General Astronomy I The Outermost Planets Chapter 24

General Astronomy I

The Outermost PlanetsChapter 24

Outermost Planets

Outermost Planets Planetary Properties

Outer Jovian planets

Uranus & Neptunethe Ice Giants

Hitchhiker’s Guide to the Jovians

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

Uranus

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

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

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

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

Uranus Discovery

first planet discovered in modern times Sir William Herschel

formally 13 Mar 1781 Observations

John Flamsteed 1690 34 Tauri

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

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

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

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°

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

atmosphere Hydrogen 83% Helium 15% Methane 1.99% Ammonia 0.01% Ethane 0.00025% Acetylene 0.00001% CO trace Hydrogen sulfide trace

Uranus

Uranus

Uranus Color

blue (cyan) due to absorbtion of red light

atmospheric methane

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

Uranus Chance discovery

William Herschel 1781 scanning the sky for

nearby objects (measurable parallax)

Uranus discovered as slightly extended object

~3.7’’ diameter

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

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

Uranus Atmosphere

cloud structures only visible after artificial computer enhancement of optical images taken from Voyager

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

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

Uranus Atmosphere

Keck images clear variability

of cloud structures

possibly seasonal changes

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

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

Uranus Magnetic field

~75% Earth’s magnetic field no metallic core

no magnetic field expected

Uranus Magnetic field

~75% Earth’s magnetic field no metallic core

Possibly due to dynamo in liquid-water/ammonia/methane solution in interior

Uranus Magnetic field

~75% Earth’s magnetic field offset from center ~30% planet’s radius inclined ~60° against axis of rotation

Uranus Magnetic field

~75% Earth’s magnetic field weak radiation belts allows

determination of rotational period 17.24 hr

Uranus Magnetic field

~75% Earth’s magnetic field rapid rotation and large inclination deform

magnetosphere into a corkscrew shape

Uranus

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?

Uranus’ Rings

Uranus Rings

similar to Jupiter’s rings consist of dark material confined by shepherd

moons

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

Uranus Rings

regularly re-supplied by dust from meteorite impacts on moons

visible in forward-scattered light

Uranus Rings

Uranus

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

Uranus Moons

Uranus’ moons

Uranus Interiors of moons

large rock cores surrounded by icy mantles

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

Uranus Surfaces of moons

Umbriel dark, cratered surface no faults or other signs of

surface activity

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

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

Uranus

Neptune

Neptune Neptune

eighth (& last) named for roman god of sea

symbol is stylized trident

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

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

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°

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

atmosphere Hydrogen 80% Helium 19% Methane 1.5% Hydrogen Deuteride 192 ppm Ethane 1.5 ppm

Neptune

Neptune Composition

55 K surface temperature increases

downward infalling matter heated now radiating slowly

Neptune Internal Structure

resembles Uranus superheated liquid interior

7000 K

Neptune

Neptune Magnetic field

similar to Uranus 47° offset at least 0.55 radii offset

Neptune Weather

blue due to methane brilliant color contrasts

with Uranus

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

Weather strong wind

despite lack of solar energy lack of turbulence

low energy

clouds methane ice crystals cast shadows

Neptune

Neptune

Neptune Adaptive Optics

Neptune in near IR

Neptune rings

unknown composition clumpy structure

small moons? unstable

Liberté may disappear in as little as 100 years

Neptune Moons

13 known Triton

largest, only one spheroidal retrograde orbit

likely captured sychronous orbit

Neptune Moons

Triton spiraling inward will disintegrate

inside Roche limit coldest object in

Solar System 38.15 K

Neptune Triton

density 2.05 g/cm3 25% water ice remainder rock

geologically active Voyager 2 – icy volcanoes

& geysers

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?

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

Neptune & Triton

Neptune’s moons Nereid

highly eccentric orbit orbital period 359.4 days

Neptune

Neptune

Pluto

Pluto

Pluto

Pluto Discovery

1930 by C. Tombaugh existence predicted from

orbital disturbances of Neptune

Pluto is too small to cause those disturbances

Pluto

Pluto Venetia Burney

Pluto Venetia Burney Phair

age 88 in 2007

Pluto Venetia Burney Phair

age 88 in 2007 6235 Burney

1987

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

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

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

Pluto Atmosphere

thin nitrogen, methane, carbon monoxide envelope in ‘summer’

much of it freezes in ‘winter’ temperature is 10 K lower than expected

Pluto Orbit

2:3 resonance with Neptune

Pluto Pluto’s moon Charon

Pluto Pluto’s moon Charon

Discovered 1978 about half the size and 1/12 the mass of

Pluto tidally locked to Pluto

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

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

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

Pluto more surprises

Pluto

Pluto

Pluto

Pluto

Pluto

2003 UB313

Next time…

Pluto Orbital

resonances Charon orbits 12

times P2 orbits 3 times P1 orbits twice

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

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

Eris Named for Greek goddess Eris

personification of strife & discord moon

Dysnomia daughter of Eris

Eris

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

Dwarf Planets Pluto Eris Ceres

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