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Two Kinds of Planets "Terrestrial" "Jovian" Two Kinds of Planets "Terrestrial" "Jovian" Mercury, Venus, Earth, Mars Jupiter, Saturn, Uranus, Neptune Distance from Sun? Size? Composition? Density? Rotation Rate? Number of moons? Rings? Heavy or light elements? Two Kinds of Planets "Terrestrial" "Jovian" Mercury, Venus, Earth, Mars Jupiter, Saturn, Uranus, Neptune Close to the Sun Small Mostly Rocky High Density Slow Rotation (1 - 243 days) Few Moons No Rings Heavy Elements (Fe, Si, C, O) Far from the Sun Large Mostly Gaseous Low Density Fast Rotation (0.41 - 0.72 days) Many Moons Rings Light Elements (H, He) What about Pluto? Origin of Pluto Now known to be just a relatively large member of a class of objects in the outer reaches of the Solar System: The Kuiper Belt Objects Questions ● ● What are some of the smaller objects (or debris) found in the solar system? What is the main reason that we study these smaller objects? (What information do they contain that the planets and moons do not?) Questions ● What are some of the smaller objects (or debris) found in the solar system? – ● Comets, asteroids, meteoroids What is the main reason that we study these smaller objects? (What information do they contain that the planets and moons do not?) – Solar system debris is unevolved => gives direct evidence of conditions during solar system formation! Comet Structure Nucleus: ~10 km ball of ice, dust Coma: cloud of gas and dust around nucleus (~106 km across) Tail: can have both ion and dust tails (~108 km long). Always points away from Sun. Solar System Formation Our solar system started out as a giant cloud of cold gas that collapsed under its own gravity. ● So, why is our solar system flat? Conservation of angular momentum! What role did dust play in the formation of our solar system? Condensation theory: 1) Dust grains act as "condensation nuclei": gas atoms stick to them => growth of first clumps of matter. Also radiate heat => help to cool gas => faster gravitational collapse. What term describes the process by which smaller particles collide and stick together to form larger clumps? Condensation theory: 1) Dust grains act as "condensation nuclei": gas atoms stick to them => growth of first clumps of matter. Also radiate heat => help to cool gas => faster gravitational collapse. 2) Accretion: Clumps collide and stick => larger clumps. Eventually, small-moon sized objects: "planetesimals". 3) Gravity-enhanced accretion: objects now have significant gravity. Mutual attraction accelerates accretion. Bigger objects grow faster => a few planet-sized objects. Atmosphere and Greenhouse Effect ● ● ● What is the greenhouse effect and how does it differ on Earth and on Venus? What type of EM radiation do the greenhouse gasses in our atmosphere trap? What type of EM radiation does the ozone layer of our atmosphere protect us from? Runaway Greenhouse Effect on Venus The Interior of the Earth ● ● How does temperature vary as we get closer to the core of the Earth? What about density? Earth's Internal Structure Temperature and density increase with increasing depth. How do we obtain information about the structure of the Earth's interior? Like all waves, seismic waves bend when they encounter changes in density. If density change is gradual, wave path is curved. S-waves are unable to travel in liquid. Measurement of seismic waves gives info about density of Earth's interior and which layers are solid/molten. The Tides ● What causes the tides? ● When are the highest (and lowest) tides seen? Tides Questions ● ● What are the two main features of the lunar surface? Which is older? How do we know? What is currently the most widely excepted theory explaining how the moon formed? The Lunar Surface Large, dark featureless areas: "maria" or "seas". More recent lava flows. ● Lighter areas at higher elevation: "highlands". ● Many craters (due to meteorite impacts). Only important source of erosion! ● Highlands have 10x the crater density of maria => Highlands are older! ● maria highlands Impact Theory Early in Solar System, when many large planetesimals around, a Mars-sized object hit the forming Earth, ejecting material from the mantle which went into orbit around Earth and coalesced to form Moon. Computer simulations suggest this is plausible. Properties of the planets Which planet exhibits retrograde rotation? How do we determine the direction and rate of a planet's rotation? Anomalous rotation of Venus • Extremely slow - Venutian day longer than Venutian year! • Retrograde - Sun rises in the west and sets in the east! • Most likely due to a collision during solar system formation Determining rotation rate of a planet • Use reflected radio waves to determine line of sight doppler shifts. Venus • Thick clouds prevent viewing of surface. (UV Image) • High temperatures and pressures, acidic gases, make it difficult to land anything on surface. • Led to much speculation. • How did we get info about surface? "Radar Mapping" technique measures altitude space probe time for signal to return tells you the altitude of surface feature. Planet Surface Properties of the Planets Which planet has the largest known volcano in the solar system? Olympus Mons Mars What is the main reason that many scientists think Mars may have once harbored life? What is the most likely origin of the two moons of Mars? Evidence for Past Surface Water "runoff channels" or dry rivers "outflow channels" standing water erosion in craters? teardrop "islands" in outflow channels Mars' Moons Phobos and Deimos Deimos: 16 x 10 km Phobos: 28 x 20 km Properties similar to asteroids. They are probably asteroids captured into orbit by Mars' gravity. Question ● ● Jupiter and Saturn were known to the ancient astronomers. Uranus, which is barely visible to the naked eye, was discovered in 1781. Neptune is not visible to the naked eye, so how was it discovered? Discovery of Neptune Neptune predicted to exist because of irregularities in Uranus' orbit. Neptune discovered in 1845 by Johann Galle. Gravitational perturbation of Uranus’ orbit by unseen planet led to discovery of Neptune! Kepler’s and Newton’s laws in action! Nice example of how the scientific method works! Jupiter's Bands Lighter-colored "zones" and darker-colored "belts". Belt Zone Shadow of a moon Great Red Spot Question ● ● ● Zones and belts are, respectively, high and low pressure systems on Jupiter. What process would you expect to be taking place between such regions? Why do the zones and belts stretch all the way around Jupiter? - Zones and belts are Jupiter's high and low pressure systems, respectively. - They mark a convection cycle. - Jupiter's rapid rotation stretches them horizontally around the entire planet. - Gas in zones and belts flow in opposite directions. Differential Rotation ● What is differential rotation and what types of objects rotate in this way? Differential Rotation Rotation period of Jovians is shorter closer to the equator: Near poles At equator Jupiter 9h 56m 9h 50m Saturn 10h 40m 10h 14m Uranus 16h 30m 14h 12m How do we know? Tracking storms at various latitudes, or using Spectroscopy and Doppler shift. The Galilean Moons of Jupiter (sizes to scale) Closest to Jupiter Furthest from Jupiter Would you expect Jupiter to be hotter or cooler very early in its history? What might this imply for the Jupiter-Galilean moon system? The Galilean Moons of Jupiter (sizes to scale) Closest to Jupiter Furthest from Jupiter The Galilean Moons resemble a miniature planetary system in many ways! Intense heat of young Jupiter played role of sun. Io's Volcanism More than 80 volcanoes have been observed. Can last months or years. Largest is bigger than Maryland - emits more energy than all Earth volcanoes combined! Io is about the size of our moon. Why is it's volcanic activity surprising? Where is the energy coming from? Io and Europa are in a resonance orbit: Start clock now Jupiter Europa Io One orbit of Io Jupiter Europa Io Two orbits of Io Jupiter Europa Io The periodic pull on Io by Europa makes Io's orbit elliptical and distorts entire moon. Jovian Rings ● How did Saturn's rings form? Origin of Saturn's Rings: If a large moon, held together by gravity, gets too close to Saturn, the tidal force breaks it apart into small pieces. The radius where this happens is called the Roche Limit. Total mass of ring particles equivalent to moon 250 km in diameter. Voyager probes found that rings divide into 10,000's of ringlets. What maintains this ringlet structure? Shepherd Moons • Some ringlets maintained by gravitational influence of “shepherd moons” • Keep particles from straying outside the path of the ringlet