How did the atmospheres of the eight planets of our solar system end up where they are today? This summary is based on some excellent lecture notes from the University of Oregon.
The primary atmosphere for every terrestrial world (Mercury, Venus, Earth and Mars) 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% Hydrogen, 5.7% Helium and everything else less than 0.1%.
However, this primary atmosphere was lost on the terrestrial planets through a combination of surface temperature, the mass of the atoms and escape velocity of the planet.
A key factor concerns the behaviour of an atom in a gravitational field. If an atom is moving less than the escape velocity for the given planet, it remains part of the planet; if it moves faster than the escape velocity for the given planet, then it eventually escapes out into space.
The mean velocity of an atom is set by the temperature of its surroundings, in this example the planet’s surface. The higher the temperature the faster the atoms will travel. Another variable is the mass of the atom concerned. For example, some atoms/molecules are low in mass, such as hydrogen and helium; some are heavy in mass, such as carbon dioxide and water. The lower mass elements will move faster than the heavier elements and another reason they can reach escape velocity.
The third variable is the surface temperature of the planet in question. The inner worlds are closer to the Sun, therefore warmer. The opposite is true of the outer planets, farther from the Sun, therefore colder.
Together these variables—mass and radius of the planet (gravity), mass of the atom (its speed of travel) and the surface temperature of the planet (its distance from the Sun) plus the effects of atmospheric heating—determine whether a planet loses or retains certain elements. The upshot is that our inner worlds (Mercury, Venus, Earth and Mars) lost most of their initial hydrogen and helium. They lost their primary atmosphere.
Note that for the outer Jovian worlds (Jupiter, Saturn, Uranus and Neptune) they held onto their primary atmospheres of hydrogen and helium.
Earth, of course, is not without atmosphere! So what happened? Essentially, as with all the inner worlds, it formed a secondary atmosphere composed of the outgassing from tectonic activity. Having lost their primary atmospheres the inner planets are left with rocky materials (iron, olivine, and pyroxene) and the icy materials such as water, carbon dioxide, methane, ammonia and silicon dioxide.
Note that the icy materials are more common in the outer Solar System. They are delivered to the inner Solar System in the form of comets.
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, including that from 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% water, 23% carbon dioxide, 13% silicon dioxide, 5% nitrogen and traces of noble gases (neon, argon and krypton). The later evolution of this outgassing is driven primarily by the surface temperature and chemistry of the planet. Although the three planets had similar secondary atmospheres they evolved in very different ways.
Note that water is the key catalyst for the evolution of a secondary atmosphere. On Earth, the temperature was just right for the formation of liquid water, which collected in oceans and so on. The carbon dioxide released by outgassing was dissolved in this liquid water to produce carbonate rocks. Thus, the earth had a “reducing atmosphere”.
On Venus there was no liquid water because it was too hot for it to collect. Therefore there was nowhere for the carbon dioxide to dissolve. If the atmosphere is reducing in carbon dioxide then the lower ranking elements become important once the carbon dioxide has gone. For Earth, this meant that the atmosphere became primarily nitrogen based, with later additions of oxygen from lifeforms. On Venus, the carbon dioxide was not reduced and stayed as the primary component of its atmosphere.
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 carbon dioxide 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 because 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 water and carbon dioxide 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.
In the case of Earth note that most of its oxygen, which was released by outgassing, was locked up in liquid water. Since oxygen is highly reactive, it must constantly be replenished. Some is released by photodisintegration with water vapour in the upper atmosphere. But most of the oxygen in today’s atmosphere is from the photosynthesis process associated with lifeforms. This occurred about one billion years after the earth was formed. The original secondary atmosphere of Earth was lacking large amounts of oxygen. But it was rich in nitrogen and carbon dioxide. Plants are needed to replenish the oxygen. Without plants all the oxygen would turn into rocks in a few hundred years!
The greenhouse effect is controlled by the amount by mass of greenhouse gases in an atmosphere. These gases are primarily water, carbon dioxide, methane, and ammonia. For secondary atmospheres on Venus, Earth and Mars, only carbon dioxide has a major contribution to the greenhouse effect (although note that the amount of methane is increasing on the earth due to the waste products of animals and agriculture).
The greenhouse effect currently raises the temperature of Mars, Earth and Venus by the following amounts:
- Mars > +5 degrees
- Earth > +35 degrees
- Venus > +500 degrees
Note that the greenhouse effect for the earth is just enough to keep us out of a perpetual Ice Age (a little greenhouse effect is good for you). Whereas for Venus, a severe runaway greenhouse effect makes it the hottest place in the Solar System. Also note that Mars probably had a stronger greenhouse effect in its distant past. But the large amounts of carbon dioxide were converted to rocks in the early Mars oceans. The atmosphere thinned too fast—stopping the greenhouse effect—and the liquid water turned to ice (cold death).
Composition of a Secondary Atmosphere
In summary, the composition of an atmosphere on a terrestrial planet will be determined by the following:
- Distance from the Sun (surface temperature of the planet)
- Mass and radius of the planet (surface gravity determines escape velocity)
- Chemical reactions (different molecules are created and destroyed in various environments, higher temperatures mean faster reactions)
- Geological activity and the amount of outgassing (the more activity the more outgassing and the thicker the atmosphere)
- Living organisms change the composition through their waste products
Based on a Lecture from Abyss.uoregon.edu [University of Oregon] – The Evolution of Planetary Atmospheres. Author as yet unidentified
Edited by Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar
Last updated: Friday, 14th February 2020