14a. Uranus & Neptune• The discovery of Uranus & Neptune

• Uranus is oddly tilted & nearly featureless

• Neptune is cold & blue

• Uranus & Neptune are like yet dislike Jupiter

• The magnetic fields of Uranus & Neptune

• The dark rings of Uranus & Neptune

• Ancient tidal heating in some Uranian moons

• Surprisingly young surface on Neptune’s Triton

Earth, Uranus, Neptune & Pluto

The Discovery of Uranus• Uranus was discovered by accident

– William Herschel sees a faint fuzzy blue object• Initial conclusion A distant comet

13 March 1781

• Final conclusion A distant planet

End of 1781

• Earlier sightings of Uranus– Uranus is plotted on at least 20 prior star charts

• It is barely visible under ideal seeing conditions

• It moves an average of only ~ 0.011° per day

Uranus Data: Numbers• Diameter: 51,120.km 4.01 . Earth

• Mass: 8.7 . 1025 kg 14.6 . Earth

• Density: 1.3 . water 0.24 . Earth

• Orbit: 2.9 . 109 km 19.19 AU

• Day: 17h.12m 00s 0.72 . Earth

• Year: 84.04 years 84.04 . Earth

Uranus Data: Special Features• Uranus is the third Jovian planet from the Sun• Uranus is the second smallest Jovian planet• Uranus has no solid surface• Uranus has a featureless blue atmosphere

– Methane (CH4) gives Uranus its light blue color• Uranus’ interior consists of three layers…

– Atmosphere Very thin– Mantle Liquid water, methane & ammonia– Core “Metal” & “rock”

• Uranus has 5 size & 10 small known moons– Largest 5 moons are ~50% “ice” & ~50% “rock”– Smallest 10 moons may be captured asteroids

Uranus Data (Table 14-1)

Uranus Is Tilted & Featureless• Axial tilt of Uranus

– Rotated ~ 98° to the plane of Uranus’s orbit• Rotational axis lies almost in the orbital plane• Slightly retrograde axial rotation

– Most exaggerated seasons of all planets in the Solar System– Each pole of Uranus can point almost directly toward the Sun

• Cloud features of Uranus seen by Voyager 2– Very faint cloud markings

Jan. 1986• Required extensive computer processing to see• Composed mostly of low altitude methane (CH4)

– Atmospheric composition• ~ 82.5 % H2 Colorless• ~ 15.2 % He Colorless• ~ 2.3 % CH4 Distinctive blue

– UV from sunlight converts some methane into hydrocarbon haze

Uranus: The Ultimate in Seasonality

Storms & Rings (HST, 1998)

The Discovery of Neptune• Observations

– Uranus’s orbit could not be accurately predicted– Discrepancies of ~ 2 arc minutes by 1830

• Uranus initially moved slightly faster than expected

• Uranus later moved slightly slower than expected

• Two possible explanations– Newtonian mechanics does not work– An undiscovered planet is causing the discrepancies

• John Couch Adams completes calculationsOct 1845

• Urbain LeVerrier completes calculations23 Sep 1846

• Johann Galle discovers Neptune23 Sep 1846

Neptune Data: Numbers• Diameter: 49,528.km 3.88 .

Earth

• Mass: 1.0 . 1026 kg 17.23 .

Earth

• Density: 1.7 . water 0.30 .

Earth

• Orbit: 4.5 . 109 km 30.06 AU

• Day: 16h.06m 36s 0.67 .

Earth

• Year: 164.8 years 164.8 .

Earth

Neptune Data: Special Features• Neptune is farthest Jovian planet from the Sun• Neptune is the smallest Jovian planet• Neptune has no solid surface• Neptune has a colorful & dynamic atmosphere

– Methane (CH4) gives Neptune its deep blue color– Great Dark Spot

• Neptune’s interior consists of three layers– Atmosphere Very thin– Mantle Liquid water, methane & ammonia– Core “Metal” & “rock”

• Neptune has 1 large, 2 medium, 5 small moons– Triton is ~25% “ice” & ~75% “rock” (like Pluto ! !)

Neptune Data (Table 14-2)

Neptune Is Cold, Blue & Dynamic• Temperature

– Neptune has an average temperature of ~ 55 K• Identical to Uranus yet ~ 50% farther from the Sun• Uranus must have a strong internal heat source

• Color– Atmospheric composition

• ~ 80 % H2 Colorless• ~ 18 % He Colorless• ~ 2 % CH4 Distinctive blue

• Storms– The Great Dark Spot

Aug. 1989• Remarkably similar to Jupiter’s Great Red Spot• Gone when viewed by HST in 1994

– Cirrus clouds• Methane ice

Great Dark Spot & Clouds

Neptune’s Banded Atmosphere• Belts & zones

– Belts Dark blue• Descending atmospheric regions

– Zones Light blue• Ascending atmospheric regions

Neptune’s Clouds (HST, 1998)

Uranus & Neptune ⇔ Jupiter• Average density

– Jupiter & Saturn Relatively low• Chemical composition very similar to the Sun

– Uranus & Neptune Relatively high• Chemical composition very different from the Sun

– Considerably deficient in H2 and He

• Hypothesized Jovian planet formation process– Planetesimals accreted to form each planet’s core– Each planet’s core accreted H2 and He

• Formation of Uranus & Neptune– Too massive to have formed so far from the Sun

• Too few planetesimals at those great distances– Did Uranus & Neptune form closer to the Sun?

• Did Jupiter/Saturn interactions fling them outward?

Uranus & Neptune Inner Structure

Uranus & Neptune Magnetic Fields• Orientation of magnetic fields

– Uranus & Neptune magnetic fields are steeply tilted• Uranus ~ 59°• Neptune ~ 47°

– Both fields are offset from center• Possible explanations

– Magnetic fields might be undergoing reversal• This happens about every 11 years on the Sun• Simultaneous reversals are highly unlikely

– Catastrophic collisions may be responsible• Much more likely for Uranus than for Neptune

– Cause of magnetic fields• Axial rotation + circulation of interior conducting liquid

– Probably ammonia dissolved in water– May be produced by multiple convection cells

Five Planetary Magnetic Fields

Uranus & Neptune Dark Rings• The rings of Uranus

– Discovered by accident

10 March 1977• Expected occultation of a faint star• Blocked out light 9 times as rings passed in front

– Voyager 2 discovers 2 more rings

January 1986– Rings are ~10 km wide & well inside the Roche limit– Ring particles are 0.1 to 10.0 m wide & dark as coal

• The rings of Neptune– Also discovered by stellar occultations

• Cause of dark rings– Methane (CH4) ice can persist that far from the Sun– Radiation darkening may be responsible

• Methane is dissociated into carbon & hydrogen atoms• Solid carbon remains & gaseous hydrogen escapes

Uranian Rings & Small Moons

Tidal Heating in Some Uranian Moons• Five moderate-sized satellites

– Average densities ~ 1.5 g . cm-3

• Consistent with a mixture of rock & ice

– All are very dark• Radiation darkening a distinct possibility

• Miranda

Unique in the Solar System– Multiple landscapes

• Abundant heavily cratered terrain

• Some dramatic terrain– Possible catastrophic impact with rearranged fragments

– Possible tidal heating that permitted rocky crust to sink

Uranus’ Remarkable Miranda

Young Surface on Neptune’s Triton• Triton is Neptune’s only large satellite

– 2,706 km diameter• Slightly smaller than Earth’s Moon

– Retrograde orbit• Almost certainly captured into that orbit

– Conspicuous absence of large craters• Mottled terrain near Triton’s south pole• Cantaloupe terrain away from Triton’s south pole

• Triton’s unusual properties– Surface temperature of ~ 38 K

• Cold enough for most N2to freeze• Warm enough for little N2atmosphere

– Wind-blown deposits in some places– Tidal forces have Triton spiraling in toward Neptune

• In ~ 100 million years Triton will be inside the Roche limit

Neptune’s Remarkable Moon Triton

• Discovery– Uranus discovered by accident

• Seen earlier but unrecognized– Neptune discovered deliberately

• Perturbations of Uranus’s orbit– Pluto discovered by accident

• No real perturbations of Neptune

• Planetary data– Uranus & Neptune

• ~ 4x Earth’s diameter & ~15x mass– Pluto

• ~ 0.2x Earth’s diameter & 0.2% mass

• Planetary characteristics– Uranus & Neptune

• Blue due to methane absorption• Relatively rich in rock & metal• Strongly tilted magnetic fields

• Unusual features– Uranus’s axis nearly in orbital plane– Neptune’s internal energy source– Pluto’s status as a true planet

• Satellites– Uranus’s Miranda

• Distinctly different terrain types– Neptune’s Triton

• Similarity to Pluto & Charon

Important Concepts