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Uranus, Neptune, and PlutoChapter 24

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.

Guidepost

I. UranusA. The Discovery of UranusB. The Motion of UranusC. The Atmosphere of UranusD. The Interior of UranusE. The Rings of UranusF. The Moons of UranusG. A History of Uranus

II. NeptuneA. The Discovery of NeptuneB. The Atmosphere and Interior of NeptuneC. The Rings of NeptuneD. The Moons of NeptuneE. The History of Neptune

Outline

III. PlutoA. The Discovery of PlutoB. Pluto as a PlanetC. The Origin of Pluto and Charon

Outline (continued)

UranusChance 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 orbital plane.

Large portions of the planet exposed to

“eternal” sunlight for many years, then

complete darkness for many years!

Possibly result of impact of a large

planetesimal during the phase of planet

formation.

19.18 AU97.9o

The Atmosphere of UranusLike 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, absorbing longer

wavelengths

Cloud structures only visible after artificial computer enhancement of optical images

taken from Voyager spacecraft.

The Structure of Uranus’ Atmosphere

Only one layer of Methane clouds (in

contrast to 3 cloud layers on Jupiter and Saturn).

3 cloud layers in Jupiter and Saturn form at relatively high temperatures that occur only very deep in Uranus’ atmosphere.

Uranus’ cloud layer difficult to see because of thick atmosphere above it.

Also shows belt-zone structure

Belt-zone cloud structure must be dominated by planet’s rotation, not by incidence angle of sun light!

Planetary Atmospheres

(SLIDESHOW MODE ONLY)

Cloud Structure of UranusHubble 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 UranusAverage 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 UranusNo metallic core no magnetic field was expected.

But actually, magnetic field of ~ 75 % of Earth’s magnetic field strength was discovered:

Offset from center: ~ 30 %

of planet’s radius!

Inclined by ~ 60o against axis of

rotation.Possibly due to dynamo in

liquid-water/ammonia/methane solution in Uranus’ interior.

Magnetosphere with weak radiation belts; allows determination of rotation period: 17.24 hr.

The Magnetosphere of UranusRapid rotation and large inclination deform magnetosphere into a corkscrew shape.

During Voyager 2 flyby: Southpole pointed towards sun; direct interaction of solar wind with magnetosphere Bright aurorae!

UV images

Uranus’s Ring Detection

(SLIDESHOW MODE ONLY)

The Rings of UranusRings of Uranus and Neptune are similar to Jupiter’s rings.

Confined by shepherd moons; consist of dark material.

Rings of Uranus were discovered through

occultations of a background star

Apparent motion of star behind Uranus

and rings

The Rings of Neptune

Made of dark material, visible in forward-scattered light.

Focused by small shepherd moons embedded in the ring structure.

Ring material must be regularly re-supplied by dust from meteorite impacts on the moons.

Interrupted between denser segments (arcs)

The Moons of Uranus5 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

Old, inactive, cratered surface,

Titania

but probably active past.Long fault across the surface.Dirty water may have flooded floors of some craters.

Largest moonHeavily cratered surface, but no

very large craters.

Active phase with internal melting might have flooded craters.

The Surfaces of Uranus’s Moons (2)

UmbrielDark, cratered surface

Ariel

No faults or other signs of surface activity

Brightest surface of 5 largest moonsClear signs of geological activity

Crossed by faults over 10 km deepPossibly heated by tidal interactions

with Miranda and Umbriel.

Uranus’s Moon MirandaMost unusual of the 5 moons detected from Earth

Ovoids: Oval groove patterns, probably associated with convection currents in the

mantle, but not with impacts.

20 km high cliff near the equator

Surface features are old; Miranda is no longer geologically active.

NeptuneDiscovered 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

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.

The “Great Dark Spot”

White cloud features of methane ice crystals

The Moons of NeptuneTwo moons (Triton and Nereid) visible from

Earth; 6 more discovered by Voyager 2Unusual orbits:

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

Triton can hold a tenuous atmosphere of nitrogen and some methane; 105 times less dense than Earth’s atmosphere.

Very low temperature (34.5 K)

Surface composed of ices: nitrogen, methane, carbon monoxide, carbon dioxide.

Possibly cyclic nitrogen ice deposition and re-vaporizing on Triton’s south pole, similar to CO2 ice polar cap cycles on Mars.

Dark smudges on the nitrogen ice surface, probably due to methane rising from below surface, forming carbon-rich deposits when

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.

PlutoDiscovered 1930 by C. Tombaugh.

Existence predicted from orbital disturbances of Neptune, but Pluto is actually too small to cause those disturbances.

Pluto as a PlanetVirtually 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

Hubble Space Telescope image

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

Tidally locked to Pluto.

Pluto and CharonOrbit 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:

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.

Mostly abandoned today since such interactions are unlikely.

Pluto and Charon formed as moons of Neptune, ejected by interaction with massive planetesimal.

occultation ovoid 

New Terms

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.

Discussion Questions

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.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 opposition 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 clouds 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 cold 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 pressure 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 gravity of passing stars.c. Much of their original hydrogen and helium was ionized and stripped away by the solar wind.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 telescopes?

a. The clumpy ring arcs of Neptune.b. The difference in the albedo 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 force 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 Roche Limit 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 forces 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 Tritonb. The thickness of nitrogen snow deposits around the geysers.c. The degree of tidal heating of Triton due to Neptune.d. Age dating of meteorites 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 Ganymede 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 temperatures.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 day.d. Directly overhead.e. Either a or b above.

Answers

1. a2. a3. e4. e5. b6. a7. e8. d9. e10. a

11. c12. c13. c14. e15. b16. c17. e18. d19. d20. d