1. Lecture Outlines Astronomy Today, Chapter 13 6th edition Chaisson
McMillan
Chapter 13
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2. Chapter 13 Uranus and Neptune
Chapter 13 opener. The outer planets have only briefly been explored as robot spacecraft glided by them. This photo of Neptune, in approximately true color, was taken by Voyager 2 when still at a distance of more than 6 million kilometers from the planet. Shown is the Great Dark Spot and its companion bright smudge, among other small clouds that persisted for as long as Voyager’s cameras could resolve them. Given other priorities of the U.S. space program—namely, sending humans back to the Moon and then on to Mars—it will likely be many years before robots return to these faraway worlds. (JPL)

3. Units of Chapter 13 13.1 The Discoveries of Uranus and Neptune
13.2 Orbital and Physical Properties
13.3 The Atmospheres of Uranus and Neptune
13.4 Magnetospheres and Internal Structure
13.5 The Moon Systems of Uranus and Neptune
13.6 The Rings of the Outermost Jovian Planets

4. 13.1 The Discoveries of Uranus and Neptune
Uranus was discovered in 1781 by Herschel; first planet to be discovered in more than 2000 years
Little detail can be seen from Earth; arrows point to three of Uranus’s moons:
Figure Uranus from Earth Details are virtually invisible on photographs of Uranus made with large Earth-based telescopes. (Arrows point to three of the planet’s moons.) (UC/Lick Observatory)

5. 13.1 The Discoveries of Uranus and Neptune
Slightly more detail can be seen in this image taken by Voyager 2 at a distance of 1 million km
Figure Uranus, Close Up This image of Uranus, taken from a distance of about 1 million km, was sent back to Earth by the Voyager 2 spacecraft as it whizzed past the giant planet at 10 times the speed of a rifle bullet. The image approximates the planet’s true color, but shows virtually no detail in the nearly featureless upper atmosphere, except for a few wispy clouds in the northern hemisphere. (NASA)

6. 13.1 The Discoveries of Uranus and Neptune
Neptune was discovered in 1846, after analysis of Uranus’s orbit indicated its presence
Details of Neptune cannot be made out from Earth either; arrows again point to moons:
Figure Neptune from Earth Neptune and two of its moons, Triton (left arrow) and Nereid (right), imaged with a large Earth-based telescope. (UC/Lick Observatory)

7. 13.1 The Discoveries of Uranus and Neptune
More detail is visible in these Voyager 2 images, also taken from a distance of 1 million km:
Figure Neptune, Close Up (a) Neptune as seen in natural color by Voyager 2, from a distance of roughly 1 million km. (b) A closer view, resolved to about 10 km, shows cloud streaks ranging in width from 50 km to 200 km. (NASA)

8. 13.2 Orbital and Physical Properties
Uranus and Neptune are very similar
Figure Jovian Planets Jupiter, Saturn, Uranus, and Neptune, showing their relative sizes compared to Earth. Uranus and Neptune are quite similar in their bulk properties, each one probably having a core about 10 times more massive than Earth. Jupiter and Saturn are both much larger, but their rocky cores are probably comparable in mass to those of Uranus and Neptune. Note the very different atmospheric features of these five worlds. (NASA)

9. 13.2 Orbital and Physical Properties
Uranus
Neptune
Mass
14.5 x Earth
17.1 x Earth
Radius
4.0 x Earth
3.9 x Earth
Density
1300 kg/m3
1600 kg/m3

10. 13.2 Orbital and Physical Properties
Peculiarity of Uranus: Axis of rotation lies almost in the plane of its orbit. Seasonal variations are extreme.
Figure Seasons on Uranus Because of Uranus’s axial tilt of 98°, the planet experiences the most extreme seasons known in the solar system. The equatorial regions have two “summers” (warm seasons, around the times of the two equinoxes) and two “winters” (cool seasons, at the solstices) each year, and the poles are alternately plunged into darkness for 42 Earth years at a time.

11. 13.3 The Atmospheres of Uranus and Neptune
Outer atmospheres of Uranus and Neptune are similar to those of Jupiter and Saturn
Uranus and Neptune are cold enough that ammonia freezes; methane dominates and gives the characteristic blue color

12. 13.3 The Atmospheres of Uranus and Neptune
Uranus is very cold; clouds only in lower, warmer layers
These images show Uranus rotating (a–c), and its ring (d):
Figure Uranus’s Rotation (a), (b), and (c) These computer-enhanced Hubble Space Telescope images, taken at roughly 4-hour intervals, show the motion of a pair of bright clouds (labeled A and B) in the planet’s southern hemisphere. (The numbers at the top give the time of each photo.) (d) A near-infrared image of Uranus shows the planet’s ring system, as well as a number of clouds (pink and red regions) in the upper atmosphere. The clouds circle the planet at speeds of up to 500 km/h. (NASA)

13. 13.3 The Atmospheres of Uranus and Neptune
Band structure of Neptune is more visible; it had a “Dark Spot” similar to Jupiter’s storms (now vanished)
Figure Neptune’s Dark Spot (a) Close-up views, taken by Voyager 2 in 1989, of the Great Dark Spot of Neptune, a large storm system in the planet’s atmosphere, possibly similar in structure to Jupiter’s Great Red Spot. Resolution in the photo on the right is about 50 km; the entire dark spot is roughly the size of planet Earth. (b) These Hubble Space Telescope views of Neptune were taken years apart (as marked) when the planet was some 4.5 billion km from Earth. The cloud features (mostly methane ice crystals) are tinted pink here because they were imaged in the infrared, but they are really white in visible light. Note that the Great Dark Spot has disappeared in recent years. (NASA)

14. 13.4 Magnetospheres and Internal Structure
Uranus and Neptune both have substantial magnetic fields, but at a large angle to their rotation axes.
The rectangle within each planet shows a bar magnet that would produce a similar field. Note that both Uranus’s and Neptune’s are significantly off center.
Figure Jovian Magnetic Fields A comparison of the magnetic field strengths, orientations, and offsets in the four jovian planets: (a) Jupiter, (b) Saturn, (c) Uranus, and (d) Neptune. The planets are drawn to scale, and in each case the magnetic field is represented as though it came from a bar magnet (a simplification, for purposes of illustration only). The size and location of each magnet represent the strength and orientation of the planetary field. Notice that the fields of Uranus and Neptune are significantly offset from the center of the planet and are much inclined to the planet’s rotation axis. Earth’s magnetic field is shown for comparison. One end of each magnet is marked N to indicate the polarity of Earth’s field.

15. 13.4 Magnetospheres and Internal Structure
Magnetic fields of Uranus and Neptune must not be produced by dynamos, as the other planets’ fields are
Interior structure of Uranus and Neptune, compared to that of Jupiter and Saturn:
Figure Jovian Interiors A comparison of the interior structures of the four jovian planets. (a) The planets drawn to scale. (b) The relative proportions of the various internal zones.

16. 13.5 The Moon Systems of Uranus and Neptune
Figure Moons of Uranus and Neptune The five largest moons of Uranus, and Proteus, the sole midsized moon of Neptune, are shown to scale, with part of Earth’s Moon (also to scale) for comparison. In order of increasing distance from the planet, the Uranian moons are Miranda, Ariel, Umbriel, Titania, and Oberon. The smallest details visible on these moons are about 15 km across. The appearance, structure, and history of Titania and Oberon may be quite similar to those of Saturn’s moon Rhea. Umbriel is one of the darkest bodies in the solar system, although it has a bright white spot on its sunward side. Ariel is similar in size, but has a brighter surface, with signs of past geological activity. (NASA; Lick Observatory)

17. 13.5 The Moon Systems of Uranus and Neptune
Uranus has 27 moons, five of which are major: Miranda, Ariel, Umbriel, Titania, and Oberon
Similar to Saturn’s medium-sized moons, except that all are much less reflective
Umbriel is the darkest

18. 13.5 The Moon Systems of Uranus and Neptune
Miranda is the most unusual moon of Uranus; origin of the cracks and grooves is unknown
Figure Miranda The asteroid-sized innermost moon of Uranus, photographed by Voyager 2. Miranda has a strange, fractured surface suggestive of a violent past, but the cause of the grooves and cracks is currently unknown. Resolution in the inset is a remarkable 2 km. The long “canyon” near the bottom of the inset is nearly 20 km deep. (NASA)

19. 13.5 The Moon Systems of Uranus and Neptune
Neptune has 13 moons, but only two can be seen from Earth: Triton and Nereid
Triton is in a retrograde orbit; Nereid’s is highly eccentric
Triton’s surface has few craters, indicating an active surface

20. 13.5 The Moon Systems of Uranus and Neptune
Nitrogen geysers have been observed on Triton, contributing to the surface features
Figure Triton The south polar region of Triton, showing a variety of terrains ranging from deep ridges and gashes to what appear to be lakes of frozen water, all indicative of past surface activity. The pinkish region at lower right is nitrogen frost, forming the moon’s polar cap. Resolution is about 4 km. The long black streaks at bottom left were probably formed by geysers of liquid nitrogen on the surface. ( NASA)

21. 13.5 The Moon Systems of Uranus and Neptune
Also, there appear to be ice volcanoes
Figure Water Ice on Triton Scientists think that this roughly circular lakelike feature on Triton may have been caused by the eruption of an ice volcano. The water “lava” has since solidified, leaving a smooth surface. The absence of craters implies that this eruption was a relatively recent event. The frozen lake is about 200 km in diameter; its details are resolved to a superb 1 km. The inset is a computer-generated view along Triton’s surface, illustrating the topography of the area. (NASA)

22. 13.6 The Rings of the Outermost Jovian Planets
Uranus and Neptune have faint ring systems, recently detected via stellar occultation
Figure Occultation of Starlight By carefully watching the dimming of distant starlight as a planet crosses the line of sight, astronomers can infer fine details about that planet. The rings of Uranus were discovered with this technique.

23. 13.6 The Rings of the Outermost Jovian Planets
Uranus’s rings are narrow
Figure Uranus’s Rings The main rings of Uranus, as imaged by Voyager 2. All nine of the rings known before the spacecraft’s arrival can be seen in this photo. From the inside out, they are labeled from 6 to Epsilon. Resolution is about 10 km, which is just about the width of most of these rings. The two rings discovered by Voyager 2 are too faint to be seen here. The inset at top shows a close-up of the Epsilon ring, revealing some internal structure. The width of this ring averages 30 km; special image processing has magnified the resolution in the inset to about 100 m—the size of a football field. (NASA)

24. 13.6 The Rings of the Outermost Jovian Planets
Two shepherd moons keep the epsilon ring from diffusing
Figure Uranian Shepherd Moons These two small moons, named Cordelia (U7) and Ophelia (U8), were discovered by Voyager 2 in They tend to shepherd Uranus’s Epsilon ring, keeping it from diffusing. (NASA)

25. 13.6 The Rings of the Outermost Jovian Planets
Neptune has five rings: three narrow and two wide
Figure Neptune’s Faint Rings In this long-exposure image, Neptune (center) is heavily overexposed and has been artificially obscured (by an instrument) to make the rings easier to see. One of the two fainter rings lies between the inner bright ring (Leverrier) and the planet. The others lie between the Leverrier ring and the outer bright ring (known as the Adams ring). (NASA)

26. Summary of Chapter 13
Uranus and Neptune were discovered in the last 350 years
Uranus and Neptune are similar: gaseous and cold
Uranus’s spin axis is almost in the plane of its orbit
Surface features are hard to discern on Uranus but are more obvious on Neptune
Uranus has no excess heat emission, but Neptune does

27. Summary of Chapter 13 (cont.)
Uranus’s midsized moons are similar to those of Saturn
Neptune’s moon Triton has a retrograde orbit
Uranus and Neptune both have faint ring systems