Terrestrial Planet Atmospheres Atmospheres in General • A layer of gas surrounding a solid/liquid body • The density decreases with height – hydrostac equilibrium • Temperature depends on energy input • May include condensates (clouds) • May have precipitaon 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 (hydrostac 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 fricon Exosphere Thermosphere Mesosphere Stratosphere Troposphere Levels of the Atmosphere • Troposphere: • temperature falls with height • Heated from below: Unstable to convecon • 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 uplih) • 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 boom • T increases with al<tude • No convecon • 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=gravitaonal acceleraon • Density falls off exponen<ally with height • Mathemacally, atmosphere never ends Atmospheric Scale Height • Ρ(h) = P(0) e-h/h0 For Earth: T = 14C (287K) • h0=kT/mg <m> = 28.9 AMU = 5 x 10-23 gm – k = Boltzmann constant g = 980 cm/s2 – T=temperature h = 8.1 km = 5 miles – m=mass of par<cle – g=gravitaonal acceleraon Examples (constant T): • Density falls off exponen<ally Denver (1 mile): P=0.8 atm with height Mauna Kea: P=0.6 atm Mt Everest: P=0.33 atm • h is the distance the pressure drop by a factor of e (2.718) 37000 h: P=0.25 atm 100 km: P=4 x 10-6 atm 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-radiaon from the planet – Inefficient radiaon 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 scaer photons. • Scaering is most efficient when the wavelength is close to the size of the scaerer – Rayleigh scaering ~ λ-4 – Blue light is scaered more efficiently than red light – The blue sky is scaered blue photons – Most Rayleigh scaering by N2 molecules – Cigareie smoke appears bluish • The sun appears yellow because blue has been scaered out • Why is the Sun red at sunrise/sunset? The sky is blue Polarizaon Rayleigh scaered 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 variaons due to wind, storms, pressure changes, etc. • Driven by convecon Climate: long term behavior • Driven by – Insolaon – atmospheric changes • Composion • circulaon Weather and Climate • Energy input is Solar • Atmospheric mo<ons are driven by local heang (convec<on) • Hot air rises (lower density); cold air falls • Precipitaon cools atmosphere Basic Atmospheric Circulaon – no rotaon (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 circulaon cell into 3 • Explains global wind paerns (consider surface air movement) Coriolis effect: convec<on cells → East-West winds. • Venus: rotaon too slow (day is longer than year) • Mars: too small • Jovian planets: Coriolis effect important. Storms Condensaon in the Atmosphere: Clouds • Allow precipitaon (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 → precipitaon • Linked to convec<on – Strong convec<on → more clouds and precipitaon – Equatorial regions: high rainfall due to more sunlight – Moisture removed by the <me convec<on reaches deserts Next: Atmospheres of the other terrestrial planets .