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Lecture 12 Overview of the Solar System Chapter 6 to be covered in another lecture - read the chapter 1877 Mars was close to Earth 2 small moons discovered by Asaph Hall Hubble Image of Mars Early photo and sketch of Mars Schiapareli reported seeing “canali” - in Italian this means grooves or channels but was interpreted to mean canals Hubble image Compare (same face) Sketches Percival Lowell in Flagstaff AZ He gave up his business in Boston to devote his life to astronomy and the search for life on Mars Novels (1917) about life on Mars inspired the young Carl Sagan and many others Northern Martian plains seen by the Phoenix lander A lifeless planet Red because of the iron content of the rocks - rust Panoramic view of Mars seen by the Opportunity Rover We are still on a quest for life - was there life on Mars in the past? Technology is the driver Galileo’s telescope Earl of Rosse - 72 inch reflector the largest telescope in the world in the 19th century Exploration by Spacecraft 1972 Apollo astronaut prospecting in Mare Serenitatis The Solar System Most information has been obtained in the last 40 years through spacecraft exploration Near circular orbits but not perfect All prograde - they orbit in the same direction Ecliptic is the plane of Earth’s orbit the line is close to the ecliptic Sizes are drawn to scale Earth about the size of the Great Red Spot on Jupiter Densities 1410 kg / m^3 = 1.410 g / cm^3 better to use numbers near unity ? Components of the solar system •! The Sun •! Mass/luminosity •! Solar Wind/Magnetic field •! Planets and their moons and ring systems •! Terrestrial planets: Mercury, Venus, Earth, Mars •! Jovian planets: Jupiter, Saturn, Uranus, Neptune •! Dwarf planets: Pluto (Ceres, Eris) •! Minor planets •! Asteroids: Asteroid Belt, Trojans, Near Earth Asteroids •! Comets: Kuiper Belt, Oort Cloud •! Dust •! Zodiacal Cloud The Sun - mass •! Vital stats: •! Mass = 1.989 x 1030 kg •! Radius = 6.95 x 108 m •! Mean density = 1410 kg/m3 •! Definition of the solar system is the material gravitationally bound to the Sun •! Everything orbits the Sun on elliptical orbits with orbital periods of 1.5 tper = a years where a=semimajor axis in AU •! The Sun’s influence extends out to ~100,000 AU (~0.5 pc), outside which galactic tides strip material from the solar system Kepler’s 3rd law a^3 / T^2 = constant This gives us the ratios of the distances based on the Earth with a distance of 1 AU and a period of 1 year For absolute numbers we need radar measurements of distances to nearby planets (Mercury and Venus) T = a^1.5 T = a^(3/2) Square both sides of the equation T^2 = a^3 Kepler 3 T must be in years a must be in AU because we are dealing with ratios and comparing with Earth Clues Comets have two tales Solar wind from one of ionized gas comets and one of dust The Sun – solar wind •! First discovered because comet ion tails always point away from the Sun; caused by fast moving ions in the corona which escape the Sun’s gravitational field •! It has a slow component (300-500km/s) at the equator (<150) and fast component (700-800km/s) at higher latitudes; at 1AU mean density is 7x106 protons/m3 (v ~ const, so ! " r-2); neutral, roughly solar composition •! Interaction of charged particles with planet magnetospheres and atmospheres -> aurorae borealis •! Solar wind interacts with the interstellar medium at the heliopause The Sun – solar wind •! First discovered because comet ion tails always point away from the Sun; caused by fast moving ions in the corona which escape the Sun’s gravitational field •! It has a slow component (300-500km/s) at the equator (<150) and fast component (700-800km/s) at higher latitudes; at 1AU mean density is 7x106 protons/m3 (v ~ const, so ! " r-2); neutral, roughly solar composition •! Interaction of charged particles with planet magnetospheres and atmospheres -> aurorae borealis •! Solar wind interacts with the interstellar medium at the heliopause Earth’s magnetic field protects us from the solar wind Mercury, Venus, Earth and Mars not to scale So, where did all these planetary systems come from? did all these planetary systems come from? where So, 4. Planet formation Jupiter, Saturn, Uranus and Neptune + Pluto (a dwarf planet) The planets – overview/mass Mass Distance Mercury 0.06 Mearth 0.39 AU Venus 0.82 M 0.72 AU earth Terrestrial Earth 1.0 Mearth 1.0 AU planets Mars 0.11 Mearth 1.5 AU Jupiter 318 Mearth 5.2 AU Saturn 98 Mearth 9.5 AU Jovian planets Uranus 15 Mearth 19.2 AU Neptune 17 Mearth 30.1 AU Dwarf planet Pluto 0.002 Mearth 39.5 AU 24 -6 11 1 Mearth = 6 x 10 kg = 3x10 Msun , 1 AU = 1.5 x 10 m Sun is nearly one million times more massive than Earth Mercury • Unusual rotation • Anomalous precession • Do Vulcanoids exist? Anomalous precession of Mercury refers to the change in the orientation of the elliptical orbit This could not be understood using Newton’s simple inverse square law of gravity It was first accounted for by Einstein’s theory of General Relativity The planets - rotation •! Mercury is in a 3:2 spin-orbit Tper, yrs Prot, hrs Obliquity resonance, probably despun by Mercury 0.241 1407.5 0.10 solar tides Venus 0.615 5832.5 177.40 Earth 1.00 23.9345 23.450 •! Venus, Uranus and Pluto have Mars 1.88 24.623 25.190 retrograde rotation, possibly as Jupiter 11.86 9.925 3.120 result of collision with proto-planet during formation (Canup 2005; Saturn 29.46 10.656 26.730 Parisi et al. 2008) Uranus 84.00 17.24 97.860 Neptune 164.80 16.11 29.560 •! NB obliquity is measured relative Pluto 247.7 153.29 119.60 to orbital plane rather than ecliptic Mostly prograde - two retrograde 3 to 2 Spin-orbit resonance Mercury spins on its axis 3 times for every 2 orbits of the Sun Produced by tidal braking of the spin caused by tides raised on Mercury by the Sun Venus • Unusual rotation • Lack of small craters • Atmosphere Carl Sagan - the greenhouse effect Spin of Venus is retrograde and very slow (-243 days) Also caused by tidal braking due to solar tides Earth • Liquid water • Atmosphere • Life! • Few impact craters Plate tectonics - the positions of the continents change The Moon • Formation? • Asymmetric faces • Synchronous • Receding • Water? Mars • Polar caps • Unusual rotation • Liquid water? • Life? • Eccentric orbit Unusual rotation does not refer to the spin (about 24 hours) but to the large, slow changes in the orientation of it’s spin axis - these change the climate The planets - orbits Aphelion Perihelion ae Three things I will say about orbits: •! Semimajor axis, a (t =a1.5) per 2a •! Eccentricity, e •! Inclination, I •! Evenly spaced -> Titius-Bode’s law a=0.4+0.3(2i), predicted Ceres (1801) a, AU e I, deg and Uranus (1781) Mercury 0.39 0.206 7.0 Venus 0.72 0.007 3.4 •! La Grande Inequalite (JS near 5:2 Earth 1.0 0.017 0.0 resonance) and NP in 3:2 resonance Mars 1.5 0.093 1.9 •! All orbit in the same direction, in the Jupiter 5.2 0.048 1.3 same plane on roughly circular orbits Saturn 9.5 0.054 2.5 Uranus 19.2 0.047 0.8 •! Except Pluto, and possibly Mercury; Neptune 30.1 0.009 1.8 high e of JS also important for formation/evolution Pluto 39.5 0.249 17.1 The planets – internal structure Density, Terrestrial planets: kg/m3 •! iron core, rocky mantle, crust (differentiated in formation) Mercury 5427 •! Mercury’s high density -> large iron core (to 0.75Rpl), perhaps caused by massive impact (Asphaug et al. 2006) Venus 5204 Earth 5515 Jovian planets: Mars 3933 •! rocky/icy (liquid) core and metallic/molecular hydrogen (JS) Jupiter 1326 or mantle of ices and H/He/CH gas (UN) 4 Saturn 687 •! mass of Jupiter’s core is model dependent 0-11Mearth, but Uranus 1318 Saturn’s core does exist 9-22Mearth (Sauron & Guillot 2004) Neptune 1638 Pluto: rocky core, ice mantle, layer of frozen methane, Pluto 2060 nitrogen and carbon monoxide Density is a clue to composition but we must allow for the effects of compression Jupiter • Gas giant • Role in orbital evolution • Satellite system • Ring system • Resonances! • Vulcanism! Planetary satellites – Galilean Moons Distance Mass Io 422,000 km 0.015 Mearth Europa 671,000 km 0.008 Mearth Ganymede 1,070,000 km 0.025 Mearth Callisto 1,883,000 km 0.018 Mearth Io: strong vulcanism caused by 100 m tides from Jupiter (synchronous rotation) and orbital resonance with Europa; sulphur gives it a red/yellow colour Europa: cracked icy surface; liquid water under surface; heated by tides; no craters Ganymede: cratered; rock and ice; maybe water; largest moon in Solar System, and larger than Mercury and Pluto Callisto: most heavily cratered; rock and ice Resonances Ratios of orbital periods are ratios of small integers Io / Europa / Ganymede = 1 / 2 / 4 Giant red spot and Galilean satellites Io Europa Ganymede Callisto The new, close-up view from space • • The space-age investigation of the solar system began in a cold war • competition between the Soviet Union, which launched the first artificial • satellite, and the United States, which won the race to the Moon. • The VoyagerVolcanic 1 and activity 2 flyby on Io spacecraft transformed our understanding of • the four giant planets, Jupiter, Saturn, Uranus and Neptune, and revealed • fascinating, unexpected aspects of their moons and rings.
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