Atmospheres of the Solid Worlds I: Origin and Composition and Magnetospheres

The atmosphere of Titan from Wikipedia
Sources of atmospheres:

Atmospheric composition

Oxidation and reduction:

It is common to refer to atmospheres as oxidizing or reducing depending on the whether their most common gasses act as oxidizing or reducing agents. The early Earth is thought to have had a reducing atmosphere of N2 and CO2. Today there is an oxidizing atmosphere of N2 and O2. Any atmosphere with substantial quantities of O2 will be oxidizing, but one could imagine possible oxidizing atmospheres in which gasses like fluorine or chlorine was the oxidizer.

Major gas components of atmospheres: From the strongest reducing agent to the strongest oxidizing agent.

The solid worlds with atmospheres: Venus, Earth, Mars, and Titan to scale
The methods of: have been used on Earth and by robot spacecraft to analyze the compositions of atmospheres, but they absolutely require physical samples. What if we are observing atmospheres at a distance? Then we rely on emission and absorption spectroscopy - the analysis of the wave lengths of light passing through the gas. An incandescent source of white light contains photons of many wavelengths. When passed through a prism or diffraction grating, that light can be broken into continuous spectra (right) because each wavelength experiences a different amount of diffraction.

Atmospheric pressure: The atmospheres of the solid worlds vary significantly in mass. The image at right shows the planets and their surface atmospheric pressures to scale.

Actual atmospheric compositions: A comparison of the compositions of the major solid world atmospheres at first shows more differences than similarities:

Volatile inventories: In part, these differences mask underlying similarities when we consider solid volatile substances near the surface and the idiosyncrasies of planetary histories: By adding the mass of volatiles in the atmosphere to that near the planet's surface, we get the planet's volatile inventory. Doing this, we find that Earth, Venus, and Mars have similar amounts of CO2 proportionally. Likewise, Earth and Mars have similar quantities of water.

Nevertheless, some differences remain when we compare volatile inventories:

Global distribution of chlorophyll from Wikipedia
Photosynthesis: Earth's atmosphere has been radically altered by its biosphere. The first living things inhabited a world with a CO2 - rich reducing atmosphere, but where oxygen was a mere trace gas. That changed with the rise of photosynthesis:

6 CO2 + 6 H2O + energy (sunlight)---> C6H12O6+ 6 O2

As a consequence:

Planetary atmospheric composition idiosyncrasies:

Venus in ultraviolet from Wikipedia


Mars dust storms


Key concepts and vocabulary:
Additional reading: