1 Lecture 8: Uranus and Neptune • 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 dark- ness – 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 1 – 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 plan- ets: 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 concentra- tion 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 2 – pressure outside the core too low to form metallic hydrogen – origin of the mag- netic 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 3 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 concentra- tion of methane, more blueish the colour 4 – 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 mag- netic 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 (al- though 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 5 – 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.