Planetary Atmospheres Earth and the Other Terrestrial Worlds

What is an atmosphere? An atmosphere is a layer of gas that surrounds a world

Earth’s Atmosphere Most gas within about 10 km of surface (height of tallest mountains) Consists mostly of molecular nitrogen: N2 -- 77% and oxygen: O2 -- 21% Most of the rest is Argon: Ar -- 1% That’s dry air: up to ~4% water vapor is held in the air too

Atmospheric Pressure Gas pressure proprotional to both density and temperature. P T Adding air molecules increases the pressure in a balloon. Heating the air also increases the pressure.

Atmospheric Pressure Pressure and density decrease with altitude because the weight of overlying layers is less Earth’s pressure at sea level is 1.03 kg per sq. meter 14.7 lbs per sq. inch 1 bar

Where does an atmosphere end? There is no clear upper boundary Most of Earth’s gas is < 10 km from surface, but a small fraction extends to >100 km Altitudes >60 km are considered “space”: sky is black, not blue

Where does an atmosphere end? Small amounts of gas are present even at > 300 km

Effects of Atmospheres Create pressure that determines whether liquids (like water) can exist on surface Absorb and scatter light Create wind, weather, and climate Interact with solar wind to create a magnetosphere Can make planetary surfaces warmer through greenhouse effect

Pop Quiz Print your name neatly (1 pt) Draw cutaway diagrams through the Earth and Mars, with relevant layers illustrated and labeled in approximately correct scales. (6 pt) What is the radius of Mars in km? (2pt) If the temperature at the surface of the earth goes up by 10% while the density of that gas stays constant, what happens to the pressure of that gas? (2pt)

Quiz Answers 2) 3) RM= 0.53  RE = 0.53  6378 km = 3380 km RM= 3397 km, but 0.5  6400 km = 3200 km is OK 4) P rises by 10% since pressure = density  temperature

Greenhouse Effect Visible light passes through atmosphere and warms planet’s surface Surface heats up, emits infrared light Gases in atmosphere absorb IR light from surface, trapping heat: its reemitted, but in random directions, so temporarily trapped CO2, H2O & CH4 So atm acts like a blanket, slowing escape of heat Greenhouse Effect Applet

Planetary Temperature A planet’s surface temperature is determined by balance between the energy of sunlight it absorbs and the energy of outgoing thermal radiation Thermal Equilibrium Applet

Temperature and Distance A planet’s distance from the Sun determines the total amount of incoming sunlight Temperature vs Radius Applet

Temperature and Rotation A planet’s rotation rate affects the temperature differences between day and night Temperature vs Day Length Applet

Temperature and Reflectivity A planet’s reflectivity (or albedo) is the fraction of incoming sunlight it reflects Planets with low albedo absorb more sunlight, leading to hotter temperatures Temperature vs Albedo Applet

“Without Greenhouse” Temperatures Venus would be 510°C colder without greenhouse effect! Earth would be 31°C colder (below freezing on average) Mars only 6 °C colder

Thought Question What would happen to Earth’s temperature if Earth were more reflective? a) It would go up. b) It would go down. c) It wouldn’t change

Thought Question What would happen to Earth’s temperature if Earth were more reflective? a) It would go up. b) It would go down. c) It wouldn’t change

Thought Question If Earth didn’t have an atmosphere, what would happen to its temperature? a) It would go up a little (less than 10° C). b) It would go up a lot (more than 10° C). c) It would go down a little (less than 10° C). d) It would go down a lot (more than 10° C). e) It would not change.

Thought Question If Earth didn’t have an atmosphere, what would happen to its temperature? a) It would go up a little (less than 10° C). b) It would go up a lot (more than 10° C). c) It would go down a little (less than 10° C). d) It would go down a lot (more than 10° C). e) It would not change.

Computing Planet Temperatures With no greenhouse gases we have the temperature depending on planet’s distance from Sun, d (in AU), & reflectivity (albedo), : Logical: T goes down if d goes up and T goes down if more light is reflected ( goes up) Example: For Mars, d = 1.524 and  = 0.16, so T = 280 K  [(1-0.16)/(1.5242)]1/4 = 280 K  [0.84/2.32]1/4 T = 280 K  [0.362]1/4 = 280 K  0.776 = 217 K = -56 C

Why do atmospheric properties vary with altitude?

Light’s Effects on Atmosphere Ionization: Removal of an electron Dissociation: Destruction of a molecule Scattering: Change in photon’s direction Absorption: Photon’s energy is absorbed

Light’s Effects on Atmosphere X-rays ionize molecules, so few get through UV light dissociates molecules Visible: Molecules tend to scatter blue light more than red Molecules can absorb infrared light

Earth’s Atmospheric Structure Troposphere: lowest layer of Earth’s atmosphere Temperature drops with altitude up to ~ 12 km Warmed by infrared light from surface and convection

Earth’s Atmospheric Structure Stratosphere: Layer above the troposphere Temperature rises with altitude in lower part to about 50 km, drops with altitude in upper part up to ~80 km This part is the mesosphere Warmed by absorption of ultraviolet sunlight by ozone

Earth’s Atmospheric Structure Thermosphere: Layer at about 100 km altitude Temperature rises with altitude X-rays and ultraviolet light from the Sun heat and ionize gases So also called Ionosphere

Earth’s Atmospheric Structure Exosphere: Highest layer in which atmosphere gradually fades into space Temperature rises with altitude; atoms can escape into space Warmed by X rays and UV light

Thought Question Why is the sky blue? a) The sky reflects light from the oceans. b) Oxygen atoms are blue. c) Nitrogen atoms are blue. d) Air molecules scatter blue light more than red light. e) Air molecules absorb red light.

Thought Question Why is the sky blue? a) The sky reflects light from the oceans. b) Oxygen atoms are blue. c) Nitrogen atoms are blue. d) Air molecules scatter blue light more than red light. e) Air molecules absorb red light.

Reminder: why the sky is blue Atmosphere scatters blue light from Sun, making it appear to come from different directions Sunsets are red because red light scatters less Also dimmer, since all blue & green light is gone, along w/ most yellow and some orange & red.

Atmospheres of Other Planets Earth is only planet with a stratosphere because of UV-absorbing ozone molecules (O3). Those same molecules partially protect us from Sun’s UV light. Sunburn & skin cancer arise from UV that gets through. No greenhouse temperatures

Earth’s Magnetosphere Magnetic field of Earth’s atmosphere protects us from charged particles streaming from Sun (solar wind)

Aurora Charged particles can enter atmosphere at magnetic poles, causing an aurora Aurora Borealis Applet

Basic Atmosphere Summary What is an atmosphere? A layer of gas that surrounds a world How does the greenhouse effect warm a planet? Atmospheric molecules allow visible sunlight to warm a planet’s surface but absorb infrared photons, trapping the heat. Why do atmospheric properties vary with altitude? They depend on how atmospheric gases interact with sunlight at different altitudes.

Weather vs. Climate Weather is the ever-varying combination of wind, clouds, temperature, and pressure Local complexity of weather makes it difficult to predict beyond a week or so Only detailed measurements plus massive computer models make predictions that good Climate is the long-term average of weather Long-term stability of climate depends on global conditions and is more predictable

Global Wind Patterns Global winds blow in distinctive patterns Equatorial: E to W Mid-latitudes: W to E High-latitudes: E to W In both troposphere and boundary of stratosphere (jet stream)

Circulation Cells: No Rotation Heated air rises at equator Cooler air descends at poles Without rotation, these motions would produce two large circulation cells just several km high

Coriolis Effect Conservation of angular momentum causes a ball’s apparent path on a spinning platform to change direction Counterclockwise spin means ball deflected right, regardless of going in or out Start here on 11/12/09