AST 111 Lecture 18(Terrestrial) Planetary Atmospheres I

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

Albedo

Does albedo warm or cool a planet?

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

Greenhouse Effect

Does the greenhouse effect warm or cool a planet?

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

Ultraviolet:

◦ Can dissociate (break apart) molecules

Interaction of Light and Atmosphere

Visible light:

◦ Usually transmitted, sometimes scattered

Interaction of Light and Atmosphere

Infrared light:

◦ Absorbed by molecules

◦ Causes rotation and vibration in molecules

Interaction of Light and Atmosphere

Interaction of Light and Atmosphere

(Page 304)

X-rays

Ultraviolet

Visible

Infrared

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

Very low density gas◦ Faster molecules

escape

Boundary between atmosphere and space

Gas very hot, but you wouldn’t feel it (low density)

The Exosphere