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Transcript
Key Concepts: Lecture 15: Jovian Planets (Gas Giants) Space Missions to Jovian Planets [You do not need to remember the details of these missions] Background, Discovery, Basic properties: rotation, orbits, seasons Internal Structure Atmosphere: clouds, winds, storms Magnetic field Voyager What are the Jovian Planets? • 4 Giant Planets in the outer solar system, beyond Mars, –Jupiter –Saturn –Uranus –Neptune Known to the ancients Discovered relatively recently with telescopes and Newton’s Laws Characteristics of Jovian Planets • Much larger and more massive than terrestrial planets • They are not solid gaseous • All have rings • All have many satellites Characteristics of Jovian Planets Questions ecliptic plane • Which of the above planets experience seasons and why? • What would the seasons be like? Rotation & Orbits Planet Rotation Period Rotate Orbital Period Period (hours) (years) Jupiter 9.9 11.9 5.2 AU Saturn 10.7 29.5 9.5 AU Uranus 17.2 84.1 19.2 AU Neptune 16.1 164.8 30.1 AU Fast rotation (i.e. quite short days) Orbital periods obey Kepler’s 3rd Law: P2=a3 • Jupiter –axis of rotation tilted by 3o - no seasons • Saturn –axis tilted by 27o - has seasons • Neptune –axis tilted by 29o - has seasons –seasonal changes slower than Saturn’s • Uranus –axis tilted by 98o - strange seasons S E A S O N S Seasons on Uranus – poles see sun for 42 years then darkness for next 42 years • What caused this extreme tilt? –Perhaps a collision with a large body during formation Composition & Structure of Jovians • Mostly Hydrogen & Helium • They have low densities • At most they have only small rocky cores • At high pressures, Hydrogen acts like a metal: conducts electricity • H and He are light and can escape more easily than heavier atoms • The gas giants are massive so they have high escape velocities • They are far from the Sun so they are cold and the atoms in their atmosphere move quite slowly gas particle at a given temperature. Northern winter solstice Southern summer solstice Escape speed from planet or speed of Why do they have H and He? • Orbit takes 84 years • Amount of sunlight varies dramatically & depends on location Internal Heat Sources • Primordial Heat from gravitational contraction associated with formation – Larger planets are heated more strongly during formation by collapse of material and rapid initial differentiation. Also it is more difficult for this heat to escape from a larger planet: takes longer to cool down. • Continued generation of heat by gradual differentiation: e.g. condensation of He in Saturn • Radioactive decay relatively unimportant because heavy elements are smaller fraction of total mass Effect of internal heat: raises the temperature of interior & atmosphere to higher values than expected from only Sun’s heating Jupiter Saturn Uranus/Neptune Clouds & Atmosphere Atmospheres • Atmosphere mostly Hydrogen & Helium • Clouds give Jovians their appearance – Jupiter & Saturn : • upper decks of frozen ammonia (NH3) • lower decks - ammonium hydrosulfide (NH4SH) - dark yellows & browns Jupiter – Uranus & Neptune: • methane - pale blue appearance • Uranus is featureless - stable atmosphere Galileo probe Winds & Circulation • Features observed are due to circulation of clouds & atmosphere • Jovian planets are fast rotators – horizontal circulation patterns parallel to equator • No solid surface – less friction for circulation patterns to lose energy – longer lived storms – can have different rotation rates at pole vs. equator • Internal heat source causes vertical convection currents Winds & Circulation • Alternating light & dark bands parallel to equator stretch around planet • Semi-permanent features –shift in intensity and position from year to year • East-west wind patterns –do not appear to change at all, even over decades Storms • Storms - local disturbances in the regular atmospheric circulation patterns • Seen on Jupiter, Neptune, Saturn –Great Red Spot - Jupiter - long lived (>=300 years) –Great Dark Spot - Neptune probably shorter lifetime –Seasonal storms on Saturn The Great Red Spot on Jupiter • A giant cyclone that has been going for centuries • ~30,000 km long • Changes in size over time The Great Dark Spot on Neptune • Dark Spot seen in Voyager images 1989 – similar to Jupiter Great Red Spot • same shape • same latitude – 10,000 km long • Dark Spot not seen in Hubble Space Telescope images 1994 – faded or disappeared? – storms may form & dissipate more quickly on Neptune Uranus/Neptune have strange magnetic field orientation Storms on Saturn • Large storms are rarer • Connected with seasons • ~ every 30 years outbreak of spots are seen in the equatorial regions • Storms resemble turbulent cloud structures • Very large - 10 times size of Earth Hubble Space Telescope image - 1990 Magnetic Fields • Jovians have strong magnetic fields • Magnetic fields generated in the same manner as on Earth –metallic liquid regions plus rotation of planet causes dynamo –Jupiter & Saturn may have liquid metallic hydrogen cores –Uranus & Neptune may have liquid metallic hydrogen mantles