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Jovian Planets Fall, 205 Astronomy 110 1 Jovian Planets • • • • Bigger & more massive Lower density, different composition Rings Numerous moons Fall, 205 Astronomy 110 2 Jovian Planet Formation • Formed beyond the frost line, planetesimals could accumulate ice. • Hydrogen compounds are more abundant than rock/metal. • Jovian planets got bigger and acquired H/He atmospheres. • The jovian cores are very similar: ~10x Earth masses • Differences are in the amount of H/He gas accumulated. Fall, 205 Astronomy 110 3 Jovian Planet Formation: How Differences Arise • TIMING: the planet that forms earliest captures the most hydrogen & helium gas. Capture ceases after the first solar wind blew the leftover gas away. • LOCATION: the planet that forms in a denser part of the nebula forms its core first. Fall, 205 Astronomy 110 4 Jovian Planets - Composition Fall, 205 2 Density (g/cc) 1.5 1 0.5 0 Ju pi te Sa r tu U rn ra N nu ep s tu ne • Jupiter & Saturn: almost all H & He, very little metal & rock (less dense) • Uranus & Neptune: <50% H & He, the rest hydrogen compounds (water, methane, ammonia), with some metal & rock (more dense) Astronomy 110 5 More massive planets could even be smaller! • Jupiter and Saturn are nearly the same size • But Jupiter is 3x more massive than Saturn Fall, 205 Astronomy 110 6 Jovian planets: Internal Structure • No solid surface. • Layers under high pressure and temperatures. • Cores (~10 Earth masses) made of hydrogen compounds, metals & rock • The layers are different for the different planets. Fall, 205 Astronomy 110 7 Layers Differ in Phase Density of water is ~1g/cm3 Metallic hydrogen conducts electricity; it is not solid. Core is hydrogen compounds, metals, rocks. But 10 x the mass of Earth inside a volume the size of Eath. Jupiter Fall, 205 Astronomy 110 8 Why different? • Less mass → less gravity → less compression. • Boundaries of the layers are deeper in less massive jovian planets. • The physical states of the cores of the less massive jovian planets are less extreme - could be liquid. Fall, 205 Astronomy 110 9 Magnetic Fields • Jupiter has a powerful magnetic field generated by its rotating, convecting layer of metallic hydrogen. Fall, 205 Astronomy 110 10 The Jovian Planets: Appearance Colorful surface features: Bands and Clouds of differing compositions Complex Weather Systems High Wind speeds Storms, some long-lived Fall, 205 Astronomy 110 11 Planet colors Jupiter’s colors • Ammonium sulfide clouds reflect red/brown. • Ammonia, the highest coldest layer, reflects white. Fall, 205 Astronomy 110 12 Planet colors Saturn’s Colors Saturn’s layers are the same, but deeper in and farther from the Sun --- more subdued. Fall, 205 Astronomy 110 13 Uranus and Neptune’s upper layers are colder still, allowing methane to condense. Methane gas absorbs red light and transmits blue light reflected by clouds Fall, 205 Astronomy 110 14 Jupiter winds and storms • Earth’s rotation makes storms ‘spin’. • Jupiter’s fast rotation stretches storms in to bands that surround the planet. • High east/west winds (up to 400 km/hr) Fall, 205 Astronomy 110 15 The Great Red Spot • twice as wide as the Earth • Has existed for at least 3 centuries Fall, 205 Astronomy 110 16 Storms Other Jovian Planets • Saturn: even faster winds • Neptune: can see storms, but not as long lived. • Uranus: dull when Voyager 2 flew by, but HST captured major storms. – Extreme tilt means that Uranus’ southern hemisphere may just now be getting sunshine for the first time in decades. Fall, 205 Astronomy 110 17 Jovian Planets: Satellites • More than 100 Jovian moons and counting • 60+ moons of Jupiter alone. • Medium and large moons mostly formed at the same time as their planets. • Small moons are mostly captured asteroids and comets. Fall, 205 Astronomy 110 18 Medium & large moons • Enough self-gravity to be spherical • Are or were geologically active. • Have substantial amounts of ice. • Formed in orbit around jovian planets. • Circular, equatorial orbits in same direction as planet rotation Fall, 205 Astronomy 110 19 Small moons • Far more numerous than the medium and large moons. • Not enough gravity to be spherical: “potato-shaped” • Captured asteroids, so orbits do not follow patterns. • Orbits can be tilted, elliptical, and even backwards! Fall, 205 Astronomy 110 20 Jupiter’s Galilean Satellites Io, Europa, Ganymede, Callisto Io has volcanoes. Europa may have an ocean under its ice. Ganymede & Callisto may also have sub-surface oceans Fall, 205 Astronomy 110 21 Io’s Volcanoes Io is the most volcanically active world in the solar system. Fall, 205 Astronomy 110 22 Tidal Heating Io is compressed and stretched as it orbits Jupiter Fall, 205 Astronomy 110 23 Europa’s Ocean: Waterworld? Fall, 205 Astronomy 110 24 Tidal stresses crack Europa’s surface ice. Fall, 205 Astronomy 110 25 Europa’s interior also warmed by tidal heating Fall, 205 Astronomy 110 26 Ganymede • Largest moon in the solar system • Clear evidence of geological activity • Tidal heating expected - but is it enough? Fall, 205 Astronomy 110 27 Callisto • “Classic” cratered iceball. • No tidal heating no orbital resonances. • But it has magnetic field Fall, 205 Astronomy 110 28 Saturn: Titan Titan is the only moon with a thick atmosphere. Titan may also have surface lakes of methane and ethane. Fall, 205 Astronomy 110 29 Titan’s Atmosphere • 90% nitrogen, the rest argon, methane, ethane, other hydrogen compounds. • Methane & ethane are greenhouse gases. • Still cold: 93 K (-180 degrees C) • Chemical reactions on Titan could produce organic chemicals. • Titan could have liquid methane and ethane lakes. • The Huygens probe from Cassini landed on Titan in January. Fall, 205 Astronomy 110 30 Saturn’s Medium-Sized Moons Fall, 205 Astronomy 110 31 Uranus’ Miranda shows huge cliffs & tectonic activity and few craters. Fall, 205 Astronomy 110 32 Neptune’s Moon Triton • Probably a captured Kuiper belt object: orbiting Neptune opposite Neptune’s direction of rotation. • Smaller than Earth’s Moon, yet has recent geological activity. Fall, 205 Astronomy 110 33 Jovian Planets: Rings Fall, 205 Astronomy 110 34 Saturn’s Rings • Made up of numerous, tiny individual particles of ice and dust • Orbit over Saturn’s equator • Many particle collisions cause rings to be very thin (tens of meters!) • Gap moons and orbital resonances create the effect of rings and gaps. Fall, 205 Astronomy 110 35 Earth-based view Fall, 205 Astronomy 110 36 Spacecraft view Fall, 205 Astronomy 110 37 Jovian Planets: Rings • Formed from dust created in impacts on orbiting moons. • Not left over from planet formation-- the particles are too small to have survived this long. • Tiny particles are constantly ejected and must be continuously replaced. Fall, 205 Astronomy 110 38 Implications • Jovian planets all have rings because they possess many small moons close-in • Impacts on these moons are random • Saturn’s incredible rings may be an ‘accident’ of our time Fall, 205 Astronomy 110 39 Mass Extinctions • Large dips in total species diversity in the fossil record. • The most recent was 65 million years ago, ending the reign of the dinosaurs. Was it caused by an impact? How would it have happened? Fall, 205 Astronomy 110 40 No dinosaur fossils in these rock layers Thin layer containing iridium from impactor Dinosaur fossils in lower rock layers Fall, 205 Astronomy 110 41 Iridium - evidence of an impact • Iridium is very rare in Earth surface rocks but often found in meteorites. • Luis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 million years ago. Fall, 205 Astronomy 110 42 Comet or asteroid about 10km in diameter approaches Earth Fall, 205 Astronomy 110 43 Fall, 205 Astronomy 110 44 Fall, 205 Astronomy 110 45 Fall, 205 Astronomy 110 46 Fall, 205 Astronomy 110 47 An iridium-rich sediment layer and an impact crater on the Mexican coast show that a large impact occurred at the time the dinosaurs died out, 65 million years ago. Fall, 205 Astronomy 110 48 Facts • Asteroids and comets have hit the Earth. • A major impact is only a matter of time: not IF but WHEN. • Major impact are very rare. • Extinction level events ~ millions of years. • Major damage ~ tens-hundreds of years. Fall, 205 Astronomy 110 49 Tunguska, Siberia: June 30, 1908 The ~40 meter object disintegrated and exploded in the atmosphere Fall, 205 Astronomy 110 50 Meteor Crater, Arizona: 50,000 years ago (50 meter object) Fall, 205 Astronomy 110 51 Impacts will certainly occur in the future, and while the chance of a major impact in our lifetimes is small, the effects could be devastating. Fall, 205 Astronomy 110 52 Hydrostatic Equilibrium: Gravity is Balanced by Pressure Fall, 205 Astronomy 110 53 Hydrostatic Equilibrium: Gravity is Balanced by Pressure Applications: •Gravity from planet • Jovian Planets Gravity External • Planetary Atmospheres •Gravity from Core + Atm •All “atmosphere” Fall, 205 Self Gravitating • Stars Astronomy 110 54