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

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

Sep 29, 2023 170 likes •899 views

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)

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Astronomy 291 1

The Earth and the Moon

  • We will discuss the

general characteristics

  • f the Earth as a point
  • f useful comparison

and contrast with other Solar System bodies.

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Astronomy 291 2

Interior Structure of the Earth

  • Core (largely Fe-Ni)

– inner solid core – outer liquid core

  • Mantle (rocky

material)

  • Crust (granite, basalts)
  • Atmosphere (mostly

N2 and O2)

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Astronomy 291 3

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.
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Astronomy 291 4

P-Waves

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Astronomy 291 5

S-Waves

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Astronomy 291 6

Seismic Waves

  • P-waves travel faster

than S-waves

– multiple stations can thus pinpoint the epicenter

  • S-waves limited to

103° from epicenter.

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Astronomy 291 7

Shadow zone 103º to 142º

Seismic Waves

  • Higher densities near

center cause seismic waves to speed up and refract

  • No direct P-waves are
  • bserved between

103° and 142°

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Astronomy 291 8

The Surface of the Earth

  • Lithosphere is broken into about a dozen

plates that move quasi-rigidly, floating on a partially molten upper mantle.

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Astronomy 291 9

The Surface of the Earth

  • Rift zone: where plates spread apart. Young

surface.

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Astronomy 291 10

The Surface of the Earth

  • Subduction zone: Collision of plates,

results in mountain building.

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Astronomy 291 11

Rift Zone

  • Mid-Atlantic ridge is

an example of a rift zone.

  • North American and

Eurasian plates are pulling apart here.

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Astronomy 291 12

Rift Zone

  • The Mid-Atlantic

Ridge runs right through Iceland, and we can actually see the two plates pulling apart.

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Astronomy 291 13

Icelandic Shield Volcano

  • “Skjaldbreidur”,
  • r “Broken

Shield”, the archetype of shield volcanoes.

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Astronomy 291 14

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.

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Astronomy 291 15

Mauna Loa

  • Mauna Loa on Hawaii is the world’s largest

shield volcano.

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Astronomy 291 16

Continental Drift Permian 225 Myrs ago Triassic 200 Myrs ago

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Astronomy 291 17

Continental Drift Jurassic 135 Myrs ago Cretaceous 65 Myrs ago

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Astronomy 291 18

Continental Drift Cretaceous 65 Myrs ago Present

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Astronomy 291 19

Continental Drift

  • The northward motion
  • f the Pacific plate

relative to the North American plate produces the San Andreas fault in California. Fault line

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Astronomy 291 20

The Earth’s Atmosphere

  • Primeval atmosphere

– Accumulated during formation. – Consists of hydrogen (H2), helium (He), ammonia (NH3), and methane (CH4).

  • Secondary atmosphere

– Outgassing during differentiation process. – Carbon dioxide (CO2) and water (H2O).

  • Keep this in mind when we discuss Venus!
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Astronomy 291 21

The Changing Atmosphere

  • Liquid water dissolves CO2 out of

atmosphere.

  • CO2 reacts with other dissolved substances

– Forms SiO2 (sand), CaCO3 (limestone), and

  • ther solid carbonates.
  • Methane and ammonia are dissociated by

solar ultraviolet radiation.

– There is no protective ozone (O3) layer.

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Astronomy 291 22

The Appearance of Life

  • About 3 billion years ago, photosynthesis

starts introduction of free oxygen (O2) into the atmosphere.

  • Oxygen is highly reactive, and must be

constantly replenished.

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Astronomy 291 23

Composition of Present Atmosphere

Percentage Constituent 75.5% N2 23.1% O2 1.3% Ar 0.05% CO2 trace Ne, He, CH4, Kr variable H2O vapor

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Astronomy 291 24

Atmospheric Structure

Equation of Hydrostatic Equilibrium

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Astronomy 291 26

Structure of the Atmosphere

  • Troposphere:

– Lowest, densest

  • level. 80-85% of

the atmospheric mass is in this layer, within 10 km

  • f ground

– Absorbs re-radiated IR emission from the ground (colder at higher altitude). – Convective motions.

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Astronomy 291 27

Structure of the Atmosphere

  • Stratosphere:

– Convective motions replaced by laminar flow. – Absorption of solar UV by ozone, so warmer at higher altitude.

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Astronomy 291 28

Structure of the Atmosphere

  • Mesosphere:

– Above ozone layer, ineffective heating because the density is so low. – Most important coolant is CO2.

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Astronomy 291 29

Structure of the Atmosphere

  • Ionosphere:

– Partially ionized (by solar extreme UV radiation) gas. – High temperature because cooling ineffective.

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Astronomy 291 30

Atmospheric Circulation

  • Hadley circulation

– warmest air at subsolar point rises – good model for a slow rotator, but not for Earth

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Astronomy 291 31

Atmospheric Circulation

  • Modified Hadley

circulation

– atmosphere coupled to the surface by friction; coriolis forces break up the Hadley cells.

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Astronomy 291 32

Effect of the Earth’s Atmosphere

  • n Astronomy
  • Atmospheric effects on light:

– scatters – absorbs – refracts All wavelength-dependent

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Astronomy 291 33

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Astronomy 291 34

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])

– Iscatter ∝ λ–4 (Rayleigh scattering) – Blue light more easily scattered – Accounts for blue skies, red sunset

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Astronomy 291 35

Scattering of Radiation

  • Case 2: L ≈ λ (scattering or red/infrared

photons by dust particles ~1 μm)

– Iscatter ∝ λ–1 – Wavelength dependence of scattering much weaker in infrared than optical

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Astronomy 291 36

Scattering of Radiation

  • Case 3: L » λ (scattering of optical light by

water droplets)

– Iscatter ∝ λ0 – Wavelength independence is why clouds are white (τ ≈ 1) or gray (τ » 1).

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Astronomy 291 37

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).

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Astronomy 291 38

The Earth’s Magnetosphere

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Astronomy 291 39

Interaction of the Solar Wind and the Earth’s Magnetosphere

Magnetopause or Stagnation Radius van Allen Belts Particle drift

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Astronomy 291 41

Aurora Borealis from the Space Shuttle

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Astronomy 291 42

The Northern Lights or Aurora Borealis

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Astronomy 291 43

Northern Lights are concentrated around the North Magnetic Pole Magnetic Pole

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Astronomy 291 44

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.

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Astronomy 291 45

Lunar Librations

  • A time lapse movie showing changing phases of the moon

along with librations

– Also notice the Moon’s changing angular size

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Astronomy 291 46

The Moon

  • 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. Maria

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Astronomy 291 47

The Moon

  • Maria concentrated on lunar near side

(maria shown in blue/violet).

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Astronomy 291 48

Clementine Mosaic of the Moon

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Astronomy 291 49

Edge of Mare

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Astronomy 291 50

Lunar Highlands

  • Lighter color.
  • Saturated with craters.
  • An older surface, not altered since heavy

cratering era of planetary formation.

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Astronomy 291 51

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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

The rock shown here is 4.4 billion years old!

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Astronomy 291 53

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

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Astronomy 291 54

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Astronomy 291 55

Cratering History of the Moon

  • Era of heavy cratering.

– ~3.5 billion years ago (formation of highlands) – Unlike Earth, erosion is inefficient at

  • bliterating craters.
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Astronomy 291 56

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Astronomy 291 57

More recent large impacts and subsequent flooding formed maria.

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Astronomy 291 58

Formation of Maria

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Astronomy 291 59

Crater Formation

1 Impact (at speeds up to 72 km s-1 for head-

  • n impact)

Solar escape speed at 1 AU = 42 km s-1 + Earth orbital speed = 30 km s-1

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Astronomy 291 60

Crater Formation

2 Deep penetration and vaporization

  • Example: 1 km

radius rock:

( ) (

)

kg 10 3 . 1 3000 10 3 4 3 4

13 3 3 3

× = = = π ρ π R M

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Astronomy 291 61

Crater Formation

2 Deep penetration and vaporization

  • Impact energy:

( )

joules 10 3 10 3 . 7 10 3 . 1 5 . 2 1

22 2 4 13 2

× = × × × × = = mV E

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Astronomy 291 62

How Much Energy Is This?

  • 1 Megaton TNT = 4.2 ×1015 joules
  • 1.3×1022 joules = 107 Megatons
  • The largest man-made nuclear weapons are

about 20 Megatons.

– Would level all of Columbus inside the I-270

  • uterbelt.
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Astronomy 291 63

Crater Formation

3 Circular crater and ejecta

– The 1 km rock produces a crater of diameter 100 km with 5 km high walls. – This is the size of some

  • f the more prominent

lunar craters such as Copernicus.

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Astronomy 291 64

Copernicus Rays

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Astronomy 291 65

Crater Formation

4 Ejecta form walls and a surrounding blanket.

– “Rebound” produces a central peak. – Surface underneath is fractured.

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Astronomy 291 66

Crater Formation

  • A “glancing blow” can

produce a chain of secondary craters.

Secondary craters

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Astronomy 291 67

Lunar Surface

  • No evidence for

volcanoes (in contrast to Earth, Venus, and Mars).

  • Rilles, domes, and

wrinkled ridges are evidence of past lava flows.

Hadley Rille

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Astronomy 291 68

Wrinkled Ridges

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Astronomy 291 69

Domes

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Astronomy 291 70

Lunar Regolith

  • Entire lunar

surface covered with dust, most of it lunar crust that has been pulverized by impacts.

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Astronomy 291 71

Differences Between Lunar and Terrestrial Rocks

1 All lunar rocks are igneous. 2 Lunar rocks do not have a trace of water.

– Earth rocks contain up to 3% water.

3 Iron in lunar rocks is not oxidized. 4 Lunar rocks are depleted in elements with low boiling points.

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Astronomy 291 72

Lunar Interior

  • Moon is

differentiated, yet geologically dead.

– Smaller than Earth, so shorter cooling time.

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Astronomy 291 73

Origin of Moon

  • Probable origin:

impact of Mars- sized protoplanet with differentiated

  • Earth. This accounts

for:

– composition differences (absence

  • f volatile elements

in lunar rocks). – small iron core of the Moon.