Slide 1Ice Giants: Uranus and NeptuneSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30PYTS/ASTR 206 – Ice Giants: Uranus and Neptune1AnnouncementsCongratulations Dr. Jones!Priyanka will deal with HW5Due ThursdayKevin will deal with HW6PYTS/ASTR 206 – Ice Giants: Uranus and Neptune2PTYS/ASTR 206 – The Golden Age of Planetary ExplorationShane Byrne – [email protected] Giants: Uranus and NeptunePYTS/ASTR 206 – Ice Giants: Uranus and Neptune3In this lecture…In this lecture…Discovering Uranus and NeptuneUnusual rotation of UranusIce giant interiorsCompositionHeat flowMagnetic fieldAtmospheresCompositionEnergy sourcesStorms and cloudsRingsDiscovery and propertiesVoyager II – 1986, 1989PYTS/ASTR 206 – Ice Giants: Uranus and Neptune4Titus-Bode lawMid 1760sA mathematical sequence that seemed to predict the sizes of the planets’ orbitsStart with 0 1 2 4 8 16 32 64 128Multiply by 0.3 0 0.3 0.6 1.2 2.4 4.8 9.6 19.2 38.4Add 0.4 0.4 0.7 1.0 1.6 2.8 5.2 10 19.6 38.8PYTS/ASTR 206 – Ice Giants: Uranus and Neptune5These numbers are similar to the sizes of the planets’ orbits in AUKnown solar system up to 1780 was relatively emptyAsteroids undiscoveredUranus undiscoveredNeptune undiscoveredPluto & Kuiper Belt undiscoveredPlanet Titus-BodePredictionObservationMercury 0.4 0.39Venus 0.7 0.72Earth 1.0 1.0Mars 1.6 1.52- 2.8 -Jupiter 5.2 5.2Saturn 10 9.5- 19.6 -- 38.8 -PYTS/ASTR 206 – Ice Giants: Uranus and Neptune6Uranus had been mistaken for a star by many peopleWilliam Hershel in 1781Had a homemade telescopeUsed high-magnification opticsUranus was a disk, not a pointHe thought he had found a cometHis reward…. £200 / yearPYTS/ASTR 206 – Ice Giants: Uranus and Neptune7Titus Bode law seemed to be working great…Uranus was discovered 1781 and fit right into this schemeCeres was discovered in 1801 and fit right into this schemePlanet Titus-BodePredictionObservationMercury 0.4 0.39Venus 0.7 0.72Earth 1.0 1.0Mars 1.6 1.52Ceres 2.8 2.8Jupiter 5.2 5.2Saturn 10 9.5Uranus 19.6 19.2- 38.8 -Was there more to come?PYTS/ASTR 206 – Ice Giants: Uranus and Neptune8PYTS/ASTR 206 – Ice Giants: Uranus and Neptune9Titus Bode law seemed turned out to be meaninglessCeres wasn’t a planet after allNeptune didn’t follow the rulePlanet Titus-BodePredictionObservationMercury 0.4 0.39Venus 0.7 0.72Earth 1.0 1.0Mars 1.6 1.52Ceres 2.8 2.8Jupiter 5.2 5.2Saturn 10 9.5Uranus 19.6 19.2Neptune 38.8 30.1PYTS/ASTR 206 – Ice Giants: Uranus and Neptune10PYTS/ASTR 206 – Ice Giants: Uranus and Neptune11Uranus NeptuneRadius 25,559 km – 4.0 x Earth 24,764 km – 3.9 x EarthMass (Earth masses) 14.5 17Distance from Sun 19.2 AU 30.1 AUCloud-top Temperature 53 K 55 KRotation period 17 hours 16 hoursNeptune is more massive – but the same size as UranusNeptune is further from the Sun – but the same temperature as UranusPYTS/ASTR 206 – Ice Giants: Uranus and Neptune12Uranus spins on its side…Moons and rings go around the equatorAxis is tilted 98° to the eclipticCompared with Earth’s 23°Perhaps a result of a giant collision early in its historyNorthPYTS/ASTR 206 – Ice Giants: Uranus and Neptune13Seasons on Uranus are very unusualOrbital period is 84 years~42 years of sunlight at each polePYTS/ASTR 206 – Ice Giants: Uranus and Neptune14Very different from Jupiter and SaturnUranus and Neptune are too small to generate the pressure needed for metallic hydrogenOuter layers are mostly hydrogen and HeliumCore is still rocky“Ice” mantle is really a hot liquidInteriors of Uranus and NeptuneInteriors of Uranus and Neptune€ 318 M⊕M95M5.14M2.17PYTS/ASTR 206 – Ice Giants: Uranus and Neptune15Rocky core is about the size of EarthSurrounded by a thick ocean of liquid water and ammoniaSurrounded by a liquid hydrogen, helium and methane atmospherePYTS/ASTR 206 – Ice Giants: Uranus and Neptune16How do you form Uranus and Neptune?On the one hand…High fraction of non- hydrogen/helium material argues for formation closer to the SunForming a planet in the tenuous outer reaches of the solar nebula is very slowBut…They’re lower mass than Jupiter/Saturn so must have formed further outOne possibility is planetary migration…Form Neptune and Uranus in the 5-10 AU range (Jupiter/Saturn range)Then migrate these bodies to the outer solar system before they grow too largeJupiter Saturn Uranus NeptuneDensity (kg m-3) 1380 687 1270 1638Mass (Earth) 318 95 14.5 17Jupiter is this dense only because its intense gravity compresses its interiorPYTS/ASTR 206 – Ice Giants: Uranus and Neptune17Neptune has a high heat flowEmits ~2.6 times what it absorbs from the SunSimilar to Jupiter and SaturnMaybe caused by breakup of methane under pressureCH4 breaks in Carbon and HydrogenThe carbon forms diamonds that sinkThe Hydrogen rises to the surface and escapesNeptune’s core might have a diamond crust… neat!Uranus has a very low heat flowPerhaps heat cannot escapePerhaps much of the primordial heat was lost in a giant impactPYTS/ASTR 206 – Ice Giants: Uranus and Neptune18Dipole fields like other planets but…Magnetic axes differ wildly from rotation axesCenter of magnetic field appears offset from center of planetNeptune and Uranus have unusual magnetic fieldsPYTS/ASTR 206 – Ice Giants: Uranus and Neptune19Fields are unusual becauseThere’s no iron coreThere’s no big envelope of metallic hydrogenFields likely caused by ionized waterConvection probably in a thin shellPYTS/ASTR 206 – Ice Giants: Uranus and Neptune20Outer atmosphere is mostly hydrogen and heliumMuch like Jupiter and Saturn…but all the ammonia froze and sank Methane levels much higherMethane absorbs red lightMakes these planets look blueAtmospheres of Uranus and NeptuneAtmospheres of Uranus and NeptuneUranus NeptuneHydrogen 83% 80%Helium 15% 18.5%Methane 2% 1.5%PYTS/ASTR 206 – Ice Giants: Uranus and Neptune21Uranus and Neptune look rather boring compared to JupiterAt Uranus (19.2 AU) – Solar power is 3.7 Wm-2At Neptune (30.1 AU) – Solar power is 1.5 Wm-2At Jupiter
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