Cosmic Connections: What’s That Smell?


Cosmic Connections is a series that explores how our experiences here on Earth aren’t so different across the universe.

It’s the height of summer. Time for a cookout. You invite the friends over, throw some protein on the grill, and light it up. Oops! A bit too much fuel this time, and you watch as the flames briefly flash higher, the smoke tracing out delicate curls in the air as you check if your eyebrows are still there. The heat more manageable now, you wait patiently for those ever-so-perfect char lines on the meat, signifying that you have mastered your grilling technique.

The smoke from the flame. The marks on the meat. That’s all carbon. Specifically, almost-but-not-quite completely combusted carbon. When you make a fire or heat something up, all the carbon in the thing about to burn would just love to hook up with all the oxygen in the air, releasing energy in the process and carrying on its merry way. But nothing’s perfect: the temperature isn’t quite hot enough everywhere, there’s not an oxygen match for every carbon atom, and so on. You’re left with carbon that got chemically rearranged, but not into the oxygen relationship it was hoping for.

Thankfully, carbon still has some friends, and hydrogen is its best buddy in the whole wide world. Carbon and hydrogen love to travel around in groups, forming a variety of complex chains known sweetly as polycyclic aromatic hydrocarbons. “Aromatic” here is a chemistry jargon term that means rings of atoms. “Polycyclic” means that those rings will repeat themselves in various ways. And no points will be awarded for guessing what “hydrocarbon” refers to.

Known as PAHs because nobody wants to say the whole thing out loud, these little compounds are ubiquitous in our environment. With over 100 different combinations, they are usually found floating in the air or stuck to surfaces, they are usually colorless (but can sometimes be pale yellowish) and by themselves tend to have a slightly sweet smell to them. You’re practically swimming in them right now as you read this.

They’re also in space.

So how to manufacture a PAH in space? Well hydrogen is element #1 when it comes to the universe: forged in the first few fiery minutes of the big bang, there’s more hydrogen in the cosmos than all other elements combined. So that part’s done.

The second part, carbon, is a little more difficult to make. Stars power themselves through the fusion of elements, starting with hydrogen (because they’re mostly hydrogen because what else are they going to be made of) and converting it into helium in their fiery inner cores. That fusion releases a little bit of energy, and repeated countless times is able to maintain a stable star for millions, billions, and sometimes even trillions of years. But as the stars age, they run out of usable hydrogen in their cores. When that happens, they switch to helium fusion instead, which doesn’t last for long, since the fusion process isn’t as efficient as it was in the good old hydrogen days.

And what do you get when you fuse helium together? Carbon. Stars like our sun, when they near the end of their lives, make carbon.

Molecules in spaaaaace. Carbon compounds are found in all sorts of nebulae. These molecules are, uh, not shown to scale.NASA

And then when the stars die they tend to either blow up or at least just barf their guts out all over their solar systems, and as you might imagine this messy process ends up with loads of carbon transferred from the insides to the outsides of the stars.

So you’ve got a) hydrogen, b) carbon, and c) energetic events that try to mix them up. Voila: PAHs, in space. I told you so.

We can see the telltale signature of PAHs in many nebulae, which leads to an intriguing thought. Earth doesn’t make carbon itself (it’s, uh, not a star), so any carbon in the air or dirt – or on your grill – was already floating around in the pre-solar nebula as the Earth formed. And since PAHs are such a common way for carbon to find its way through space, did the Earth start with its carbon store in the form of such PAHs?

And since life itself depends on complex arrangements of carbon, is that how life got started?

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