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Uranus and Neptune
Uranus: The god of the
heavens—The son and husband
of Gaia—he hated his sons which
angered Gaia. She fashioned a
sickle which Saturn, his son,
used to castrate him.
Neptune: The god of the sea—
brother of Jupiter and Pluto
The Solar
Uranus, Neptune, Pluto & TNO’s
What We Learn About Uranus,
Neptune, Pluto & TNO’s
How Uranus and Neptune were discovered
The unusual properties of the axis, orbit and
atmosphere of Uranus
What gives Neptune’s clouds and atmosphere
their distinctive appearance
The internal structures of Uranus and Neptune
The unique orientations of the magnetic fields
of Uranus and Neptune
Why the rings of Uranus and Neptune are hard
to see
What We’ve Learned About
Uranus, Neptune, Pluto & TNO’s
What could have powered geologic activity on
Uranus’s moderate-sized moons
Why Neptune’s satellite Triton is destined to be
torn apart
How Pluto came to be discovered
What shapes the orbits of Pluto and the
thousands of other objects that orbit beyond
What TNO’s are and why Pluto, Charon, and
Triton are part of this population.
Orbit of Uranus
Uranus—Voyager 2, 1986
False Color
True Color
South Pole
Hubble Image 2004
Ultraviolet, Visible,
infrared composite
Springtime on Uranus
Uranus’ Structure
Atmosphere: H2 , He, CH4
Outer Mantle:
Liquid H2, He ≈ 4 ME
Inner Mantle:
Liquid H2O, NH3, CH4
≈ 11 ME
Core: Fe, Ni and
Mg silicates ≈ 1 ME
Discovery of Uranus’ Rings–1977 Stellar Occultation
Uranus Rings
• Rings are thin and dark
• Typically 0.1–10 m size
• larger particles than
Saturn’s rings
• Methane ice—
undergoes radiation
Uranus rings
Hubble Image of Rings
• Short exposure to reveal planet and inner rings
• Long exposure to reveal outer rings
How were the rings of Uranus discovered?
A.By Galileo who originally saw them but
thought the looked like ‘ears’ sticking
out on the sides of Uranus, which he
mistakenly thought were moons.
B.By a Cornell astronomer who hoped to
measure the size of Uranus during an
anticipated stellar occultation in 1977,
when he surprisingly observed that light
from the target star blinked on and off
before and after it was actually occulted
by Uranus.
C.By Voyager I, when its electronics failed
as it passed through Uranus’ rings.
D.By Voyager II, during its 1986 flyby.
Uranus’ Major Moons
Uranus Rings and Small Moons
Miranda Mosaic
Surface of Miranda
Miranda initially in resonance orbit with Umbriel
Tidal heating melted Miranda causing differentiation
Differentiation failed to complete when tidal heating ceased
Neptune Data
Discovered by Johann Gotfried Galle Sept 23, 1846
Mass (Earth = 1) 17.135
Equatorial radius (Earth = 1) 3.8799
Mean density (gm/cm 3)1.64
Mean distance from the Sun (Earth = 1) 30.0611
Rotational period (hours) 16.11
Orbital period (years) 164.79
Orbital eccentricity 0.0097
Tilt of axis (degrees)29.56
Orbital inclination (degrees) 1.774
Equatorial surface gravity (m/sec 2) 11.0
Albedo 0.41 (Magnitude 7.84)
Mean cloud temperature -193 to -153°C
Atmospheric pressure (bars) 1-3
Atmospheric composition Hydrogen 85%, Helium 13%, Methane 2%
When the English student, John Couch Adams, and the
French astronomer, Urbain Le Verrier, independently
predicted the existence of Neptune on the basis of measured
irregularities of Uranus’ orbital motion, what factor ultimately
led to Neptune’s discovery by Johann Galle at Berlin
Observatory rather than by James Challis at Cambridge,
England, even though Challis was told of the prediction much
earlier than Galle?
A. better eyesight
B. Galle used photography, while Challis had to
rely on eye observation
C. The Berlin Observatory had better star charts
and Galle took LeVerrier seriously while Challis
and Sir George Biddell Airy failed to take
Adams seriously
D. less cloud cover
Neptune Structure
H2 , He, CH4 ≈ 2
Mantle: Liquid
H2O, NH3, CH4
≈ 14 ME
Core: Fe, Ni
and Mg
silicates, ≈ 1
Magnetic Fields of Jovian
Neptune Storms
Neptune Rings
Jovian Planet Differences
Triton and Neptune
• Triton in retrograde orbit
• Orbit inclined 23o from
Neptune’s equatorial
• How does satellite co-form
this way?
• Diameter 2706 km
• Largest captured
Triton and Neptune
Absence of large craters—why?
Triton Volcanic Surface
Frozen ‘lakes’—
calderas of
Triton initially in highly elliptical orbit
Tidal flexing as on Io until orbit ‘circularized’
Triton’s Surface
• Pink material—
N2 frost?
What future awaits Triton, the largest
satellite of Neptune?
A.eventual escape from Neptune as it
gradually spirals outward
B.tidal breakup as it slowly spirals closer
to Neptune and falls within its Roche
C.probable destruction when it eventually
collides with Pluto
D.vaporization as it eventually falls into
the Sun
Pluto and Charon
Pluto: The god of the
Underworld—brother of
Charon: The ferryman who carried the
doomed souls across the River Styx into
Discovery of Pluto—1930
Pluto shifts about 1
arcmin/day relative to
background stars
Pluto and Charon
discovered in
1978, James
Pluto and Charon
“He gave way to the queer, savage feeling that
sometimes takes by the throat a husband twenty
years’ married, when he sees, across the table, the
same face of his wedded wife, and knows that, as
he has sat facing it, so must he continue to sit
until the day of its death or his own.”
Rudyard Kipling
Pluto Data
Discovered by Clyde W. Tombaugh, Feb 18, 1930
Mass (kg)1.27 x 10 22
(Earth = 1) 2.125 x 10 -3
Radius (km) 1,137
(Earth = 1) 0.1783
Mean density (gm/cm -3) 2.05
Mean distance from the Sun (Earth = 1) 39.5294
Rotational period (days) 6.3872
Orbital period (years) 248.54
Orbital eccentricity 0.2482
Tilt of axis (degrees) 122.52
Orbital inclination (degrees )17.148
Equatorial surface gravity (m/sec2) 0.4
Albedo 0.3 (Magnitude 15.12)
Atmospheric composition Nitrogen Methane
Pluto, Charon and (Hydra and
HST discovery of Nix and
Pluto…Light & Dark Regions
Mercator Map of Pluto
New Horizons … 2015
The orbital and rotational periods of the
Pluto-Charon system bear what
relationship (if any) to each other?
A.Charon and Pluto orbit each other’s
common center of mass with identical
orbital and rotational periods.
B.Charon orbits Pluto twice, while Pluto
orbits Charon once.
C.There is no relationship between
rotation period of Pluto and orbital
period of Charon.
D.Charon orbits Pluto once each month.
Kuiper Belt and Oort Cloud
• Kuiper Belt extends
from about 30 to 60 AU
• Oort cloud, a spherical
distribution, extends to
about 100,000 AU!
‘Kuiper Belt’ Around HD 139664
57 Ly Away
Known KBO’s
• Green–main belt
• Orange–scattered
TNO Distribution
KBO Resonances
Mauna Kea—Site of CFHT
Orbits before
Jupiter Saturn 2:1
note: Neptune
formed closer in
than Uranus
Orbits after
Jupiter/Saturn 2:1
causes Uranus
& Neptune to
migrate outward
Neptune scatters
into TNO’s
(Kuiper Belt, etc)
Largest (TNO’s)
Eris … The Largest Dwarf
Eris was named after the Greek goddess of discord and
strife. She stirs up jealousy and envy to cause fighting and
anger among men. At the wedding of Peleus and Thetis all
the gods were invited with the exception of Eris, and,
enraged at her exclusion, she spitefully caused a quarrel
among the goddesses that led to the Trojan war.
Orbit of Eris – Largest TNO
The Kuiper Belt consists _______.
A.very soft leather used by Gerard
Kuiper to make designer belts.
B.asteroids that pass closer to the
Sun than Earth's orbital distance.
C.a region in the asteroid belt where
all the Kuiper family objects orbit
the Sun.
D.mostly of small, icy planetesimals
that formed just beyond Jupiter and
were scattered outward just beyond
the orbit of Neptune by gravitational
forces exerted on them by the
Jovian planets.
Key Ideas
Discovery of the Outer Planets: Uranus was
discovered by chance. Neptune’s existence was
predicted via Newtonian mechanics. Pluto was
discovered after a long search.
Atmospheres of Uranus and Neptune: Uranus and
Neptune have atmospheres composed of hydrogen,
helium, and a small percentage of methane.
Methane absorbs red light, giving Uranus and Neptune
their greenish-blue color.
No white ammonia clouds are seen on Uranus or
Neptune. Low temperatures caused ammonia to
precipitate into planetary interiors. Clouds are composed
of methane.
Key Ideas
Much more cloud activity is seen on Neptune than on
Uranus. Uranus lacks a substantial internal heat source.
Interiors and Magnetic Fields of Uranus and Neptune:
Uranus and Neptune have rocky cores surrounded by a
mantle of water and ammonia. Electric currents in these
mantles generate the magnetic fields of the planets.
The magnetic axes of both Uranus and Neptune are
steeply inclined from their axes of rotation. The magnetic
and rotational axes of all the other planets are more
nearly parallel.
The magnetic fields of Uranus and Neptune are also
offset from the centers of the planets.
Key Ideas
Uranus’s Unusual Rotation: Uranus’ 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 planet-like object early in the history of our solar
system. Such a collision could have knocked Uranus on
its side.
Ring Systems of Uranus and Neptune: Uranus and
Neptune are both surrounded by systems of thin, dark
rings. The low reflectivity of the ring particles may be due
to radiation-darkened methane ice.
Key Ideas
Satellites of Uranus and Neptune: Uranus has five
satellites similar to the moderate-sized moons of Saturn,
plus at least 22 more small satellites. 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.
Key Ideas
Pluto and Charon:
Pluto and its moon, Charon, move together in a highly
elliptical orbit steeply inclined to the plane of the ecliptic.
They are rocky, icy worlds that belong to a class of
objects called Trans Neptunian Objects (TNO’s)
Key Ideas
Beyond Neptune:
Trans-Neptunian Objects (TNO’s) are icy worlds that
mostly lie in a band called the Kuiper belt that extends
from 30 to 50 AU from the Sun.
The Kuiper Belt and the Oort Cloud contain billions of
small, icy, rocky objects in orbit around the Sun
More than a thousand icy worlds have been discovered
beyond Neptune. Pluto and Charon are part of this
Neptune’s gravity shapes the orbits of objects within the
Kuiper belt.