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 objects with measurable parallax. Herschel saw Uranus as slightly extended object, about 3.7 arc seconds in diameter.

audio link

The Motion of Uranus• Unusual rotation axis,

almost on its side.

• One hemisphere has sunlight for decades, then darkness for decades. Sun rises and sets only at equinox.

• Axial tilt is possibly the result of large impact during planet formation.

19.18 AU97.9o

The Atmosphere of UranusLike other gas giants it has no surface, but a gradual transition from gas phase to fluid interior.

83% hydrogen, 15% helium, 2% methane, ammonia & water vapor.

Optical view from Earth shows blue 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

• Upper layer of methane clouds makes planet look blue.

• Clouds of ammonia, ammonia hydrosulfide and water occur in deeper, warmer layers.

• Lower cloud layers hard to see because of thick atmosphere above it.

• Subtle belt-zone structure must be dominated by planet’s rotation, not by direction 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.

Could be the result of seasonal changes of the cloud structures?

The Interior of UranusAverage density is 1.29 g/cm3, so it has a larger portion of rock and ice than Jupiter and Saturn.

Ices of water, methane, and

ammonia, mixed with

hydrogen and silicates

Uranus and Neptune

often called “ice giants” because of icy interior

Under intense pressure/temperature, methane can form pure carbon diamonds!

The Magnetic Field of UranusNo metallic core so no magnetic field was expected.

But actually, magnetic field of around 75% of Earth’s magnetic field strength was discovered.

Magnetic field is offset from center and inclined

from axis of rotation.

Magnetic field possibly from dynamo effect in layer of liquid-water/ammonia/methane near the surface

Rotation period of 17.24 hr measured by fluctuation in radiation

belts trapped in magnetosphere.

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

During Voyager 2 flyby the south pole was pointed towards the sun. Interaction of solar wind with magnetosphere showed bright

aurora.

UV images

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

Confined by shepherd moons and consist of dark material.

Rings of Uranus were discovered through

occultations of a background star

Apparent motion of star behind Uranus

and rings

Uranus’s Ring Detection

(SLIDESHOW MODE ONLY)

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 are 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 moon

Heavily cratered surface, but no very large craters.

Active phase with internal melting might have flooded craters.

The Surfaces of Uranus’s Moons (2)

Umbriel

Dark, cratered surface

Ariel

No faults or other signs of surface activity

Brightest surface of 5 largest moons

Clear 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.

Blue-green color from methane in the atmosphere (like Uranus.)

4 times Earth’s diameter; 4% smaller than Uranus.

The Atmosphere of Neptune

Cloud-belt structure with high-velocity winds, but 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

Rotation period of 16 hr measured by Great Dark Spot

moving counterclockwise.

The Magnetic Field of NeptuneHeavy element core, like Uranus, does not produce a magnetic field.

Magnetic field about 50% of Earth’s field, probably caused by dynamo effect in fluid mantle.

Magnetic field is offset from center and

inclined from axis of rotation.

The Rings of Neptune

Made of dark dusty 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 NeptuneTwo moons (Triton and Nereid) visible from Earth;

6 more discovered by Voyager 2Unusual orbits:

Triton: Only satellite in the solar system orbiting clockwise, (“backward”).

Nereid: Highly eccentric orbit; very long orbital period (359.4 d).

Astronomers think a violent collision or capture may caused these strange orbits.

The Surface of Triton

Triton can hold a thin atmosphere of nitrogen and some methane

Very low temperature at near absolute zero!

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)One of few moons in solar system with geologic activity: surface features probably less than 100 million years old.

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

Icy version of greenhouse effect may be one of the heat sources for Triton’s geological activity.

PlutoDiscovered 1930 by Clyde Tombaugh.

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

Pluto as a Planet• No surface features visible from Earth.

• about 65% of size of Earth’s Moon.

• Elliptical orbit (30-50 AU); occasionally closer to the sun than Neptune.

• Orbit highly inclined (17o) to other planets’ orbits

• Neptune and Pluto will never collide; they are in 3:2 orbital resonance.

• Surface covered with nitrogen ice, frozen methane, and carbon monoxide.

• Daytime temperature (50 K) enough to vaporize some nitrogen and carbon dioxide to form a very thin 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 in a nearly circular orbit.

Two other small moons found by Hubble in 2005.

Pluto and CharonOrbit highly inclined against orbital plane.

From orbital distance and orbital period, Pluto’s mass is found – about one-fifth of Earth’s mass.

Density is about 2 g/cm3 for both Pluto and Charon.

Surface is about 35% ice and 65% rock.

Large orbital inclinations cause big seasonal changes, like Uranus.

The Origin of Pluto and Charon

Probably very different history than neighboring Jovian planets.

Modern theory suggests Pluto and Charon are members of Kuiper Belt of small, icy objects.

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

Theory mostly abandoned today since such interactions are unlikely.

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

New Horizons spacecraft will reach Pluto in July, 2015.

occultation ovoid 

New Terms