The primary atmosphere for every terrestrial world was composed mostly of light gases that accreted during initial formation. These gases are similar to the primordial mixture of gases found in the Sun and Jupiter. That is 94.2% H, 5.7% He and everything else less that 0.1%.

However, this primary atmosphere was lost on the terrestrial planets. Why? a combination of surface temperature, mass of the atoms and escape velocity of the planet. What determines if a particular atom is retained by a planet's gravitational field? if the atom is moving less than the escape velocity for the planet, it stays. If it moves faster than escape velocity, it escapes into outer space.

From the kinetic theory of gases, we know that the mean velocity of a bunch of atoms is set by the temperature of the planet's surface. Remember our microscopic description of macroscopic quantities such as pressure and temperature. Higher temperatures translate into higher velocities for the atoms.

Now consider of mix of elements in an atmosphere. Some atoms/molecules are low in mass (H, He) some are heavy (CO₂, H₂O, etc). The light elements are moving faster than the heavy elements and can reach escape velocity.

The second variable is the surface temperature of the planet. The inner worlds are closer to the Sun, therefore warmer. The opposite is true of the outer planets, farther from the Sun, therefore colder.

Combining the variables of escape velocity (mass, radius of planet) and surface temperature (distance from Sun plus effects of atmosphere heating) produces the following diagram. For key elements, lines are draw to show where the element escapes from the planet. If a planet is below that line, that element will escape.

Secondary Atmospheres:

For the warmer terrestrial worlds, the light, gaseous elements (H, He) are lost. The remaining elements are grouped into the rocky materials (iron, olivine, pyroxene) and the icy materials (H₂O, CO₂, CH₄, NH₃, SO₂). The icy materials are more common in the outer Solar System, they are delivered to the inner Solar System in the form of comets (see later lecture).

The rocky and icy materials mix in the early crust and mantle. If the planet cools quickly, there is little to no tectonic activity and the icy materials are trapped in the mantle (see for example the Galilean moons). If the planet has a large mass (which means lots of trapped heat from formation), then there is a large amount of tectonic activity -> volcanos.

The icy materials are turned to gases in the warm mantle and returned to the planet surface in the form of outgassing to produce a secondary atmosphere. The atmospheres of Venus, Earth and Mars (and in some sense, Titan) are secondary atmospheres.

The composition of outgassing is similar for Venus, Earth and Mars and is composed of 58% H₂O, 23% CO₂, 13% SO₂, 5% N₂ and traces of noble gases (Ne, Ar, Kr). The latter evolution of this outgassing is driven primarily by the surface temperature and chemistry of the planet. Study this table closely, for all three planets had similar secondary atmospheres which evolved in very different ways.

On Venus there was no liquid water (too hot) and, therefore, no place for the CO₂ to dissolve. If the atmosphere is reducing in CO₂ than lower ranking elements become important once the CO₂ is gone. For the Earth, this meant that the atmosphere became primarily N₂ based, with later additions of O2 from lifeforms. On Venus, CO₂ was not reduced and stayed as the primary component to their atmospheres.

On Mars there was a period of liquid water very soon after formation. But there was insufficient temperature for this water to remain as a liquid, so it froze out leaving CO₂ as the primary component in the atmosphere.

Also note how the noble gases are good traces of the amount of evolution an atmosphere undergoes. Noble gases do not react with other elements (they are inert). An atmosphere that is thin and undergoes sharp changes in mass has a high percentage of noble gases. In this case, Mars has had most of its atmosphere frozen out in the form of H₂O and CO₂ ice, leaving a high amount of noble gases. Thick atmospheres, such as Venus, have small percentages of noble gases since most of the outgassing material remains on the planet surface.

Earth's Oxygen:

Note that most of the O₂ released by outgassing is locked up in liquid H₂O. Since O₂ is highly reactive, it must constantly be replenished. Some is released by photodisintegration with H₂O vapor in the upper atmosphere.

But most of the O₂ in today's atmosphere is from the photosynthesis process associated with lifeforms. This occurred about 1 billion years after the Earth formed. The original secondary atmosphere of the Earth was lacking large amounts of O₂ and was rich in N₂ and CO₂. Plants are needed to replenish O₂, without plants all the oxygen would turn into rocks in a few 100 years.

Greenhouse changes:

The greenhouse effect is controlled by the amount (by mass) of greenhouse gases in an atmosphere. These gases are primarily H₂O, CO₂, CH₄, NH₃. For secondary atmospheres on Venus, Earth and Mars, only CO₂ has a major contribution to the greenhouse effect (although note that the amount of CH₄ is increasing on the Earth due to the waste products of animals and agriculture).

The greenhouse effect currently raises the temperature of Venus, Earth and Mars by the following amounts: