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?
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
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
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.
Also - further agents of climate change are...
......... all of which are involved in various feedback systems...