Graphite pencil lead probably triggered the first prebiotic molecules

Graphite – better known as ordinary pencil lead – probably triggered the appearance of the first prebiotic molecules here on earth. But there was nothing simple about it. The creation of enough graphite to push the required chemical complexity into the vector towards life was undoubtedly due to a series of giant accidental impactors that hit the Earth starting about 4.3 billion years ago.

In a paper submitted to the journal MDPI ZOE, Paul Rimmer and Oliver Shorttle, two planetary astrochemists at the University of Cambridge in the UK detail their laboratory simulations, indicating that graphite offers a possible route to prebiotic chemistry. That is, the basic chemistry that enables the initiation of this mysterious “factor X” we call “life”.

There was probably a lot of organic tar on the early Earth, and our new model shows warming that tar will likely produce molecules for the building blocks of life, Rimmer told me over the phone.

The authors’ heating mechanism for the tar was modeled using a hypothetical Hades-era surface vent dating back to our planet’s first 500 million years.

“We model a surface vent fed by nitrogen-rich volcanic gas from high-temperature magmas flowing through graphite-saturated crust,” the authors write. Rimmer and Shorttle note that their models assumed that the Earth during this Hades phase had surface pressures a hundred times greater than that of our present atmosphere with temperatures reaching 1700 degrees Celsius. Or about eight times that of a conventional kitchen oven.

The tar is created as a result of a massive impact plus photochemistry and atmospheric rain, Rimmer says. The tar ends up in the crust and is heated by the magma, he says.

Giant Impactors made the difference

About 4.3 billion years ago, Earth was likely struck by an iron-rich object about the size of our Moon, the authors note. This was about 200 million years after the Mars-sized impactor that created our Moon in the process. This earlier shock also rearranged the chemical distribution of Earth’s nascent surface, giving today’s inhabitants access to precious and rare metals such as gold.

The iron in this giant impactor would have reacted with ocean water, producing large amounts of hydrogen, Rimmer and Shorttle write. At the high temperatures of this post-impact atmosphere, the hydrogen would have reacted with carbon dioxide and nitrogen to produce methane and ammonia, the authors note.

Once this thick tar gouge rained down and deposited on the surface, some of it was heated by the magma, Rimmer says. When it did, the tar was mostly converted to graphite. The remaining gas was hydrogen, carbon monoxide, nitrogen and then nitrile, he says.

Nitriles are molecules with carbon and nitrogen held together by a triple bond, Rimmer says. They are naturally very stable (cyanide is a nitrile) and are also chemically reactive, he says. But take five cyanide molecules and put them together and you get adenine, one of the bases for RNA and DNA, Rimmer says. And they react to form many of the building blocks of life, he says.

These nitriles can start to produce formaldehyde, Rimmer says. The formaldehyde itself isn’t much sugar, but it’s almost there, he says. And formaldehyde can make sugar by finding other formaldehyde, Rimmer says.

But chemical diversity can be a very good thing

Many environments have hundreds of thousands of other chemicals, Rimmer says. Typically, the chemistry that produces nitriles creates a “mess”: many thousands to millions of other molecules, he says. No known scenario contains many nitriles with only a handful of other (mostly unreactive) molecules, he says. We found a way to do this for the first time, Rimmer notes.

What’s next?

These are experiments that are run on things like internal combustion engines, places where you have a lot of hydrogen and carbon and nitrogen, Rimmer says. The next steps will be to actually heat up part of that gun and see if it converts in the way our model predicts, he says.

But Prebiotic Chemistry is still in luck

In chemistry, you want two molecules to react with each other, not with a bunch of other molecules, Rimmer says. Think of it like a deck of 52 cards. there are two on deck and you want them to be next to each other, he says.

That’s very possible in a four-card deck, but highly unlikely in a million-card deck, Rimmer says.