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


Solar wind and magnetospheres

Solar wind: The stream of ionized particles (plasma) flowing outward from the sun. Their flow is controlled by the sun's magnetic field. The Sun has tremendous magnetic activity. Near the sun this is manifested in sunspots, solar flares and related phenomena. Farther away we can compare its magnetic field to a dipole, like a bar magnet and call it the interplanetary magnetic field (IMF). The particles of the solar wind follow its field lines. What happens when this magnetically guided wind encounters planets?

No atmosphere or magnetic field: In the case of a body like the moon, with no atmosphere or magnetic field, the solar wind simply impacts the dayside surface, and cases a plasma shadow on the night side.

No atmosphere but has magnetic field: What if the planet is airless but has a magnetic field like Mercury? The IMF interacts with Mercury's magnetic field. We describe this in terms of two limits:

No magnetic field but has atmosphere: What if the planet has no magnetic field but a thick atmosphere, like Venus? The solar wind strikes the upper atmosphere directly, heating the thermosphere and ionizing its gasses. The resulting layer of ionized gasses is the ionosphere. The ionized gasses of the ionosphere can deflect the solar wind. This barrier is the ionopause. As before, the solar wind decelerates approaching the ionopause, yielding a bow shock. Here the region between the ionopause and the bow shock is the magnetosheath.

Remember: Gasses in the thermosphere are effected by interactions with the solar wind. The disassociation of water molecules into hydrogen and oxygen in Venus' thermosphere is thought to account for its low global volatile inventory of water.

Both: Earth has both a substantial atmosphere (with ionosphere) and magnetic field. Because the magnetic field extends farther out, it dominates interaction with the solar wind, yielding a bow shock and magnetopause like Mercury's, only larger. The geometry of the magnetic field is such that charged particles following magnetic field lines strike the upper atmosphere mostly at the poles. These collisions cause ions and atoms to emit light that we see as auroras.

Earth's magnetic field also traps ions in two torus-shaped loops - the Van Allen Radiation Belts:

Key concepts and vocabulary:
Additional reading: