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. 10 25 kg 14.6. Earth Density: 1.3. water 0.24. Earth Orbit: 2.9. 10 9 km 19.19 AU Day: 17 h.12 m 00 s 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 (CH 4 ) 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 (CH 4 ) Atmospheric composition ~ 82.5 % H 2 Colorless ~ 15.2 % He Colorless ~ 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 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 calculations Oct 1845 Urbain LeVerrier completes calculations 23 Sep 1846 Johann Galle discovers Neptune 23 Sep 1846 Neptune Data: Numbers Diameter: 49,528.km 3.88. Earth Mass: 1.0. 10 26 kg 17.23. Earth Density: 1.7. water 0.30. Earth Orbit: 4.5. 10 9 km 30.06 AU Day: 16 h.06 m 36 s 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 (CH 4 ) 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 % H 2 Colorless ~ 18 % He Colorless ~ 2 % CH 4 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 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 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 (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 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 N 2 to freeze Warm enough for little N 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 Important Concepts 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