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Lecture 07 The Air We Breathe.pptx - The Air We Breathe:...

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The Air We Breathe:Earth’s AtmosphereAstronomy 1141
The Earth’s atmosphere today is composedof mostly Nitrogen and Oxygen.78%N221%O21%H2O(Water Vapor)0.93%Argon0.038%CO2Traces ofCH4(methane),Inert Gases (Ne, He, Kr, Xe)Particulatessilicate dustsea saltsulfatesNASA
In class discussion:1, Why is there no H and He?2, What is origin of of O2, CO2, and H2O?
The Earth’s atmosphere is divided vertically into fiveThermal Layers.Exosphere690 km
Atmospheric Pressure drops by 50% for every 5.6 kmyou go up in altitude.[Wikimedia Commons]Sea Level: Pressure ~1 kg/sq-cm(14 pounds/sq-in)Pressure dropsexponentiallyby ~50% for every5.6 km of altitude gain:5.6 km-50%~11km-25%56 km-0.1%Mt. Everest:Altitude: 8850 mPressure:~1/3 sea-level
About 50% of the atmosphere is below 5.6 km (18000 ft)About 90% of the atmosphere is below 16 km (52000 ft)
Earth’s gravity is too weak and it is too warm to retainhydrogen and helium in its atmosphere.H2HeH2HeH2HeAt Earth’s temperature, H & Hemovefasterthan Earth’s escapespeed and arelostto space.H2OO2CO2N2O3N2O2Heavier molecules (H2O, N2, CO2, O2,etc.) moveslowerthan the escapespeed and aretrapped.Thermal Escape
The ability of a planet to retain an atmosphere isdetermined by itsmassandtemperature.Moremassiveplanets havefasterescape speedsAt highertemperatures, molecules movefaster.But, at a given temperature,heaviermolecules moveslower.Massive planets retain an atmosphere even when hot.Smaller planets must becolderto retain an atmosphere.vesc=2GMRvth=3kTm
Background image
In class activity: calculating the escaping velocity forEarthvesc=2GMR
Giant, cool planetsretain all molecules.Tiny, hot planetslose all molecules.Mid-sized planets onlyretainmassivemolecules.
Question:Why is an atmosphere important for life?High air pressure is necessary forliquid waterto exist;all life on Earth requires liquid water.
5.5 kilometers up, water boils at 82°C (180°F).When air pressure dropsto0.006atmospheres, theboiling point and freezingpoint become the same!No liquid water, only ice orvapor, exists at pressuresbelow 0.006 atmospheresAt sea level, waterboilsat 100°C (212°F) andfreezesat 0°C (32°F).Liquid water cannot exist without an atmosphere.IceLiquidVapor5.5 km above sea-levelsea-level
Kepler 445cLP-791-18cGJ 143 bPi Men cTOI 270 cTOI 270 d20050010005000Temperature (K)-1012345678log Pressure (bar)321LiquidVapourSolidSupercriticalIhVIVIIXH2O adiabatsHHB lociTint= 25KTint= 50K456789101112

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Term
Fall
Professor
Tendrup

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