Earth Astronomy 311 Professor Lee Carkner Lecture 12

Terra -- The Earth Goddess  Greeks and Romans personified the Earth as a mother goddess

Earth Facts  Size: km diameter   Orbit: 1 AU (1.5 X 10 8 km)  Description: wet, temperate, inhabited

Earth’s Celestial Motions  Earth is tilted on its axis by 23 1/2 degrees   Earth has large satellite, The Moon    May stabilize tilt of axis

Earth’s Atmosphere  Composition:    small amounts of water vapor, CO 2 other gasses  Very different from other atmospheres  

Early Atmosphere  Where did the original atmosphere come from?    Early composition:  Carbon dioxide (CO 2 )  Water (H 2 O)  Methane (CH 4 )  Ammonia (NH 3 )

Formation of Atmosphere  Start with CO 2, H 2 O, CH 4, NH 3   C, H, O form H 2 O, CO 2  CO 2 dissolves in H 2 O, H 2 O rains out to form oceans, N 2 left behind   Final atmosphere -- O 2 and N 2

Earth’s Atmosphere

Temperature  Temperature and composition of Earth’s atmosphere is regulated by the carbonate-silicate cycle    Surface temperature stays near the point where water is mostly liquid

Climate  The atmosphere circulates due to convection (hot thing rise, cool things sink)  Two basic sources of convection    The Earth’s rotation breaks the atmosphere up into convection cells that keep the air circulating, producing global weather patterns

The Earth’s Surface  What shapes the Earth’s surface?    Also, volcanoes and cratering  Unlike other planets the Earth does not have many craters  Craters on Earth are largely obliterated by erosion

Plate Tectonics  The two top layers of the Earth are the crust and the mantle    Crust is broken up into plates that float on the upper mantle  Plates move around and crash into each other forming trenches and mountains 

How Plate Tectonics Work

Plate Boundaries

Water  The Earth has more liquid water on its surface than any other planet  Water is present on Earth in all three phases (ice, liquid water, vapor) and continuously cycles between them    

Water on the Earth

Effects of Surface Alteration  Plate tectonics and erosion alter the Earth’s surface      Earth’s surface is constantly changing

Plate Collision -- The Himalayas

The Grand Canyon

Meteor Crater

The Earth’s Interior  The crust and atmosphere are very thin compared the rest of the planet (like the skin of an orange)   We learn about them by studying the seismic waves from earthquakes

Seismic Waves  Types of waves:  P waves: pressure or compression wave   S waves: shear waves   The different densities of the inner earth refract the waves   When an earthquake occurs we can measure the strength of S and P waves all over the Earth

Earthquake Studies of the Earth’s Interior

Seismic Waves and the Earth’s Interior  No S waves detected on opposite side of Earth    There is a shadow zone where no P or S waves are detected   Very faint P waves detected in shadow zone  Refracted by solid inner core

Structure of the Earth  Crust:  surface to 35 km   Mantle  km   Outer core  km   Inner core  km   Density increases toward the center

Structure of the Earth

Earth’s Magnetic Field  Rotation and the motions of the liquid iron core produce a magnetic field via the dynamo effect   This magnetic field helps shield the Earth from the solar wind

The Earth’s Magnetosphere

The Habitable Zone  In order to support life a planet must be in the habitable zone   Width of zone determined by the effectiveness of the carbonate-silicate cycle  Inner Edge --  Outer Edge --  For our solar system habitable zone width is about AU

Hypothetical Habitable Zone Too hot, water is destroyed can’t remove CO 2 Too cold, try to warm up with more CO 2 but CO 2 forms clouds and blocks sunlight Just right, temperature kept stable at ~273 K (water is liquid)

Why is the Earth Habitable?  It has the right temperature for liquid water (~273 K) because:    Has both water and plate tectonics for carbonate-silicate cycle

Summary  Earth is unique for at least two reasons  Large amounts of liquid water   Large amounts of free oxygen   Earth has liquid water and life because it is in the habitable zone

Summary: Atmosphere  Earth’s initial atmosphere composed of CHON      Mild temperature maintained by carbonate-silicate cycle

Summary: Surface  Solid iron inner core, liquid iron outer core, solid mantle and crust   Plate tectonics and erosion constantly alter surface