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14a. Uranus, Neptune & Pluto
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 A surprisingly young surface on Neptune’s Triton Pluto’s amazing discovery Pluto & Charon are unique planetary objects
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Earth, Uranus, Neptune & Pluto
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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 Earlier sightings of Uranus It 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
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Uranus Data: Numbers Diameter: 51,120.km 4.01 . Earth
Mass: kg Earth Density: water Earth Orbit: km AU Day: 17h.12m 00s Earth Year: years Earth
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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 an almost featureless blue atmosphere Methane (CH4) gives Uranus its light blue color Uranus’ interior probably consists of three layers… Atmosphere: Very thin Mantle: Liquid water, methane & ammonia Core: “Metal” & “rock” Uranus has 5 medium size & 10 small known moons Largest 5 moons are ~50% “ice” & ~50% “rock” Smallest 10 moons may be captured asteroids
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Uranus Data (Table 14-1)
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Uranus Is Tilted & Nearly 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 methane (CH4) at relatively low altitudes Atmospheric composition ~ 82.5 % H2 Colorless ~ 15.2 % He Colorless ~ % CH4 Distinctive blue UV from sunlight converts some methane into hydrocarbon haze
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Uranus: The Ultimate in Seasonality
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Uranian Storms & Rings (HST, 1998)
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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 J. J. LeVerrier completes calculations 23 Sep. 1846 Johann Gottfried Galle discovers Neptune 23 Sep. 1846
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Neptune Data: Numbers Diameter: 49,528.km 3.88 . Earth
Mass: kg Earth Density: water Earth Orbit: km AU Day: 16h.06m 36s Earth Year: years Earth
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Neptune Data: Special Features
Neptune is the farthest Jovian planet from the Sun Neptune is the smallest Jovian planet Neptune has no solid surface Neptune has a colorful & dynamic blue atmosphere Methane (CH4) gives Neptune its deep blue color Great Dark Spot Neptune’s interior likely 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 ! !)
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Neptune Data (Table 14-2)
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Neptune Is Cold, Blue & Dynamic
Temperature Neptune has an average temperature of ~ 55 K This is 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
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Neptune’s Great Dark Spot & Clouds
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Neptune Has a Banded Atmosphere
Belts & zones Belts Dark blue Descending regions of the atmosphere Zones Light blue Ascending regions of the atmosphere
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Neptune’s Cloud Bands (HST, 1998)
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Uranus & Neptune Compared to 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 formation process of Jovian planets Planetesimals accreted to form each planet’s core Each planet’s core accreted H2 and He Formation of Uranus & Neptune They are too massive to have formed so far from the Sun Too few planetesimals at those great distances Uranus & Neptune may have formed closer to the Sun Interactions with Jupiter & Saturn may have pushed outward
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Interior Structure of Uranus & Neptune
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Magnetic Fields of Uranus & Neptune
Orientation of magnetic fields Fields of both Uranus & Neptune are steeply tilted Uranus ~ 59° Neptune ~ 47° Fields of both Uranus & Neptune 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 + interior circulation Probably ammonia dissolved in water May be produced by multiple convection cells
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Five Planetary Magnetic Fields
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The Dark Rings of Uranus & Neptune
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 & as 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 The solid carbon remains & the gaseous hydrogen escapes
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Uranian Rings & Small Moons
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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
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Uranus’s Remarkable Moon Miranda
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A Young Surface on Neptune’s Triton
Triton is Neptune’s only large satellite 2,706 km diameter Slightly smaller than the 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 N2 to freeze Warm enough for very small amounts of N2 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
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Neptune’s Remarkable Moon Triton
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Pluto Data: Numbers Diameter: 2,290.km 0.18 . Earth
Mass: kg Earth Density: water Earth Orbit: km AU Day: 6d.09h 17m 51s Earth Year: years Earth
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Pluto Data: Special Features
Pluto is the farthest planet from the Sun Pluto is the smallest planet Pluto has a very thin atmosphere near perihelion Pluto is much smaller than the Moon Pluto has only ~ the mass of the Moon Pluto has only ~ diameter of the Moon Pluto’s interior likely consists of two layers An “icy” mantle (~25% of Pluto’s mass) A “rocky” core (~75% of Pluto’s mass) Pluto is extremely difficult to observe from Earth Pluto is extremely small & extremely far from the Sun Pluto’s moon Charon has ~ the mass of Pluto
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Pluto Data (Table 14-5)
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Pluto’s Amazing Discovery
The reason for a search Supposed discrepancies in the predicted orbit of Neptune Actually no unaccounted perturbations of Neptune The actual search Percival Lowell Urged construction of a wide-field astronomical camera Camera was completed in 1929 Clyde Tombaugh worked at Lowell Observatory Discovered Planet X on 18 February 1930 Announced discovery on 13 March 1930 Some obvious problems Much more dim & small than expected More highly elliptical orbit than any other planet More steeply inclined orbit than any other planet
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Pluto & Charon Are Unique Objects
Planetary patterns Terrestrial planets H2 & He poor planets with solid surfaces Satellites much smaller than parent planet Jovian planets H2 & He rich planets with no solid surfaces Pluto’s patterns Composition Mixture of ices & rock with a solid surface Satellite Closest in mass & diameter of all Solar System pairs
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The Discovery of Charon
U.S. Naval Observatory James W. Christy 1978 Examined existing photographs of Pluto Noted a bulge on one side Examination of other photos confirmed existence of a moon Fundamental characteristics Orbital period of ~ 6.4 days < 5 % the Earth-Moon distance Mutual synchronous axial rotation One side of Charon always faces Pluto One side of Pluto always faces Charon
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Determining Surface Characteristics
An extremely rare alignment Charon’s line of nodes points directly toward Earth Throughout the years 1985 to 1990 Mutual eclipses of Pluto & Charon Determined most accurate sizes of Pluto & Charon Determined generalized brightness patterns of Pluto & Charon The Hubble Space Telescope Also helpful in determining surface brightnesses
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Kuiper Belt Objects (KBO’s)
Hypothesized Gerard Kuiper 1951 Proposed a source region for some comets Discovered David Jewitt & Jane Luu 1992 Found 1992 QB1 ~ 42 AU from the Sun Spectrally very similar to Pluto & Charon More than 300 KBO’s have been discovered Quaoar discovered June 2002 Quaoar measured September 2002 ~ 1,300 km in diameter & in a nearly circular orbit Implications Pluto & Charon may simply be closest & largest KBO’s Should we still consider Pluto a planet ? ? ?
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Computer-Derived Views of Pluto
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Three Faces of Pluto
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Colors of Pluto (Video)
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Important Concepts Discovery Planetary data Planetary characteristics
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 Blue due to methane absorption Relatively rich in rock & metal Strongly tilted magnetic fields Mixture of ices & rock 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 Pluto’s Charon Relatively large & close to Pluto Kuiper Belt Objects
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