EART 160: Planetary Science 15 February 2008

Last Time Planetary Interiors –Cooling Mechanisms Conduction Convection –Rheology Viscoelasticity

Today Elastic Flexure Paper Discussion – Titan Atmosphere –Tobie et al., 2005 Planetary Atmospheres –General Description –Atmospheric Structure –General Circulation –Origin / Geochemistry –Thermal Balance

Elastic Flexure The near-surface, cold parts of a planet (the lithosphere) behaves elastically This lithosphere can support loads (e.g. volcanoes) We can use observations of how the lithosphere deforms under these loads to assess how thick it is The thickness of the lithosphere tells us about how rapidly temperature increases with depth i.e. it helps us to deduce the thermal structure of the planet The deformation of the elastic lithosphere under loads is called flexure EART163: Planetary Surfaces

Flexural Stresses In general, a load will be supported by a combination of elastic stresses and buoyancy forces (due to the different density of crust and mantle) The elastic stresses will be both compressional and extensional (see diagram) Note that in this example the elastic portion includes both crust and mantle Elastic plate Crust Mantle load

Flexural Parameter Consider a load acting on an elastic plate: TeTe mm load ww The plate has a particular elastic thickness T e If the load is narrow, then the width of deformation is controlled by the properties of the plate The width of deformation  is called the flexural parameter and is given by  E is Young’s modulus, g is gravity and is Poisson’s ratio (~0.3)

If the applied load is much wider than , then the load cannot be supported elastically and must be supported by buoyancy (isostasy) If the applied load is much narrower than , then the width of deformation is given by  If we can measure a flexural wavelength, that allows us to infer  and thus T e directly. Inferring T e (elastic thickness) is useful because T e is controlled by a planet’s temperature structure 

Example This is an example of a profile across a rift on Ganymede An eyeball estimate of  would be about 10 km For ice, we take E=10 GPa,  =900 kg m -3, g=1.3 ms -2 Distance, km 10 km If  =10 km then T e =1.5 km So we can determine T e remotely This is useful because T e is ultimately controlled by the temperature structure of the subsurface

T e and temperature structure Cold materials behave elastically Warm materials flow in a viscous fashion This means there is a characteristic temperature (roughly 70% of the melting temperature) which defines the base of the elastic layer 110 K 270 K elastic viscous 190 K E.g. for ice the base of the elastic layer is at about 190 K The measured elastic layer thickness is 1.4 km (from previous slide) So the thermal gradient is 60 K/km This tells us that the (conductive) ice shell thickness is 2.7 km (!) Depth 1.4 km Temperature

T e in the solar system Remote sensing observations give us T e T e depends on the composition of the material (e.g. ice, rock) and the temperature structure If we can measure T e, we can determine the temperature structure (or heat flux) Typical (approx.) values for solar system objects: BodyT e (km)dT/dz (K/km) BodyTeTe dT/dz (K/km) Earth (cont.) 3015Venus (450 o C) 3015 Mars (recent) 1005Moon (ancient) 1530 Europa240Ganymede240

Planetary Atmospheres Atmosphere – The layer of gases surrounding a planet Determines present surface conditions Controls long-term climatic history and evolution Greenhouse Effect Global Warming Image Credit Brett Wilson

Planetary Atmospheres in the Solar System Nothing to write home about Moon, Mercury (He, Ar, Na, O) MicrobarsIo, Triton, Europa, Pluto (S, N, C, H, O) MillibarsEarth, Mars, Venus, Titan (C, H, O, N, Ar) Gas BagsJupiter, Saturn, Uranus, Neptune (H, He, O, S, N, C)

Composition P = 1 bar 78% N 2 21% O 2 1% Ar 1%H 2 O T = 287 K P = 92 bars 96.5% CO 2 3.5% N 2 T = 737 K P = 6 millibars 95% CO 2 3% N 2 2% Ar T = 218 K This Planet is too hot! This Planet is too cold!This Planet is just right!

Earth Same amount of Carbon as Venus –Where did it all go? Climate controlled by water vapor –Clouds, Rain, Oceans Near Triple-Point –Ice, Liquid, Vapor all present Image Credit Fabio Grasso Image Credit Luca Galuzzi

Venus Similar to Earth’s bulk properties but VERY different Hot enough to melt lead No water at all – where did it go? Clouds of Sulfur, Rain of Sulfuric Acid Surface not visible MASSIVE CO 2 greenhouse Slow rotation  stagnant

Mars Water vapor clouds Polar Ice Caps Liquid water unstable Warmer, wetter in past? CO 2 cycle Dust storms –Massive Dust Devils!

Titan The only Moon with an atmosphere Haze prevented Voyager from seeing the surface P = 1.5 bar –98% N 2 –2% CH 4 T = 95 K

Atmospheric Compositions Isotopes are useful for inferring outgassing and atmos. loss EarthVenusMarsTitan Pressure1 bar92 bar0.006 bar 1.5 bar N2N2 77%3.5%2.7%98.4% O2O2 21%--- H2OH2O1%0.01%0.006%- Ar0.93%0.007%1.6%0.004% CO %96%95%~1ppb CH 4 1.7ppm-?1.6% 40 Ar6.6x10 16 kg1.4x10 16 kg4.5x10 14 kg3.5x10 14 kg H/D N/ 15 N

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 (Thermosphere keeps going)

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 AssumingIsothermal 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 Structure 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?

Next Time Planetary Atmospheres –General Circulation –Origin / Geochemistry –Thermal Balance