The Origin of the Air We Breathe
By: Debbie Patskowski
Tags: Space, Astronomy, Climate Change, Earth Science, Origin Of Life

I want you to travel through time for a moment. Not for minutes, or seconds, or hours, but eons, what geologists call Deep Time, billions of years in the past. Such a vast amount of time almost defies human imagination. 4.5 billion years ago, Earth was in its infancy, rocked in the cradle of the early solar system by constant bombardments. Planets vied with one another for supremacy and not all of them survived.

The fate of our solar system was ultimately decided by gravity and orbital mechanics. Earth emerged as one of the winners by accruing enough mass to hold itself together during the heavy bombardment period. Theia, in theory, was one of the losers. Theia, a planetesimal roughly the size of Mars, collided with the early Earth. Most of the impacts during this period are thought to have been glancing blows, leaving the two jousting planetary objects battered, but intact.

The collision between Earth and Theia, however, was a devastating impact, nearly a direct hit. It was enough to melt our planet, and the resulting debris, after being scattered into Earth’s orbit, eventually coalesced to form our moon. This is what we think happened based on the best data and computer models we have. We can’t really prove it, though, without the godlike power to crash one planet into another in a cosmic-scale experiment.

A hypothetical impact between Earth and a Mars-sized planetesimal. Source: William Hartmann

That’s what makes deep time so mysterious. It erases the data and our ability to reconstruct geologic history. To think we can even theorize about events that happened billions of years ago — before the first flicker of life even existed on this planet — is astounding. Yet we continue to devise new experiments to improve our understanding of these distant occurrences. As we begin to lift the veil on Earth’s own history, we come closer to some fundamental questions about the cosmos: Why Earth? Why us? Why didn’t life also evolve on Venus and Mars? Or maybe it has and we just haven’t found evidence of it yet. We seem to be converging on an answer.

The air we breathe

The giant impact theory for the creation of our moon has a relative advantage when it comes to the problem of deep time: the moon is still here. Look up at the sky on a cloudless night, and you’ll see evidence of Theia’s passing. Now consider instead the air you breathe. It seems fleeting, nearly invisible, and ever-changing. The air you breathe in differs in composition from the air you breathe out. Ask yourself this: if you wanted to know what the air was like during Earth’s early history, say right after the impact with Theia, where would you start? Recently, scientists devised an ingenious experiment to answer this question. The answer, it turns out, has been hidden deep below the surface of the Earth all along.

The impact with Theia would have reset the compositional clock on Earth’s atmosphere, blasting its proto-atmosphere into space. The impact was powerful enough to also melt Earth’s mantle, which is mostly composed of peridotite. For thousands of years afterward, Earth was an ocean of liquid rock. Gasses bubbled out of this primordial stew and formed a new, secondary atmosphere, one which was much hotter and thicker than today.

Scientists came to this conclusion by simulating the interaction between molten peridotite and Earth’s early atmosphere. Normally, rocks in the mantle are tucked away below Earth’s crust and don’t interact with the atmosphere at all. It takes a massive impact, like the one from Theia, to melt both the crust and the mantle on a planetary scale. As odd as that sounds today, it was common during the heavy bombardment period for the rocky planets of our inner solar system to undergo such transformations. Earth was not alone in that regard.

Eventually, the heavy bombardment period ended, and Earth cooled, the evidence of its early atmosphere locked in stone. Peridotite samples found today have a specific ratio of iron oxides that reflects their equilibrium with the atmosphere billions of years ago, one these rocks helped to create.

Olivine crystal, the principal component of peridotite. Source: Wikipedia

Paolo Sossi, a senior research fellow at ETH Zurich and the NCCR PlanetS, devised an experiment to determine the original composition of Earth’s atmosphere. Starting with a simulated peridotite magma in a sealed chamber, he then adjusted the air composition until he and his team successfully recreated the ratio of iron oxides we see in Earth’s mantle today. Like a key fitting a lock, only one combination of gasses resulted in the present-day mantle composition. This is what the early atmosphere on Earth had to look like, one that was rich in carbon dioxide and nitrogen.

If it sounds simple, it isn’t. Peridotite melts at temperatures in excess of 1800 C (3272 F), hotter than most laboratory furnaces. The scientists also needed to precisely control the air surrounding the molten rock. They accomplished this by melting the peridotite with a CO2 laser and fluxing the gas-mixture around it. The flux created enough buoyancy to levitate the bead of magma before it was quenched to preserve its composition.

Let that sink in. Levitating magma is a thing! When asked if that’s as cool as it sounds, Sossi said it’s “very cool indeed.”

What Earth was like billions of years ago

To imagine what that early the Earth was like, look no further than Venus. Sossi’s experiment shows that at this point in Earth’s history it looked a lot like Venus does today — hot and dense. The atmosphere of Mars also contains similar proportions of carbon dioxide and nitrogen but at a fraction of the pressure. If these three planets once had similar atmospheres, why are they so different today?

When it comes to planets, mass matters. More massive planets have a stronger gravitational pull which helps them hold onto their atmospheres. Earth is more massive than Venus (although not by much) which is more massive than Mars.

Proximity to the sun also engenders a certain pecking order among the terrestrial planets. Venus, being closer to the sun, has a hotter surface temperature which promotes atmospheric loss. Looking at hydrogen, in particular, Venus and Mars are most prone to losing this light gas to outer space. According to Sossi, “The strongest evidence for Venus and Mars having had significantly more water in the past is found in their Deuterium/Hydrogen (D/H) ratios. Since Hydrogen has half the mass of Deuterium, it is lost during atmospheric escape more readily than is Deuterium, which is more strongly bound gravitationally. Thus, the higher the D/H ratio of the planet, the greater the water (hydrogen) loss that planetary body experienced.”

Earth, on the other hand, has been able to hold onto its hydrogen for billions of years. Hydrogen is critically important to our planet’s evolution because it combines with oxygen to form water, and liquid water is the reactive brew that makes life possible. Venus and Mars lost their gaseous hydrogen and quite possibly the chance to harbor life along with it, although Mars was able to maintain some hydrogen in the form of water ice in its polar ice caps where it has lain dormant for 3.8 billion years.

Liquid water on Earth also played an important role in the evolution of Earth’s atmosphere. Without it, we might still have a hostile atmosphere like Venus. Instead, CO2 from Earth’s early atmosphere dissolved in the oceans and eventually became part of the oceanic crust. These crustal rocks would then be recycled into the upper mantle through the process of plate tectonics. Once removed from the atmosphere, the greenhouse effect was reduced, allowing the Earth to cool and life to flourish. Neither Venus nor Mars was able to sustain liquid water or tectonic activity for long enough periods to develop a hospitable atmosphere.

The spark of life

This research sheds light on how life may have evolved on our planet in another subtle, but profound way. In 1952, Stanley Miller and Harold Urey of the University of Chicago conducted an experiment to prove how lightning strikes may have given rise to life itself — the proverbial spark of life. They used a mixture of gasses which they thought approximated Earth’s early atmosphere: water vapor, methane, ammonia, and hydrogen. An electric spark simulated a lightning strike which produced 20 different amino acids from the concoction. It was a simple experiment that caught the imagination of the public, but there was only one problem: Earth’s early atmosphere didn’t look anything like that. The water vapor was there, but the Miller-Urey experiment should have contained carbon dioxide and nitrogen instead of methane and ammonia, as Sossi’s experiment showed. The early Earth was not as hospitable to life as Miller and Urey thought. The spark of life would have to wait.

Sources:

Asphaug, Erik. When the Earth Had Two Moons: Cannibal Planets, Icy Giants, Dirty Comets, Dreadful Orbits, and the Origins of the Night Sky. HarperCollins, 2019.

Sossi, Paolo A., Antony D. Burnham, James Badro, Antonio Lanzirotti, Matt Newville, and Hugh St C. O’Neill. “Redox state of Earth’s magma ocean and its Venus-like early atmosphere.” Science Advances 6, no. 48 (2020): eabd1387.