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This set of slides. • This material covers an overview of our solar system, some comparative planetology, the Jovian planets (Jupiter, Saturn, Uranus and Neptune), planetary magnetic fields. • Units covered: 32, 42, 43, 44 Components of the Solar System • The vast majority of the Solar System’s mass resides in the Sun. – All the planets, asteroids and comets make up less than 1/700 of the mass of the Solar System! • • • • The rocky inner planets (Mercury, Venus, Earth and Mars) are called the terrestrial planets. The gaseous outer planets (Jupiter, Saturn, Uranus and Neptune) are the Jovian planets. An asteroid belt lies between the inner and outer planets. The outermost icy planet, Pluto, is in a class called Trans-Neptunian Objects (TNO). It’s a dwarf planet. The Kuiper Belt • Outside the orbit of Neptune lies the Kuiper Belt. – Located about 40 AU from the Sun. – Home of TNO’s – Many objects smaller and larger than Pluto have been found here. • So is Pluto a planet or not? How to Be a Planet • Once upon a time, be a wanderer in the night sky. • Since 2006, – Be massive enough that your own gravity pulls you into a spheroid shape. – Be the dominant mass in your orbital neighborhood. • Pluto makes the cut in the first category but not the second. • Meet the first criterion, you can be a dwarf planet. The Oort Cloud • The Solar System is surrounded by a cloud of cometary bodies. – Located about 50,000 AU from the Sun. – Gravitational influences from passing stars occasionally send comets into the Solar System. Rotation and Revolution in the Solar System • Because of the conservation of angular momentum, all planets revolve around the Sun in the same direction and in more or less the same plane. – Mercury’s orbit is tipped by 7 degrees. – Pluto’s is tipped by 17 degrees. • Most of the planets rotate in the same direction. – Counterclockwise as viewed from above. – Venus rotates clockwise as viewed from above. – Uranus and Pluto’s rotational axes are tipped significantly. • Any model of solar system formation must explain all of these oddities. Composition of the Solar System Objects • Spectrum analysis shows us the Sun is 71% hydrogen, 27% helium, 2% everything else. • Jovian planets have similar composition. Much in ice, frozen methane, ammonia, and water. • Inner planets are rocky, silicon oxide, aluminum, etc. • Spectroscopy tells us surface composition. We need other info to determine below the surface structure. Calculating a Planet’s Density • Calculate the planet’s mass (M) by observing its satellite’s orbital distance (d) and period (P). • Use Newton’s modified form of • If we know the distance to the planet, we can measure its angular diameter and calculate its linear diameter (or radius, R), and then its volume: 4 V R 3 3 Kepler’s 3rd Law: 4d M GP 2 3 • The planet’s average density, , is then: M V Average Density tells us a lot • Inner planets have high average densities (~5 kg/liter) – Small bodies – Mostly rock and iron • Outer planets have lower densities (~1 kg/liter) Again, any model of solar system formation must explain all of this! – Larger bodies – Gasses, ices and other volatiles The Role of Mass and Radius • Mass and size of a planet help determine its environment. – Small planets cool quickly, leading to dead worlds with little activity. – Small planets also have trouble holding an atmosphere. (low gravity) – Larger planets hold on to their heat, and have active interiors and surfaces. – Mars is right in the middle, not too large, and not too small. • Once had water and an active surface. • Now is cold and dead. The Role of Water and Biological Processes • The presence or absence of water helps determine the nature of the atmosphere. – Water acts as a sink for carbon dioxide, removing it from the atmosphere. – Water helps lock CO2 into rocks as well. – Too much CO2 can lead to a runaway greenhouse effect (as with Venus). – Too little CO2 can lead to cooling (as on Mars). • Biological activity impacts the environment, too. – Animals remove oxygen from the atmosphere (and get carbon from plants), and release CO2 (and methane.) – Plants remove CO2 from the atmosphere, and with sunlight and water, converts it into our food, and release oxygen. – Burning (wood, fossil fuels) releases CO2 into the air. The Role of Sunlight • A planet’s distance from the Sun determines how much sunlight it receives. – Venus receives ¼ of the energy per square meter that Mercury does. – Planets in eccentric orbits receive varying amounts of sunlight. – The axial tilt of a planet determines its seasons. • Sunlight warms a planet, but the atmosphere has an impact, too – Venus’s atmosphere warms the surface to 750 K, but it would be very warm even without the CO2 – Mercury is closer to the Sun, but still cooler than Venus. – The Moon is cooler than the Earth, even though they are at the same distance from the Sun. • Sunlight also determines the makeup of the planets. – Inner planets are rocky. (iron) – Outer planets are gaseous. The Outer Planets • Far from the Sun, temperatures are cold enough that water vapor can condense into ices. • Beyond this frost line, planets are primarily composed of hydrogen and ices. • The low temperatures allowed the outer planets to capture hydrogen and helium gas, and to grow to immense sizes. • The outer planets have no surfaces. – Pressures steadily climb (moving inward), turning gases into liquids and eventually metals. Equatorial Bulges • The outer planets rotate much faster than their terrestrial cousins. – These faster rotational speeds make the outer planets much wider at the equator. Other Differences • Each gas giant has a set of rings. – Some are easy to see, like Saturn’s. – Others are harder, like Neptune’s. • The gas giants have many more moons, as well. – The number of moons discovered goes up all the time. Jupiter and Saturn • Jupiter – 5 AU from the Sun – 11x Earth’s diameter – 300x Earth’s mass • Saturn – 9.5 AU from the Sun – 9.5x Earth’s diameter – 100x Earth’s Mass The Appearance of Jupiter • Parallel bands of clouds – Dark belts – Light zones • 90% H2, 10% He, traces of methane, ammonia and water. • Outer atmosphere has a temperature of 160K. • Rotates once every 9.9 hours. • Visibly flattened. The Appearance of Saturn • Parallel bands of clouds. – Similar to Jupiter’s, but not as distinct. • 96% H2, 4% He, traces of hydrogen-rich compounds. • Outer atmosphere has a temperature of 130K. • Rotates once every 10.7 hours. • Even flatter than Jupiter. The Interiors of the Gas Giants Coriolis Effect • Coriolis Effect is due to the different rotational speeds at different latitudes. A spinning sphere rotates at higher speed at the equator than north or south. • Coriolis Forces DO cause weather patterns (for example) to move in the directions they do – hurricanes and tornadoes turn counterclockwise in the Northern hemisphere, clockwise in Southern. • Coriolis Forces are too small and insignificant to the water in your toilet bowl. Winds • Rapid rotation gives rise to strong Coriolis forces, and very high winds. – Measured max wind speeds of 500 km/hr at Jupiter, and faster at Saturn. • Bands of clouds move in opposite directions, creating very large wind shears. The Great Red Spot • On Jupiter, these wind shears give rise to enormous vortices, or storms, seen as white, brown or red ovals in its clouds. • The Great Red Spot on Jupiter is one such vortex. – Rises 50 km above surrounding clouds – Wind speeds of 500 km/hr. • The Great Red Spot is a storm that has lasted for at least 300 years. – Galileo saw it, and it hasn’t changed much. Storms on Saturn • Saturn, though it appears calmer, has storms as well – Higher wind speeds than Jupiter – Storms are deeper in its atmosphere Magnetic Fields • The liquid metallic hydrogen in Jupiter and Saturn can carry electrical currents, similar to the liquid core of the Earth. • These currents generate very large magnetic fields. – Jupiter’s is 20,000 times as strong as Earth’s, and if it were visible, would appear larger than the full Moon in our sky. – Saturn’s field is 500 times as strong as Earth’. • Both Jupiter and Saturn experience auroras. The Discovery of Uranus • In 1781 a new planet was discovered by W. Herschel – Originally thought to be a comet. – Herschel named it Georgium Sidus (George’s Star) after King George III. – Name changed to Uranus to stay consistent with the mythological names of the other planets. A New Method of Discovery • Uranus was not following its calculated orbit. – Another planet must be effecting its orbit. – Scientists calculated where the unseen planet should be. – Astronomers looked at this location, and found Neptune. – Galileo saw Neptune but didn’t realize what it was. The Atmospheres of Uranus and Neptune • The atmospheres of both Uranus and Neptune are rich in hydrogen and helium. – Both have larger amounts of methane, giving them their blue color. – Methane crystals scatter blue light, and methane gas absorbs red light. • Both planets are very cold – Uranus: 80K – Neptune: 75K • Densities: – Uranus: 1.3 kg/liter – Neptune: 1.6 kg/liter • Their interiors are probably ordinary water mixed with methane and ammonia, surrounding a core of rock and iron-rich material. Interior of Uranus Storms • High winds lead to storms on Neptune. • Neptune has a Great Dark Spot, which disappeared recently. Uranus’s Axial Tilt • Uranus is tipped almost 90 degrees to the ecliptic plane. • Possible that a collision early in its history tipped the axis, and broke out material that formed its moons. • This inclination means that for half of Uranus’ orbit, one hemisphere is in uninterrupted daylight, while the other hemisphere is in darkness. Odd Magnetic Fields • Both Uranus and Neptune have strong magnetic fields. – Uranus: 47xEarth – Neptune: 25xEarth – Possibly generated by currents in the liquid water in their interiors. – Not centered on the center of the planet and tipped in odd directions. Earth’s Magnetic Field • Earth’s magnetic north pole and the “north pole” (i.e., north end of axis) are not in the same location. • Earth’s magnetic north (and south) pole aren’t fixed but change over time. • The poles have “flipped” throughout history. • We may be “due” for a flip again. • Results not likely to be catastrophic but could be interesting if so…