Download 1 Lecture 8: Uranus and Neptune

Lecture 8: Uranus and Neptune
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• Read chapter 13 in the textbook
• Exercises: Do all “Review and Discussion” and all “Conceptual Self-Test”)
1.1
Uranus
• Preliminaries
– Uranus: gas giants (jovian planet) like Jupiter and Saturn
– discovered by British William Herschel in 1781, first discovery of a planet in over
2000 years
– apparent magnitude is at the edge of the naked eye’s ability to see, if you know
exactly where it is – discovered through optical telescope
– orbital semimajor axis: 19.19 AU, mass: 14.54 earth masses (8.68 × 1025 kg,
radius: 4.01 earth radius (25, 559 km), mean surface temperature 58 K
– Voyager 2 probe flew by Uranus in 1986 giving the first close up pictures of the
planet.
• Physical properties:
– nearly 20 AU from the sun – twice as far as Saturn
– orbital period 83.75 earth years
– gas giant, but much smaller than Jupiter and Saturn
– SHOW RELATIVE SIZE PICTURE
– like other jovian planets, Uranus has a short rotation period, 17.2hours
– like all planets, lies close to the ecliptic plane inclined at 0.77◦
– unlike any other planet, the axis tilt is 98◦ – Uranus is nearly on its side
– during the Uranian solstices, the northern (southern) hemisphere points toward
the sun, leaving almost the entire southern (southern) hemisphere in total darkness
– SHOW ORBIT DIAGRAM
– not understood why Uranus axis tilt is so large (perhaps the result of collisions
during the formation of the solar system)
– Uranus appears a blue-green colour – colour of the upper atmosphere
– the atmosphere rotates differentially, like Jupiter and Saturn, (but rotates faster
at the poles)
• The atmosphere of Uranus
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– spectroscopic studies of reflected sunlight
– outer atmosphere similar composition to Jupiter and Saturn
– H2 (84%), He (14%), CH4 (2%), almost no ammonia NH4
– abundances of ammonia and methane vary systematically across the jovian planets: Jupiter has much more gaseous ammonia than methane, but as we move out
the amount of ammonia decreases relative to methane
– temperature is the reason: ammonia freezes into crystals at 70 K, since the upper
atmosphere of Uranus is 58 K, ammonia does not exist as a gas
– methane is a good absorber at longer wavelengths (red light) – higher concentration of methane, more blueish the colour
– blue-green colour of Uranus results from methane in the atmosphere
– Unlike Jupiter, Uranus lacks an internal heat source: clouds at low-lying, warmer
altitudes
– low amount of high level clouds means that weather patterns cannot readily be
seen as they are blocked out how high atmospheric haze
– high winds 200 km/h to 500 km/h do form bands like Jupiter, but are buried
deeper in the atmosphere
– Hubble Space Telescope discovered small dark spot indicating a storm
– SHOW PICTURE OF SPOT
– Uranian atmosphere is efficient at transporting energy around the planet – winter
and summer sides differ by only a few K
– wind speeds near the poles higher than at the equator probably due to higher
amount of sunlight these regions receive
• Magnetosphere and internal structure
– Uranus has a fairly strong magnetic field, about 100 times stronger than the
earth’s
– creates a substantial magnetosphere for Uranus
– Uranian magnetic field inclined 60◦ relative to the rotation axis and is not centered
– SHOW MAGNETIC FIELD PICTURE
– misalignment with rotation different from Jupiter and Saturn – suggests different
physics is responsible
– theoretical models suggest that Uranus has a rocky core similar to Jupiter and
Saturn (about 10 earth masses)
– rocky core of Uranus makes up a large fraction of the planet as compared to
Jupiter and Saturn
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– pressure outside the core too low to form metallic hydrogen – origin of the magnetic field is therefore probably very different than Jupiter and Saturn
– possible “slushy” outer core with ammonia dissolved in water creating a thick
electrically conducting layer – circulating currents far from Uranus’s centre and
rotation axis
– SHOW CORE PICTURES
• Moon and ring system of Uranus
– As of 2007: 27 moons – most tiny, less than 25 km across
– five major moons: Titania, Oberon, Ariel, Umbriel, and Miranda
– all orbit close to the equatorial plane of the Uranus – almost perpendicular to the
ecliptic plane
– Uranian moons similar to the midsized moons of Saturn – densities 1.1 g/cm to
1.7 gm/cm
– The five moons range from 1600 km in diameter to the smallest, Miranda at 480
km
– SHOW MOON COMPARISON PHOTO
– Uranus has no moons as large as the Galilean satellites or Saturn’s moon Titan
– the outermost of these five moons, Titania and Oberon – heavily cratered, no
geologic activity
– radiation darkening (chemical reaction from high energy particles) thought to
darken the surface of Uranus’s moons – less reflective than Saturn’s midsized
moons
– Ariel, close to Uranus – tidal stress, some geological activity
– Miranda – very odd moon, many surface feature inconsistent with quiet geology:
perhaps the result of catastrophic disruption, impacts partially breaking the moon
up and with reformation
– SHOW MIRANDA PHOTO
– in 1977, discovery of rings around Uranus from star occultion
– SHOW OCCULTATION PICTURE
– rings very different from Saturn and Jupiter: rings dark and narrow with wide
space between them
– ring system, inside the Roche limit
– density of Uranus’s rings similar to the A and B rings of Saturn
– rings are much less reflective than Saturn’s
– rings made up of particles all about 1cm in size
– narrow rings of Uranus require shepherd moons (like Saturn’s F ring), Cordelia
and Ophelia are two shepherd moons
– SHOW SHEPHERD MOON DIAGRAM
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1.2
Neptune
• Preliminaries
– Neptune: gas giants (jovian planet) like Jupiter and Saturn – near twin of Uranus
– discovered by theoretical predictions: British physicist John Couch Adams, French
physicist Urbain Le Verrier – discovered in with a telescope by German astronomer
Johann Galle
– apparent magnitude is too dim for the naked eye
– orbital semimajor axis: 30.07 AU, mass: 17.15 earth masses (1.02 × 1026 kg,
radius: 4.01 earth radius (25, 559 km), mean surface temperature 59 K (internal
heat source
– Voyager 2 probe flew by Uranus in 1989 giving the first close up pictures of the
planet.
• Physical properties:
– about 30 AU from the sun – three times as far as Saturn
– orbital period 163.7 earth years
– gas giant, but much smaller than Jupiter and Saturn
– SHOW RELATIVE SIZE PICTURE
– like other jovian planets, Neptune has a short rotation period, 16.1hours
– like all planets, lies close to the ecliptic plane inclined at 0.77◦
– the axis tilt similar to earth 29.6◦
– Neptune appears a blue in colour – colour of the upper atmosphere
– the atmosphere rotates differentially, like the other gas giants
– Neptune has an internal heat source that causes Neptune to radiate more than 2
times as much energy as it receives from the sun
• The atmosphere of Neptune
– spectroscopic studies of reflected sunlight
– outer atmosphere similar composition to Jupiter and Saturn
– H2 (83%), He (14%), CH4 (3%), almost no ammonia NH4
– as we move out of the solar system, the amount of ammonia decreases relative to
methane
– temperature is the reason: ammonia freezes into crystals at 70 K, since the upper
atmosphere of Uranus is 58 K, ammonia does not exist as a gas
– methane is a good absorber at longer wavelengths (red light) – higher concentration of methane, more blueish the colour
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– blue-green colour of Uranus results from methane in the atmosphere
– Neptune’s internal heat source gives Neptune pronounced weather patterns
– high level clouds observed, has several storm systems clearly visible from space
– Great Dark Spot observed by Voyager 2 – disappeared in recent years
– SHOW DARK SPOT AND RECENT PHOTOS
– wind speeds in excess of 1,500 km/h – almost half the speed of sound in Neptune’s
upper atmosphere
– stormy atmosphere arises in part from the internal heat source
• Magnetosphere and internal structure
– Like Uranus, Neptune has a fairly strong magnetic field, about 100 times stronger
than the earth’s
– creates a substantial magnetosphere
– Neptune’s magnetic field inclined 40◦ relative to the rotation axis and is not
centered
– SHOW MAGNETIC FIELD PICTURE
– misalignment with rotation different from Jupiter and Saturn – again suggests
different physics is responsible
– theoretical models suggest that Neptune has a rocky core similar to Jupiter and
Saturn (about 10 earth masses)
– rocky core of Uranus makes up a large fraction of the planet as compared to
Jupiter and Saturn
– pressure outside the core too low to form metallic hydrogen – origin of the magnetic field, like Uranus, is therefore probably very different than Jupiter and
Saturn
– possible “slushy” outer core with ammonia dissolved in water creating a thick
electrically conducting layer – circulating currents far from Neptunes’s centre and
rotation axis
– SHOW CORE PICTURES AGAIN
• Moon and ring system of Neptune
– as of 2007: 13 moons
– Neptune has only one large moon, Triton and one midsized moon, Proteus (although Nereid is not much smaller)
– unlike other jovians, no regular moon system – orbits are inclined to the equator
and Triton has a retrograde orbit
– suggests that some moons and particularly Triton are captured Kuiper objects
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– information on Triton mostly limited to the Voyager 2 probe
– Triton shows a lacked mark of cratering
– SHOW TRITON PICTURES
– Voyager 2 observed geysers of nitrogen gas erupting
– Triton has a thin atmosphere consiting mostly of nitrogen
– Triton covered in nitrogen frost – solar heating causes eruptions
– Triton has evidence of water ice lakes – the water erupts as lava and then solidifies
– evidence of tidal stress geologic activity in the past as the result of Neptune’s
gravity making the orbit more circular
– retrograde orbit with tidal interactions will cause Triton to gradually move inward
– it will be doomed once it approaches the Roche limit, Neptune will have end
up with a ring system like Saturn!
– Neptune’s rings conclusively discovered by Voyager 2
– Five dark rings – narrow like Uranus
– Again, undiscovered moonlets and the inner satellites of Neptune play a role in
confining the narrow rings.
– The rings are inside the Roche limit.
– Rings composed mostly of small dust particles – low reflectivity
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