Introduction to the atmosphere

Introduction to the atmosphere1. Some features of atmospheres2. The Earth’s atmosphere1. Composition, Layers and bounds2. Radiation budget, circulation and chemistry.3. Origin and evolution of the atmosphere3. A brief look at other atmospheres in the SolarSystem4. The HeliosphereVíctor J. Herrerovherrero@iem.cfmac.csic.es

Atmosphere• “Gaseous envelope surrounding a celestial body andretained to it by gravity”

Pressure and gravity• Gravity imposes a roughly exponential decline of pressure(or density) with height:p⎛ mgz ⎞z= ⎜ − ⎟ ⎜⎝ kT ⎠ ⎝ l⎛ ⎞( z) p(0)exp= p(0)exp − ⎟ ⎠z is the altitudel is a characteristic scale heightl=(kT/mg )In the troposphere l ≈ 8.5 km

Gravity and escape• Molecules can escape the upper atmosphere of a body iftheir kinetic energy exceeds their gravitationalattraction (Jeans escape)• The escape velocity at the exobase (i.e. at the altitudewhere collisions become negligible) is given by:v es= 2grFor the Earth:v es ≈ 11.2 km s -1• Diffusion can often limit the access to the exobase

Gravity and escapeEscape velocityv es (km/s) v es /6Earth 11.2 1.87Moon 2.4 0.4Jupiter 59.5 9.9Related escape mechanisms:Average speed (km/s)100 K 300 K 1500 KH 2 1.02 1.77 3.96CH 4 0.36 0.62 1.40N 2 0.27 0.47 1.06Light species can drag heavy molecules and help themescape (hydrodinamic escape)Fast atoms or molecules can be produced by charge exchangein collisions with ions

Atmospheres

The Earth’s atmosphere:CompositionDry air:Gas %N 2 78.08O 2 20.95Ar 0.93CO 2 0.038•Trace gases : noble gases, CH 4 , H 2 , NH 3 , O 3 , N and S oxides …• Water vapour: 1‐4%• Particulate matter• Clouds (water droplets, ice particles)•Aerosols•Primary (dust, sea‐salt, volcanic ashes, soot)•Secondary (H 2 SO 4 , sulphates, nitrates, organic matter)

The Earth’s atmosphere:Layers

Troposphere• Temperature drops with height (≈‐6.5 K/km) .• Located between surface (T≈ 300 K) and tropopause (9‐18 km,p ≈ 200 mbar, T≈ 210 K), where the temperature drop stops.• Contains 80% of the total atmospheric mass and 99% ofwater and particualte matter.• Divided into two regions:• Boundary layer: quiet region, close to the surface (< 1 km).• Free troposphere: turbulent region, between boundarylayer and tropopause.

Stratosphere• Temperature increases with height (≈ 2 K/km), due to ozonephotochemistry• Located between tropopause (9‐18 km, p≈ 200 mbar, T≈ 210 K)and stratopause (≈50 km, p ≈ 1 mbar, T≈ 270 K )• Thermal inversion hampers vertical air movements and leads tostratification• The tropopause is an effective (but not impermeable) barrier fortransport between troposphere and stratosphere.

Mesosphere• Temperature drops with altitude (≈‐3 K/km)• Located between stratopause (≈ 50 km, p ≈ 1 mbar, T≈ 270 K)and mesopause (≈ 80‐90 km, p ≈ 1 μbar, T≈ 170 K)• The mesopause is the coldest place in the atmosphere.• The highest atmospheric clouds (noctiluent clouds) are formedin this region.

Thermosphere• Characterized by a rapid temperature increase (≈ 10‐20 K/km)with height due to O 2 photodissociation.• Located between mesopause (≈ 80‐90 km, p ≈ 1 μbar, T≈ 170 K)and thermopause ( 400‐1000 km, daytime T≈ 600‐2000 K)• Properties very sensitive to the insolation received• Density very low. The International Space Station operates inthis region (≈ 350 km)

Exosphere• Starts at the exobase (400‐1000 km), where collisionsbecome negligible.• H and He atoms begin to dominate.• Its upper limit is ill defined

Atmospheric bounds• The atmosphere has no clear bounds. There is a gradualtransition between atmosphere and “outer space”• For certain purposes limits can be defined.Magnetopause:(> 60,000 km)Influence of Earth oncharged particlesKármán Line(100 km)Viscous effects inaeronautics.

Radiation budget•The solar radiation is partlyreflected and partly absorbedand re‐emitted by the Earth.•The emission spectrum of theSun has a maximum in thevisible•The emission spectrum of theEarth has a maximum in the IR

Radiation budget :Temperature of the EarthAbsorbed incoming radiation:Emitted radiationS: solar constantA: average albedo(Stefan‐Boltzmann)Emission temperatureT e ≈ 254 K in accord withsatellite measurements, but theaverage temperature at theEarth´s surface is ≈ 288 K

Greenhouse effectGasContribution toT increase (K)H 2 O 20.6 62CO 2 7.2 22O 3 trop. 2.4 7N 2 O 1.4 4CH 4 0.8 2.5Other ≈ 0.6 2.5Sum 33 100Contribution togreenhouseeffect (%)Part of the energyIs absorbed by gasesin the loweratmosphere and reemittedtowards thesurface

Atmospheric circulation• Buildup of convective cells

Atmospheric circulation• Coupling of convection and Coriolis forces

Atmospheric chemistry• Is driven by the small UV component of the Sun’sradiation• Has distinct characteristics in the various layers• Troposphere: Dominated by oxidation processes• Stratosphere: Ozone chemistry• Thermosphere: Significant dissociation and ionization(ionosphere)The most complex atmospheric chemistry takes placein the tropospheric boundary layer

Tropospheric Chemistry• Oxidation reactions• Main daytime oxidant: OH radical, produced by:• O 3 +hν (λ>290 nm) → O( 1 D) +O 2• O( 1 D) + H 2 O → 2 OH• Main oxidant at night: NO 3radical, produced by:• O 3 + NO 2 → NO 3 + O 2

Tropospheric chemistry:urban smog•“London type”•Reducing character•Sulphur compounds•“Los Angeles type”•Photochemical•Oxidizing character•NO x and VOCs

Urban smog: Madrid

• Ozone photochemistry• “Oxygen only” cycle :• O 2 + hν (λ

The ionosphere• Atomic ions• Molecular ions• Cluster ions

Ions at high latitudes• Close to the poles, charged particles can enter and leavethe atmosphere along open magnetic lines• Polar wind: net outflow of ions (H + , He + , O + )• Polar aurorae: collisions of solar wind particles (mostlyelectrons) with atoms and molecules (≈ 100 km)

Evolutionof theEarth’satmosphere• Primieval atmosphere ?• Formed together with the planet (≈ 4500 Myr bp)• Species from the original nebula• Unlikely retention of H 2 and He (too light)• Main volatiles light hydrogenated compounds (CH 4 , NH 3 ,H 2 O)•Present isotopic abundance on Earth very different fromthe solar nebulaThe original atmosphere was lost (enhancedsolar wind, heat liberated in gravitationalcollapse…)

Evolution of the Earth’satmosphere• Secondary atmosphere• generated by outgassing• Probably formed by H 2 O and CO 2• Minor gases: NH 3 , CH 4 , N 2 …• Planet cooling → condensation of H 2 O→ oceans• CO 2 + H 2 O (liquid) → sedimentary carbonates (3800 Myr bp)• CH 4 and NH 3 are photolyzed (NH 3 → N 2 )With most H 2 O and CO 2 in condensedphases, the dominant atmospheric gas3800 Myr bp was probably N 2

Evolution of oxygen and lifeMost atmospheric O 2 was produced by photosynthesisFe 2+ →Fe 3+

Planets• Rocky planetsIce line• Formed below the frost line (≈ 3 AU)• Rich in metal and silicates. Small sizes• Unable to retain H 2 or He. Thin atmospheres• Gaseous planets• Formed beyond the frost line• Condensation of volatiles. Large sizes• Large atmospheres of H 2 and He

Atmosphere of Venus• Composed of CO 2 (96%) and N 2 (3%)• Minor gases: SO 2 , H 2 O, CO• H 2 SO 4 clouds• Surface temperature: 700 K• Surface pressure: 90 bar• “Superrotation” of zonal windsVery large greenhouse effect

Atmosphere of Mars• Composed of CO 2 (96%) and N 2 (3%)• Minor gases: O 2 , H 2 O, CH 4• some H 2 O and CO 2 trapped in ice caps• Average surface temperature: 227 K• Surface pressure: 6 mbar• Wind and dust storms

Atmospheres of the rocky planetsVenus Earth Mars

Atmosphere of Titan• Composed of N 2 (96%) and CH 4 (5% at surface)• Minor gases: CO, CO 2 , H 2 O, hydrocarbons• Photochemical aerosol layer of organic compounds• Average surface temperature: 94 K• Surface pressure: 1.5 barPossibly similar to some stage of the primitiveEarth’s atmosphere in composition andpressure

Atmospheres of thegaseous giants• Mostly composed of H 2and He• Minor gases: H 2O, NH 3, CH 4• Large and dense atmospheres• Probably no definite surface• Presence of clouds and photochemical aerosols

Atmospheres of Jupiter and Saturn

Atmospheres of Uranus andNeptune“Icy giants”UranusNeptune

Solar wind andheliosphere• The solar wind is a stream of highly energetic particles(mostly electrons and protons) emitted by the Sun.• The expansion of the solar wind ends in a shock structure.The region of space within this shock structure is calledthe heliosphere. It is a kind of very thin “atmosphere” forthe whole Solar System.• The termination shock, at ≈ 90 AU, has been recentlyaccessed by the Voyager 1 and Voyager 2 space missions.

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