ASTRONOMY 340FALL 200711 October 2007

Class #12

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

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

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

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”

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?

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

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

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

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?

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

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 balanceBetween dissociation and recombination which depend on

temperature. Temperature depends on altitude.

Venus (importance of trace elements)

HCl + hν H + Cl SO2 + hν SO + O

SO2 + O SO3

SO3 + H2O H2SO4 (this stuff condenses pretty easily)

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)

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?

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

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?

What’s geochemistry all about?

What do elemental ratios tell us about the history of accretionand differentiation processes?

It’s not the absolute abundances, it’s the relative abundances of different regions.

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)