Chapter 24 Uranus, Neptune, and Pluto 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 planets in our solar system, 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 planetary system 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 Atmosphere of Uranus D. The Interior of Uranus E. The Rings of Uranus F. The Moons of Uranus G. A History of Uranus II. Neptune A. The Discovery of Neptune B. The Atmosphere and Interior of Neptune C. The Rings of Neptune D. The Moons of Neptune E. The History of Neptune Outline (continued) III. Pluto A. The Discovery of Pluto B. Pluto as a Planet C. The Origin of Pluto and Charon Uranus Chance discovery by William Herschel in 1781, while scanning the sky for nearby objects with measurable parallax: discovered Uranus as slightly extended object, ~ 3.7 arc seconds in diameter. The Motion of Uranus Very unusual orientation of rotation axis: Almost in the 97.9o orbital plane. 19.18 AU Possibly result of impact of a large planetesimal during the phase of planet formation. Large portions of the planet exposed to “eternal” sunlight for many years, then complete darkness for many years! The Atmosphere of Uranus Like other gas giants: No surface. Gradual transition from gas phase to fluid interior. Mostly H; 15 % He, a few % Methane, ammonia and water vapor. Optical view from Earth: Blue color due to methane, Cloud structures only visible after artificial absorbing longer computer enhancement of optical images wavelengths taken from Voyager spacecraft. Cloud Structure of Uranus Hubble Space Telescope image 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 density ≈ 1.29 g/cm3 larger portion of rock and ice than Jupiter and Saturn. Ices of water, methane, and ammonia, mixed with hydrogen and silicates The Magnetic Field 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! Magnetosphere with weak radiation belts; allows determination of rotation period: 17.24 hr. The Magnetosphere of Uranus Rapid rotation and large inclination deform magnetosphere into a corkscrew shape. UV images During Voyager 2 flyby: Southpole pointed towards sun; direct interaction of solar wind 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 star behind Uranus discovered through and rings occultations of a background star The Rings of Neptune Ring material must be regularly re- supplied by dust from meteorite 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 Moons of Uranus 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) Oberon Titania Old, inactive, cratered surface, Largest moon 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 Ariel 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 Miranda and Umbriel. Uranus’s Moon Miranda Most unusual of the 5 moons detected from Earth Ovoids: Oval groove patterns, 20 km high cliff near the equator probably associated with convection currents in the Surface features are old; Miranda is mantle, but not with impacts. no longer geologically active. Neptune Discovered in 1846 at position predicted from gravitational disturbances on Uranus’s orbit 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 “Great Dark Spot” Cloud-belt structure with high-velocity winds; origin not well understood. Darker cyclonic disturbances, similar to Great Red Spot on Jupiter, but not long-lived. White cloud features of methane ice crystals The Moons of Neptune Unusual orbits: Two moons (Triton and Nereid) 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 orbital period (359.4 d). The Surface of Triton Very low temperature (34.5 K) Triton can hold a tenuous atmosphere of nitrogen and some methane; 105 times less dense than Earth’s atmosphere. Surface composed of ices: nitrogen, methane, carbon monoxide, carbon dioxide. 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 polar cap cycles on exposed to sun light. Mars. 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 mass of Pluto itself. Tidally locked to Hubble Space Telescope image Pluto. Pluto and Charon Orbit highly inclined against orbital plane. From separation and orbital period: Mpluto ~ 0.2 Earth masses. 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 Kuiper belt 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 occultation 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 atmospheres are composed of mostly hydrogen and helium. b. Their atmospheres are composed of mostly carbon dioxide. c. Their atmospheres are composed of mostly nitrogen. c. Their atmospheres contain some ammonia.