So there I was, fresh off the heels of receiving my PhD in quantum chemistry, having a chat with my supervisor, Jamie. As we talked about my next steps, I couldn’t help but express my burning desire to dive into the world of astrochemistry. To me, it was the stuff of dreams: studying the vast expanse of the cosmos and contributing to our collective understanding of the universe. But Jamie didn’t share my enthusiasm. “It’s just chemistry in vacuo,” he said. No little green friends out there.
Well, at the end, my academic path led me to the investigation of systems of biological relevance, which I found, and still do, immensely gratifying. Yet, a small voice inside me couldn’t help but wonder: what if I had pursued astrochemistry after all?
Astrochemistry: Molecules in Space
Have you ever stopped to think about galaxies? Billions of stars and a bunch of interstellar matter out there. And it’s not just random fluff floating around: these massive clouds give birth to entire star systems.
For the longest time, scientists, just like my supervisor, thought that space was just a boring, empty vacuum with only hydrogen and maybe some helium. A snooze-fest. But then, about fifty years ago, some gutsy researchers started looking deeper, and they found something that blew everyone’s minds.
Molecules. Molecules all around.
Radio astronomy was just getting started back in the 1930s, and it wasn’t until 1937 that we even knew there were interstellar molecules. Three years after, in 1940, while Frank Sinatra was debuting his singing in the Tommy Dorsey Orchestra, McKellar detected the first emission lines from the interstellar space attributed to CH and CN molecules. This was a significant discovery, as it provided evidence that molecules could exist in the harsh and mostly empty environment of interstellar space.
If these identifications are proved true, they are of considerable interest and importance in that they constitute the first definite evidence of the existence of molecules in interstellar space
McKellar (1940)
From there, things only got more moleculary.
Over the next few decades, we – as in they smart scientists – found all sorts of funky compounds, like the first polyatomic molecule, CH2O, and ammonia, in the center of the galaxy.
Astrochemistry and Astrochemists
Astrochemists are a unique breed – 15% astronomer, 25% astrophysicist, 60% chemist, my personal non-verified estimation – and their work takes them to the far reaches of space, where they study the molecular makeup of the universe. These intrepid explorers don’t just gaze up at the stars and wonder – as I used to. No, they use telescopes to detect the electromagnetic radiation emitted by celestial objects, and through a combination of experimental and theoretical models, including computer simulations, they unlock the secrets of the universe’s chemical composition.
And yes, James Webb is astrochemists’ newest, most powerful, and expensive toy.
So, how do they exactly do it?
In the vast field of astrochemistry, the most reliable way to detect the presence of molecules in astronomical environments is through molecular spectroscopy.
Molecular spectroscopy is a fancy way of investigating how light interacts with molecules. Think Pink Floyd’s Dark Side of the Moon. Astrochemists use different types of light like infrared, ultraviolet, and visible to record how molecules absorb, emit, or scatter light in their own special way. It is like shining a flashlight on a group of molecules and seeing how they respond.
Every molecule has its own unique response, the spectral fingerprint.
To identify different species and their molecular properties, astrochemists usually compare laboratory- or computer-generated spectra with astronomical measurements.
Complex Molecules of the Universe
It turns out, who would have guessed, that molecules are everywhere. Even though dark matter and dark energy make up most of the universe, it’s not just an empty Darth Vader wasteland. Atoms and molecules only amount to 5% of the universal matter, but there’s still a ton of variety in the great beyond. We’ve found over 200 different molecular species in the interstellar medium and circumstellar shells.
It totally shatters the old idea that the universe is a boring empty space only with hydrogen and helium.
These “newly detected” molecules have relatively complex structures, with carbon as their rockstar core. Take, for example, glycolaldehyde, the first sugar, or acetamide, particularly intriguing because it contains a peptide bond, the molecular glue of proteins. This is a big deal, as these discoveries fuel theories about the extraterrestrial origin of DNA bases and/or proteins.
And that’s why we’re so motivated in the hunt for more interstellar molecules with direct biological implications, such as glycine, the simplest amino acid ever discovered… in a comet tail. Glycine has been extensively searched for in the interstellar medium, but so far, no luck.
No surprise here. These are the harshest conditions you could imagine (as cold as -260 °C!), in which carbon-based molecules must survive in order to evolve into more complex structures and thus serve as the initial building blocks of life.
We still have so much to learn. The universe’s molecular inventory is like a big ol’ mystery novel, and we’ve barely read the first pages.
The Central Role of Quantum Chemistry
Now, my beloved quantum chemistry and the investigation of the stars are like Batman and Robin. Already in the nineteen-freaking-90’s, 1992 to be specific, Y. Ellinger concluded that quantum chemistry had reached such a level of accuracy to provide reliable information on chemical problems of astrophysical interest.
The state of the art in Quantum Chemistry has now reached a point such that the wave functions determined by advanced computational techniques are accurate enough to provide reliable informations useful for a basic understanding of a number of chemical problems of astrophysical interest
Y. Ellinger (1992)
Over the years, quantum chemistry has advanced to a point where it can predict the spectroscopic properties of molecules to guide the experimental identification of spectral fingerprints or shed light on the formation and behavior of complex molecules in the interstellar medium.
Welcome to solving a cosmic jigsaw puzzle with quantum chemistry.
Since 1996’s Y. Ellinger paper, the progress is nothing short of impressive, with incredibly accurate predictions of gas-phase thermodynamic properties, rotational constants, and vibrational signatures (better than 1 kJ mol-1, better than 0.1%, and a mean absolute error of 10 cm-1, respectively).
Now, use these numbers with caution.
They may not mean much to you, but if you whisper them to your local computational chemist, they will be yours forever ❤️
The 3 Challenges of Astrochemistry
It is not all roses and flowers in the world of astrochemistry. In a 2020 article published in Frontiers, Cristina Puzzarini highlights the grand challenges of the field:
1 – On the Hunt for Molecules
Although we’ve discovered over 200 molecular species, there are still many features in radio astronomical spectra that have yet to be identified. We’re not even close to a complete census of interstellar molecules, and that raises some intriguing questions. How many molecules have we missed? And just how complex is the chemistry of space?
2- Chemical Reactivity
Back in the 70s, scientists used gas-phase ion-molecule reactions to explain the molecular abundances found in interstellar clouds. But as our technology improved, we started finding molecules in places where gas-phase reactions just couldn’t explain their formation. Some scientists believe they’re the product of dust-grain surfaces, while others still argue that gas-phase chemistry is where it’s at.
3- The Origin of Life
This is the big one. There are two main theories about how life emerged on Earth: exogenous delivery (meaning life was brought here from somewhere else) and endogenous synthesis (meaning life arose on Earth from scratch). But we’re still far from figuring out which one (or both) is actually true. We’re talking about the building blocks of life DNA and proteins… the hunt for the molecules of life is not over yet.
A Final Personal Touch
True story. When I was a snot-nosed kid, I’d gaze out the window at night and get all tripped out by the flickering stars. I was both creeped out and stoked, hoping beyond hope that a little green friend would make an appearance and blow my tiny curly mind.
Did life come from space?
Despite all our attempts to find amino acids or DNA bases lurking in the interstellar void, we haven’t found squat yet. We’re just getting started in our chemical cosmic hunt.
Maybe that little green friend is around the corner, waiting to high-five us with its slimy little hand.
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