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Astronomy 110 Announcements: • Midterm on Monday – mostly conceptual short answer questions, some MC, no calculators allowed Chapter 8 Jovian Planet Systems • Observing tonight if weather is OK. Meeting at Watanabe at 7:30pm, then at Kapiolani Park by 8pm. Check your e-mail around 7pm to check for possible cancellation. Rain date is Friday—same place and time. Call me if you’re lost. Jovian Planets A Different Kind of Planet • • • • Bigger & more massive Lower density, different composition Rings Numerous moons What are jovian planets made of? • 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) Why are they different? Jovian planet formation • Beyond the frost line, planetesimals could accumulate ICE. • Hydrogen compounds are more abundant than rock/metal so jovian planets got bigger and acquired H/He atmospheres. • The jovian differences are in the amount of H/He gas accumulated. Differences in Jovian planet formation • 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. Why did that amount differ? 2 1.5 1 0.5 pi t S a er tu U rn ra N nu ep s tu ne 0 Ju Density (g/cc) Density Differences Uranus and Neptune are denser than Saturn because they have less H/He, proportionately But that explanation doesn’t work for Jupiter…. Jupiter’s “Metallic” Hydrogen • Jupiter and Saturn are nearly the same size • But Jupiter is 3x more massive than Saturn – Mass yields large pressure, which compresses Jupiter to the point where H atoms are touching (electron clouds are overlapping! degenerate) – This results in metallic H – electrons are free to move from atom to atom – Increasing the mass of Jupiter further would not increase its size because the atoms just continue to get pressed together. What are jovian planets like on the inside? Layers Differ in Phase Notes: [Density of liquid water is ~ 1g/cm3.] • 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. WHY? Metallic hydrogen conducts electricity; it is not solid. Core is hydrogen compounds, metals, rocks. But not in a form you’d recognize… 10 x the mass of Earth inside a volume the size of Earth. Jupiter Why different? Magnetic Fields • Jupiter has a powerful magnetic field generated by its rotating, convecting layer of metallic hydrogen. 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 jovians are less extreme (could be liquid). What is the weather like on jovian planets? Colorful surface features reveal: – Clouds of different compositions – Wind speeds – Storms, some long-lived (Great Red Spot on Jupiter) Planet colors Jupiter’s colors • Ammonium sulfide clouds reflect red/brown. • Ammonia, the highest coldest layer, reflects white. Planet colors Saturn’s Colors Saturn’s layers are the same, but deeper in and farther from the Sun --- more subdued. 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 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) The Great Red Spot • twice as wide as the Earth • Has existed for at least 3 centuries What have we learned? • What are jovian planets made of? • Jupiter and Saturn – mostly H, He • Uranus and Neptune – H compounds mixed w/ metal and rock. • Originated from ice-rich planetesimals of about the same size, but captured different amounts of hydrogen and helium gas from the solar nebula. What have we learned? • What are jovian planets like on the inside? • All have a core about 10 times as massive as Earth, consisting of hydrogen compounds, metals, and rock. They differ mainly in their surrounding layers of hydrogen and helium. What have we learned? • What is the weather like on jovian planets? • The jovian planets all have multiple cloud layers that help determine the colors of the planets, fast winds, and large storms. Some storms, such as the Great Red Spot, can apparently rage for centuries or longer. Many moons… • More than 100 jovian moons and counting… • 60+ moons of Jupiter alone… What kind of moons orbit the jovian planets? • Medium and large moons mostly formed at the same time as their planets. • Small moons are mostly captured asteroids and comets. Medium & large moons • Enough self-gravity to be spherical • Are or were geologically active. • Have substantial amounts of ice. Medium & large moons • Formed in orbit around jovian planets. • Circular, equatorial orbits in same direction as planet rotation. Small moons • Captured asteroids, so orbits do not follow patterns. • Orbits can be tilted, elliptical, and even backwards! Small moons • Far more numerous than the medium and large moons. • Not enough gravity to be spherical: “potatoshaped” What makes Jupiter’s Galilean moons unusual? Io, Europa, Ganymede, Callisto What makes Jupiter’s Galilean moons unusual? Io’s Volcanoes • Io has volcanoes. • Europa may have an ocean under its ice. • Ganymede & Callisto may also have subsurface oceans. …but they’re so small??? Shouldn’t they be cold & dead? Orbital Resonances Tidal Heating Io is squished and stretched as it orbits Jupiter Io is the most volcanically active world in the solar system. HOW? But why is its orbit so elliptical? Every 7 days, these 3 moons line up. The tugs add up over time, making all 3 orbits elliptical. Europa’s Ocean: Waterworld? Tidal stresses crack Europa’s surface ice. Europa’s interior also warmed by tidal heating What makes Titan different from other moons? • Titan is the only moon with a thick atmosphere (90% N2). • Titan may also have surface lakes of methane and ethane. • Methane and ethane are greenhouse gases, but it’s still cold (93 K). • possible life? 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. 8.3 Jovian Planet Rings Our Goals for Learning • What are Saturn’s rings like? • Why do the jovian planets have rings? Why are small icy moons more geologically active than small rocky planets? • Hot interiors are necessary for geological activity. • Ice deforms more easily than solid rock, so less internal heat is required, and smaller objects can be geologically active • Tidal heating is not important for rocky planets. What are Saturn’s rings like? • They are made up of numerous, tiny individual particles • They orbit over Saturn’s equator • They are very thin Earth-based view Spacecraft view Artist’s interpretation of a close-up view Saturn’s rings • Many particles of ice and dust orbit Saturn. • Many particle collisions cause the rings to be very thin (tens of meters!) • Gap moons and orbital resonances create the effect of rings and gaps. Why do the jovian planets have rings? • They formed from dust created in impacts on moons orbiting those planets How do we know that? 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 How do we know? • Rings aren’t leftover from planet formation-the particles are too small to have survived this long. • There must be a continuous replacement of tiny particles. • The most likely source is impacts with the jovian moons.