Uranus & Neptune: The Ice Giants. Discovery of Uranus. Bode s Law. Discovery of Neptune

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1 Uranus & Neptune: The Ice Giants Discovery of Uranus Discovery of Uranus & Neptune Properties Density & Composition Internal Heat Source Magnetic fields Rings Uranus Rotational Axis by William Herschel (1781) with a 7-inch telescope, while mapping the sky a curious either nebulous star or perhaps a comet Subsequent observations were used to derive a distance from the Sun of 19 AU, which is consistent with Bode s Law 40-foot telescope Discovery of Neptune Bode s Law The motion of Uranus could not be accounted for by the gravitational influence of the known planets Position of Neptune was determined mathematically by Adams & Leverrier Neptune s distance from the Sun (30 AU) is not consistent with Bode s Law

2 Properties of Uranus Note: bluish color is due to methane Composition: 84% hydrogen, 14% helium, 2% methane (CH 4 ) + others Average Distance from the Sun: 2.7 light hours = 19.2 AU Rotation Rate 17 h Magnetic Field 16 h Clouds Orbital Period: 84 Earth years Axis Tilt: 98 (!!!) Number of Moons: ~ 27 Properties of Neptune Note: Bluish color due to methane Composition: ~84% (?) hydrogen, ~14% (?) helium, 2% methane + others Average Distance from the Sun: 4.2 light hours = 30.1 AU Rotation Rate 16.8 h Equatorial Clouds Orbital Period: 165 Earth years Axis Tilt: 27 Number of Moons: ~ 13 Storms on Neptune Neptune - High Elevation Ice Clouds Great dark spot Clouds: Ice crystals (cirrus clouds) of Methane (??) Banding presently less prominent on Uranus due to axis tilt - pole continuously heated, producing extended haze

3 Density & Composition Jovian Planetary Interiors Densities are high ~ (Uranus)1.2 & (Neptune)1.7 g cm -3 Composition cannot be same as Jupiter 1.3 & Saturn 0.7 g cm -3 - large core of rock & ice - thin outer envelopes of liquid & gaseous hydrogen - no metallic hydrogen (too small) Ice-rich planetesimals make up 3% and 10% of Jupiter s and Saturn s mass, respectively. They make up 2/3 of Uranus and Neptune s mass. Cores are a mixture of ice, rock and metal Appended Formation Ice & rock core built - Mass = 10 Earth Masses Hydrogen & helium from surrounding solar nebulae was gravitationally captured - i.e, planets that are massive & cold enough can retain hydrogen & helium Solid grains in surrounding nebula form satellites Uranus & Neptune are less massive because - particles more spread out in outer solar nebulae - and thus accretion was slower. Jupiter has a density similar to Uranus because - Very massive planets compress their interiors to high densities Uranus - No. Neptune Internal Heat Sources - Yes. - Why? Primordial heat of contraction of ice & rock?? - Consequence: suppression of seasons due to internal heat relative to heat received by the Sun. Remember that Neptune has an axis tilt similar to the Earth

4 Magnetic Fields Magnetic poles are not aligned with rotational poles & are off-axis Cause: - pressure ionized ice surrounding rocky core(??) - Oceans of hydrogen(??) Jovian Magnetic Fields Source of particles in Magnetic Fields: Jupiter: solar wind & volcanically active Io. Saturn: solar wind Uranus: primarily protons & electrons from hydrogen escaping the planet s atmosphere Neptune: protons & electrons from hydrogen escaping the planet s atmosphere & nitrogen from Triton s atmosphere Source of Magnetic Field Jupiter & Saturn: metallic hydrogen (motion of free electrons Uranus & Neptune: pressure-ionized ice / oceans of hydrogen Rings of Uranus Determining structure of Ring occultation events

5 Occultations Uranus - the Video Video HST visible & near-infrared light Rings wobble Cause(s): Bulge of Uranus disk + Gravitational tug of moons Rings: Backlit view & Shepherd Moons Rings of Neptune: small particles Rings were seen from Earth to be incomplete rings Voyager 2 detected dust sheets between rings of both Neptune and Uranus

6 Tilt of Uranus Rotational Axis Air Circulation of Uranus Sunlight fast polar winds slow equatorial winds Follows rotation, not Hadley cells, similar to Neptune Wind speeds highest near poles, not equator. If poles continuously heated, the winds should increase towards the equator. Neptune shows similar rotational structure. Temperature nearly the same at both poles Seasons last for ~ 20 years Cause of Tilt: glancing collision with planet? We have no clue why any of this is observed