Terrestrial Planet Atmospheres Atmospheres in General •  A layer of gas surrounding a solid/liquid body •  The density decreases with height –  hydrosta<c equilibrium •  Temperature depends on energy input •  May include condensates (clouds) •  May have precipita<on Terrestrial Atmosphere Stats Physics of Atmospheres •  Pressure balance (hydrosta<c equilibrium) •  Gas pressure ~ nT –  N: gas density –  T: gas temperature •  Pressure is the result of collisions between atoms/molecules •  Thermal energy kT = kine<c energy ½ mv2 Atmospheric Pressure Earth standard atmosphere: •  1 bar = 106 dynes/cm2 = 105 N/m2 = 105 Pa •  1 bar = 750 tor (mm of Hg) = 29.5 in •  1 bar = 14.7lbs/sq in •  1 atmosphere = 1.03 bar •  Density ~ 1019 molecules/cm3 Atmospheric Pressure Top of the Atmosphere •  No clear upper boundary –  ρ and P decrease with al<tude. –  On Earth, above ~ 60 km, “edge of space” –  Low density gas extends for several hundred km more. •  Satellites in low Earth orbits experience fric<on Exosphere Thermosphere Mesosphere Stratosphere Troposphere Levels of the Atmosphere •  Troposphere: •  temperature falls with height •  Heated from below: Unstable to convec<on •  Stratosphere: •  temperature rises with height •  Heated in-­‐situ by solar UV •  Exosphere: •  essen<ally the vacuum of space •  Heated by X-­‐rays •  Includes the ionosphere Troposphere •  Sunlight heats Earth's surface •  Earth re-­‐radiates in IR •  Troposphere greenhouse gases absorb IR –  T decreases with height. –  The adiabat –  Convec<on and weather The Adiabat On an adiabat heat is not transferred •  Rising air: pressure decreases, volume increases, temperature falls (orographic uplid) •  Falling air: pressure increases, volume decreases, temperature increases (chinook winds) •  If temperature increases faster than surroundings, air becimes buoyant (convec<ve instability) •  If temperature falls faster than surroundings, air falls (convec<vely stable) This drives weather Stratosphere •  O3 absorbs UV photons •  Top of stratosphere absorbs more UV than boeom •  T increases with al<tude •  No convec<on •  stagnant •  Every terrestrial planet? Thermosphere •  All gases absorb X-­‐rays •  Solar X-­‐rays → absorbed by top of thermosphere –  T increases with al<tude •  Gas: (mostly) ions + free electrons –  Ionosphere reflects radio broadcasts Exosphere •  High T, low density gas –  Collisions rare •  Some ar<ficial satellites orbit in Earth's exosphere •  Atmospheric gases escape from Earth's exosphere Comparison of Terrestrial Planets Atmospheric Scale Height •  Ρ(h) = P(0) e-­‐h/h0 •  h0=kT/mg –  k = Boltzmann constant –  T=temperature –  m=mass of par<cle –  g=gravita<onal accelera<on •  Density falls of exponen<ally with height •  Mathema<cally, atmosphere never ends The Magnetosphere The Aurora Van Allen Belts •  Charged par<cles (protons and electrons) trapped in Earth’s magnetosphere –  Inner belt: 1600 – 13000 km –  Outer belt: 19,000 – 40,000 km Atmospheric Escape •  Thermal energy kT = kine<c energy ½ mv2 •  vT = √(2kT/m) •  Escape velocity vesc= √(2GmMp/r) The Greenhouse. I. •  A bare rock radiates ~ like a blackbody. •  The atmosphere modifies the equilibrium –  Clouds increase the reflec<vity (albedo) –  Greenhouse gases absorb IR re-­‐radia<on from the planet –  Inefficient radia<on èhigher than expected T •  Atmospheres warm planetary surfaces Energy Balance
(1-­‐a) πR⊕2 (L¤ / 4π d2) ⇒ ⇓ 4πR⊕2σT⊕4 The Greenhouse. II. •
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Earth mean temperature: 287 K Earth equilibrium temperature (a=0): 280 K Earth equilibrium temperature (a=0.39): 247 K Greenhouse effect: 40K Why is the Sky Blue? •  Molecules and dust sca,er photons. •  Scaeering is most efficient when the wavelength is close to the size of the scaeerer –  Rayleigh scaeering ~ λ-­‐4 –  Blue light is scaeered more efficiently than red light –  The blue sky is scaeered blue photons –  Most Rayleigh scaeering by N2 molecules –  Cigareee smoke appears bluish •  The sun appears yellow because blue has been scaeered out •  Why is the Sun red at sunrise/sunset? The sky is blue Polariza<on Rayleigh scaeered light is polarized Dusk Why is the Eclipsed Moon Red? Why is the Mar<an Sky Red? Mar<an Sunset Weather and Climate Weather: local varia<ons due to wind, storms, pressure changes, etc. •  Driven by convec<on Climate: long term behavior •  Driven by –  Insola<on –  atmospheric changes •  Composi<on •  circula<on Weather and Climate •  Energy input is Solar •  Atmospheric mo<ons are driven by local hea<ng (convec<on) •  Hot air rises (lower density); cold air falls •  Precipita<on cools atmosphere Basic Atmospheric Circula<on – no rota<on (Hadley cells) What Winds do for a Planet •  Equator heated more than poles •  Hadley cell transport heat poleward –  Earth's poles warmer than otherwise would be Coriolis Forces Coriolis Forces: A result of mo<on in a moving reference frame Terrestrial Winds What drives the wind? Coriolis effect breaks each circula<on cell into 3 •  Explains global wind paeerns (consider surface air movement) Coriolis effect: convec<on cells → East-­‐West winds. •  Venus: rota<on too slow (day is longer than year) •  Mars: too small •  Jovian planets: Coriolis effect important. Storms Condensa<on in the Atmosphere: Clouds •  Allow precipita<on (rain, snow, hail, ...) •  Alter energy balance –  Sunlight reflected—cools planet (increases albedo) –  Made of greenhouse gases—warms planet •  Water vapor carried to high al<tude condenses –  Large droplets fall → precipita<on •  Linked to convec<on –  Strong convec<on → more clouds and precipita<on –  Equatorial regions: high rainfall due to more sunlight –  Moisture removed by the <me convec<on reaches deserts Thursday: Atmospheres on the other terrestrial planets