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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 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.
• Axial tilt is possibly
the result of large
impact during planet
formation.
• One hemisphere
has sunlight for
decades, then
darkness for
decades. Sun rises
and sets only at
equinox.
19.18 AU
97.9o
The Atmosphere of Uranus
Like 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 Uranus
Hubble 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 Uranus
Average 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
Under intense pressure/temperature,
methane can form pure carbon diamonds!
Uranus and
Neptune
often called
“ice giants”
because of
icy interior
The Magnetic Field of Uranus
No 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 possibly
Magnetic field is offset
from dynamo effect in
from center and inclined
layer of liquidfrom axis of rotation.
water/ammonia/methane
Rotation period of 17.24 hr
near the surface
measured by fluctuation in radiation
belts trapped in magnetosphere.
The Magnetosphere of Uranus
Rapid rotation and large inclination deform
magnetosphere into a corkscrew shape.
UV images
During Voyager 2 flyby the south pole was pointed towards the sun.
Interaction of solar wind with magnetosphere showed bright aurora.
The Rings of Uranus
Rings of Uranus and Neptune are similar to Jupiter’s rings.
Confined by shepherd moons and consist of dark material.
Apparent motion of
star behind Uranus
and rings
Rings of Uranus were
discovered through
occultations of a
background star
Uranus’s Ring Detection
(SLIDESHOW MODE ONLY)
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 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,
but probably active past.
Long fault across the surface.
Dirty water may have flooded
floors of some craters.
Titania
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
No faults or other signs of
surface activity
Ariel
Brightest surface of 5 largest moons
Clear signs of geological 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
no longer geologically active.
mantle, but not with impacts.
Neptune
Discovered 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
The “Great
Dark Spot”
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.
White cloud features of
methane ice crystals
Rotation period of 16 hr
measured by Great Dark Spot
moving counterclockwise.
The Magnetic Field of Neptune
Heavy 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
Ring material must
be regularly resupplied by dust
from meteorite
impacts on the
moons.
Interrupted between
denser segments (arcs)
Made of dark
dusty material,
visible in
forwardscattered light. Focused by small shepherd
moons embedded in the
ring structure.
The Moons of Neptune
Two moons (Triton and Nereid) visible from Earth;
6 more discovered by Voyager 2
Unusual 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
Very low temperature
at near absolute zero!
Triton can hold a thin
atmosphere of nitrogen
and some methane
Surface composed of
ices: nitrogen, methane,
carbon monoxide,
carbon dioxide.
Possibly cyclic nitrogen
ice deposition and revaporizing 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.
Pluto
Discovered 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
Discovered
in 1978;
about half
the size and
1/12 the
mass of
Pluto itself.
Tidally locked
to Pluto in a
nearly
circular orbit.
Hubble Space Telescope image
Two other small moons found by Hubble in 2005.
Pluto and Charon
Orbit 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.
Older theory suggests that Pluto and Charon formed as moons
of Neptune, ejected by interaction with massive planetesimal.
Theory mostly abandoned
today since such
interactions are unlikely.
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.
New Horizons spacecraft will reach Pluto in July, 2015.
New Terms
occultation
ovoid