Download Uranus and Neptune

The Outer Worlds
Uranus was discovered by chance
• Uranus recognized as a planet
in 1781 by William Herschel
while scanning the
sky for nearby
objects with
measurable parallax:
discovered Uranus
as slightly extended
object, ~ 3.7 arc
seconds in diameter.
Neptune’ Discovery
Discovered in 1846 at
position predicted from
gravitational
disturbances on
Uranus’s orbit by John
Couch Adams and
Urbain Jean Leverrier.
(But don’t forget Galle!)
Blue-green color from
methane in the
atmosphere
4 times Earth’s
diameter; 4 % smaller
than Uranus
The Atmospheres of Uranus and Neptune
Outer atmospheres of Uranus and Neptune are
similar to those of Jupiter and Saturn
Uranus and Neptune are cold
enough that ammonia
freezes; methane dominates
and gives the characteristic
blue color
The Atmospheres of Uranus and Neptune
Uranus is very cold; clouds only in lower,
warmer layers.
Few features are
visible:
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,
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!
The Atmospheres of Uranus and Neptune
Neptune has storm systems similar to those on
Jupiter, but fewer:
The Atmospheres of Uranus and Neptune
Band structure of Neptune is more visible, and
Neptune has internal heat source of unknown
origin:
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.
Neptune is a cold, bluish world with Jupiter-like
atmospheric features
• No white ammonia clouds are
seen on Uranus or Neptune
• Presumably the low
temperatures have caused
almost all the ammonia to
precipitate into the interiors of
the planets
• All of these planets’ clouds are
composed of methane
• Much more cloud activity is
seen on Neptune than on
Uranus.
• This is because Uranus lacks
a substantial internal heat
source.
Neptune’s Clouds
• Much more cloud
activity is seen on
Neptune than on
Uranus
• This is because
Uranus lacks a
substantial internal
heat source
Exaggerated Seasons On Uranus
• Uranus’s axis of rotation
lies nearly in the plane of
its orbit, producing greatly
exaggerated seasonal
changes on the planet
• This unusual orientation
may be the result of a
collision with a planetlike
object early in the history
of our solar system. Such
a collision could have
knocked Uranus on its
side
The Motion of Uranus
Very unusual
orientation of rotation
axis: Almost in the
orbital plane.
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!
19.18 AU
97.9o
Uranus and Neptune contain a higher proportion
of heavy elements than Jupiter and Saturn
• Both Uranus and Neptune may have a rocky core
surrounded by a mantle of water and ammonia
• Electric currents in the mantles may generate the
magnetic fields of the planets
Magnetospheres and Internal Structure
Comparison of the interiors of the Jovian planets.
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
against axis of
of planet’s
liquid-water/ammonia/methane
rotation.
radius!
solution in Uranus’ interior.
Magnetosphere with weak radiation belts; allows
determination of rotation period: 17.24 hr.
Magnetospheres and Internal Structure
Uranus and Neptune both have substantial
magnetic fields, but at a large angle to their
rotation axes.
The rectangle
within each
planet shows a
bar magnet that
would produce a
similar field. Note
that both
Uranus’s and
Neptune’s are
significantly off
center.
The Magnetosphere of Uranus
Rapid rotation and large inclination deform
magnetosphere into a corkscrew shape.
UV images
During Voyager 2 flyby: South pole pointed towards sun; direct
interaction of solar wind with magnetosphere  Bright aurorae!
Uranus and Neptune each have a system of thin,
dark rings
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
star behind Uranus
and rings
Rings of Uranus were
discovered through
occultations of a
background star
Uranus’s rings are narrow:
Two shepherd moons
keep the epsilon ring
from diffusing:
The Rings of Neptune
Neptune has five rings, three narrow and two
wide:
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
material,
visible in
forwardscattered
light.
Focused by small shepherd
moons embedded in the
ring structure.
The moons of Uranus – 27 at present
• The first two were discovered by William
Herschel in 1787, and named, by his son, after
characters from Shakespeare’s A Midsummer
Nights Dream, Titania and Oberon.
• Two more moons were found by William Lassell
in 1851 and named Ariel and Umbriel
• G. Kuiper discovered Miranda in 1948.
• All the moons of Uranus are named after
characters from Shakespeare or Alexander
Pope.
• Voyager 2’s flyby in January 1986 led to the
discovery of another 10.
• Six additional moons have since been
discovered by telescope.
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.
Moons of Uranus
Interiors of Uranus’s Moons
Large rock cores surrounded by icy mantles.
The Surfaces of Uranus’s Moons
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
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.
MIRANDA
The Moons of Neptune
Unusual orbits:
Triton: Only
satellite in the
solar system
orbiting clockwise,
i.e. “backward”.
Nereid: Highly
eccentric orbit;
very long orbital
period (359.4 d).
Two moons (Triton and Nereid) visible from
Earth; 6 more discovered by Voyager 2
Triton is a frigid, icy world with a young surface
and a tenuous atmosphere
• Neptune has 13 satellites,
one of which (Triton) is
comparable in size to our
Moon or the Galilean
satellites of Jupiter
• Triton has a young, icy
surface indicative of
tectonic activity
• The energy for this activity
may have been provided
by tidal heating that
occurred when Triton was
captured by Neptune’s
gravity into a retrograde
orbit
• Triton has a tenuous
nitrogen atmosphere
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 reDark smudges on the nitrogen ice surface,
vaporizing on Triton’s
south pole, similar to CO2 probably due to methane rising from below
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.