Key Concepts: Lecture 11 Earth s Atmosphere and Greenhouse Effect Blackbody Radiation and Temperature Earth s Oceans Earth s Magnetic Field and Aurora The Green House Effect Temperature set by balancing energy in and energy out Energy In: From sunlight: 50% reaches the ground (the rest reflect back to space) Energy Out: Infrared light from the heated surface Without Atmosphere T(average)=-18C = 255 K With Atmosphere T(average)=+15C = 288 K Atmosphere is transparent to optical sunlight Water and Carbon Dioxide are opaque to infrared from surface - energy is trapped and thus heats the Earth s surface Note Kelvin Temperature scale: Absolute zero: 0K = -273 Celsius (C) 0C = +273K So -18C = 273-18=255K and +15C = 273+15=288K Earth s Atmosphere Atmospheric Composition 78% Nitrogen 21% Oxygen 1 % Argon, 0.03% Carbon dioxide, Water Protects the surface Blocks ultraviolet radiation from Sun Blocks cosmic rays and radiation Blocks small impacts Regulates Surface Temperature Clouds reflect sunlight Atmosphere traps and holds heat Atmospheric circulation moderates temperatures For Kelvin (K) temperature scale, see next slide Recall the Electro-Magnetic (EM) Spectrum Electro-magnetic waves of different wavelengths. Visible light is only a very small part of the whole EM spectrum.
Light is emitted from all objects depending on their temperature: Blackbody Radiation (see Ch. 2.4 in textbook) All objects with a temperature above absolute zero (-273C; -459F; 0K) emit light: the light waves carry energy. Temperature is a measure of how quickly the atoms in an object are moving and vibrating. Hotter objects emit more light energy because the motions of the electrons in their atoms are more violent: larger amplitude of waves in electric field. Hotter objects emit shorter wavelength light because the frequency of oscillation of electrons is higher. Question? Since the industrial revolution humans have been burning more trees and fossil fuel. This has caused the amount of CO 2 to increase in the atmosphere. What effect would you expect this to have on temperatures on Earth? Earth s temperature is about 300K 400 ppm was breached on May 9, 2013 400 Room temperature objects (i.e. T~300K) emit light that peaks in the infrared, with wavelengths ~10 microns. This is about 20 times longer than visible light and is not visible to our eyes.
Astro News Atmospheric Escape Properties of a gas behaves like a group of hard balls moving and colliding Speed depends on the temperature Light gas particles move faster When they collide with more massive particles they recoil at higher speeds The gravity of a planet holds the gas in More massive planets can hold lighter gasses Equal forces Larger accel The Primordial Atmosphere The original atmosphere - we expect this was mostly hydrogen and helium These are the most common elements in the universe Came from formation of Earth Atmosphere was lost Light atoms move faster than heavy Hydrogen and Helium have low mass Their velocity is above escape velocity (11km/s) Evolution of the Atmosphere Secondary atmosphere from volcanic outgassing Volcanoes emit CO 2, SO 2, H 2, N 2, water (H 2 O), methane (CH 4 ), ammonia (NH 3 ) Removing the carbon dioxide Dissolves in the oceans and is subducted Ends up in rocks 3 billion years ago mostly methane, H 2, SO 2 Formation of N 2 and CO 2 Ultraviolet sunlight breaks up methane and ammonia Nitrogen from ammonia - CO 2 from methane and water Hydrogen escapes into space
Oxygen (O 2 ) in the Atmosphere Very little primordial oxygen Almost no oxygen 2 billion years ago 1 billion years ago 2% of atmosphere was O 2 600 million years ago sudden increase Origin of oxygen Biological activity started ~3.5 billion years ago Plants convert CO 2 into O 2 and trap carbon Dead biomass is trapped in the Earth and subducted -> oil Origin of the Ocean After formation Almost no liquid water Water trapped inside the Earth Water from inside the Earth Volcanoes emit gasses and steam Steam cools and condenses into water Oceans in 4.5 billion years at current rate Water also came from space Earth is continual hit by comets Comets contain a large amount of water ice Water can be removed into the Earth by subduction The Ocean Water covers a large fraction of the Earth 71% of surface area Average depth 4 km Helps regulate and moderate temperatures Important for evolution of atmosphere Must have liquid water for life Contains much of the life on Earth Question? If the Earth had no water and therefore life had never occurred, what would the Earth s atmosphere be like? If you were trying to locate a planet with life on it, what atmospheric constituents would you look for?
The Magnetic Field The solid core rotates faster than the surrounding liquid core. This Dynamo acts like a generator. The field is roughly aligned with the rotation axis. The field flips every 10000 years or so. Rotating, electrically conducting, i.e. metallic, liquid core is necessary to produce a strong magnetic field. Question If the Earth had no magnetic field. What would happen to the Aurora? The Sun emits a large number of charged particles from solar activity These particles are trapped in the Earth s magnetic field The field channels them near the pole When they hit the atmosphere it causes the air to glow The Aurora