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 Atmospheres are Insignificant
• Earth’s atmosphere % by mass: 0.00009 Atmospheres are Insignificant
• Earth’s atmosphere % by mass: 0.00009
NOT! 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 upli ) • 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 bo om • 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 off exponen ally with height • Mathema cally, atmosphere never ends Atmospheric Scale Height
• Ρ(h) = P(0) e-h/h0 For Earth: T = 14C (287K) • h0=kT/mg
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 èhigher than expected T • Atmospheres warm planetary surfaces Energy Balance
2 2 (1-a) πR⊕ (L¤ / 4π d ) ⇒
⇓ 2 4 4πR⊕ σT⊕ The Greenhouse. II.
• 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. • Sca ering is most efficient when the wavelength is close to the size of the sca erer – Rayleigh sca ering ~ λ-4 – Blue light is sca ered more efficiently than red light – The blue sky is sca ered blue photons
– Most Rayleigh sca ering by N2 molecules – Cigare e smoke appears bluish • The sun appears yellow because blue has been sca ered out • Why is the Sun red at sunrise/sunset? The sky is blue Polariza on
Rayleigh sca ered 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 pa erns (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 Next:
Atmospheres of the other terrestrial planets