EART 160: Planetary Science

1. EART 160: Planetary Science 15 February 2008

2. Last Time • Planetary Interiors • Cooling Mechanisms • Conduction • Convection • Rheology • Viscoelasticity

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

4. 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

5. Flexural Stresses load • 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 Crust Elastic plate Mantle

6. Flexural Parameter rw load Te a • Consider a load acting on an elastic plate: rm • The plate has a particular elastic thicknessTe • If the load is narrow, then the width of deformation is controlled by the properties of the plate • The width of deformation a is called the flexural parameter and is given by E is Young’s modulus, g is gravity andn is Poisson’s ratio (~0.3)

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

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

9. Te 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 • 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 (!) 110 K 270 K 190 K 1.4 km Depth elastic viscous Temperature

10. Te in the solar system • Remote sensing observations give us Te • Te depends on the composition of the material (e.g. ice, rock) and the temperature structure • If we can measure Te, we can determine the temperature structure (or heat flux) • Typical (approx.) values for solar system objects:

11. 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

13. Planetary Atmospheres in the Solar System

14. Composition This Planet is just right! This Planet is too cold! This Planet is too hot! P = 92 bars 96.5% CO2 3.5% N2 T = 737 K P = 1 bar 78% N2 21% O2 1% Ar 1% H2O T = 287 K P = 6 millibars 95% CO2 3% N2 2% Ar T = 218 K

15. 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 Luca Galuzzi Image Credit Fabio Grasso

16. 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 CO2 greenhouse • Slow rotation  stagnant

17. Mars • Water vapor clouds • Polar Ice Caps • Liquid water unstable • Warmer, wetter in past? • CO2 cycle • Dust storms • Massive Dust Devils!

18. The only Moon with an atmosphere Haze prevented Voyager from seeing the surface P = 1.5 bar 98% N2 2% CH4 T = 95 K Titan

19. Atmospheric Compositions Isotopes are useful for inferring outgassing and atmos. loss

20. Atmospheric Structure Exobase – Height at which 1/e particles can escape (Thermosphere keeps going) • Free electrons, ions • Affects radio wave propagation Low density, heated by X-rays Shooting stars burn up T increases with alt. Stable to convection Cools by radiation Convection, Weather, Clouds

21. Atmospheric Pressure • Atmosphere is hydrostatic: • Ideal Gas Law: • Combining these two: P Pressurer Densityg Gravityz HeightV VolumeN Number of MolesR Gas ConstantT Temperaturem Mass of one Mole Assuming Isothermal Atmosphere Constant Gravity

22. Let H = RT/gm : P = P0 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 Mc = P/g Scale Height

23. 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 Cp is the specific heat capacity of the gas at constant pressure • Combining these two equations with hydrostatic equilibrium, we get the dry adiabatic lapse rate: • On Earth, the lapse rate is about 10 K/km • What happens if the air is wet?

24. Next Time • Planetary Atmospheres • General Circulation • Origin / Geochemistry • Thermal Balance