the outermost planets - university of...

The Outermost Planets

• The 7 ‘Wanderers’ known since Antiquity.• Uranus and Neptune distinctly Blue-ish!

Uranus

• Discovered by William Herschel 1781.(Accidentally!) First new planet for > 3000 years!

• Featureless planet, just a few wispy clouds!View from Voyager 2 1986.

Uranus and 3 of its moons, barely visible from Earth.

Neptune

• Elliptical Orbit of Uranus is perturbed: There must be another planet!

• Adams 1845 and Le Verrier 1846 predicted Neptune's position.

• First Seen by Galle in 1846.

Property Uranus NeptuneRadius 25559km (4.0 RE) 24766km (3.88 RE)

Mass 17.15 ME

Average Density 1271kg/m3 (0.23) 1638kg/m3 (0.297)

Gravity 0.91 Earth’s 1.14 Earth’s

Escape Speed 21.3km/s 23.5km/s

Temperature (at Cloud tops)

59K / -214C

14.54 ME

56K / -217C

Eccentricity 0.047 0.009

Property Uranus NeptuneSidereal

Orbital Period83.75 years 163.7 years

Sidereal Rotation Period

-0.72 daysRetrograde!

0.67 days

Axial Tilt 97.92° 29.6°

# Moons 27…. 13…

Magnetic axis tilt

46°58.6°

Similar – but with significant differences!

Uranus’ Peculiar Rotation

• Axis of rotation nearly in the orbital plane!• Extreme seasons and weird days!

Reason for such a strange tilt not known.

Consequences• In Winter: Darkness for 42 years!• In Summer: No night, sun moves in circles,

with altering radii.

Other Rotational Features• Differential Rotation (~17.2 hours)• Atmosphere moves faster at Poles

(~14.2 hrs) than at Equator (16.5hrs) .• No zones or belts: Wind in same direction.

The Atmospheres Compositionof Uranus and Neptune

• H2 ~84%• Helium ~14%• Methane ~2% (Uranus); 3% Neptune

x10 more than on Jupiter and Saturn.Less ammonia and more methane as distance from Sun increases:

Ammonia freezes out at ~70K.

Neptune is “Bluer” as it has more Methane.

Weather on Uranus• South pole region is

reflecting sunlight -highest wind speeds ~500km/hr.

• Spots: high clouds or storms ~1000km across.

• Temperature variation from pole to equator is small : Efficient heat transfer.

• No internal source of Heat.

False colour Infra-red view from Hubble (1998)

Ring System!

Weather on Neptune 1989

Voyager 2 sees ‘Great Dark Spot’ storm system as big as Earth. (Similar to Jupiter)

CH4 ‘ice’ clouds

Weather on Neptune in 1998

• Dark spot disappeared!• New storms appeared...• Wind speeds greater

than 1500km/hr, driven by internal heat from gravitational collapse.

Still shrinking!• Faint zones and belts:

Convection cycles.

Interiors of Uranus and Neptune

• No metallic Hydrogen – not enough pressure.• Relatively larger cores - (than Jupiter and Saturn.)• Slushy interiors of conducting ammonia in H20.

Very similar

structures.

Strange Magnetic Fields.

Strange Magnetic Fields II.

Magnetic fields aligned at strange angles to rotation axis. Why? (Don’t really know!)

Field in transition of flipping?Fields generated differently? (No metallic H)

Presumably via the slushy conducting water.

The Moons of Uranus…and its rings

The Moons of Uranus• Oberon

and Titaniadiscovered in 1789 by William Herschel.

• Mirandaby Gerard Kuiper in only 1948.

Earth’s Moon

The five largest Moons – to scale.

General Features of Major Moons

• Diameters from ~500-1600km.• All have average densities of ~1500kg/m3

ice and rock.• Geologically dead, but scarred and cratered. • Dull – less reflective. “Radiation Darkening”:

Light induced chemistry of surface molecules to give dark organic material.

• Voyager 2 found 10 more moons……. And now the moon count is up to 27.

Moons of Uranus

Titania

Oberon Umbriel

Ariel

Miranda

• The strange inner moon (~5 RUranus).

• Signs of a painful geological history:surface ridges, valleys,

faults, craters. • Tidal heating re-

melting? Collisions?Lots to discover.

Surface detail: cliffs, stripes, few craters

Miranda

Neptune’s Main Moons

• Triton discovered in 1846. Diameter 2710km, 0.29 Moon mass, Orbits at ~14.3 RNeptuneHas retrograde orbit of 5.88 days.

• Nereid discovered by Kuiper in 1949.Diameter 340km, orbit period ~360 daysOrbits at ~223 Rneptune Eccentric orbit.

Very different moons!Neptune has 13 known moons...

Triton

TritonRetrograde motion means:

1. Origin may be ‘capture’.

2. But spiralling into Neptune: Doomed to tidal break-up in ~100 million years.

New ring system!

Triton

• Captured into elliptical orbit; Neptune’s tidal forces made orbits circular over time.

• Resultant stresses melted interior – ice volcanoes, geysers.

• Temperature: -235C.• Thin Nitrogen atmosphere.

FrozenIce

water Lake

The Rings of Uranus

Discovered in 1977 by “Stellar Occultation”.

Thin, Well-separated Rings

• 9 thin rings discovered by ground-based observations.

• In 1986, Voyager 2 discovered 2 more…

• Narrow ~10km wide rings, only 10’s m thick.

• Not very reflective – dark.

Image enhanced view of the rings

of Uranus

…revealing ‘fine’ structure

Outer Epsilon Ring• Also has 2 ‘Shepherd’ satellites:

Cordelia and Ophelia.• Others proposed for other thin

rings – but not yet observed.• Keeps ring narrow.• Close-up shows

internal structure.~30 km wide

Neptune’s Rings

• 5 rings: Galle*, Leverrier, Lassel*, Arago and Adams: * Broader – ‘Saturn-like’ – rings.

• Cold, faint rings contain methane ice, radiation darkened into other carbon compounds.

Like Uranus’rings, first seen

by Stellar Occultation

View from Voyager 2

Pluto

• By late 1800’s Neptune’s and Uranus’ orbits were apparently sufficiently perturbed to postulate the existence of another planet.

• Eventually discovered in 1930 by Tombaugh.• Despite Lowell’s earlier predictions, Pluto is

too small to account for the irregularities!• Hence it was a ‘Lucky’ discovery!

Pluto – Roman ‘god of the dead’

Pluto’s Strange 248-year Orbit

High ~0.25 Eccentricity

Orbital plane

inclined at ~17.2° to Ecliptic

Perihelion: 29.7AU; Aphelion 49.4 AU

Other Properties• Orbit period is exactly 1.5x that of Neptune.

Planets orbits locked in a 3:2 resonance:2 Pluto orbits = 3 Neptune orbits

• No chance of a collision!

• Mass: 0.0021 ME ; Radius: 1137km (0.18 RE)• Gravity: 0.067 GE ; Escape Speed: 1.2km/s• Axial Tilt: 118° ; Temperature: ~50K, –223C• Rotation period: -6.387 days – Retrograde!

Pluto’s Moon: “Charon”

• Discovered 1978• ‘Charon’: Mythical

‘boatman’ who ferried the dead to Hades – Pluto’s realm.

• Radius ~650km, Mass ~1/6of Pluto. Relatively Big!

• Orbits at 19700km, ~17.3RP

View from Hubble (1990)

More images of Pluto and Charon!

Pluto and Charon….and now Nix and Hydra

• Strongly ‘Tidally’ locked to each other!Pluto’s ~ 6.4 day rotation = Charon’s orbitalperiod and its rotational period.

• Always keep same face to each other.• Moon does not rise or set! (Fixed in Sky)• Two more moons, Nix and Hydra, have

recently been confirmed!

Charon’s Synchronous Orbit

Pluto is Similar to Triton

• Density of Pluto 2100 kg/m3 (like Triton) Rock and ‘Ice’

• Solid ices cover the planet: Nitrogen (N2), Methane (CH4), Carbon Monoxide (CO).

• Like Triton, has a very thin atmosphere. • Charon has ‘water’ ice surface – polar caps.

Pluto’s Surface Structure

• Hubble’s ‘enhanced’UV/ Visible images.

• Icy polar caps• Darker regions:

Craters? Old dirty snow?

• Charon also has ‘contrasts’

The Origin of Pluto-Charon

• Co-formation or Capture theories both possible.• Could have been formed along with the other

planets….• But icy space debris beyond Neptune is known

to exists – hence collisions possible.(Such debris origin of short-period comets.)

At these large distances….more speculation than hard evidence.