General Astronomy

TerrestrialAtmospheres

Cosmic AbundancesPer million H Percentage

Hydrogen H 1000000 92.4608%Helium He 80000 7.3969%Carbon C 450 0.0416%Nitrogen N 92 0.0085%Oxygen O 740 0.0684%Neon Ne 130 0.0120%MagnesiumMg 40 0.0037%Silicon Si 37 0.0034%Sulfur S 19 0.0018%Iron Fe 32 0.0030%

Element

Cosmic Abundances

HHe

All others

0.0000%

10.0000%

20.0000%

30.0000%

40.0000%

50.0000%

60.0000%

70.0000%

80.0000%

90.0000%

100.0000%

Hydrogen and Helium

Cosmic Abundances

C N O Ne Mg Si S Fe

0.0000%

0.0100%

0.0200%

0.0300%

0.0400%

0.0500%

0.0600%

0.0700%

All the remaining appreciable elements

Terrestrial Atmospheres

• As far as the inner worlds are concerned, Mercury and the Moon have essentially no atmosphere at all

• Venus has a very dense, hot atmosphere composed primarily of CO2

• Earth has a dense atmosphere composed primarily of N2

• Mars has a thin, cold atmosphere composed of CO2

Terrestrial Atmospheres• Mercury & Moon – No atmospheres• Venus

– Thick atmosphere, can’t see surface in visible light– 96% CO2 with sulfuric acid clouds and no water– Surface conditions

• Pressure = 90 bars• Temperature = 750 K = 900 °F

• Earth– Relatively thin atmosphere, transparent to visible

light– 78% N2 and 21% O2 with water clouds– Surface conditions

• Pressure = 1 bar• Temperature = 290 K = 60 °F

• Mars– Very thin atmosphere, transparent to visible light– 96% CO2 with thin carbon dioxide clouds– Suffers severe dust storms– Surface conditions

• Pressure = 0.007 bar = 7 mbar• Temperature = 130 K – 290 K = -220 °F – 70 °F

This is about 3000 meters of water;The crush depth for a human is about 350 meters

Terrestrial Atmospheres

Terrestrial Atmospheres

Terrestrial Temperatures

DistanceAU

Albedo No GreenhouseK F

With Greenhouse

K F

Mercury 0.38 0.11 440 +330 700(day)100(night)

+800-280

Venus 0.72 0.72 230 -45 740 +873

Earth 1.00 0.36 250 -9 288 +60

Moon 1.00 0.07 273 0 400 (day)100 (night)

+261-280

Mars 1.52 0.25 218 -67 223 -58

Weather & Climate• Weather is defined by local short-term, day-to-day

atmospheric changes– Examples include thunderstorms, fronts, and tornados

• Climate is defined by regional long-term atmospheric/surface changes– Examples:

• Arizona has had a hot, dry desert climate for over 100 years• Brazilian rainforests are hot and humid year in and year out

• Climate change represents a significant shift in the average weather patterns for a region over long time scales– Examples:

• Antarctica is a cold desert now, but was a tropical paradise 200 million years ago indicating a climate change

• Mars is a very cold desert, but may have been lush, wet, and warm 4 billion years ago

• Ice ages• Global warming

Weather & Climate• Mercury & Moon

– No atmospheres– Climates consist of extreme temperatures between day and

night• Venus

– Extremely dense atmosphere transports heat efficiently– Sulfuric acid clouds– Surface breezes– Mild weather

• Earth– Relatively thin atmosphere but very active– Hurricanes, tornadoes, thunderstorms, high & low pressure

systems– Extreme variety

• Mars– Very thin atmosphere– Carbon dioxide & water clouds– Significant dust storms

Atmospheric Circulation• Earth’s atmosphere is in a

constant state of change– Sunlight heats surface– Surface heats atmosphere producing

convection cells– Warmer air at equator moves

poleward as cooler air circulates to the equator (Hadley cells)

• Sunlight causes warm air to expand (high pressure); cooler air gets compressed (low pressure)– Wind drives weather away from high

pressure and into low pressure– Hurricanes, typhoons, and tornados all

form in low pressure systems

• Atmosphere provides a thermal blanket to moderate Earth’s surface temperature

Coriolis Effect• The Coriolis effect is caused by Earth’s

rotation – Earth rotates on its axis– Projectile fired from N. pole will be deflected to

the right– Projectile fired from equator northward travels

faster than Earth rotates, is deflected to the right

– Projectile fired in northern hemisphere is always directed to the right

– Projectiles fired in southern hemisphere are always deflected to the left

• Examples– WW II – German ships in South Atlantic did not

account for Coriolis force, but British did. Oops!

– Hurricanes in northern hemisphere spin counter-clockwise; opposite in southern hemisphere

• Bathtub vortex myth– Toilets and drains do not spin “backwards” in

southern hemisphere

Coriolis Effect

Hurricane Isabel in the northern hemisphere is shown in the upper left.

A cyclone in the southern Indian ocean is shown in the upper right.

Note the opposite circulation directions!

Ice Ages• Ice age causes

– Feedback loops– Polar land masses tend to hold

glaciers better than polar ocean masses

– Milankovitch Cycles• Eccentricity (100 kyr)• Obliquity (41 kyr)• Precession (26 kyr)• Unknown why Milankovitch cycles work

Milankovitch Cycles

Ice ages versus Warm ages

Greenhouse Effect• The greenhouse effect is the result of the atmosphere

trapping IR light– Atmosphere is transparent to visible light, but translucent to IR– Ground absorbs visible light and heats up; ground re-radiates IR, but atmosphere

reflects some of it– Without greenhouse effect, water on Earth would freeze (~ 33 ºC added by

Earth’s greenhouse)

• Causes of Greenhouse Effect– CO2 and H2O from volcanic outgassing– CO2 from fossil fuels– Trees that remove CO2 are being cut down

• Possible results– Ice caps melt and flood coastlines– Drought as more water is evaporated

• Possible Mitigating Factors– More clouds (except water vapor is a greenhouse gas)– Increased rainfall from increased evaporation?

The Goldilocks or 3 Bears Problem

Venus: Hot Stuff

Mars: Cold Stuff

Earth: Just right!

Early calculations of Solar System "habitable zone" 0.95 < D < 1.01 AU

First order equilibrium temperature of a planet depends primarily on distance from the Sun

Earth's temperature is regulated by:- solar input (energy applied)- greenhouse effect (energy retained)- internal heating (geothermal heat flow)

The Goldilocks or 3 Bears Problem

A few degrees makes a lot of difference!

If Earth was closer or farther from Sun, we might have radically different (inhospitable) conditions.

Too close: oceans boil awaysuper dense atmospheregreenhouse effect kicks inhot, dry like Venus

Too far: temperatures drop rapid glaciation

Runaway greenhou

se

Runaway refrigerat

or

CO2 in air!~90 Bar!

Atmosphere and Gravity• The gases of the atmosphere are affected by

– Temperature– Gravity

• The average speed of a gas molecule is proportional to the square root of the temperature and inversely proportional to its mass– The more massive the slower it moves– The higher the temperature, the faster it moves– For example, at 300K a hydrogen molecule will move at

about 2.5 Km/sec on the average; if we increase the temperature to 600K, the speed will increase by x 2.5 = 3.5 Km/sec

– If, instead, we were to use Oxygen (M = 16 MH), the speed would decrease by ¼ or 0.6 Km/sec

2

Atmosphere and GravityGravity comes into play by

– Determining the escape velocity of the planet.– As the distance from the surface increases, the atmosphere thins

• Deep in the atmosphere molecules tend to collide with each other frequently, where the air is thin, it is more likely that a molecule will not collide and escape (if moving fast enough)

– As a rule, atoms of a gas will escape if their average velocity is 1/6 of escape velocity

• The outer atmosphere of most planets is at or less than 500K– Hydrogen 3.2 Km/sec– Helium 1.6 Km/sec– Oxygen 0.8 Km/sec– Nitrogen 0.9 Km/sec

Mercury

Venus Earth Moon Mars Jupiter Saturn Uranus Neptune

Pluto

0.5 1.7 1.9 0.4 0.8 9.9 5.9 3.6 3.9 0.2

1/6 Escape Velocities in Km/sec

Atmospheric Evolution• Mercury & Moon

– Little evidence of volcanism and outgassing on Mercury– Moon may have been active and outgassed more than Mercury– Both planets’ small sizes and low escape velocities lead to thermal escape

of any gases that did happen to be outgassed

• Venus– Volcanically active early in its history outgassing CO2, CH4, H2O, NH3, SO2

– Proximity to Sun allows gases to reach altitudes high enough for photodissociation.

– Molecules are broken into constituent atoms. H escapes easily because it is light.

– Atoms recombine leaving a dominant CO2 atmosphere with traces of N2 and H2SO4

– Since all water is dissociated, no water is left to scrub CO2 from the atmosphere

– Massive greenhouse effect develops and bakes all remaining CO2 from surface rocks

Atmospheric Evolution• Earth

– Volcanically active early in its history outgassing CO2, CH4, H2O, NH3, SO2

– Distance from Sun prevents photodissociation of CO2 and H2O.– Remaining molecules broken into constituent atoms. H escapes easily.– Atoms recombine leaving a dominant CO2 atmosphere with significant N2

– Since water is not dissociated, water scrubs CO2 from the atmosphere– CO2 gets locked into surface rocks as CaCO3 (limestone)– Cyanobacteria begin converting CO2

• Plants scrub remaining CO2 leaving O2 – Hydrologic cycle keeps the water content of the atmosphere variable

• Mars– Volcanically active early in its history outgassing CO2, CH4, H2O, NH3, SO2

– Atmosphere probably looked like Earth’s early in its history, but was probably blown to space in a giant impact about 4 billion years ago

– Mars’ cold temperatures combined with low atmospheric pressure allow CO2 and H2O to freeze out