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Atmosphere:◦ Layer of gas that
surrounds a world
Thin for terrestrial planets◦ 2/3 of air within 10
km of Earth’s surface
Atmosphere
So what do atmospheres do?
◦ Pressure allows liquid phase of water
◦ Absorb and scatter light “Radiation shield” Ozone in Earth’s atmosphere absorbs UV radiation
◦ Wind and weather
◦ Can trap heat and warm the planet
Atmospheres and Planets
Molecules move fast and collide◦ 500 m/s on Earth◦ They therefore push on surfaces
Aside: Why don’t they travel across a room that fast?
Molecular Motion
Consider how gravity acts on a bunch of molecules in motion
◦ They “pile up” toward the surface
◦ The atmosphere below supports the atmosphere above
Pressure
Planets are able to hold onto their atmospheres longer if:
◦ They are large (stronger gravity)
◦ The temperature of the atmosphere is low
Molecules don’t try as hard to escape
Atmospheres and Planets
Distance from the Sun
Albedo: Reflectivity of surface and atmosphere
Greenhouse Effect: Trapping a planet’s emitted radiation
Factors That Determine Planetary Climate
Sunlight rejected by planet◦ Low Albedo:
Darker: absorbs more Soil, trees, etc.
◦ High Albedo: Lighter: reflects more
Cloud, ice caps, etc.
If the sunlight is reflected, it can’t warm the planet
Albedo
Surface Typical Albedo
Fresh Asphalt 0.04
Worn Asphalt 0.12
Coinifer Forest 0.09 to 0.15
Bare Soil 0.17
Green Grass 0.25
Desert Sand 0.40
Concrete 0.55
Fresh Snow 0.80 – 0.90
Different materials respond differently to different frequencies of light!
Clouds reflect visible light. They do not reflect UV.
Albedo
Visible light from Sun absorbed by the ground
Ground returns absorbed radiation as a continuous spectrum.
◦ Peaks in the infrared
Greenhouse gases absorb these infrared photons◦ Water Vapor◦ CO2 (Carbon dioxide)
◦ CH4 (Methane)
Greenhouse Effect
Keeps the lower atmosphere and ground warm
◦ Energy from the photons can be “exchanged” for kinetic energy through collisions
Cloudy nights can be warmer than clear nights!
Greenhouse Effect
Be thankful for it…
◦ The infrared radiation emitted by Earth would escape straight back into space if not for the greenhouse effect.
◦ Earth would be at 3 oF if not for the greenhouse effect.
◦ We wouldn’t have liquid water.
Greenhouse Effect
Venus has a high albedo and reflects 75% of incoming light. Why is it so hot (800 oF)?
Do Mercury and the Moon have a greenhouse effect? Why or why not?
Earth’s atmosphere is mostly diatomic nitrogen and oxygen (poor infrared absorbers). How would the temperature change if they were good infrared absorbers?
Examples
Planetary climates are modeled as follows:
◦ Calculate Effective Temperature Assumes planet absorbs all radiation, emits freely
◦ Calculate Albedo Temperature Assumes that planet can reflect incident radiation
◦ Calculate Atmospheric Temperature Assumes atmosphere can inhibit radiation emission
by the planet
Models for Planetary Climates
Variation of temperature with height
Due to how atmospheric gas interacts with sunlight
Atmospheric Structure
X-rays:
◦ Can remove electrons from atoms(Ionizes them)
◦ Can dissociate (break apart) molecules
Interaction of Light and Atmosphere
Infrared light:
◦ Absorbed by molecules
◦ Causes rotation and vibration in molecules
Interaction of Light and Atmosphere
The atmosphere scatters visible light◦ Think in terms of
light rays◦ If no scattering,
would see stars with Sun in view
Blue light scattered more than red◦ Red sunsets
Visible Light and the Surface
Troposphere gets the infrared light emitted by Earth
◦ Temperature drops farther from surface
◦ Has convection and storms Dense air Surface heat
Infrared Light and the Troposphere
Infrared not significant here
UV light absorbed by ozone here
◦ UV light from Sun
◦ The top layer absorbs more than the bottom
◦ Gets hotter with height to a point
No convection, no weather
Ultraviolet Light and the Stratosphere
Most gases absorb X-rays
They get absorbed by the first dense gas they encounter
◦ Exosphere not dense enough
This is the thermosphere
Gets hotter higher up
X-Rays and the Thermosphere