We can thank our atmosphere for quite a lot, particularly our ability to, well, survive. Earth’s atmosphere keeps us from freezing (or boiling), it provides the air we breathe, it stops (some) space debris from clobbering our planet, and it filters out harmful radiation from the Sun. Without the atmosphere, life would be very different or nonexistent. So, how did our atmosphere form in the first place? In general, here’s how atmospheres form:
1. You need the right circumstances during formation.
Way back when the solar system formed, the rocky and outer planets distinguished themselves from one another by the materials they collected. As the name suggests, the inner planets had more rocky material to use during formation, and the outer planets had more gaseous particles. While there wasn’t enough gas for the inner planets to become gas giants, there was plenty to form an initial atmosphere of hydrogen and helium. Gases also became trapped within the rocky planets and would later be released.
2. Stored gases are released through mechanisms like volcanic eruptions.
The hydrogen and helium atmospheres of the rocky planets didn’t last long. Gravity held these gases close to the planets initially, but once the sun began to heat up, they were able to escape. That’s when secondary atmospheres kicked in and began to form. Secondary atmospheres arise from the gases within the planet as well as gases that arrive with impacts from things like comets. CO2, H2O, nitrogen, and maybe even some methane were also released during volcanic eruptions, which sent these gases flying up into the atmosphere where gravity held them in its grip.
3. Atmospheres don’t always stick around.
Of course, sometimes gravity alone isn’t enough to hold onto an atmosphere. Earth’s molten core generates a magnetic field that protects our atmosphere from being blown away by solar winds, which is likely what befell Mars’s atmosphere. In the case of Venus, its atmosphere most definitely did stick around, but it got a bit too carried away and is essentially cooking the planet. Atmospheres are great - when circumstances are right, such as on Earth.