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 Themagnetic fieldsof Uranus & Neptune Thedark ringsof 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 InitialconclusionA distant comet13 March 1781 FinalconclusionA distant planetEnd 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.km4.01. Earth Mass: kg14.6. Earth Density:1.3. water0.24. Earth Orbit: km19.19 AU Day:17 h.12 m 00 s Earth Year:84.04 years 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 (CH 4 ) gives Uranus its light blue color Uranus’ interior consists of three layers… –AtmosphereVery thin –MantleLiquid 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 markingsJan Required extensive computer processing to see Composed mostly of low altitude methane (CH 4 ) –Atmospheric composition ~ 82.5 % H 2 Colorless ~ 15.2 % HeColorless ~ 2.3 % CH 4 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 Uranusinitiallymoved slightlyfasterthan expected Uranuslatermoved slightlyslowerthan expected Two possible explanations –Newtonian mechanics does not work –An undiscovered planet is causing the discrepancies John Couch Adamscompletes calculationsOct 1845 Urbain LeVerriercompletes calculations23 Sep 1846 Johann Galle discovers Neptune23 Sep 1846

Neptune Data: Numbers Diameter:49,528.km3.88. Earth Mass: kg Earth Density:1.7. water0.30. Earth Orbit: km30.06 AU Day:16 h.06 m 36 s Earth Year:164.8 years 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 (CH 4 ) gives Neptune its deep blue color –Great Dark Spot Neptune’s interior consists of three layers –AtmosphereVery thin –MantleLiquid 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 % H 2 Colorless ~ 18 % HeColorless ~ 2 % CH 4 Distinctive blue Storms –The Great Dark SpotAug 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 –BeltsDark blue Descendingatmospheric regions –ZonesLight blue Ascendingatmospheric regions

Neptune’s Clouds (HST, 1998)

Uranus & Neptune ⇔ Jupiter Average density –Jupiter & SaturnRelatively low Chemical composition verysimilar tothe Sun –Uranus & NeptuneRelatively high Chemical composition verydifferent fromthe Sun –Considerably deficient in H 2 and He Hypothesized Jovian planet formation process –Planetesimals accreted to form each planet’s core –Each planet’s core accreted H 2 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 accident10 March 1977 Expected occultation of a faint star Blocked out light 9 times as rings passed in front –Voyager 2 discovers 2 more ringsJanuary 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 (CH 4 ) 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 MirandaUnique 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 MottledterrainnearTriton’s south pole Cantaloupeterrainaway fromTriton’s south pole Triton’s unusual properties –Surface temperature of ~ 38 K Cold enough formostN 2 to freeze Warm enough forlittleN 2 atmosphere –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