of the narrow Keeler Gap. Finally, numerous gravitational resonances give the A ring a grooved or striped appearance in this view. The image was taken in visible light with the Cassini spacecraft narrow angle camera on Jan. 17, 2005, at a distance of approximately 1.2 million kilometers (746,000 miles) from Saturn. The image scale is 7 kilometers (4 miles) per pixel. Mimas as viewed by the Cassini spacecraft. Mimas is seen here with the thin F ring in the background. The Enke gap in the outer A ring is 320 km wide. It is believed that it is caused by the gravitational disturbances of the 20-km-diameter satellite Pan orbiting within it. The gravitational tug of Pan on nearby ring particles pushes them into orbits with greater separation from Pan, thus clearing out the Enke gap in the ring. This counter counter-intuitive intuitive behavior of the ring particles is a result of the special nature of Keplerian motion, which is illustrated on the following slides. Solid body rotation compared with motion due to gravity. Pair of satellites near Saturn’s F ring -- Sky & Telescope, 1/81, p. 12 10 The inner moon shown orbits faster than the ring particles outside it. Therefore its gravitational attraction tends to speed up the orbital motion of these ring particles. As the ring particles begin to orbit faster, they experience a greater centrifugal force away from the planet, and they therefore move to more distant orbits. The outer moon shown orbits slower than the ring particles inside its orbit. orbit Therefore its gravity tends to slow down the orbital motion of these particles. As these ring particles begin to orbit more slowly, they experience a reduced centrifugal force. As a result, Saturn’s gravity pulls them closer to it, and they move to orbits closer to the planet. These effects of a moon on the ring particles inside and outside its own orbit explain the clearing of the Encke gap. Two co-orbiting satellites at Saturn’s F ring, viewed from 25 million km Two images of Saturn’s eleventh moon, a trailing co-orbital satellite, viewed from 177,000 km. Saturn’s fourteenth satellite, just inside the F ring, viewed from 7 million km Saturn’s braided F ring from 750,000 km. Prometheus (63 miles across) and Atlas (12 miles across) orbit between Saturn’s A and F rings. (view from Cassini spacecraft) 11 Saturn’s moon Pandora (52 miles across) is viewed by Cassini with the F ring in the foreground. Pandora is lit by reflected light from Saturn. Saturn’s moon Prometheus (63 miles across) is seen here making a new diagonal gore in the tenuous material inside Saturn’s F ring. Prometheus creates a new gore each time it comes closest to the F ring, and the memory of these is preserved from previous passes. Saturn’s moon Prometheus (63 miles across), followed by Pandora, is seen here making a new diagonal gores in the tenuous material inside Saturn’s F ring. (This is a movie clip that was shown in class.) Moons visible in this image: Mimas (398 kilometers, or 247 miles across) at right, Pandora (84 kilometers, or 52 miles across) near center and Janus (181 kilometers, or 113 miles across) in the lower left corner. Mimas' orbit inclination of 1.6 degrees relative to Saturn's equator is enough to make it appear as if it orbits just beyond the F ring when viewed from this vantage point of 5 degrees below the rings. In fact, it is 34,000 kilometers (21,000 miles) more distant than Janus. One of the shepherd satellites of Saturn’s rings Cassini confirms that a small moon is orbiting within the narrow Keeler gap, near the outer edge of Saturn’s A ring. (This is a movie clip that was shown in class.) 12 Cassini view of Titan with its normal haze. Cassini view of Titan on its closest flyby, April 16, 2005. False color: green areas are on the surface, and red areas are high in the atmosphere. A huge annular feature with an outer diameter of approximately 440 kilometers (273 miles) appears in this image taken with Cassini's Titan radar mapper. It resembles a large crater or part of a ringed basin, either of which could be formed when a comet or asteroid tens of kilometers in size slammed into Titan. This is the first impact feature identified in radar images of Titan. The surface of Titan appears to be very young compared to other Saturnian satellites. In Titan's case, debris raining down from the atmosphere or other geologic processes may mask or remove the craters. The pattern of brightness suggests that there is topography associated with this feature; for example, in the center of the image there appear to be mounds each about 25 kilometers (15 miles) across. Since they are dark on their lower edges that face away from the radar and bright on the opposite face, they must be elevated above the surrounding terrain. Cassini sees a river on Titan. First impact feature found on Titan by Cassini’s radar mapper. First color view of Titan’s surface. Initially thought to be rocks or ice blocks, they are more pebble-sized. The two rock-like objects just below the middle of the image are about 15 centimeters (about 6 inches) (left) and 4 centimeters (about 1.5 inches) (center) across respectively, respectively at a distance of about 85 centimeters (about 33 inches) from Huygens. The surface is darker than originally expected, consisting of a mixture of water and hydrocarbon ice. There is also evidence of erosion at the base of these objects, indicating possible fluvial activity. 13 A radar map of the north polar region of Titan made by the Cassini spacecraft in 7 flybys. It rains ethane and methane, making hydrocarbon lakes and seas, some as large as Lake Superior on Earth. A spectacular landslide within the low-brightness region of Iapetus’s surface known as Cassini Regio is visible in this image from Cassini. Iapetus is one of the moons of Saturn. The landslide material appears to have collapsed from a scarp 15 kilometers high (9 miles) that forms the rim of an ancient 600 kilometer (375 mile) impact basin. Unconsolidated rubble from the landslide extends halfway across a conspicuous, 120-kilometer diameter (75-mile) flat-floored impact crater that lies just inside the basin scarp scarp. Iapetus close-up Saturn’s irregularly shaped moon Iapetus The most unique, and perhaps most remarkable feature discovered on Iapetus in Cassini images is a topographic ridge that coincides almost exactly with the geographic equator. The ridge is conspicuous in the picture as an approximately 20kilometer wide (12 miles) band that extends from the western (left) side of the disc almost to the day/night boundary on the right. On the left horizon, the peak of the ridge reaches at least 13 kilometers (8 miles) above the surrounding terrain. Along the roughly 1,300 kilometer (800 mile) length over which it can be traced in this picture, it remains almost exactly parallel to the equator within a couple of degrees. The physical origin of the ridge has yet to be explained. It is not yet clear whether the ridge is a mountain belt that has folded upward, or an extensional crack in the surface through which material from inside Iapetus erupted onto the surface and accumulated locally, forming the ridge. Cassini view of Dione, with Saturn and its rings in the background. The color in this image, g , including that of Dione, is roughly as it would appear to the human eye. 14 Saturn’s moon Rhea in natural color. 15