Chapter 24 , , and

Guidepost In the three previous chapters, we have used our tools of comparative planetology to study other worlds, and we continue that theme in this chapter. A second theme running through this chapter is the nature of astronomical discovery. Unlike the other in our , Uranus, Neptune, and Pluto were discovered, and the story of their discovery helps us understand how science progresses. As we probe the outer fringes of our in this chapter, we see strong evidence of smaller bodies that fall through the solar system and impact planets and satellites. The next chapter will allow us to study these small bodies in detail and will give us new evidence that our solar system formed from a solar nebula. Outline

I. Uranus A. The Discovery of Uranus B. The Motion of Uranus C. The of Uranus D. The Interior of Uranus E. The F. The of Uranus G. A History of Uranus

II. Neptune A. The B. The Atmosphere and Interior of Neptune C. The D. The E. The History of Neptune Outline (continued)

III. Pluto A. The Discovery of Pluto B. Pluto as a C. The Origin of Pluto and Uranus Chance discovery by in 1781,

while scanning the sky for nearby objects with measurable : discovered Uranus as slightly extended object, ~ 3.7 arc seconds in diameter. The Motion of Uranus

Very unusual orientation of axis: Almost in the 97.9o orbital plane. 19.18 AU

Possibly result of impact of a large during the phase of planet formation.

Large portions of the planet exposed to “eternal” for many years, then complete darkness for many years! The Atmosphere of Uranus Like other gas : No surface. Gradual transition from gas phase to fluid interior. Mostly H; 15 % He, a few % , and vapor.

Optical view from : Blue color due to methane, structures only visible after artificial absorbing longer computer enhancement of optical images wavelengths taken from Voyager . Cloud Structure of Uranus Hubble Space of Uranus shows cloud structures not present during Voyager’s passage in 1986.

 Possibly due to seasonal changes of the cloud structures. The Interior of Uranus Average ≈ 1.29 g/cm3  larger portion of and ice than and .

Ices of water, methane, and ammonia, mixed with and The of Uranus No metallic core  no magnetic field was expected. But actually, magnetic field of ~ 75 % of Earth’s magnetic field strength was discovered:

Offset from o center: ~ 30 % Inclined by ~ 60 Possibly due to dynamo in of planet’s against axis of liquid-water/ammonia/methane rotation. solution in Uranus’ interior. radius! with weak radiation belts; allows determination of : 17.24 hr. The Magnetosphere of Uranus Rapid rotation and large inclination deform magnetosphere into a corkscrew shape.

UV images

During : Southpole pointed towards ; direct interaction of solar with magnetosphere  Bright aurorae! The Rings of Uranus Rings of Uranus and Neptune are similar to Jupiter’s rings.Confined by shepherd moons; consist of dark material.

Apparent motion of Rings of Uranus were behind Uranus discovered through and rings of a background star The Rings of Neptune Ring material must be regularly re- supplied by dust from impacts on the moons.

Interrupted between denser segments (arcs)

Made of dark material, visible in forward- scattered Focused by small shepherd light. moons embedded in the ring structure. The 5 largest moons visible from Earth.

10 more discovered by Voyager 2; more are still being found.

Dark surfaces, probably ice darkened by dust from meteorite impacts.

5 largest moons all tidally locked to Uranus. Interiors of Uranus’s Moons

Large rock cores surrounded by icy mantles. The Surfaces of Uranus’s Moons (1)


Old, inactive, cratered surface, Largest but probably active past. Heavily cratered surface, but no Long fault across the surface. very large craters. Dirty water may have flooded Active phase with internal melting floors of some craters. might have flooded craters. The Surfaces of Uranus’s Moons (2)

Umbriel Dark, cratered surface Brightest surface of 5 largest moons No faults or other signs of Clear signs of geological activity surface activity Crossed by faults over 10 km deep Possibly heated by tidal interactions with and . Uranus’s Moon Miranda Most unusual of the 5 moons detected from Earth

Ovoids: Oval groove patterns, 20 km high cliff near the probably associated with currents in the Surface features are old; Miranda is , but not with impacts. no longer geologically active. Neptune Discovered in 1846 at position predicted from gravitational disturbances on Uranus’s by J. C. Adams and U. J. Leverrier. Blue-green color from methane in the atmosphere 4 times Earth’s diameter; 4 % smaller than Uranus The Atmosphere of Neptune

The “

Cloud-belt structure with high-velocity ; origin not well understood. Darker cyclonic disturbances, similar to on Jupiter, but not long-lived. White cloud features of methane ice crystals The Moons of Neptune

Unusual : Two moons ( and ) visible from Earth; 6 more discovered by Voyager 2 Triton: Only satellite in the solar system orbiting clockwise, i.e. “backward”.

Nereid: Highly eccentric orbit; very long (359.4 d). The Surface of Triton

Very low (34.5 K)

 Triton can hold a tenuous atmosphere of and some methane; 105 times less dense than Earth’s atmosphere. Surface composed of ices: nitrogen, methane, , .

Possibly cyclic nitrogen ice deposition and re- vaporizing on Triton’s Dark smudges on the nitrogen ice surface, south pole, similar to CO probably due to methane rising from below 2 surface, forming carbon-rich deposits when ice cap cycles on exposed to sun light. . The Surface of Triton (2)

Ongoing surface activity: Surface features probably not more than 100 million years old.

Large basins might have been flooded multiple times by liquids from the interior.

Ice equivalent of greenhouse effect may be one of the heat sources for Triton’s geological activity. Pluto Discovered 1930 by C. Tombaugh.

Existence predicted from orbital disturbances of Neptune, but Pluto is actually too small to cause those disturbances. Pluto as a Planet Virtually no surface features visible from Earth. ~ 65 % of size of Earth’s Moon. Highly elliptical orbit; coming occasionally closer to the sun than Neptune. Orbit highly inclined (17o) against other planets’ orbits  Neptune and Pluto will never collide.

Surface covered with nitrogen ice; traces of frozen methane and carbon monoxide. Daytime temperature (50 K) enough to vaporize some N and CO to form a very tenuous atmosphere. Pluto’s Moon Charon

Discovered in 1978; about half the size and 1/12 the of Pluto itself.

Tidally locked to image Pluto. Pluto and Charon

Orbit highly inclined against orbital plane.

From separation and orbital period:

Mpluto ~ 0.2 Earth .

Density ≈ 2 g/cm3 (both Pluto and Charon)

 ~ 35 % ice and 65 % rock.

Large orbital inclinations  Large seasonal changes on Pluto and Charon. The Origin of Pluto and Charon

Probably very different history than neighboring Jovian planets.

Older theory: Pluto and Charon formed as moons of Neptune, ejected by interaction with massive planetesimal. Mostly abandoned today since such interactions are unlikely.

Modern theory: Pluto and Charon members of of small, icy objects (see Chapter 25).

Collision between Pluto and Charon may have caused the peculiar orbital patterns and large inclination of Pluto’s rotation axis. New Terms ovoid

Discussion Questions

1. Why might it be unfair to describe William Herschel’s discovery of Uranus as accidental? Why might it be unfair to describe the discovery of the rings of Uranus as accidental?

2. Suggest a single phenomenon that could explain the inclination of the rotation axis of Uranus, the orbits of Neptune’s satellites, and the existence of Pluto’s moon. Quiz Questions

1. How do the seasons on Uranus differ from seasons on Earth? a. Seasons on Uranus are 84 times longer and more extreme than on Earth. b. Seasons on Uranus are 84 times longer and less extreme than on Earth. c. Seasons on Uranus are 21 times longer and more extreme than on Earth. d. Seasons on Uranus are 21 times longer and less extreme than on Earth. e. Seasons on Uranus are longer, more extreme, and in reverse order of the seasons on Earth. Quiz Questions

2. What is our current best hypothesis as to how the whole Uranian system came to have such a large inclination? a. A large impact during the latter stages of planet building tipped Uranus on its side. b. Tidal interactions between Uranus and the other Jovian planets pulled Uranus onto its side. c. Magnetic interactions between the Sun and Uranus flipped Uranus onto its side. d. Uranus formed outside of the Solar System and was captured later. e. The slow rate of rotation of Uranus gives it such little stability that its rotation axis precesses wildly. Quiz Questions

3. Both Uranus and Neptune have a blue-green tint when observed through a telescope. What does this tell you about their composition? a. Their are composed of mostly hydrogen and . b. Their atmospheres are composed of mostly carbon dioxide. c. Their atmospheres are composed of mostly nitrogen. c. Their atmospheres contain some ammonia. e. Their atmospheres contain some methane. Quiz Questions

4. How do we get an accurate measurement of the rotational period of Uranus? a. We measure the time for one orbit of a dark spoke in the rings. b. We measure the time for the Great Dark Spot to travel once around. c. We measure the time for a particular cloud to rotate once around the planet. d. We measure the time from one of Uranus to the next opposition of Uranus. e. We measure the period of the cyclic fluctuation in the synchrotron radiation emitted by Uranus. Quiz Questions

5. In the atmospheres of Jupiter and Saturn we see ammonia, ammonia hydrosulfide, and water in three distinct layers. Why don't we see these same three cloud layers in the atmospheres of Uranus and Neptune? a. Farther from the Sun it is too for these three layers of clouds to form. b. These three layers are likely hidden beneath a higher layer of methane clouds. c. These chemicals are not present in the atmospheres of Uranus and Neptune. d. These condensates form one layer in the atmospheres of Uranus and Neptune. e. Uranus and Neptune have no atmosphere. Quiz Questions

6. Which interior zone of Uranus and Neptune do we suspect contains the electrically conducting fluid that is responsible for planetary magnetic fields? a. The zone of liquid water with dissolved ammonia and methane. b. The liquid metallic hydrogen zone. c. The liquid hydrogen-helium zone. d. The liquid outer iron core. e. The heavy element core. Quiz Questions

7. In what way is Uranus different than the other Jovian planets? a. Uranus has no rings. b. Uranus has no moons. c. Uranus has a higher density. d. Uranus has no metallic hydrogen. e. Uranus has little remaining heat of formation. Quiz Questions

8. Why is there no liquid metallic hydrogen zone in the interior of Uranus or Neptune? a. The temperature is not low enough for hydrogen to become a superconductor. b. The hydrogen does not contain sufficient amounts of deuterium. c. Uranus and Neptune do not contain hydrogen and helium. d. The is too low for hydrogen to be metallic. e. No fusion occurs in Uranus and Neptune. Quiz Questions

9. How did Uranus and Neptune come to have less hydrogen and helium than Jupiter and Saturn? a. The mass fraction of light elements like hydrogen and helium in the solar nebula decreases with distance from the Sun. b. Much of their original hydrogen and helium was stripped away by the of passing . c. Much of their original hydrogen and helium was ionized and stripped away by the . d. Much of their original hydrogen and helium was ionized and stripped away by interstellar winds. e. Uranus and Neptune took longer to form than Jupiter and Saturn. Quiz Questions

10. What difference in the rings of Uranus and Neptune was first revealed in observations from Earth-based ? a. The clumpy ring arcs of Neptune. b. The difference in the of the ring particles. c. The difference in the size of the ring particles. d. The elemental composition of the ring particles. e. The difference in the dust-to-ice ratio of ring particles. Quiz Questions

11. What evidence indicates that the rings of Uranus have little dust and the rings of Neptune contain a lot of dust? a. The camera lens of Voyager 2 was dust-free until it passed through the rings of Neptune. b. The rings of Uranus appear bright in forward-scattered light, and the rings of Neptune appear dark in forward-scattered light. c. The rings of Uranus appear dark in forward-scattered light, and the rings of Neptune appear bright in forward-scattered light. d. The rings of Uranus appear bright in back-scattered light, and the rings of Neptune appear dark in back-scattered light. e. The rings of Uranus appear dark in back-scattered light, and the rings of Neptune appear bright in back-scattered light. Quiz Questions

12. How does a thin planetary ring retain its shape? a. The tidal of the planet on the ring particles keeps them together. b. The magnetic field of the planet traps the ring particles in a well-defined orbit. c. Small moons orbiting just inside and outside the rings shepherd the ring particles. d. The gravitational attraction of the ring particles on one another keeps the ring together. e. The electrostatic attraction of the ring particles on one another keeps the ring together. Quiz Questions

13. What keeps small shepherd moons from breaking apart within the of a planet? a. Gravitational attraction of the moon's material. b. Gravitational attraction by the ring particles. c. Electrostatic bonds of the moon's material. d. Gravitational attraction of larger moons. e. Tidal by the planet. Quiz Questions

14. The discoveries of Uranus, Neptune, and Pluto all came long after the death of Isaac Newton. How was Newton involved in the discovery of a new planet? a. It was the application of Newtonian gravity to the problem of the orbit of Uranus that led to the discovery of Neptune. b. It was with a reflecting telescope (the type invented by Newton) that the planet Uranus was discovered. c. It was through perceived perturbations by Newtonian gravity on the orbit of Neptune that a search for a ninth planet was begun, which eventually resulted in the discovery of Pluto. d. Both b and c above. e. All of the above. Quiz Questions

15. We could divide the Jovian planets into two subclasses, the Gas Giants and the Ice Giants. Into which groups should we place the four Jovian planets? a. The Gas Giants are Uranus & Neptune, and the Ice Giants are Jupiter & Saturn. b. The Gas Giants are Jupiter & Saturn, and the Ice Giants are Uranus & Neptune. c. The Gas Giants are Saturn & Uranus, and the Ice Giants are Jupiter & Neptune. d. The Gas Giants are Jupiter & Neptune, and the Ice Giants are Saturn & Uranus. e. The Gas Giants are Saturn & Neptune, and the Ice Giants are Jupiter & Uranus. Quiz Questions

16. What is peculiar about the orbits of Neptune's moons Triton and Nereid? a. Triton's orbit is around Neptune and Nereid's orbit is around Triton. b. Triton's orbit is large and very elliptical, and Nereid's orbit is very small and circular. c. Triton's orbit is in the retrograde direction, and Nereid's orbit is large and very elliptical. d. Triton's orbit places it inside the Roche Limit of Neptune, and Nereid's orbit is large and very elliptical. e. They share similar orbits, and gravitational interactions cause them to switch orbits each time they meet. Quiz Questions

17. The surface age of Triton is thought to be about 100 million years. What is the evidence for such an age determination? a. The relationship between the rotational and orbital periods of Triton b. The thickness of nitrogen snow deposits around the . c. The of of Triton due to Neptune. d. Age dating of from Triton. e. The density of impact craters. Quiz Questions

18. How can worlds like Triton and Pluto have atmospheres when a larger world such as has none? a. Impacts vaporize ices on these cold bodies. b. Tidal heating releases gases on these cold bodies. c. In cold environments, gas molecules have more mass. d. Gas molecules move more slowly at low . e. More frozen gases exist in the colder outer solar system. Quiz Questions

19. What evidence do we have that Pluto and Charon are made of mixtures of rock and ice? a. Spectra show that both bodies have some surface ices. b. Both bodies have a density of 2 grams per cubic centimeter. c. The Hubble Space Telescope detected active nitrogen geysers on Pluto. d. Both a and b above. e. All of the above. Quiz Questions

20. If you visited Pluto and found Charon a full moon directly overhead, where would Charon be in the sky when it was at First Quarter phase? a. At the west point on the horizon. b. At the east point on the horizon. c. It depends on the time of . d. Directly overhead. e. Either a or b above. Answers

1. a 11. c 2. a 12. c 3. e 13. c 4. e 14. e 5. b 15. b 6. a 16. c 7. e 17. e 8. d 18. d 9. e 19. d 10. a 20. d