Chapter 10.1 Planetary Atmospheres

10.1 Atmospheric Basics Our goals for learning What is an atmosphere? How do you obtain an atmosphere?

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

How do you obtain an atmosphere? Gain volatiles by comet impacts outgassing during differentiation Ongoing outgassing by volcanoes

Keeping an Atmosphere Atmosphere is kept by the world’s gravity Low mass (small) planets = low gravity =almost no atm. High mass (large) planets = high gravity = thick atm. Gravity and pressure Air pressure depends on how much gas is there (i.e. The atmospheric thickness.)

Gravity and Atmospheric Pressure The stronger the gravity, the more gas is held by the world and the greater the weight of atm. on a point

Earth’s Atmosphere About 10 km thick Consists mostly of molecular nitrogen (78%) and oxygen (21%)

Atmospheric Pressure Gas pressure depends on both density and temperature. Adding air molecules increases the pressure in a balloon. Heating the air also increases the pressure.

What 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

Temperatures and composition change with Height giving structure to an atmosphere

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

Earth’s Atmospheric Structure Stratosphere: Layer above the troposphere Temperature rises with altitude Warmed by absorption of ultraviolet sunlight in the Ozone layer

Earth’s Atmospheric Structure Mesosphere: Layer above the stratosphere Temperature decreases with altitude Coldest layer of the atmosphere

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

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

What have we learned? How do you obtain an atmosphere? What is an atmosphere? A layer of gas that surrounds a world How do you obtain an atmosphere? comet impacts. outgassing by differentiation, volcanoes, Why do atmospheric properties vary with altitude? They depend on how atmospheric gases interact with sunlight at different altitudes.

Atmospheric Processes 1 Our goals for learning What are the key processes? How does a planet gain or lose atmospheric gases? How does the greenhouse effect warm a planet?

Atmospheric Processes Gaining and losing atmosphere Gains: volcanic outgassing, impacts, evaporation. Losses: gas escape,impacts,condensation,surface reactions Greenhouse Effect Infrared energy is re-radiated back to the ground by CO2 Atmospheric circulation (convection) Convection cells move gas from equator to pole and back. Coriolis Effect Wind direction is deflected by the rotation rate of the world.

Atmospheric Gains

Atmospheric Losses

Greenhouse Effect

Air Movement and Flow

What have we learned? There are 3 ways of adding to atmosphere and 4 ways of depleting it. Gas molecules can transfer out to space or down to the ground. 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.

Atmospheric Processes 2 Weather and Climate Our goals for learning What creates wind and weather?

Air Movement Gas molecules move from high density to lower density

Atmospheric Pressure Gas pressure depends on both density and temperature. Adding air molecules increases the pressure in a balloon. Heating the air also increases the pressure. (molecules more energetic)

Atmospheric Circulation (convection) Heated air rises at equator Cooler air descends at poles Maximum Sun warming

Coriolis Effect

Coriolis effect deflects north-south winds into east-west winds

Coriolis Effect breaks up Global Circulation On Earth the large circulation cell breaks up into 3 smaller ones, moving diagonally Other worlds have more or fewer circulation cells depending on their rotation rate

Coriolis Effect Winds blow N or S Winds are diagonal Winds blow W or E Venus Earth Mars Jupiter, Saturn Neptune, Uranus(?)

Total Atmosphere Circulation

Intense radiation at the equator warms the air Air cools as it rises, moisture condenses and falls as rain Warm air rises, collecting moisture Lots of rain in the tropics!

some of the rising air flows north some of the rising air flows south Rising air is now dry… some of the rising air flows north some of the rising air flows south Dry air descends at around 30º N …and at around 30º S Deserts Deserts The descending air flows N and S

Circulation patterns repeat at 30-60º and 60-90º… Hadley cells Dry Wet Hadley cells Dry Polar cells (60-90º) Wet Ferrell cells (30 - 60º) Dry These are called circulation cells – the basic units of Vertical atmospheric circulation Wet Dry

(vertical circulation) (horizontal circulation, Air rises and falls in Hadley, Ferrel, and Polar cells (vertical circulation) Circulation cells explain global distribution of rainfall Earth’s rotation determines wind direction (horizontal circulation, Coriolis force)

What have we learned? What creates wind and weather? Atmospheric heating and Coriolis effect. Solar warming creates convection cells. The coriolis effect drags winds sideways and breaks up the cells The faster a planet spins, the more E-W gas movement there is

Weather and Climate Weather is the ever-varying combination of wind, clouds, temperature, and pressure Local complexity of weather makes it difficult to predict Climate is the long-term average of weather Long-term stability of climate depends on global conditions and is more predictable

Factors that can Cause Long-Term Climate Change Brightening of Sun Changes in axis tilt Changes in reflectivity Changes in greenhouse gases

Changes in Greenhouse Gases Increase in greenhouse gases leads to warming, while a decrease leads to cooling

Most major greenhouse gases are increasing in atmospheric concentrations

Global Warming

Seasons Our goals for learning What creates the seasons on Earth?

For this reason, it’s warmer near the equator than at the poles. Climatic Variation & Seasons on Earth Uneven heating of Earth’s surface causes predictable latitudinal variation in climate. Why? - Angle of incidence… equator vs. poles North Pole Equator Earth South Pole Thus, radiation is more intense near the equator compared to the poles. For this reason, it’s warmer near the equator than at the poles.

Uneven heating of Earth’s surface causes atmospheric circulation Greater heating at equator than poles 1. sun’s rays hit more directly (perpendicular) 2. less atmosphere to penetrate Therefore 1. Net gain of energy at equator 2. Net loss of energy at poles Now let’s look at regional variations in Earth’s energy budget

What about seasons? Why do we have them? Earth’s distance from the sun varies throughout the year – doesn’t that cause the seasons? Tilt! Because of the tilt of Earth’s axis, the amount of radiation received by Northern and Southern Hemispheres varies seasonally A. Northern Hemisphere has summer when it tilts toward the sun, winter when it tilts away B. Southern Hemisphere has summer when it tilts toward the sun, winter when it tilts away

Earth’s Seasons Earth North Pole Equator South Pole Tilt of the Earth’s axis towards or away from the sun creates the seasons When the north pole tilts toward the sun, it gets more radiation – more warmth during the summer SUMMER (Northern Hemisphere) North Pole Equator Earth When the north pole tilts toward the sun, the south pole tilts away So when it’s summer in the north, it’s winter in the south South Pole WINTER (Southern Hemisphere)

Earth’s Seasons Earth North Pole Equator South Pole Tilt of the Earth’s axis towards or away from the sun creates the seasons When the north pole tilts toward the sun, it gets more radiation – more warmth during the summer North Pole WINTER (Northern Hemisphere) Equator Earth When the north pole tilts toward the sun, the south pole tilts away So when it’s summer in the north, it’s winter in the south South Pole SUMMER (Southern Hemisphere)

Earth North Pole Equator South Pole Earth’s Seasons Tilt of the Earth’s axis towards or away from the sun creates the seasons WINTER (Northern Hemisphere) When the north pole tilts away from the sun, it gets less radiation – So it’s colder during the winter North Pole Equator Earth South Pole When the north pole tilts away from the sun, the south pole tilts toward it… When it’s winter in the north, it’s summer in the south SUMMER (Southern Hemisphere)