Reading

• Ch6 of Goody & Walker.

Topics

• Where did the Volatiles come from?
• The Goldilocks Effect - Earth, Venus, Mars
• Cycles - Resevoirs and transfer processes
• Coupled Systems - Feedback

Where did the Volatiles that form the Terrestrial Planet Atmospheres come from?

Many clues to their origins come from the abundance of various isotopes and noble/inert gases (Ne, Ar, Kr, Xe).

Possible sources:

• Solar nebula
  • if came with Earth as formed - as minerals or trapped in rock/metal interior of terrestrial planets
  • would produce solar abundance of all heavy (not H or He) elements
  • would prodict too little N, C (which do not form minerals)
  • would predict too little Ar, Kr, Xe
  • implies external source of volatiles
• Meteorites
  • would predict Kr~Xe
  • Earth Kr~20x Xe - hence, meteorites not the answer
• Comets or Icy Planetesimals
  • lab tests (condensing a fake gas of solar composition onto a "cold finger" and then re-heating it up and measuring the gas coming off) show that for T~10K Ar, Kr, Xe condense and then re-evaporate at original proportions
  • for same labe experiments with T>25K Ar, Kr, Xe are in same proportion as observed in Earth's atmosphere
  • Where did comets form?? T<10K? T>25K? Need to measure inert gases of comets
• Jupiter - Galileo probe (elemental abundances, isotope ratios) showed
  • all elements ~x3 solar nebula
  • N isotopes suggest N condensed as N2 (means T<10K since N2 is hard to trap, because very volatile)
  • suggesting the original core of Jupiter was form from "cold" icy planetesimals that condensed with T~10K (early in SS formation? BEFORE terrestrial planets? kinda makes sense if Jupiter stirs up the planetesimals and sends them to Earth)
• D/H in Earth's oceans suggests ~half of Earth's H2O came from T>25K "warm" comets/icy planetesimals, ~half of H2O came from solar nebula composition, probably outgassing of volatiles from deep down
• N2, CO2 on Earth
  • came from "warm" comets/icy planetesimals that formed with T>25K (e.g. Kuiper Belt)
  • delivered as compounds (not N2), then dissociated/combined to N2

Cycles - Resevoirs and transfer processes

Starting with the most global, general case - 3 phases of atmospheric evolution

The Goldilocks Effect

1 bar = 10⁵ Pa, 1 kPa = 0.01 bar = 10 mbar

Or maybe the perpendicular version is more familiar....

So, let's summarize what happened in each case....

Venus

• A little warmer -> water could not condense
• CO2 (and CH4) keeps Venus warm
• H2O dissociated, H (and O) lost via thermal escape and solar wind ionization and stripping
• Evidence? D/H ratio in Venus' atmosphere (~150 times greater than in Earth's oceans) - H was preferentially lost compared with heavier isotope Deuterium

Mars - in detail Thursday

• A little colder -> water freezes
• Why less CO2 (and CH4)? Less outgassing? Lost? Frozen? Carbonates?
• Atmospheric H2O dissociated, H (and O) lost via thermal escape and solar wind ionization and stripping

Earth

• Started with CO2 atmosphere - thicker to compensate for Faint Early Sun?
• More CH4 early on? e.g. H escape less (recent Tien & Toon paper) and/or early life (methanogens) produced methane. In any case, with lots of methane and H early on, then conditions good for starting/evolving life.
• Temperature just right - water says liquid on the surface, clouds increase albedo
• CO2 -> oceans and LIMESTONE
• Photosynthesis CO2 -> O2

Coupled Systems - Feedback

In most atmospheric systems there are feedback loops - NEGATIVE and POSITIVE - the pointed arrows mean a direct relationship, the open circle arrows mean an inverse relationship.

..

Key to stability is whether the negative loops overcome the positive feedback systems.

The Carbon Cycle (resevoirs/inventories in gm, transfers in gm/year)

Another version of the carbon cycle - the numbers are in gigatonnes for the resevoirs and gigatonnes per year for transfer processes

The CARBON-SILICATE cycle in more detail

The Oxygen Cycle - inventories in gm, transfers in gm/year

Note lack of moderating influence of oceans - possible large fluctuations