Class 20 - Atmospheric Evolution 1

Reading

• Ch6 of Goody & Walker.
• Global Warming - Houghton - .pdf - can you open it? may be protected

Topics

• Volatile Inventory
• Source & Loss Processes
• What Can Cause Atmospheres to Change?

Volatile Inventory

RED = primary constituent, BLUE = secondary, GREEN = third component

WHY these constituents? Partly an issue of cosmic abundances of elements

... and partly an issue of a planet's history.

Stages of planetary formation

1. small rocky objects collide with each other to form planetary cores.
2. ices condense and add to rocky cores.
3. Jupiter and Saturn capture nebula
4. Uranus and Neptune, in less dense outer part of nebula, form slightly too late to capture nebular gas.

Where are these elements in planetary materials?
Rocky solids-

1. dominate terrestrial planets
2. existing examples are the unprocessed meteorites

"ices"-H2O, NH3,CH4 - WAM,"clathrates"

1. add this to rocks and you get a lot of Uranus and Neptune.Probably contributed significantly to late evolution of the terrestrial planets.
2. existing examples are comets

nebular gas-H2, He

1. add this to rocks and ices and you get Jupiter and Saturn
2. existing examples are the sun

Source & Loss Processes

Source processes

• Outgassing -
  • major source - mostly from volcanoes
  • gases stored up inside the planet
  • gases brought in by comets
  • gases recycled through plate tectonics
• Evapulation/sublimation - particularly for smaller, colder objects
• bombardment by micrometeorites, energetic particles and photons - particularly for smaller objects where above processes less active

Loss Processes

• Thermal escape
  • the faster gas molecules have speeds greater than the escape speed
  • molecules are heated by interior sources, solar radiation, energetic particles, meteorites
• Non-thermal escape - ionized particles stripped off by solar wind
• condensation
• chemical reactions (e.g. oxidation, CO2-> limestone, )
• giant impacts blow off atmosphere - e.g. early periods

What Can Cause Atmospheres to Change? What elements of this diagram could change?

Orbital Parameters - Milankovitch theory - and observations

Left - sunshine in summer in Joules/m2/day (2.3 -> 27 watts/m2)

Right - corresponding estimate of thickness of ice, based on left plot

Theory.... each of the orbital components

Eccentricity cycle ~413,000 yr

Precession cycle ~19-23,000 yr

Obliquity cycle ~100,000 yr

Solar Variability

The Faint Young Sun

this means that for Earth to have been warm (as necessary for development of life 3.7 BY) we need greenhouse gases....

Changes in greenhouse gases

Variations in CO2 and O2 with time

This is the MODELED amount of CO2 needed to compensate for the lower solar flux (see 2 figures up). The lower bound of the shaded region corresponds to having just enough CO2 to keep temperatures just at/above freezing. The upper curve corresponds to a global ocean. The two vertical bars correspond to 2 major periods of glaciation (0.65 and 2.3 BY ago) when the geological record suggests there was an ice sheet all over the globe - "Snowball Earth" - resulting in a major change in albedo....

Changes in Albedo

HOW do we know the temperatures this far back? Good question. It is tough to go back more than a few hundred million years. Mostly the evidence is from a few of the oldest rocks, their chemical composition and the micro-fossils therein.

More recently, in the past few 100 million years we can infer the amount of CO2 and O2, again from interpreting the fossil record, which seem to be anti-correlated.

Note:

• High CO2 and low O2 before ~400 MY.
• Sharp drop in atmospheric CO2 and increase in O2 around 350 MY - this is when the large fossil fuel deposits (mostly coal) were layed formed.

Also - further agents of climate change are...

Oceans

Biosphere

......... all of which are involved in various feedback systems...