Planets - Chapters 8 12 The Solar System 1 star 8 planets several dwarf planets many moons asteroids comets meteoroids What is a Planet Due to the

Planets - Chapters 8 12 The Solar System 1 star 8 planets...

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Unformatted text preview: Chapters 8 - 12 – The Solar System • 1 star • 8 planets • several dwarf planets • many moons • asteroids, comets, meteoroids What is a Planet? Due to the discovery of additional solar system bodies beginning in the late 1990s, it became necessary to reexamine the term “planet”. At the August 2006 International Astronomical Union meeting in Prague, a new definition was passed. RESOLUTION 5A: (1) A planet is a celestial body that (a) orbits the Sun, (b) has enough mass to form a spherical shape, and (c) has cleared the neighborhood around its orbit. (2) A dwarf planet is a celestial body that (a) orbits the Sun, (b) has enough mass to form a spherical shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite. Thus, Pluto is now a Dwarf Planet mainly because it has not “cleared its neighborhood” sufficiently. There are other objects in similar orbits as Pluto - Neptune, Pluto’s moon Charon, Kuiper Belt objects. The Planets The Planets 1st Law 2nd Law 3rd Law The laws of planetary motion were determined by Johannes Kepler in 1609. The planets’ orbits obey these three laws based on the effects of gravity. The Sun’s gravitational pull dominates the motions of all planets. The Planets The distances to planets are determined from Kepler’s Laws Sizes are determined from angular size and distance The Planets Masses – observe the gravitational effect of the planet on some nearby object (moons, nearby planets, satellites) M = 4π2a3/(GP2) where a and P are the semi-major axis and period or the satellite’s orbit Densities ρ = mass/volume = 3M/(4πR3) • Planets orbit the sun counterclockwise as seen from the North Celestial Pole. • All planets are roughly in the same orbital plane EXCEPT Mercury (and the dwarf planet Pluto). The Planets The 8 planets appear to fall into two distinct groups based on various observed properties Terrestrial Planets •Mercury, Venus, Earth and Mars •Close to Sun •Small masses, radii •Rocky, solid surfaces •High densities •Slow rotation •Weak magnetic field •No rings •Few moons Jovian Planets •Jupiter, Saturn, Uranus, and Neptune •Far from Sun •Large masses and radii •Gaseous surface •Low densities •Fast rotation •Strong magnetic field •Many rings •Many moons Other Solar System Objects…. Comets Icy fragment of Solar System “dirty snowballs” – dust/rock in methane, ammonia and ice All light is reflected from the Sun - the comet makes no light of its own The nucleus is a few km across with coma up to 10000 km across and tail up to 1 AU Halley’s Comet in 1986 •Long period comets take up to 1 million years to orbit the Sun (may originate in the Oort cloud) •Short period comets orbit the Sun in 200 years or less (e.g. Halley’s comet) – likely originate in the Kuiper belt and were kicked into an eccentric orbit that brings them close to the Sun Other Solar System Objects…. Comet Temple 1 image obtained from Stardust satellite flyby on Feb 14, 2011 •Crater with a small mound in the center indicates cometary nucleus is fragile and weak. •Caused by impactor from Deep Impact mission in 2005 – found comet to be less icy and more dusty than expected... Other Solar System Objects…. Meteoroids – interplanetary Meteor showers rocky objects smaller than ~300m (down to grain size). Consist mainly of iron and nickel with some carbon. • called a meteor as it burns in the Earth’s atmosphere. • Can be traveling up to 70 km/s. • Enters at heights ~ 90 km • Usually about 1 cm in size • if it makes it to the ground, it is a meteorite • Must be at least 3 cm to keep from completely burning up in the atmosphere Meteor crater near Winslow, AZ - the culprit was probably 50 m across weighing 200,000 tons! are the result of Earth passing through the orbit of a comet which has left debris along its path Spring Meteor showers: Lyrids – Apr 21/22 Eta Aquarids – May 5/6 Other Solar System Objects…. Asteroids - rocks with sizes greater than 300m across Most asteroids remain in the Asteroid belt between Mars and Jupiter but about 2000 have orbits that cross Earth’s path. Based on known Earth-crossing asteroid orbits, it is estimated that 3 asteroids impact Earth every 1 million years! Other Solar System Objects…. Asteroid Watch (Near Earth Object Program) Other Solar System Objects…. Asteroids range in size from ~300m to ~1000km They are composed of carbon or iron and other rocky material. The Asteroid belt is a group of material that appear to have never joined to make a planet •Too little mass •Different chemical compositions •Planet formation probably effected by nearby Jupiter’s strong gravity Masses and distances from the Sun for planets, asteroids, KBO’s, dwarf planets Solar System Formation Our Sun and the planets began from a dense, cold (T~10K) nebula of dust and gas As the cloud contracts due to gravity, the Sun is formed at the center (highest density) As the cloud contracts, its rotation increases due to conservation of angular momentum The cloud forms a flattened disk known as a protoplanetary disk Smaller condensations within this disk become the planets Dust grains are the condensation nuclei upon which the accretion process forms planets within the gas cloud. Solar System Formation • Solar nebula contracts and flattens into a disk. • Condensation nuclei form clumps that grow into moon-size planetesimals. • Solar wind from star formation blows out the rest of the gas. • Planetesimals collide and grow coalescence. • Growing planetesimals form the planets over about 100 million years. • The more massive proto-planets are also able to sweep up large amounts of gas to become the Jovian planets. Why the difference between inner and outer planets? TEMPERTURE! •Rocky inner planets: The type of the material that condensed out of the nebular cloud at higher temperatures was rocky in nature (lowvolatility materials such as metals/silicates) . •Gaseous, Bigger outer planets: Both rock and gas (higher-volatility materials) condenses out of the cloud at lower temperatures. Why are they gaseous? – H and He gas are present and outer planets have large enough masses to retain these light gasses. Why are they bigger? - accretion onto the planet started sooner because they are further from the Sun, less effected by solar wind Temperature of a Planet A planet’s temperature is determined by balancing energy absorbed from the Sun and energy radiated away – Equilibrium Temperature Energy received per second per unit area from Sun is F(d) = L/4πd2 Projected surface area of planet (from Sun perspective) = πRp2 Rate of energy absorbed by planet Wabs is F(d) x surface area x fraction of light Wabs = L(1 – A) Rp2/4d2 where A is albedo (fraction of reflected light) Rate of energy radiated by planet Lrad = 4πRp2σTp4 In equilibrium, rate emitted (Lrad) = rate absorbed (Wabs) Equate the two and solve for T Tp = [L(1 – A)/(16πd2σ)]1/4 Substituting with solar luminosity and constants give Tp = 279 K (1 – A)1/4 (d/1 AU) -1/2 eq. 8.10 The Terrestrial Planets The Terrestrial Planets Basic Properties 0.06 The Earth The Moon • solid inner core, liquid outer core • atmosphere ~ 50km thick • magnetosphere – charged particles in magnetic field • no hydrosphere, atmosphere or magnetosphere • similar interior regions as Earth but no liquid core Evolution of the Solid Earth •Accretion- material comes together to make the planet ~4.5 billion years ago (during formation of the Sun). Earth was bombarded by interplanetary debris (coalescence) high temps. •Differentiation - Earth was molten, allowing higher-density material (nickel, iron) to sink to the core and lighter materials (silicate rocks) to rise to the upper levels (e.g. mantel, crust) •Crustal Formation - cooling and thickening of crust occurred ~3.7 billion years ago Seismology – studying Earth’s interior • Earth’s interior structure is probed by studying how waves travel through it (we can only drill so far! – deepest holes ~12km) • Earthquakes generate seismic waves • Certain types of waves reflect off different materials and travel through these materials at different speeds (e.g. waves travel faster through higher density material). Earth’s Interior Crust - 30 km thick (8 km under ocean - 20-50 km under continents) Mantle - 2900 km thick (80% of planet volume) Core (3500 km to edge of outer core -1300 km inner core) - High central density suggests the core is mostly nickel and iron. Only 16% of volume but 31% of mass. Density and temperature increase with depth. Density jumps between mantle and core due to changes in composition (becomes less metallic) but smoothly increases between inner and outer core. The Surface of the Earth •The Earth is still active today: earthquakes, volcanoes… •Sites of activity outline surface plates - plate tectonics •Continental drift - few cm/year •Plates collide head on (mountains) or shear past (earthquakes) •Some plates are separating (rift zones – under oceans) - new mantle material wells up between •Convection of warm mantle material responsible for tectonics Sites of earthquakes & volcanoes in the past 100 years Earth’s Atmosphere •Protects the surface •Regulates temperature nitrogen •nitrogen (78%) – N2 oxygen •oxygen (21%) – O2 •argon (0.9%) - Ar •carbon dioxide (0.04%) Variables that describe a gas (P, ρ, T) are related by the ideal gas law P = ρkT/(μmp) Pressure at Earth’s surface (bottom of atmosphere) = 1 atm (or 1 bar) 1 atm = 105 N/m2 (Like 100 people standing on a square meter!) μ = average mass per particle 0.78(2x14) + 0.21(2x16) + 0.01(40) ~ 29 T = 290 K at surface ρ = 1.1 x 10-3 g/cm3 at surface Pressure Distribution Vertical distribution of pressure and density – hydrostatic equilibrium Treat atmosphere as a plane parallel layer Hydrostatic equilibrium dP/dr = -ρg Combine with ideal gas law and assume that T, μ and g are constant dP/dr = -(μmpg/kTo)P eq. 9.12 Integrating over the pressure from Po (surface) to P gives P/Po = exp[-(μmpg/kTo)r] The scale height is defined as H = (kTo/μmpg) and has units of length. Can rewrite as P/Po = exp (-h/H) where we replace r with h At an altitude of just 2km the pressure drops to 80% of the surface value - distance above sea-level eq 9.14 Earth’s Magnetosphere •Field generated by the rotation of the planet coupled with the electrically conducting liquid metal core = dynamo effect •Magnetic field lines run from the south to north magnetic poles •Magnetic axis tilted ~12 degrees from rotational axis and moves ~10 km/yr •The field is distorted by and deflects many of the charged particles of the solar wind Aurora Borealis Northern Lights – caused when charged particles from Sun and collide with Earth’s atmosphere near the poles The Earth and Moon System - Tides Tidal effects of the Moon are greater than those from the Sun due to its close proximity. Gravitational force exerted by the Moon is differentiated across the Earth. Force between Earth and Moon is greater on the side of the Earth closer to the Moon and weaker on side further from Moon This creates two bulges in the Earth’s hydrosphere - one on the side facing the moon and one on the opposite side As the Earth rotates. the surface moves through both bulges and thus experiences two high (and two low) ocean tides per day Over time, tides have the following effects on the Earth and the Moon • Slowing the Earth’s rotation - the day is increasing by 0.002 sec/century. • Increasing the size of the Moon’s orbit - its distance from the Earth is increasing by 4 cm/year (2 inch/year) • The moon is tidally locked to the Earth - the same side of the moon is always facing us (moon rotation period is the same as its orbital period) Moon Surface - lack of atmosphere and water preserves surface features Maria – mantle material • “seas” - darker areas resulting from earlier lava flow • Basaltic, iron rich, high density (3300 kg/m3). •Younger surface – 3.5 Gyr Highlands – crust material • elevated many km above maria • Aluminum rich, low density (2900 kg/m3). •Older surface – 4 Gyr Craters • Caused by meteoroid impacts at speeds up to 73 km/s • Pressure to the lunar surface heats the rock and deforms the ground. Heat vaporizes meteoroid and surrounding lunar rock • Explosion pushes rock layers up and out • The ejecta blanket surrounds the crater • The rate of cratering on the Moon is determined from the known ages of the highland and maria regions. • The Moon (and other solar system bodies) experienced a sharp drop in the rate of meteoritic bombardment about 3.3 – 3.5 billion years ago (end of accretion era) • The rate of cratering has been roughly constant since that time Formation of the Moon The Moon has an overall composition and density different from the Earth, but resembles the material in the Earth’s mantle (not very metallic like the Earth’s core). This observation has led to the Impact Theory for the formation of the Moon. • Mars-sized body hit the molten Earth about 4.5 Gyr ago • Parts of the mantle (of colliding planet and Earth) blew off and later formed the Moon • Earth (and colliding planet) had differentiated so the mantle was already metal poor Lunar Eclipses Earth’s shadow passes over the Moon Mercury •Mercury is not tidally locked to the sun in the same way as the moon-earth system •Sun’s gravity and Mercury’s eccentric orbit brought it into semi-synchronous orbit These images from the MESSENGER satellite (Jan 2008) show a surface similar to the Moon •Mercury presents the same face to the sun every OTHER time around (rotational period is 59 days and orbital period is 88 days). Solar day is 176 earth days! •Old surface with no apparent plate tectonics •Craters flatter and have thinner ejecta rims than lunar craters due to higher gravity on Mercury than the moon •Almost non-existent atmosphere - high surface temperature and low escape velocity (drastic temperature changes 800 F (day) to -280 F (night) Venus •Rotation direction is retrograde (opposite that of other terrestrial planets)! •243 day rotation period •Axis is almost exactly perpendicular to orbit •Why? ____________________________ •Much more massive atmosphere than Earth’s – air pressure 92 atm on surface •Carbon dioxide (96.5%), Nitrogen (3.5%) •No H20 - clouds are H2SO4 (sulfuric acid) •Fast moving (400km/hr!) clouds in upper atmosphere – very little wind at the surface •740K temperature due to greenhouse effect of CO2 Mariner 10 spacecraft image taken 1974 Venus reflected radio waves reveal the surface topology •Elevated “continents” make up 8% of the surface (25% on Earth) •Mostly rolling plains with some mountains (up to 14 km) •No tectonics •Volcanoes resurface the planet every ~300 million years •Shield volcanoes are the most common (like Hawaiian Islands) • A caldera (crater) is formed at the summit when the underlying lava withdraws •Venus is thought to still be volcanically active today A few Soviet spacecraft have landed on Venus in 1970s •Survived only an hour before burning up •little evidence of erosion - young surface •rocks are basaltic and granite •Cloud cover makes Venus seem like a heavily overcast day on Earth all the time! Mars •Slightly smaller than Earth with similar rotation period of 24.6 hours and inclination from ecliptic (24 degrees) •Very little atmosphere - 1/150 the pressure of Earth - CO2 (95.3%),nitrogen (2.7%), argon, oxygen, CO, water vapor •Two very small moons (Phobos and Deimos) •1965 – Mariner 4, 6 and 7 “fly by” Mars •1971 – Mariner 9 orbits and maps Mars in detail •1976 – Viking 1 lands on Mars •1997 - Pathfinder and Sojourner rovers - highly successful mission which sent back lots of pictures of the Martian landscape (+ soil and atmosphere analyses) – revealed iron rich soil – Mars is “rusting” •2004 – rovers Opportunity and Spirit – look for evidence of water on Mars (Opportunity still running!) •2008 – Phoenix mission at north polar region Latest rover Curiosity landed in August 2012 to look for evidence of and conditions favorable for microbial life This scene looking back at where Curiosity crossed a dune at "Dingo Gap" combines several exposures taken This mosaic of images from Curiosity's Mast by the Navigation Camera (Feb 10, 2014) Camera (Mastcam) shows geological members of the Yellowknife Bay formation (Dec 24, 2012) Curiosity selfie from two months ago…. Mars Surface •Polar ice caps •frozen CO2 •Northern hemisphere •rolling volcanic planes •Like lunar maria but larger •Few craters – young (3 billion yrs old) •Southern hemisphere •heavily cratered highlands •Older (4 billion yrs old) Some interesting features: Tharsis Bulge - Roughly the size of North America, sits on the equator - 10km high, less heavily cratered (i.e. young surface; 2-3 billion yrs) Valles Marineris - extends one-fifth of the way around the planet at the equator, up to 120 km across and 7 km deep, the Grand Canyon would fit into one of its side "tributary" cracks, probably produced from stretching and cracking when Tharsis bulge formed Volcanoes on Mars •Largest volcanoes in the solar system are here (Mars has a surface gravity only 40 percent that of Earth, and its volcanoes rise roughly 2.5 times as high) •Shield volcanoes (e.g. Olympus Mons) •None appear currently active but eruptions occurred 100 million years ago Water on Mars About 4 billions years ago Mars had a thicker atmosphere, warmer surface, and liquid water Runoff channels •Found in southern highlands •Extensive river systems (like Earth) •Carried water from highland to valleys Mars Earth Outflow channels •Caused by flooding •Found at the equator •Formed about 3 billion years ago Internal Structure of the Terrestrial Planets All terrestrial planets are likely to be differentiated like Earth. The larger planets take longer to cool and therefore still have active mantles and liquid cores. Mercury •Interior dominated by large iron core (high density and weak magnetic field present) •Solid mantle prevents volcanoes and tectonics •As core formed and cooled, shrinking caused the surface to contract (scarps,fissures, etc) *Venus and Earth are believed to have similar interiors Venus •No magnetic field (slow rotation) •surface of Venus resembles that of the young Earth, at an age of perhaps a billion years •Never developed plate tectonics – possibly because of high surface temp and soft crust Mars •Very weak magnetic field – non liquid or non metallic core? •large-scale tectonic activity almost started but was stifled by rapidly cooling outer layers The Jovian Planets Space Craft Exploration of Jovian Planets •Voyager 1and 2 left Earth in 1977 and reached Jupiter in March and July of 1979 • Used Jupiter’s strong gravity to send them on to Saturn - gravity assist • Voyager 2 used Saturn’s gravity to propel it to Uranus and then on to Neptune • Studied planetary magnetic fields and analyzed multi-wavelength light • Both are now headed out into interstellar space Galileo - launched in 1989 and reached Jupiter in December 1995 • Two components: atmospheric probe and orbiter • Probe descended into Jupiter’s atmosphere and orbiter went through moon system Cassini mission to Saturn in 2004 • orbiter continues to orbit Saturn and its moons • Huygens probe launched from the orbiter in 2005 to study Saturn’s moon Titan. Jovian Planet Properties •Most of their mass is Hydrogen and Helium – light elements = low densities •High surface gravity allows their atmospheres to retain these light elements •Dense, solid core at the center •Differential Rotation – outer regions rotate at a different rate than the inner regions and rotation can be different at the poles than at the equator Jovian planets - axis tilt and magnetic fields •All Jovian planets have strong magnetic fields - rapid rotation and liquid conductive cores •Note that magnetic field axes not always aligned with rotational axis or centered in the planet Jupiter’s Atmosphere • molecular hydrogen – 86% • helium – 14% • small amounts of methane, ammonia, and water vapor •Darker belts lie atop downward moving convective cells •Lighter zones are above upwar...
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