General Astronomy

1. General Astronomy Terrestrial Atmospheres

2. Cosmic Abundances

3. Cosmic Abundances Hydrogen and Helium

4. Cosmic Abundances All the remaining appreciable elements

5. 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

6. 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

7. Terrestrial Atmospheres

8. Terrestrial Atmospheres

9. Terrestrial Temperatures

10. 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

11. 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

12. 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

13. 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

14. 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!

15. 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

16. Milankovitch Cycles Ice ages versus Warm ages

17. 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?

18. The Goldilocks or 3 Bears Problem Why?  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)

19. 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 away • super dense atmosphere • greenhouse effect kicks in • hot, dry like Venus • Too far: • temperatures drop • rapid glaciation

20. Runaway greenhouse Runaway refrigerator CO2 in air! ~90 Bar!

21. 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

22. Atmosphere and Gravity Gravity 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 1/6 Escape Velocities in Km/sec

23. 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

24. 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