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Uranus & Neptune
Lecture 25
Featureless Atmosphere of Uranus
Uranus from Voyager 2
This image looks nearly
straight down onto Uranus’s
south pole.
None of the Voyager 2 images
of Uranus show any
pronounced cloud patterns.
The color is due to methane
in the planet’s atmosphere,
which absorbs red light but
reflects green and blue.
No feature (storm)  lack of
energy
Uranus in UV, Visible, and IR
Uranus from the Hubble
Space Telescope Images made
at ultraviolet, visible, and
infrared wavelengths were
combined and enhanced to
give this false-color view of
cloud features on Uranus.
82.5% H, 15% He, 2.3% CH4,
very small amount of NH3
(because it got frozen out).
Extreme seasons in Uranus
For Uranus the rotation axis is tilted by 98° from the perpendicular. This causes severely
exaggerated seasons.
For example, during midsummer at Uranus’s south pole, the Sun appears nearly overhead
for many Earth years, while the planet’s northern regions are in continuous darkness.
Half an orbit later, the seasons are reversed.
Wind flows to the rotation direction in northern/southern hemispheres
excepting for the equatorial region (backward).
Strong seasonal mix + lack of strong zonal winds  featureless Uranus
Neptune
Neptune from Voyager 2
Nearly identical atmospheric
composition to Uranus and
the same temperature (55K0
at top of atmo.
The white clouds are thought
to be composed of crystals of
methane ice.
Great Dark Spot = measured
about 12,000 by 8000 km,
comparable in size to the
Earth  similar to the
Jupiter’s Great Red Spot (but
has shorter timescale).
Neptune : Methane world
Cirrus Clouds over Neptune
Voyager 2 recorded this
image of clouds near
Neptune’s terminator.
Like wispy, high-altitude cirrus
clouds in the Earth’s
atmosphere, these clouds are
thought to be made of ice
crystals. The difference is
that Neptune’s cirrus clouds
are probably methane ice, not
water ice as on Earth
(because NH3 and H2O got
frozen out long before).
Neptune’s Banded Structure
Neptune further away than
Uranus (and same size), yet
Neptune shows weather
pattern!
White areas denote highaltitude clouds; the very
highest clouds (near the top
of the image) are shown in
yellow-red. The green belt
near the south pole is a
region where the atmosphere
absorbs blue light, perhaps
indicating a different chemical
composition there.
Internal Structures
 Both Uranus and Neptune have a rocky core, resembling a terrestrial planet; a
mantle of liquid water with ammonia dissolved in it; and an outer layer of liquid
molecular hydrogen and liquid helium
Origin of Uranus and Neptune
? at these large distances, the solar nebula was too sparse to form these massive
planets. Takes long, long time to grow from low density material before gas is being
blown away by the young Sun.
1. Formed closer : Uranus and Neptune were formed at 4-10AU (i.e., same region as
Jupiter and Saturn), then they were “ejected” to the outer part stopped growing
further due to lacking material
2. Disk instability model : They were formed in situ directly from gas and later
accreted icy material. Nearby passing star “evaporate” away H+He and prevented
them from growing further.
Enigmatic magnetic fields of Uranus and Neptune
 lower gravity  no metallic hydrogen  no magnetic field.
 But, they have magnetic field and they are off centered!!
Rings of Uranus
 Uranus’s Rings viewed from Voyager 2
Discovery of Uranus’ rings from Earth
During 1977 occultation of Uranus, astronomers try to determine the precise
radius of Uranus and Atomsphere  the star “blinked” several times!
Neptune’s ring was discovered from stellar occultation also.
Uranus’s Rings and Small Satellites
They all lie within 86,000 km
of the planet’s center (only
about one-fifth of the
distance from the Earth to our
Moon) and all are less than
160 km (100 mi) in diameter.
The arcs show how far each
satellite moves around its
orbit in 90 minutes
Miranda : Moon of Uranus
This composite of Voyager 2
images shows that part of
Miranda’s surface is ancient and
heavily cratered, while
other parts are dominated by
parallel networks of valleys and
ridges.
At the very bottom of the
image—where a “bite” seems
to have been taken out of
Miranda—is a range of
enormous cliffs that jut upward
to an elevation of 20 km, twice
as high as Mount Everest.
One idea is that the entire
moon was broken apart by a
massive impact, only to have
the fragments reassemble
themselves.
Triton: Surprising possibility of potential habitability
largest moon among 13 Neptune’s moons
 Retrograde orbit
 captured satellite
 23° tilted from the Netune’s equator
Triton’s cantalope skin  Possibly formed by
diapirism (i.e., slow
boiling pattern)
 Soon after the capture, its
orbit was quite elliptical.
Overtime, it got circularized
through tidal interaction
(ongoing heating!)
Triton: Surprising possibility of potential habitability
 Continuing tidal heating : because of
retrograde motion (spiraling inward). In ~100
Myr, Triton will be inside of the Roche limit.
 Crater count  Triton’s surface is 10-100
million years old.
 Active ice geysers!!
 Remnant internal heat from the capture may
drive the geological activity…
possible subsurface liquid ocean
even at -230°C, possible habitable world!
Jovian Planets in Summary
Uranus and Neptune are not simply smaller versions of Jupiter and Saturn.
Jovian Planets in Summary
Uranus and Neptune are not simply smaller versions of Jupiter and Saturn.
Jovian Planets in Summary
Uranus and Neptune are not simply smaller versions of Jupiter and Saturn.
Jovian Planets in Summary
Uranus and Neptune are not simply smaller versions of Jupiter and Saturn.
Jovian Planets in Summary
Uranus and Neptune are not simply smaller versions of Jupiter and Saturn.
Jovian Planets in Summary
Uranus and Neptune are not simply smaller versions of Jupiter and Saturn.
In summary…
Important Concepts
Important Terms
 Uranus and Neptune are not the
smaller version of Jupiter and
Saturn
 Captured satellites.
 Energy source of Triton
 Occultation
 Cryovolcanism
Chapter/sections covered in this lecture : sections 14-1 through 14-8