general astronomy i the outermost planets chapter 24
TRANSCRIPT
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General Astronomy I
The Outermost PlanetsChapter 24
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Outermost Planets
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Outermost Planets Planetary Properties
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Outer Jovian planets
Uranus & Neptunethe Ice Giants
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Hitchhiker’s Guide to the Jovians
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Hitchhiker’s Guide to the Jovians much larger than terrestrial planets
thick atmospheres, but very inhospitable no solid surfaces mostly hydrogen and helium strong atmospheric circulation
(winds/storms) cloud belt patterns rings multiple moons
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Uranus
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Uranus, Neptune & others Uranus
Discovery of Uranus Motion of Uranus Atmosphere of Uranus Interior of Uranus Rings of Uranus Moons of Uranus History of Uranus
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Uranus Ice Giant Uranus
7th planet from the Sun gas giant
third largest by diameter fourth largest by mass 27 known satellites
five of particular interest
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Uranus Ice Giant Uranus
named after Uranus, the Greek god of the sky and progenitor of the other gods.
astrological symbol derived from discover’s name
astronomical symbol is a combination of the devices for the Sun and Mars
Uranus was the personification of heaven in Greek mythology, dominated by the light of the Sun and the power of Mars
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Uranus Missions
Voyager 2 is the only spacecraft to have visited the planet
launched in 1977 closest approach 24 Jan
1986 continued journey to
Neptune no other visits currently
planned
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Uranus Discovery
first planet discovered in modern times Sir William Herschel
formally 13 Mar 1781 Observations
John Flamsteed 1690 34 Tauri
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Uranus Discovery
other planets (from Mercury to Saturn) known since ancient times
expanded the boundaries of the solar system for the first time in modern human history
first (only) planet discovered using technology (a telescope) rather than the naked eye
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Uranus Visibility
magnitude +5.5 to +6 4 arcseconds from earth with larger telescope (12”+)
pale blue disk with limb shading visible two of larger satellites visible
no details with any earth-based telescope except IR studies & AO
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Uranus Historically correct
pronunciation stressed syllable is properly the
first, antepenultimate syllable Latin the penultimate vowel a is
short in an open syllable such syllables are never
stressed in Latin contrast adjective Uranian
& element uranium
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Saturn – physical data Orbit
19.19 AU 19.19 billion km
orbital inclination 0.77° orbital period 84.07 years period of rotation –17h14m24s axial tilt 97.77°
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Saturn – physical data Characteristics
Equatorial diameter 51,118 km 4.007 Earths
Mass 8.683x1025 kg 14.5 Earth density 1.318 g/cm3 surface gravity 0.886 gee
mean cloudtop temperature: 55 K albedo 0.51
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atmosphere Hydrogen 83% Helium 15% Methane 1.99% Ammonia 0.01% Ethane 0.00025% Acetylene 0.00001% CO trace Hydrogen sulfide trace
Uranus
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Uranus
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Uranus Color
blue (cyan) due to absorbtion of red light
atmospheric methane
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Uranus Axial tilt
97.77° one pole faces sun while
other faces away perhaps collision with
protoplanet? equatorial regions remain
hotter than poles underlying mechanism
unknown
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Uranus Chance discovery
William Herschel 1781 scanning the sky for
nearby objects (measurable parallax)
Uranus discovered as slightly extended object
~3.7’’ diameter
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Uranus very unusual
orientation of rotation axis almost in orbital plane
(97.9° inclination) possibly result of
impact of large planetesimal during planet formation
“eternal” sunlight for many years, then complete darkness
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Uranus Gas Giant
gradual transition from gas phase to fluid interior
mostly hydrogen 15% helium few % methane, ammonia,
water vapor, acetylene view from Earth
blue color due to methane, absorbing longer wavelengths
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Uranus Atmosphere
cloud structures only visible after artificial computer enhancement of optical images taken from Voyager
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Uranus Atmosphere
one layer of methane clouds vs 3 cloud layers Jupiter/ Saturn
3 cloud layers form at relatively high temperatures
occur only very deep in Uranus’ atmosphere
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Uranus Atmosphere
cloud layer difficult to see because of thick atmosphere above it
Also shows belt-zone structure
structure dominated by planet’s rotation
not incident angle of sunlight
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Uranus Atmosphere
Keck images clear variability
of cloud structures
possibly seasonal changes
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Uranus bland atmosphere
Uranus’ core roughly 7000 K vs 30,000 K in Jupiter
cooler & generates less heat convection currents formed in
the Uranian atmosphere are not as strong
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Uranus Interior structure
density ≈ 1.29 g/cm3 more rock & ice
than Jupiter & Saturn
ice/rock layer ices of water,
methane, ammonia ices mixed with hydrogen and silicates
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Uranus Magnetic field
~75% Earth’s magnetic field no metallic core
no magnetic field expected
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Uranus Magnetic field
~75% Earth’s magnetic field no metallic core
Possibly due to dynamo in liquid-water/ammonia/methane solution in interior
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Uranus Magnetic field
~75% Earth’s magnetic field offset from center ~30% planet’s radius inclined ~60° against axis of rotation
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Uranus Magnetic field
~75% Earth’s magnetic field weak radiation belts allows
determination of rotational period 17.24 hr
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Uranus Magnetic field
~75% Earth’s magnetic field rapid rotation and large inclination deform
magnetosphere into a corkscrew shape
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Uranus
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Uranus Rings
similar to Jupiter’s rings consist of dark material confined by shepherd moons one is reddish one is blue
ice refracting light from Uranus?
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Uranus’ Rings
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Uranus Rings
similar to Jupiter’s rings consist of dark material confined by shepherd
moons
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Uranus Rings
regularly re-supplied by dust from meteorite impacts on moons
visible in forward-scattered light
Focused by small shepherd moons embedded in structure
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Uranus Rings
regularly re-supplied by dust from meteorite impacts on moons
visible in forward-scattered light
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Uranus Rings
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Uranus
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Uranus Moons
5 largest visible from Earth
Voyager 2 discovered 10
more being found dark surfaces
probably ice darkened by dust from meteorites
5 largest tidally locked
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Uranus Moons
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Uranus’ moons
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Uranus Interiors of moons
large rock cores surrounded by icy mantles
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Uranus Surfaces of moons
Oberon old, inactive, cratered surface
probably active past long fault across the surface dirty water may have flooded some
craters
Titania largest heavily cratered surface, none very
large internal melting might have flooded some
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Uranus Surfaces of moons
Umbriel dark, cratered surface no faults or other signs of
surface activity
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Uranus Surfaces of moons
Ariel brightest surface of 5 largest
Clear signs of geological activity crossed by faults over 10 km deep
Possibly heated by tidal interactions with Miranda and Umbriel
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Uranus Surfaces of moons
Miranda most unusual of 5 largest ovoids: oval groove patterns
probably associated with convection currents in mantle
not with impacts 20 km high cliff near the
equator Surface features old
no longer geologically active
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Uranus
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Neptune
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Neptune Neptune
eighth (& last) named for roman god of sea
symbol is stylized trident
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Neptune Discovery
Galileo observed 1612/1613 1821 Bouvard saw unexpected
deviations in Uranus’ orbit 1843 Adams predicts orbit Calculations & searches
1846 Le Verrier/Galle 1846 Herschel/Challis
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Neptune Discovery
1846 Heinrich d’Arrest student at Berlin Observatory compared new chart with old
one in area of de Verrier’s prediction
Neptune discovered that night within 1° of Le Verrier
prediction within 10° of Adams’ prediction
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Neptune – physical data Orbit
30.07 AU 4.5 billion km
orbital inclination 1.77° orbital period 164.9 years period of rotation –16h6m36s axial tilt 28.3°
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Neptune – physical data Characteristics
Equatorial diameter 49,530 km 3.88 Earths
Mass 1.024x1026 kg 14.94 Earth density 1.64 g/cm3 surface gravity 1.14 gee
mean cloudtop temperature: 53 K albedo 0.51
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atmosphere Hydrogen 80% Helium 19% Methane 1.5% Hydrogen Deuteride 192 ppm Ethane 1.5 ppm
Neptune
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Neptune Composition
55 K surface temperature increases
downward infalling matter heated now radiating slowly
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Neptune Internal Structure
resembles Uranus superheated liquid interior
7000 K
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Neptune
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Neptune Magnetic field
similar to Uranus 47° offset at least 0.55 radii offset
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Neptune Weather
blue due to methane brilliant color contrasts
with Uranus
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Neptune Weather
highest wind in Solar System
2000 kph Great Dark Spot
Voyager 2 in 1989
not seen by HST now thought
hole in cloud cover
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Weather strong wind
despite lack of solar energy lack of turbulence
low energy
clouds methane ice crystals cast shadows
Neptune
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Neptune
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Neptune Adaptive Optics
Neptune in near IR
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Neptune rings
unknown composition clumpy structure
small moons? unstable
Liberté may disappear in as little as 100 years
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Neptune Moons
13 known Triton
largest, only one spheroidal retrograde orbit
likely captured sychronous orbit
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Neptune Moons
Triton spiraling inward will disintegrate
inside Roche limit coldest object in
Solar System 38.15 K
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Neptune Triton
density 2.05 g/cm3 25% water ice remainder rock
geologically active Voyager 2 – icy volcanoes
& geysers
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Neptune Triton
geologically active surface features
probably not more than 100 million years old
large basins might have flooded multiple times by liquids from interior
seasonal heating?
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Neptune Triton
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
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Neptune & Triton
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Neptune’s moons Nereid
highly eccentric orbit orbital period 359.4 days
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Neptune
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Neptune
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Pluto
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Pluto
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Pluto
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Pluto Discovery
1930 by C. Tombaugh existence predicted from
orbital disturbances of Neptune
Pluto is too small to cause those disturbances
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Pluto
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Pluto Venetia Burney
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Pluto Venetia Burney Phair
age 88 in 2007
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Pluto Venetia Burney Phair
age 88 in 2007 6235 Burney
1987
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Pluto – physical data Orbit
30.07 AU 5.9 billion km eccentricity 0.249
29.7 AU to 49.3 AU
orbital inclination 17.14° orbital period 248 years period of rotation –6d9h17m36s axial tilt 119° to orbit/113° to ecliptic
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Pluto – physical data Characteristics
Equatorial diameter 2390 km 0.19 Earths
Mass 1.3x1022 kg 0.0021 Earth density 2.03 g/cm3 surface gravity 0.059 gee
mean cloudtop temperature: 44 K albedo 0.49-0.66
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Pluto Appearance
virtually no surface features visible from Earth
~65% size of Earth’s Moon
fainter than 14th magnitude
light brown with very slight yellow tint
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Pluto Atmosphere
thin nitrogen, methane, carbon monoxide envelope in ‘summer’
much of it freezes in ‘winter’ temperature is 10 K lower than expected
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Pluto Orbit
2:3 resonance with Neptune
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Pluto Pluto’s moon Charon
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Pluto Pluto’s moon Charon
Discovered 1978 about half the size and 1/12 the mass of
Pluto tidally locked to Pluto
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Pluto Charon’s orbit
highly inclined against orbital plane
large seasonal changes on Pluto and Charon
MPluto ~ 0.2 Earth masses from separation & orbital
period density ≈ 2 g/cm3
Pluto & Charon ~35 % ice, 65% rock
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Pluto History
Probably very different history than neighboring Jovian planets
Older theory Pluto and Charon formed as moons of
Neptune, ejected by interaction with massive planetesimal
Mostly abandoned today since such interactions are unlikely
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Pluto History
Modern theory Pluto and Charon members of Kuiper
Belt Chapter 25 – small, icy objects Pluto caught in orbital resonances with
Neptune Collision between Pluto and Charon
may have caused peculiar orbital patterns & large inclination of Pluto’s rotation axis
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Pluto more surprises
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Pluto
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Pluto
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Pluto
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Pluto
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Pluto
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2003 UB313
Next time…
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Pluto Orbital
resonances Charon orbits 12
times P2 orbits 3 times P1 orbits twice
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2003 UB313
Discovery July 29 2005 48” scope at Palomar & 8m at Mauna Kea extreme 44° orbital inclination
only 2004 DG77 more inclined 97 AU orbit
eccentric Pluto 40 AU
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Eris (2003 UB313) Named for Greek goddess Eris
personification of strife & discord spectrum dominated by frozen methane not reddish like Pluto
breakdown of tholins by methane
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Eris Named for Greek goddess Eris
personification of strife & discord moon
Dysnomia daughter of Eris
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Eris
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Dwarf Planets IAU definition
in orbit around the Sun has sufficient mass for its self-gravity to
overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
has not cleared the neighbourhood around its orbit
not a satellite
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Dwarf Planets Pluto Eris Ceres
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Homework Go home
WebCT on the way… study hard