Astronomy 340 Fall 2005 6 October 2005 Class
Astronomy 340 Fall 2005 6 October 2005 Class #10
Terrestrial Atmospheres n n Composition Structure Energy Balance Atmospheric Circulation q q n Formation and evolution of clouds Effect on thermal properties Origin and Evolution
Composition of Terrestrial Atmospheres n Venus q q n Earth q q q n CO 2 86. 4 bar, 95% N 2 3. 2 bar, 3. 5% Ar 0. 0063 bar, 0. 007% H 2 O 0. 009 bar, 0. 01% Note differences in total pressure… CO 2 0. 000355 bar, 0. 035% N 2 0. 78 bar, 77% Ar 0. 0094 bar, 0. 94% H 2 O 0. 01 bar, 1% O 2 0. 21 bar, 21% Mars q q CO 2 0. 0062 bar, 95% N 2 0. 00018 bar, 2. 7% Ar 0. 0001 bar, 1. 6% H 2 O <0. 00001 bar, 0. 006% What about the rocks?
Fate of Solar Radiation (Earth) n n n 22% absorbed in atmosphere 24% absorbed by ground 47% scattered by clouds q q n n n 26% reflected back into space 21% scattered to ground 7% reflected back into space by ground Total 45% gets to ground total albedo ~ 33% Venus 2. 5% gets to ground
Basic Structure n n n Troposphere q Closest to ground, highest density q T decreases with increasing altitude Stratosphere q Up to ~50 km q Constant T on Earth, Mars, decreasing with altitude on Venus Mesosphere q 50 km < H < 100 km q Constant T on Earth, Mars, decrease flattens out on Venus Thermosphere q Diurnal variations q T dramatically increases with altitude on Earth exosphere
Saturates n n Gases reach maximum concentration of vapor that the atmosphere can hold and then they condense Result = clouds affect energy balance via reflection of incoming radiation q q q H 2 O on Earth H 2 O, CO 2 on Mars condensation then sublimation when it warms up H 2 SO 4 on Venus
Atmospheric Circulation driven by temperature gradients… n Vertical circulation convection q q q n Isothermal approximation apply to one layer at a time? q n Packet of gas rises, pressure is less, packet expands and cools, or packet is compressed, sinks, and warms Redistribute energy vertically through atmosphere Surface heating via solar radiation more thermal mixing “scale height” height over which pressure/density decreases by e. Variation with latitude – equatorial regions warmer than the poles warm air moves towards poles q q Mixing over latitudes in largescale cells Hadley cells. Combine with rotation trade winds, etc
Mercury & The Moon n n n H 200 cm-3 He 6000 cm-3 O <40, 000 cm-3 Na 20, 000 cm-3 K 500 cm-3 Ar 107 cm-3 n n n H <17 cm-3 He 2000 -4000 cm-3 O <500 cm-3 Na 70 cm-3 K 16 cm-3 Ar 40000 cm-3 Transient, easily ionized via solar radiation; lieftimes of hours/days Originate in impacts, solar wind Re-freeze on surface or escape
Origin of Terrestrial Atmospheres n n n Hydrated minerals in “planetesimals” q Asteroids up to 20% by mass of H 2 O q N 2 also found in asteroids comets Atmosphere by accretion q Accretion heating/releasing gas thick primordial atmosphere n n n q q Initial outgassing Continued outgassing via differentiation Ongoing volcanism/tectonics Condensation only after end of accretion Geochemistry – 40 Ar/36 Ar ratio n n 40 Ar product of radioactive decay of 40 K 36 Ar “primordial”
Evolution of Terrestrial Atmospheres n n CO 2 feedback removed from atmosphere deposited onto ocean floor as carbonates eventually recycled via subduction/volcanism Life O 2 production Runaway greenhouse (Venus) increase temp, increase H 2 O evaporation, increase atmospheric density, increase temp H 2 O dissociates, H escapes, total net loss of water from Venus Mars loss of atmosphere, big cooldown q q q Impact ejects atmosphere Solar wind stripping (particularly of ionized particles) Is it all in the rocks?
- Slides: 10
