Why does earth have an atmoshphere?

Actually, im talking about planets in general. Question again is: Why does earth have an atmoshphere?


Put very simply: gases came out as volcanic eruptions when the Earth was cooling, and the Earth is big enough that gravity stopped the heavier gases from escaping.

The lighter bits bubbled to the top… solid, liquid, gas, thats how it works.

Some think that planet formation is violent enough to drive off most (or all) primordial atmosphere they might start with, and it’s replaced with gases from comets after the planets cool a bit. There may have been more comets around early in the solar system’s history than there are now, and we’ve seen at least one hit Jupiter.

Because life is one big party. Without atmosphere who would want to stick around after the first free beer?


rjk is right; it seems the Earth probably lost all it’s first atmosphere in a collision with a Mars sized object 4.5 billion years ago,
and the water and air has been replaced since;
both by more outgassing from volcanoes, and impacts with a class of comets from around about Jupiter’s orbital distance which have now all disappeared into interstellar space or been absorbed.

SF worldbuilding at

Are there not vocanos on other planets? Obviously the distance from the sun has given people problems with heat or cold, but they could hold some form of life, if they have volcanos.

I think it is naive to think that we are the only planet with life forms with intelligence. How could we, out of all the universe be the only one with vocanos and the right amount of heat.

Actually, Purple, the other terrestrial planets don’t have anywhere near the tectonic and volcanic activity the Earth does. Some of Jupiter’s moons are heavily active.

It’s certainly reasonable to expect there is life elsewhere, but it’s equally reasonable to believe that there doesn’t happen to be any other life in this particular solar system.

There is a second problem due to solar luminosity. As the sun ages it gets hotter. That means that planets closer to the sun (or their own star) may have had Earth like atmospheres, however as the planet heats up, more and more water vapour appears. This vapour reacts to incident light by splitting into oxygen and hydrogen. Now due to hydrogen’s small mass it tends to get achieve escape velocity and leaves you with an increasingly oxidizing atmosphere. The reduction potential of oxygen reacts with rock forming oxides. For a living example look at Venus, though current thinking is that it arrived in the “hot house” state by default.

That being said there is no need for tectonic activity to produce an atmosphere. Look at Venus, Pluto, or the moon Titan. It may, however be needed to lock away carbon and dampen climate variations.

And there are other factors. In the Earth’s case, there’s its magnetic field, which protects the atmosphere from being blown away by the solar wind, as has (largely) happened with Mars.

Well Venus has no magnetic field and it has a massive atmosphere, so a strong magnetic field is not necessarily required.

OK – Atmospheres on the inner planets:
Mercury: virtually nonexistent
Venus: Too *&^%$# much atmosphere. Mostly CO2, with sulfuric acid clouds. Surface pressures several hundredfold those on Earth
Earth: as we know it. 78% N2, 20% O2, miscellaneous other gases.
Mars: very thin atmosphere, barely enough to have winds and seasons, but something like a fraction of a hundredth of the Earth’s pressure, mostly CO2.

Notice that in Mars and Venus, as Grey suggested, the surfaces are mostly oxidized and the atmospheres are mostly CO2 – with the differences apparently caused by the different temperature zones, and by Mars’ smaller size meaning (a) less atmosphere to start with (b) less gravity to hold light gases and © the core cooling and solidifying quicker, thus ceasing outgassing and shutting down the magnetic field.
In the particular case of Earth, the current gas balance depends heavily on the biological processes contained therein, and indeed the % of O2 started increasing as photosynthesis evolved – continued planetary geological AND biological activity keeps replenishing this atmosphere.

But don’t fall into the trap of thinking that the plants on Earth today are responsible for maintaining the oxygen level in our atmosphere; this oxygen is mostly balanced by prehistoric plant material now incorporated in the rocks as fossil fuels and carbon rich strata.

The oxygen in our atmosphere is largely a historical fossil, and would not change very much if all the plants on Earth ceased to photosynthesise…
a couple of percent at most.
The nitrogen bulk of the atmosphere would also remain about the same.
Only the CO2 level would be affected to any great extent, and this would increase the greenhouse effect and greatly change the temperature of our world.

So to sum up.

  1. Some original stuff locked inside a planet
  2. Most stuff from comet/asteroid impacts
  3. Sufficient gravity to prevent the majority of gases escaping

As for modern Earth like you need to tack on a few extras
4. Carbon sequestering mechanism (plate tectonics)
5. Moderate solar energy maintaining a non “Hot House” state
6. Anaerobic production of oxygen in the past by emergent plant life

I think that covers most of what we discussed.

In fact, the more interesting question might be why Earth has so little atmosphere. Venus is lighter and hotter than Earth, and has no appreciable magnetic field, yet it has a much thicker atmosphere. So why isn’t our atmosphere that thick? The last I heard on the subject, the Moon-forming event is likely to blame, but I’m not sure how proven that is.

I think this is the third time I’ve recommended this book in a month, but here goes: Worlds Without End by John S. Lewis is a thorough and well-written layman’s summary of the current state of planetary science, including formation and development. One chapter has a really interesting thought experiment that follows the evolution of an Earthlike planet, and two copies that are identical but for size (one is like half as big, the other is twice as big), to see how something as simple as mass and gravity affects how the planet comes out. This includes atmosphere. It’s a good book; you should look for it.