The Earth and the Moon We will discuss the general characteristics - PowerPoint PPT Presentation

The Earth and the Moon • We will discuss the general characteristics of the Earth as a point of useful comparison and contrast with other Solar System bodies. Astronomy 291 1

Interior Structure of the Earth • Core (largely Fe-Ni) – inner solid core – outer liquid core • Mantle (rocky material) • Crust (granite, basalts) • Atmosphere (mostly N 2 and O 2 ) Astronomy 291 2

The Interior of the Earth • Observations of the interior structure depend on propagation of seismic waves. • Seismic waves come in two varieties: – S-waves : “Shear waves” • Matter is displaced perpendicular to direction of propagation. – P-waves : “Pressure waves” • Compressional waves, like sound waves. Astronomy 291 3

4 P-Waves Astronomy 291

5 S-Waves Astronomy 291

Seismic Waves • P-waves travel faster than S-waves – multiple stations can thus pinpoint the epicenter • S-waves limited to 103° from epicenter. Astronomy 291 6

Seismic Waves • Higher densities near center cause seismic waves to speed up and refract • No direct P-waves are observed between 103° and 142° Shadow zone 103º to 142º Astronomy 291 7

The Surface of the Earth • Lithosphere is broken into about a dozen plates that move quasi-rigidly, floating on a partially molten upper mantle. Astronomy 291 8

The Surface of the Earth • Rift zone : where plates spread apart. Young surface. Astronomy 291 9

The Surface of the Earth • Subduction zone : Collision of plates, results in mountain building. Astronomy 291 10

Rift Zone • Mid-Atlantic ridge is an example of a rift zone. • North American and Eurasian plates are pulling apart here. Astronomy 291 11

Rift Zone • The Mid-Atlantic Ridge runs right through Iceland, and we can actually see the two plates pulling apart. Astronomy 291 12

Icelandic Shield Volcano • “Skjaldbreidur”, or “Broken Shield”, the archetype of shield volcanoes. Astronomy 291 13

Hawaii: Drifting over a Hot Spot • The Hawaiian Island chain is formed by a plate drifting over a permanent hot spot in the Earth’s mantle. Astronomy 291 14

Mauna Loa • Mauna Loa on Hawaii is the world’s largest shield volcano. Astronomy 291 15

Continental Drift Triassic Permian 225 Myrs ago 200 Myrs ago Astronomy 291 16

Continental Drift Cretaceous Jurassic 135 Myrs ago 65 Myrs ago Astronomy 291 17

Continental Drift Cretaceous Present 65 Myrs ago Astronomy 291 18

Continental Drift Fault line • The northward motion of the Pacific plate relative to the North American plate produces the San Andreas fault in California. Astronomy 291 19

The Earth’s Atmosphere • Primeval atmosphere – Accumulated during formation. – Consists of hydrogen (H 2 ), helium (He), ammonia (NH 3 ), and methane (CH 4 ). • Secondary atmosphere – Outgassing during differentiation process. – Carbon dioxide (CO 2 ) and water (H 2 O). • Keep this in mind when we discuss Venus! Astronomy 291 20

The Changing Atmosphere • Liquid water dissolves CO 2 out of atmosphere. • CO 2 reacts with other dissolved substances – Forms SiO 2 (sand), CaCO 3 (limestone), and other solid carbonates. • Methane and ammonia are dissociated by solar ultraviolet radiation. – There is no protective ozone (O 3 ) layer. Astronomy 291 21

The Appearance of Life • About 3 billion years ago, photosynthesis starts introduction of free oxygen (O 2 ) into the atmosphere. • Oxygen is highly reactive, and must be constantly replenished. Astronomy 291 22

Composition of Present Atmosphere Percentage Constituent 75.5% N 2 23.1% O 2 1.3% Ar 0.05% CO 2 trace Ne, He, CH 4 , Kr variable H 2 O vapor Astronomy 291 23

Atmospheric Structure Equation of Hydrostatic Equilibrium Astronomy 291 24

Structure of the Atmosphere • Troposphere : – Lowest, densest level. 80-85% of the atmospheric mass is in this layer, within 10 km of ground – Absorbs re-radiated IR emission from the ground (colder at higher altitude). – Convective motions. Astronomy 291 26

Structure of the Atmosphere • Stratosphere : – Convective motions replaced by laminar flow. – Absorption of solar UV by ozone, so warmer at higher altitude. Astronomy 291 27

Structure of the Atmosphere • Mesosphere : – Above ozone layer, ineffective heating because the density is so low. – Most important coolant is CO 2 . Astronomy 291 28

Structure of the Atmosphere • Ionosphere : – Partially ionized (by solar extreme UV radiation) gas. – High temperature because cooling ineffective. Astronomy 291 29

Atmospheric Circulation • Hadley circulation – warmest air at subsolar point rises – good model for a slow rotator, but not for Earth Astronomy 291 30

Atmospheric Circulation • Modified Hadley circulation – atmosphere coupled to the surface by friction; coriolis forces break up the Hadley cells. Astronomy 291 31

Effect of the Earth’s Atmosphere on Astronomy • Atmospheric effects on light: – scatters – absorbs All wavelength-dependent – refracts Astronomy 291 32

33 Astronomy 291

Scattering of Radiation • Depends on characteristic size L relative to wavelength λ • Case 1: L « λ (scattering by small particles, such as aerosols [small airborne particles]) – I scatter ∝ λ –4 (Rayleigh scattering) – Blue light more easily scattered – Accounts for blue skies, red sunset Astronomy 291 34

Scattering of Radiation • Case 2: L ≈ λ (scattering or red/infrared photons by dust particles ~1 μ m) – I scatter ∝ λ –1 – Wavelength dependence of scattering much weaker in infrared than optical Astronomy 291 35

Scattering of Radiation • Case 3: L » λ (scattering of optical light by water droplets) – I scatter ∝ λ 0 – Wavelength independence is why clouds are white ( τ ≈ 1) or gray ( τ » 1). Astronomy 291 36

The Earth’s Magnetosphere • Magnetic field generated by convective motion in molten core. – Deflection of solar wind around Earth. – Trapping of particles in “van Allen Belts” – Interaction between solar particles produces aurorae. – Magnetic field reverses polarity every million years or so (geological evidence). Astronomy 291 37

38 The Earth’s Magnetosphere Astronomy 291

Interaction of the Solar Wind and the Earth’s Magnetosphere Magnetopause or Stagnation Radius van Allen Belts Particle drift Astronomy 291 39

Aurora Borealis from the Space Shuttle Astronomy 291 41

The Northern Lights or Aurora Borealis Astronomy 291 42

Northern Lights are concentrated around the North Magnetic Pole Magnetic Pole Astronomy 291 43

The Moon • The Moon is in synchronous rotation because of tidal friction. • On account of librations, only 59% of the Moon’s surface can be seen from Earth. Astronomy 291 44

Lunar Librations • A time lapse movie showing changing phases of the moon along with librations – Also notice the Moon’s changing angular size Astronomy 291 45

The Moon Maria • Moon’s surface has lighter (higher elevations) and darker (lower elevations). • Galileo noted that the darker areas are relatively smooth. He called them “seas”. – Mare (singular) – Maria (plural) • Maria concentrated on lunar near side. Astronomy 291 46

The Moon • Maria concentrated on lunar near side (maria shown in blue/violet). Astronomy 291 47

Clementine Mosaic of the Moon Astronomy 291 48

49 Astronomy 291 Edge of Mare

Lunar Highlands • Lighter color. • Saturated with craters. • An older surface, not altered since heavy cratering era of planetary formation. Astronomy 291 50

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Age of Lunar Surface • Apollo samples (some 400 kg of rock returned by six lunar missions) show: – Age of highlands: ~ 4 billion years The rock shown here is 4.4 billion years old! Astronomy 291 52

Age of Lunar Surface • Apollo samples (some 400 kg of rock returned by six lunar missions) show: – Age of highlands: ~4 billion years – Age of lowlands: ~3.5 billion years Astronomy 291 53

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Cratering History of the Moon • Era of heavy cratering. – ~3.5 billion years ago (formation of highlands) – Unlike Earth, erosion is inefficient at obliterating craters. Astronomy 291 55

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More recent large impacts and subsequent flooding formed maria . Astronomy 291 57

58 Formation of Maria Astronomy 291

Crater Formation 1 Impact (at speeds up to 72 km s -1 for head- on impact) Solar escape speed at 1 AU = 42 km s -1 + Earth orbital speed = 30 km s -1 Astronomy 291 59

Crater Formation 2 Deep penetration and vaporization • Example: 1 km radius rock: π R 4 = ρ 3 M 3 π ( ) ( 4 ) 3 = 3 10 3000 3 = × 13 1 . 3 10 kg Astronomy 291 60