EART 160: Planetary Science 20 February 2008

Last Time Elastic Flexure Paper Discussion – Titan Atmosphere –Tobie et al., 2005 Planetary Atmospheres –General Description

Today Homework 4 due, 5 out –Oh, huzzah. Planetary Atmospheres –Atmospheric Structure –General Circulation –Thermal Balance –Origin / Geochemistry

Atmospheric Structure Convection, Weather, Clouds T increases with alt. Stable to convection Cools by radiation Shooting stars burn up Low density, heated by X-rays Free electrons, ions Affects radio wave propagation Exobase – Height at which 1/e particles can escape Heating due to UV absorption by O 3

Atmospheric Pressure Atmosphere is hydrostatic: Ideal Gas Law: Combining these two: P Pressure  Density g Gravity z Height V Volume N Number of Moles RGas Constant TTemperature  Mass of one Mole Assuming: Isothermal Atmosphere Constant Gravity

Scale Height Let H = RT/g  : P = P 0 e -z/H H is the scale height of the atmosphere –Distance over which P drops by 1/e Mass of a column of atmosphere –M c = P/g VenusEarthMarsJupiterSaturnUranusNeptune H (km)

Atmospheric Temperature Of course, temperature actually does vary with height If a packet of gas rises rapidly (adiabatic), then it will expand and, as a result, cool Work done in cooling = work done in expanding Combining these two equations with hydrostatic equilibrium, we get the dry adiabatic lapse rate: C p is the specific heat capacity of the gas at constant pressure On Earth, the lapse rate is about 10 K/km What happens if the air is wet?

General Circulation Zonal Mean Circulation –Wind parallel to lines of latitude Fastest Transport What drives zonal winds? –Angular Momentum Conservation! –dv = -2  dR R RR

Zonal Winds Westerlies –Near-surface winds that blow from the west in mid-latitudes Trade Winds –Light winds that blow from the east in the tropics Winds alternate between prograde (eastwards) and retrograde (westwards)

Hadley Circulation Parcels of atmosphere rise and fall due to buoyancy (equator is hotter than the poles) The result is that the atmosphere is broken up into several Hadley cells Each cell makes a band of zonal wind

Rotation  Cells How many cells depends on the Rossby number (i.e. rotation rate) Venus Ro = Cell / hemisphere Ro = 4 3 Cells / hemisphere EarthJupiter Ro = 30 5 Cells / hemisphere R: planet radius,  : rotation rate,  : latitude, v: wind speed

Zonal Winds on Giant Planets The reason Jupiter, Saturn, Uranus and Neptune have bands is because of rapid rotations (periods ~ 10 hrs) The winds in each band can be measured by following individual objects (e.g. clouds)

Atmospheric Dynamics Solve a set of coupled dynamical, radiative and possibly chemical equations. –General Circulation Models (GCM) Coriolis Force –Objects moving on a rotating planet get deflected Geostrophic Wind –Balance Between Pressure Gradients and Coriolis Force

Geostrophic balance In some situations, the only significant forces acting are due to the Coriolis effect and due to pressure gradients The acceleration due to pressure gradients is The Coriolis acceleration is 2  v sin  In steady-state these balance, giving: The result is that winds flow along isobars and will form cyclones or anti-cyclones What are wind speeds on Earth? L L H isobars pressure Coriolis wind

Next Time Planetary Atmospheres –Thermal Balance –Climate Change –Origin and Loss