Earth's atmosphere is enormous, so far reaching that it even affects the International Space Station's route. But how did this giant gaseous envelope form?
That is, why does Earth have an atmosphere?
In short, our atmosphere is here because of gravity. When Earth formed, about 4.5 billion years ago, the molten planet barely had an atmosphere. But as the world cooled, its atmosphere formed, largely from gases spewed out of volcanoes, according to the Smithsonian Environmental Research Center (SERC). This ancient atmosphere was very different from today's; it had hydrogen sulfide, methane and 10 to 200 times as much carbon dioxide as the modern atmosphere does, according to SERC. [Infographic: Earth's Atmosphere Top to Bottom]
"We believe the Earth started out with an atmosphere a bit like [that of] Venus, with nitrogen, carbon dioxide, maybe methane," said Jeremy Frey, a professor of physical chemistry at the University of Southampton in the United Kingdom. "Life then began somehow, almost certainly in the bottom of an ocean somewhere."
After around 3 billion years, the photosynthetic system evolved, meaning that single-celled organisms used the sun's energy to turn molecules of carbon dioxide and water into sugar and oxygen gas. This dramatically increased oxygen levels, Frey told Live Science. "And that is the biggest pollution event, you might say, that life has ever done to anything, because it slowly transformed the planet," he said.
Nowadays, Earth's atmosphere consists of approximately 80 percent nitrogen and 20 percent oxygen, Frey said. That atmosphere is also home to argon, carbon dioxide, water vapor and numerous other gases, according to the National Center for Atmospheric Research (NCAR).
It's a good thing these gases are there. Our atmosphere protects the Earth from the harsh rays of the sun and reduces temperature extremes, acting like a duvet wrapped around the planet. Meanwhile, the greenhouse effect means that energy from the sun that reaches Earth gets waylaid in the atmosphere, absorbed and released by greenhouse gases, according to the NCAR. There are several different types of greenhouse gases; the major ones are carbon dioxide, water vapor, methane and nitrous oxide. Without the greenhouse effect, Earth's temperature would be below freezing.
However, today, greenhouse gases are out of control. As humans release more carbon dioxide into the atmosphere, Earth's greenhouse effect gets stronger, according to NCAR. In turn, the planet's climate gets warmer.
Intriguingly, no other planet in the universe has an atmosphere like Earth's. Mars and Venus have atmospheres, but they cannot support life (or, at least, not Earth-like life), because they don't have enough oxygen. Indeed, Venus' atmosphere is mainly carbon dioxide with clouds of sulfuric acid, the 'air' is so thick and hot that no human could breathe there. According to NASA, the thick carbon dioxide atmosphere of Venus traps heat in a runaway greenhouse effect, making it the hottest planet in our solar system. Surface temperatures there are hot enough to melt lead.
"The fact that Earth has an atmosphere is extremely unusual in respect of the planets in the solar system, in that it's very different from any of the other planets," Frey said. For example, the pressure of Venus is about 90 atmospheres, the equivalent to diving 3,000 feet (914 meters) beneath the ocean on Earth. "The original Russian spaceships that went there [to Venus] just recorded for a few seconds and then got crushed," Frey said. "Nobody ever really understood how hot it was."
So, Earth's atmosphere is life — and without it, life as we know it wouldn't exist. "Earth needed the right atmosphere [for life] to get started," Frey said. "It has created that atmosphere, and it has created circumstances to live in that atmosphere. The atmosphere is a totally integral part of the biological system."
Originally published on Live Science.