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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
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