1. (Terrestrial) Planetary Atmospheres I

2.  Atmosphere: ◦ Layer of gas that surrounds a world  Thin for terrestrial planets ◦ 2/3 of air within 10 km of Earth’s surface

3.  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

4.  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?

5.  Consider how gravity acts on a bunch of molecules in motion ◦ They “pile up” toward the surface ◦ The atmosphere below supports the atmosphere above

6.  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

7.  Distance from the Sun  Albedo: Reflectivity of surface and atmosphere  Greenhouse Effect: Trapping a planet’s emitted radiation

8.  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 SurfaceTypical Albedo Fresh Asphalt0.04 Worn Asphalt0.12 Coinifer Forest0.09 to 0.15 Bare Soil0.17 Green Grass0.25 Desert Sand0.40 Concrete0.55 Fresh Snow0.80 – 0.90

9.  Different materials respond differently to different frequencies of light!  Clouds reflect visible light. They do not reflect UV.

10. Does albedo warm or cool a planet?

11.  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 ◦ CO 2 (Carbon dioxide) ◦ CH 4 (Methane)

12.  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!

13. Does the greenhouse effect warm or cool a planet?

14.  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 o F if not for the greenhouse effect. ◦ We wouldn’t have liquid water.

15.  Venus has a high albedo and reflects 75% of incoming light. Why is it so hot (800 o F)?  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?

16.  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

17.  Variation of temperature with height  Due to how atmospheric gas interacts with sunlight

18.  X-rays: ◦ Can remove electrons from atoms (Ionizes them) ◦ Can dissociate (break apart) molecules

19.  Ultraviolet: ◦ Can dissociate (break apart) molecules

20.  Visible light: ◦ Usually transmitted, sometimes scattered

21.  Infrared light: ◦ Absorbed by molecules ◦ Causes rotation and vibration in molecules

22. (Page 304) X-rays Ultraviolet Visible Infrared

23.  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

24.  Troposphere gets the infrared light emitted by Earth ◦ Temperature drops farther from surface ◦ Has convection and storms  Dense air  Surface heat

25.  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

26.  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

27.  Very low density gas ◦ Faster molecules escape  Boundary between atmosphere and space  Gas very hot, but you wouldn’t feel it (low density)