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Chapter 1

Chapter 1 - Tour of the Solar System.pdf

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Western University
Astronomy 1021

Chapter 1 - Tour of the Solar System Wednesday, January 22, 2018:17 PM • What is the solar system? → The solar system consists of:  The sun (star)  All those objects that are in orbit around the Sun: - Plants, comets,asteroids… • Defining a planet → A plant is a celestial body that  Is in orbit around the Sun  Has sufficient mass for its self-gracity to overcomerigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape - i.e. is large enough to have settled into a round shape  Has cleared the neighborhood around its orbit (this is why Pluto is NOT a planet) → 8 planets  Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune • What about Pluto? → A dwarf planet is a celestial body that  Is in orbit around the Sun  Has sufficient mass for its self-gracity to overcomerigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape - i.e. is large enough to have settled into a round shape  Has NOT cleared the neighborhood around its orbit (this is why Pluto is NOT a planet)  Is not a satellite (i.e. a moon) → All other bodies except satellites that orbit the sun shall be referred to as small solar-systembodies → Planetary body  Planets, moon, asteroids, comets,etc. • Planetary scale → AstronomicalUnit (AU): the mean distance between the centers of the Sun 11 and the Earth = 1.5x10 m(150 million km) • Planets → Orbit counterclockwise in the approx same plane → Inclination rances 0 < inc (°) < 7  Measured relative the ecliptic → Orbits are almost circular → The orbital periods (i.e. the length of their "year") is longer for planets forther out • The terrestrial planets → Mercury, Venus, Earth, Mars → Mercury, Venus, Earth, Mars → The inner planets are dense, rocky, little gas, warm → Warm due to proximityto Sun • Mercury → Smallest terrestrial planet → Closest to the Sun → No atmosphere(very tenuous) → Surface temperature variations are most extremein the solar system (80K to 740K) → No moon → Each day lasts 1.5 Earth Years → Studied by Mariner 10 spacecraft (1974-1975)amd Messenger mission (2011) • Venus → Most similar to Earth in size (95% of diameter, 80% of mass) → Surface temperature 740K,pressure = 90 bars → Atmosphere97% CO 2  Clouds of 2O ,SH4 O rain → Most intense greenhouse effect in the Solar system (surface temp hotter than Mercury) → No moons → Retrograde rotation;each day lasts longer than one year (243 Earth days cs. ~225) → Surface of Venus:  Numerous major volcanos  Many lava flows and extensivelava fields  Pancake-likedomes (viscous lava)  "young" surface (500 million years old) … perhaps a violent resurfacing event?  No direct evidence for ongoing volcanic activity  New evidence in 2010 from venus express suggests that may have been recently active! • The moon → Surface dominated by craters and smoothless-cratered dark areas called "maria" → Diameter= 3476km (Earth = 12756km) → Distance from Earth = 384400km → Rotation period and orbit period match (~27 days) • Mars → Most Earth-like planet in the solar system (even seasons) → Thing CO -rich atmosphere(6 millibar) 2 → Average surface temp = 223K → Winds can kick up dust storms → Olympus Mons: 24 km high, and Valles Marineris: up to 11 km deep → Two small moons:Phobos, and Deimos(captured asteroids) → Most missions since 1965 • NOTE: 1 sol = 1 Martian day • NOTE: 1 sol = 1 Martian day • Mars continued… → Olympos Mons  24 km high, width = 600 km  3 craters at summit are > 2 km deep  Similar in slope to Mauna Lowa  Flows along flank → Water on Mars  It is believed that Mars used to have water and in fact it was a lot like Earth in its youth • Jovian Planets → The outer planets are gas giants that contain mostlyhydrogen and helium, but have solid cored → Jovian = like jupiter → Lower densities than terrestrial planets (saturn would float on water) → Much more massive → Have rings • Jupiter → All that we see is the top of the atmosphere → More massive than the rest of the planets combined → 143 000 km in Diameter → In 1994,Comet Shoemaker-Levy9 collided with Jupiter → Primarily composedof H and He with a core of heavier elements → Great red spot (~20 000 km across) has persisted for > 300 years → 67 moon! → Satellites of Jupiter:  lo: Most volcanicallyactive body in the solar system, rocky in composition- terrestrial-like body  Europa: ice surface showing many cracks, no craters, young surface, possibility of ocean of melted ice underneath icy surface, cryovolcanism(cold slurries of ice and liquid erupt and flow)  Ganymede, Callisto: icy bodies, heavenly cratered → Many of the Jovian satellites are larger than small planets!  The terrestrial planets only have 3 moons between them, Jovian planets have lots • Saturn → 2nd largest planet in the solar system → Rings are > 250 000 km in diameter, less than 1km thick → Postulated to have formed from the break-up of a moon → Ring gaps are from the gravitational influence of the other moons → 62 known Moons → 62 known Moons → Titan  Second largest moon in the solar system (bigger than Mercury)  Only moon in the solar system with a thick Nitrogen-rich atmosphere (1.5 bar)  Temperaturenear surface is 94K Cassini radar and infrared pierce the clouds revealing drainage  channels and lakes • Uranus → Severe axial tilt at 98°, probably from a huge impact event → Atmospheresimilar to Jupiter and Saturn, but higher proportion of Methane → Pressure and temperaturenear core creates diamond hail → Rings less pronounced than Saturn's - consists of darker particles → 27 known Moons • Neptune → White cloud streaks and Great Dark Spot (now vanished) → 13 known Moons → Triton is the most interesting moon with a surface of Nitrogen, carbon monoxide,methane and carbon dioxide ices → Captured moon (pluto-like object) • Satellite or Moon systems → Any small body orbiting a more massive parent body → Some small solar-systembodies have satellites → Moons either from in situ with the planet or are captured  Mars: Phobos and Deimosare captured asteroids → Surface characteristics  Impact craters: from asteroids and cometscauses cratering on surfaces  The lack of impact craters is due to - Resurfacing: volcanismand lava flows on terrestrial planets and cryovolcanismon icy satellites - Weathering: if a planet has an atmosphere,impact craters get weathered away with time → Asteroids  Small lump of rock  Left over from the formationof the solar system:they didn't get the chance to form planets, but were too large to get blown away by the sun's radiation  Sizes up to a few hundred km, although most are much smaller than that  Have impact craters  Main asteroid belt - Distribution: 1.6 < d < 5.1 - Inclination range: 0 < inc. < 30 - Circular orbits - Between Mars and Jupiter  Near Earth Asteroids - Distribution: 0 < d < 5 - Confined to the ecliptic - Highly elliptical orbits - Can potentially collide with the Earth → Kuiper Belt Objects (KBOs): →  Large ice/silicate(cometary)bodies  Distribution: 40 < d < 10  Named after Gerard Kuiper, who suggested the belt might exist in 1951 - techanically K.E. Edgeworth first suggested the idea in 1949  Also called trans-Neptunian objects (TNOs)  Diametersup to several 10 km  Partially stable, circular orbits in the ecliptic plane  ~1000known (eg. Pluto) → comets  Lumps of ice, with small particles of dust and rock  Originate from the Oort cloud, a collectionof cometsin the far outer regions of the solar system  The sun's gravity acts only weakly at the distance of the Oort cloud (~10 000 - 100 000 AU), so comets get easily perturbed by the small tugs of nearby stars or aligned planets. This causes them to get displaced and they can approach the sun on closer orbits  A cometconsists of an icy nucleus and gaseous tail  The tail is caused as the cometgets close enough to the sun that the ice starts to get vaporized  Some cometsare very regular in their periods, others don't live very long and are either vaporized by the sun, or can get pulled in by massive planets → The Oort cloud: a cometarygraveyard → Meteoroids  A solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom or molecule  Solid particles made of rock and organic compounds  Derived from cometsand asteroids  Sizes: 10 m to 10 m → Meteors  Any small particle that enters the Earth's atmosphere from space  Mostly comefrom comet trails  Occasionally, a slightly larger lump of rock will actually survive its passage through the atmosphereand land as a meteorite  As cometsevaporateduring their orbits, they leave trails of dust behind them. As the Earth passes through these dust trails, tiny particles of dust hit the Earth's atmosphere.The friction caused as they enter the atmosphereheats them to high temperatures,so they look very bright, even though they are tiny. These are meteors, shooting/fallingstars  The Earth's orbit passes through these trails everyyear, so we have predictable meteor showers - Eg: the Leonide shower in Novemberis one of the biggest showers, up to 600 meteorsper hour as the Earth passes through the trail of cometTempel-Tuttle • Also between the planets → Radiation field  Electromagneticradiation from Sun (and stars) → Solar wind → Solar wind  High speed, charged particle "gas" emanating from Sun → Magnetic fields  Solar magnetic field and planetary magnetic fields → Cosmic rays  Ionized atoms traveling with very high velocities Most originate from beyond the solar system,but someare  produced by the Sun • Formationof the solar system → Temperaturedistribution:  Close to the sun = warm - Small icy particles cannot survive - Accretion of rocky and metallic material - Terrestrial planets  Further out: cold - Icy particles do survive and form planets - Ices melt during formationbut is retained as gas due to gravitational forces - Giant planets Chapter 2 - The internal Structure of the terrestrial Planets Tuesday, January 28,12:18 PM • Compositionof Earth's crust • Compounds I → Elements combine (via chemical bonds) to form multi-elementcompounds → Properties of compounds depend on type of bonds, elemental composition → Refractory compounds: high condensation temperature,high melting temperature and high boiling temp → Volatile compounds: low condensationtemps, melting temps, boiling temps • Compounds II → Alloy: metallic compound made of more than one metallic chemical element, e.g. a Fe-Ni alloy → A mineral is an inorganic chemical compound that is normally crystalline and that has been formed as a result of geological processes (naturally occurring)  Most common mineral involve elements which are abundantand which bond stably (O, Si, Mg, and Fe) • Silicate Mineral → Fledspar, mica, quartz, olivvine,pyroxene • Minerals to Rocks → Most planetary materials are not pure minerals; mixtures of several major minerals locked together in grains → These "lumps" of mixed minerals are rocks → 3 major categories of rocks found on Earth  Igneous  Metamorphic  Sedimentary  Sedimentary • Rock Types I: Igneous Rocks → Rocks that solidified directly from molten silicates (magma, lava)  Either within the Earth's crust (intrusive or plutonic)  Or at the Earth's surface (extrusive or volcanic) → e.g. granite, basalt, pumice • Rock Types II: Sedimentary Rocks → Compactedand cementationof particles (sediment) derived from the erosion of other rocks → When minerals precipitate directly from water (e.g. cave stalactites) or from the compactionand cementationof shells or other fossil fragments → e.g. limestone,chalk, sandstone • Rock Types III: Metamorphic → Form when other rocks are deformed and/or transformeddue to intense pressures and temperatureor by reactions to other fluids → e.g. slate, marble, quartzite • The Rock Cycle • Inner structure of the Earth → Crust → Mantle → Core • The Earth → Rock samples of the interior:  Mines - max depth 15 km  Mines - max depth 15 km  Xenoliths, plutonic rocks → Radius: 6371 km and mass: 5.97x10 kg → Interior if earth more dense than that of surface materials → Self-compression: an effect of pressure within a planetary body, as a result of its own gravity, causing its internal density to be greater than it would be in the absence of any internal pressure → Compositional variation: transform to a more compact structure, chemical reaction between the constituent mineral within a rock to produce a more dense assemblage of minerals → Accreted from planetesimal 4.56 billion years ago and run-away growth formed planetary embryo then planet → We assume that the initial bulk compositionof the Earth was the same as the solar system as a whole (minus the most-volatileelements: H, He, C, N, Ne)  Dominatedby O, Mg, Si, Fe, Al, Ca  Basically the same as in a class of meteorites:Cl chondrites → Major elements: measured in percentages often expressed as weight percent oxide → Trace elements: expressed in parts per million (ppm) or ppb → Higher abundance of many major elements and smaller concentrationsof MgO in oceanic and continental crust compared to xenoliths → Also depletion/enrichmentwhen comparing with continental crust • Cl Chondrites → Meteorite:small extra-terrestrialbody found on Earth → Three main groups: stony, stony-irons,irons → Stony meteorites:  Must abundant  Closest to rocks found on Earth  Cl carbanaceous chondrites: most primitive • Earth vs Chondrites → Fe and Ni are severely depleted in the Earth's mantle and crust → Si, Ti, Ba, Nb, Th, U and Zr are far moreabundant in the earth's mantle and crust → This shows that: Chemical segregation as occurred   Compositional differences with depth  Bulk Earth has differentiated (fractionated - separated into different portions) • Inner structure of the Earth → Density, composition and material distributionin the Earth's interior can only be indirectly inferred from seismic data → Earthquakes produce different types of seismic disturbances which travel through the Earth → Measurementsof the velocities, types and propagation paths of seismic waves can be used to measure density, phase state and distribution of matter in the Earth's interior • Seismic Waves → Any large release of energy produces waves which travel through the Earth: i.e. Earthquake, explosion → Two Types:  P (push-pull) - Waves: compression waves (like sound) that can travel through solids, liquids and gases  S (shear) - waves: transverse waves (like waves on a string) that can travel only through solids → P and S waves also travel at different velocities in solids (P-waves have higher velocities) → Refraction: change in speed and direction of the wave as it moves between media of differingdensities differingdensities → Seismic waves refract through different layers in the Earth  The major regions: mantle and core  Based on the recorded locations of P and S waves relative to a seismic source we know that the outer core of the Earth is liquid since S waves do not pass through it (and P-waves moveslower across it)  Inner core is solid iron which we know since P-wavestravel at a different (higher) velocity through it than the outer core  MANTLE IS SOLID • Liquid outer core → Other indirect evidence for liquid outer core: Earth has a magnetic field → No known solid materials with magnetic properties above 1200°C → Core considerably hotter than this → Outer core is constantly stirred up • Inner structure of the Earth → Crust → Mantle → Core • Oceanic crust and continental crust • The mantle: → Composedof silicate material, but chemicallydistinct from the crust → Dominant minerals are olivine and pyroxene → Upper mantle is divided into  The lithosphere (rocky)  Asthenosphere (without strenth) • The core: → Very dense → Inner core: alloy (mixture of iron with about 4% nickel), solid → Outer core: broadly similar in compositionbut including lighter elements such as S, K, O, liquid • Origins of planets and planetary layering • Origins of planets and planetary layering → Condensation in the solar nebula: 4.6 Ga ago → As soon as the nebula has cooled down so that some elements/compoundscould condense  The most refractorysubstances cooled first  Progressivelymore volatile componentscooled as temp dropped more → = condensation sequence → Similar progression of condensing compositionsaway from the center • Origins of planets and planetary layering → Accretions:  Clumps 1-10 m  Primitivechondrites → Planetesimals  Gravitational focusing  Temp increases due to impacts  Less primitivematerials More collisions →  Few 1000 years to incorporate planetesimals → Planetary embryos(a few 100) → Giant impacts  Large temperatureincrease in the molten mantle  Differentiations → 10 million years: half mass, 100 million years: full growth → Layering of Earth  Differentiation:separating out different constituents as a consequence of their physical or chemical behavior  Rain-out model:core to mantle; fast process → Mantle vs. crust: element partitioning  The effect during partial melting: some elements becomemobilized • Planetary differentiation → Processby which a homogenousbody becomesheterogeneous → Driven by density → Homogeneousbody separates into layers with different properties • Magma ocean and crust formation → As Fe-Ni sank to form the core, silicate-rich magma rose to form a magma ocean → Cooling of magma ocean = layer of basaltic crust (present-day oceanic crust) • Origin of the moon → Moon lacks a large core → Lunar rocks reveal very similar geochemicaland isotopic signatures to the analogous rocks on Earth → Different compositionbetween Earth and Moon since moon is:  Depleted in volatile elements  Enriched in refractory elements  Severely depleted in siderophile elements(when molten, have an affinity to or combine with iron and so are removedfrom the silicate during differentiation process → Giant impact → Much of the debris from a fragmented impactor, plus someejecta from the larger "target" body ended up in orbit around the larger body → Larger body cooled and became Earth → Orbiting debris accreted to form the Moon → Final giant impact to have affected Earth → Theory has importantimplications for the origin and timing of layering in both bodies - requires partial different in both embryos → Depletion of siderophile elements and small size of Moon's core (if any) - depleted outer layers of embryos embryos → Depletion of volatile elements - accretion fo partially vaporized debris with more volatile elements able to escape • Origins of planets and planetary layering → Large core of Mercury: giant impact of differentiated embryos with part of the vaporized mantle materials dispersed and lost in space → Earth's depletion of volatile elementswrt chondrites  Light elements segregated to surface in embryos  Vaporized during subsequent impacts  Lost in space • How to cook a planet → Planets need to be heated in order to differentiate → Primordial heat:  Developedin the early stages of planetary evolution  Associated with accretion, collision and core formation All planets experienced accretional heating   Intensity of the bombardmentdecreases over time  Impacts are smaller and less frequent once planets are formed  Only heats the newly formed crust  Early curst on magma oceans created and destroyed many times due to convectionin the underlying moltenfraction, escape of gases, new impacts, infall of impact debris  There are remnants of early crust on planetary body's that are not resurfaced  Heat of impact - Differentiationof core-mantle - Sinking of Fe, Ni towards the core releases gravitational potential energy that is convertedto kinetic and then thermal energy - additional source of primordial heating  No continuous source of heating  Still contribute significantly to internal heat of planets → Tidal heating  Distortionof shape resulting from mutual gravitational attraction  Deformationcauses heating of the planet in the crust and mantle  Almost an insignificant source of heating → Radiogenic heating  Due to radioactivedecay  Amount of heating depends on concentrationand t
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