Pompey Theatre, 15th March, 44 A.D, the middle of the Ides.
Stream of blood is covering the floor, the smell of death is dramatically spreading across the stage.

Julius Caesar is dead. Stabbed and wounded to death by his very own friends and allies.

Et Tu Brut?

Caesar did not see this coming.

Heck, just two months ago, he was named dictator perpetuo, dictator for life, by the Senate. What a title to have, huh?

His last, dying breath has, for some strange historical and creative reason, become a classic teaching material in high schools and colleges — for the sake of science.

When Caesar exhaled, he released a great number of air molecules, mostly nitrogen and carbon dioxide.

By chemists’ estimation and calculation: 25 sextillions.
Sounds gargantuan?
It is.
21 zeros!

When Caesar exhaled that last breath, all those molecules got spread across the Earth, first in a band of prevailing winds around the same latitude as Italy, then over the northern hemisphere. In the course of about two years, given air currents and circulation, it would probably have spread across the entire world.

Some scientists figured: some of Caesar’s air is absorbed by plants, some by animals, some by water — and a large portion would float free and spread themselves all around the globe in a pattern so predictable that (this is the fun part) if you take a deep breath right now, at least one of the molecules entering your lungs literally came from Caesar’s last breath.

It is a slim chance, but it’s still possible.

Some knowledgeable people are more optimistic and say that we take in three of Caesar’s molecules per breath, or eight. Some say 10, even.

It all depends on your assumptions about the size of a breath, the size of the atmosphere, the location of the breather (on a mountain, or at sea level?)
Even though these calculations apply to any breath exhaled long ago — Shakespeare’s, Cleopatra’s, Hitler’s, Einsteins’s. your great-great-grandma’s — you may still want to take a moment today to share with Caesar. Just take a deep breath and share Caesar’s molecule.

Nitrogen and oxygen are the main ingredients of air, making up 99 percent, but that extra 1 percent is still really important. It’s like a glass of wine: most of the wine is alcohol and water, but there are all these extra overtones and flavors, too. In the air, this 1 percent is responsible for all of the global warming as well as all scents and perfumes. It includes carbon dioxide, nitrous oxide (laughing gas), assorted pollutants and volcano exhaust.

The ingredients of air reveal the world’s entire history. Some of them have been around since the early days of the planet, while some only arose with the arrival of life, or with human civilization.

Where did our atmosphere come from?

We’ve actually had four different atmospheres in the Earth’s history.

The first was a wispy leftover from our planet’s formation, and soon got blown away. The next came from the ground, seeping out of cracks in the Earth’s surface – mostly carbon dioxide and water vapor, but also gases such as sulfur dioxide and hydrogen sulfide. Atmosphere number three was dominated by nitrogen, emitted from volcanic vents in relatively small quantities, but capable of sticking around for a long time. And finally, oxygen began to build up in the atmosphere thanks to early, photosynthesizing life forms. This paved the way for an oxygen-rich atmosphere that could support complex life.

If you wanted to travel back in a time machine to the Earth’s distant past and take a deep breath outside, you’d only be able to go back a few hundred million years – it’s only very recently in our planet’s history that there’s been enough oxygen to sustain us.

How is the air we breathe changing?

The atmosphere is like a living thing – it’s constantly evolving. The rates of carbon dioxide and other greenhouse gases are increasing, and the air is more radioactive now because we’re still dealing with the fallout from the 1950s nuclear weapons tests.

We also see a lot more complex, human-made molecules in the air today. If aliens were to look at our planet’s atmosphere, the presence of these gases would be a good sign that the Earth harbors life. Likewise, the next generation of telescopes should allow us to start looking for these complicated gases in the atmospheres of distant exoplanets, helping us to search for the best candidates for extraterrestrial life.

I think it’s also inevitable that we’ll find an exoplanet with a great mix of gases for us to survive on. The hard part will be figuring out how to get there.

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