Astronomy 340 Fall 2007

1. Astronomy 340Fall 2007 11 October 2007 Class #12

2. Where are we? • Midterm  October 25 (not Oct 18) • HW #3 will be handed out today, due Oct 23 • Project assignment will be handed out on Oct 16 • Reading • Planetary atmospheres: Chap 4 (except 4.5, 4.7) • Planetary interiors: Chapter 6

3. Recall our conditions for calling something an “atmosphere” • Gravitationally bound • Exist as gases above planetary surface (volatile at local temperatures) • Physics dominated by collisions • Moon, Mercury  “collisionless”, transient

4. Basic Structure • Troposphere • Closest to ground, highest density • T decreases with increasing altitude • Stratosphere • Up to ~50 km • Constant T on Earth, Mars, decreasing with altitude on Venus • Mesosphere • 50 km < H < 100 km • Constant T on Earth, Mars, decrease flattens out on Venus • Thermosphere • Diurnal variations • T dramatically increases with altitude on Earth • exosphere

5. Origin of Terrestrial Atmospheres • Hydrated minerals in “planetesimals” • Asteroids  up to 20% by mass of H2O • N2 also found in asteroids • comets • Atmosphere by accretion • Accretion  heating/releasing gas  thick primordial atmosphere • Initial outgassing • Continued outgassing via differentiation • Ongoing volcanism/tectonics • Condensation only after end of accretion • Geochemistry – 40Ar/36Ar ratio • 40Ar product of radioactive decay of 40K • 36Ar“primordial”

6. Evolution of Terrestrial Atmospheres • CO2 feedback  removed from atmosphere  deposited onto ocean floor as carbonates  eventually recycled via subduction/volcanism • Life  O2 production • Runaway greenhouse (Venus)  increase temp, increase H2O evaporation, increase atmospheric density, increase temp  H2O dissociates, H escapes, total net loss of water from Venus • Mars  loss of atmosphere, big cooldown • Impact ejects atmosphere • Solar wind stripping (particularly of ionized particles) • Is it all in the rocks?

7. Greenhouse Effect • Simple  high opacity to radiation leads to increased surface temperature • Solar radiation re-radiating in near-IR part of spectrum (think about a blackbody with T ~ 300K) • Atmospheric constituents have high optical depth in near-IR (molecules like methane, carbon dioxide, etc) • Think back to radiative transfer equations!!! • Radiation can’t escape  temperature increases

8. CO2 • Earth – most of it resides in rocks in the form of CaCO3 – depleted from atmosphere via rain • Released via subduction/volcanism • Venus • Without H2O there is no mechanism to remove CO2 from atmosphere  CO2 is mostly in the atmosphere • Lack of H2O  increase viscosity of mantle  no convection  no tectonics  rigid “lid” • Mars • No means of releasing CO2 from rocks (no tectonics) • Loss of water via dissociation/escape

9. Atmospheric Composition • Noble gases • Ne, Ar, Kr, Xe • All depleted relative to solar • For Venus/Earth  Kr and Xe are similar (Ne and Ar a little more depleted on Earth • Everything much more depleted on Mars • Mars? Similarity between atmosphere and glassy bits of some meteorites

10. Evolution from Primordial Atmospheres • Gravitational gas capture • Solar wind implantation • Impact-degassing of accreting matter • Atmospheric ejection of accreting matter • Escape Which of these will preserve compositional signatures? Which will fractionate elements/isotopes? What would you expect to see in the relative composition of Ne (20), Ar (36), Kr (84), Xe (132) between Mars, Venus, and the Earth?

11. Evolution of Primordial Atmospheres • Escape-fractionation • Moon-forming event (Earth) • Loss of magnetic field (Mars) • Solar UV radiation (Venus, Earth, Mars) • Note various reactions in 4.6.2.1 • Planetary degassing • Products of radioactive decay

12. Atmospheric Chemistry • Consider molecular oxygen • O2 + hν O + O (λ≤1750 A) • If recombination is faster than escape • O + O  O2 + hν • O + O + catalyst  O2 (or O3) + catalyst • Recombination rate is k = σvmean ~ 2 x 10-10 (T/300)1/2 Total oxygen abundance is determined by the balance Between dissociation and recombination which depend on temperature. Temperature depends on altitude.

13. Venus (importance of trace elements) • HCl + hν H + Cl • SO2 + hν  SO + O • SO2 + O  SO3 • SO3 + H2O  H2SO4 (this stuff condenses pretty easily)

14. Atmospheric Escape • We did the simple bit last time • Thermal energy (kT)  kinetic energy mv2 • Compare v(kinetic) to v(escape) • What really is the velocity distribution? • Maxwellian (eqn 4.78)  leads to a measure of the rate of escape at a given temperature (eqn 4.80)

15. Probing Planetary Interiors • Mean density/surface density • Moment of inertia (I = kMR2) • Magnetic field  how? • Geological activity (e.g. seismic data) • Mean crater density • Physics of matter at various densities • How large/small can you make a gravitationally bound object out of H and He? Si? Fe?

16. Distribution of Elements in Earth • Starting assumption  accretion (of what?) • Net loss of volatiles • Chemical differentiation (but can you estimate the degree to which the Earth was molten enough to allow for differentiation?) • Current best guess • Inner core – largely Fe • Outer core – lower density, maybe some K? • Mantle – partially molten, chemically inhomogeneous

17. Mantle Geochemistry • What’s a “basalt”? • Mid-Ocean Ridge Basalt (MORB) • Depleted in “incompatibles” • Differentiated  primitive? • Ocean Island Basalt (OIB) • Less depleted/not primitive • Originates deeper in mantle  recycled crust?

18. What’s geochemistry all about? What do elemental ratios tell us about the history of accretion and differentiation processes? It’s not the absolute abundances, it’s the relative abundances of different regions.

19. Sources of Heating • Accretion – kinetic energy of impacts  melting • Differentiation – release of gravitational thermal heat • Radioactivity (particularly K, Ur, Th) • 238U  206Pb + 8He + 6β (4.5 Gyr, ~3 J g-1 yr-1) • 235U  207Pb + 7He + 7β (0.7 Gyr, ~19 J g-1 yr-1) • 237Th, 40K yield (~0.9 J g-1 yr-1)