Earth Astronomy 311 Professor Lee Carkner Lecture 12

Terra -- The Earth Goddess   We use the old English names for the Earth, Sun and Moon, rather than the Latin ones

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

Earth’s Atmosphere  Composition:   21% O 2   Very different from other atmospheres   Outer planets: mostly H 2

Early Atmosphere  Where did the original atmosphere come from?   Bombardment of icy planetesimals (comets)  Early composition:   Water (H 2 O)   Ammonia (NH 3 )

Formation of Atmosphere   Ultraviolet light breaks up some molecules (four key elements: C,H,O,N)   CO 2 dissolves in H 2 O, H 2 O rains out to form oceans, N 2 left behind  Life (plants) forms and produces O 2  Final atmosphere -- O 2 and N 2

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 -- where water is lost by UV dissociation  Outer Edge --   Note: planet needs to be large enough to have atmosphere and plate tectonics

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)

Planet Temperature  Can estimate a planet’s temperature by radiation balance:   Emit radiation back into space   Note: this assumes the planet is just a big uniform ball with no atmosphere or climate   Remember 0 K is absolute zero and 300 K is about room temperature   T F = 1.8T K - 460

Radiative Power  Hot objects absorb and emit energy based on the Stefan-Boltzmann law P =  AT 4  Where:    is a constant = 5.67 X (W/(m 2 K 4 ))   for a sphere A = 4  r 2  T is the temperature of the object (in K)

Energy Balance  If we compare the energy the Earth emits with the energy it gets from the Sun, we can find its temperature T E = [R S /(2 D S )] ½ T S  Where: TETE  T S is the temperature of the Sun RSRS  D S is the distance from the Earth to the Sun

Plate Tectonics  The two top layers of the Earth are the crust and the mantle   Crust is hard and rigid   Plates move around and crash into each other forming trenches and mountains   Plate tectonics and water resurface the Earth  Most other planets resurfaced by volcanism and cratering

How Plate Tectonics Work

Plate Boundaries

Plate Collision -- The Himalayas

Seismic Waves  Types of waves:  P waves: pressure or compression wave  example:  S waves: shear waves  example:  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   Core must be liquid  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  composed of silicates and heavier material  Outer core  km   Inner core  km  composed of solid iron 

Next Time  Read Chapter 8  Just the moon parts

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

Summary: Atmosphere  Earth’s initial atmosphere composed of CHON  H and O form water -- oceans  C and O form carbon dioxide -- rock  N stays in atmosphere  Plants produce oxygen  Mild temperature maintained by carbonate-silicate cycle

Summary: Surface  Solid iron inner core, liquid iron outer core, solid mantle and crust  Crust is broken up into plates which slide around on the upper mantle  Plate tectonics and erosion constantly alter surface