Astronomy 311: Lecture 4 - Terrestrial Planetary Atmospheres• An atmosphere is a layer of gas that surrounds a planet.• On Earth, atmosphere is about 10km thick: O2, N2, H2O, CO2.• Atmospheric pressure decreases as you go up. Principle of hydrostatic equi-librium applies.• Ideal gas law: P = ρT × constant.• Average pressure at sea level is 1.03 kg/m2which is 1 bar o f pressure.• Atmospheres create pressure that can determine whether water is in liquidform on the surface.• Atmospheres can absorb and scatter light: daytime skies are blue in worldswith atmospheres.• Atmospheres create winds and weather.• Atmospheres can contain gases which make planetary surfaces warmer thanthey would be otherwise using the greenhouse effect.• Greenhouse effect– Visible light from Sun absorbed by the ground.– Ground reradiates this radiation in the form of I=infra-red (IR) lightbecause this is the appropriate wavelength for a body of that temper-ature (Wien’s law).– Then some gases in lower atmosphere ”trap” this IR radiation for awhile and are heated by it.– These greenhouse gases are water vapor (H2O, CO2, and CH4).– These gases aborb an IR photon coming from the ground and thenreradiate it in a r andom direction. Further collisions occur.– Escape of t his IR radiation is slowed down but ensuing molecular mo-tion heats the surrounding air.– The more greenhouse gases that are present the greater the degree ofsurface warming.– These gases abosrb IT because their molecular structure makes themprone to begin rotating or vibrating when struck by a IR photon.– Energy balance is not altered.• Without the greenhouse effect, planet’s temperature much cooler.1• Planet’s temperature without greenhouse a ff ected by distance from Sun(inverse square law) and the planet’s reflectivity.• Black absorbs more than white.• Rflectivity depends on surface composition and color.• For example, the temperature of a planet’s surface is roughlyT = 280K × 4s(1 − ref)d2.• X rays ionize most gases and dissociate molecules.• UV rays dissociate molecules.• Visible light: blue light is scattered more than red light.• IR photons abosrbed by molecules causing them to rotate and vibrate.• Troposphere, Stratosphere, Thermosphere and Exosphere.• Sky is blue because blue light is scattered more than red light.• Sunsets are red because you ar e watching the beam r ather than the scat-tered light.• Greenhouse effect warms the troposhere and because more is absorbedcloser to the ground, temperature drops with altitude in troposhere.• Without greenhouse effect not such a difference in temp. between mountaintops and sea level.• Convection in troposphere: storms, weather etc.• At top of troposphere IR photons no longer collide and go into space.• Now only the effect of sunlight on atmosphere.• Now primary source of atmospheric heating is UV photons from Sun.• This is stronger at higher altitudes so temp increases with altitude in lowerstratosphere.• Convection cannot occur in stratosphere because of this temp. inversion.• No weather and no rain: pollution stays here.2• Planet can only have a stratosphere if its atmosphere contains moleculesthat are good at absorbing UV photons: Ozone, O3plays this role on Earth.• Earth is the only terrestrial world with a stratosphere.• Nearly all gases are good X-ray absorbers so X rays are absorbed by thefirst gases they encounter in atmosphere.• Desnity in exosphere too low so not much absorption there.• Most X rays absorbed in thermosphere. Virtually no Xrays penetrate belowthermosphere.• Part of thermosphere is ionized: ionosphere. This layer reflects radio wavesand hence important for communications.• Exosphere is low-density boundary between a t mosphere and space. Hightemperature but low density.• Mercury and Moon have contain tiny atmpsheres: very small, no structure,essentially an exosphere.• Mars, Earth and Venus have a warm troposhere at the base caused by thegreenhouse effect and a warm thermosphere at the top caused by X-rayabsorption.• Only Earth has a stratosphere.• Venus, Earth and Mars have all experienced and are experiencing the green-house effect in order of severity, starting with the most severe.• Solar wind: outpouring of subatomic charged particles from Sun.• Earth’s strong magnetic field creates a magentosphere that deflects thesecharged particles around the planet.• Some enter through the poles and reside in the Van-Allen belts encirclingEarth.• The interaction of these charged particles produces aurora eg. Northernlights.• Climate Change– Solar brightening due to the Sun brightening somewhat during its mainsequence lifetime.3– Tilt of the earth’s axis can change due to gravitational tugs from otherplanets: Earth’s axis changes from 22 to 25 degrees. Greater tilt meansmore severe seasons. These tilt changes are called Milankovitch cycles.Mars experiences more extreme changes in axis tilt than the Earth.– Changes in reflectivity of a planet. One possible cause for changes inEarth’s reflectivity is human activity, deforestation, smog etc. Smog isa good reflector and so one theory is that increased smog has actuallyreduced increasing temperatures due to increased greenhous gas emis-sion becuase the smog reflects a lot of sunlight back. Hence efforts toclean up smog pollution may actually accelerate greenhouse warming.– Changes in greenhouse gas abundance.• Origin of Atmospheres– During their births, planets captures some H,He but the planetesimalswere too small to retain these primitive a t mospheres during the solarT-Tauri phase.– It is thought that volcanic outgassing has been the primary source forterrestrial atmospheres: terrestrials made up of metal a nd rock butimpacts of ice-rich planetessimals from beyond the frost line broughtin water and gas that became trapped in the interiors. This was further”solidified” when the crust formed and the mantle started to cool fromthe outside in. Volcanic outgassing releases H2O, CO2, N2, H2S, SO2.It is thought that this occurred either during the molten mantle stageor as a consequence of a catastrophic collision f or the Earth.– Recent work though suggests that argon can stay trapped in the mantleeven at very high temperatures, with mantle convection and volcanism.– This suggests its is difficult for the Earth to outgass all the argontrapped in the mantle. Argon is a lso a