Graphite —- better known as ordinary pencil lead —- likely triggered the onset of the first prebiotic molecules here on earth. But there was nothing simple about it. The creation of enough graphite to push along the needed chemical complexity to vector towards life was arguably due to a series of giant random impactors that hit Earth beginning some 4.3 billion years ago.
In a paper submitted to the MDPI journal Life, Paul Rimmer and Oliver Shorttle, two planetary astrochemists at Cambridge University in the U.K. detail their laboratory simulations indicating that graphite offers a potential route towards prebiotic chemistry. That is, the basic chemistry that enables the onset of that mysterious ‘X factor’ we call ‘life.’
There probably was a lot of organic tar on the early Earth and our new model shows the heating that tar will likely produce molecules for the building blocks of life, Rimmer told me via phone.
The authors’ heating mechanism for the tar was modeled using a hypothetical Hadean-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 passing through graphite-saturated crust,” the authors write. Rimmer and Shorttle note that their models assumed that the Earth during this Hadean phase had surface pressures a hundred times that of our present atmosphere with temperatures reaching 1700 degrees Celsius. Or some eight times that of a conventional kitchen oven.
The tar is created as a result of a giant impact plus photochemistry and atmospheric rainout, says Rimmer. The tar ends up in the crust and is heated by magma, he says.
Giant Impactors Made The Difference
Some 4.3 billion years ago, Earth was likely hit by an iron-rich object roughly the size of our Moon, the authors note. This was roughly 200 million years after the Mars-sized impactor that created our Moon in the process. That earlier impactor also rearranged the chemical distribution of Earth’s nascent surface, giving today’s inhabitants access to precious and rare metals like gold.
The iron in this follow-on giant impactor would have reacted with ocean water, producing large amounts of hydrogen, Rimmer and Shorttle write. In the high temperatures of this post-impact atmosphere, hydrogen would have reacted with carbon dioxide and nitrogen to produce methane and ammonia, the authors note.
Once this thick tar gunk rained out and was deposited on the surface, some of it got heated by magma, says Rimmer. When it did, the tar mostly turned into graphite; the leftover gas was hydrogen, carbon monoxide, nitrogen, and then nitriles, he says.
Nitriles are molecules with a carbon and nitrogen joined by a triple-bond, says Rimmer. They are physically 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, says Rimmer. And they react to form many of life’s building blocks, he says.
These nitriles can start making formaldehyde, says Rimmer. Formaldehyde itself isn’t quite a sugar, but it’s almost there, he says. And formaldehyde can make sugar by finding other formaldehyde, says Rimmer.
But Chemical Diversity Can Be Too Much Of A Good Thing
A lot of environments have hundreds of thousands of other chemicals, says Rimmer. Typically, the chemistry that produces nitriles produces a “mess”: many thousands to even millions of other molecules, he says. No known scenario gets lots of nitriles with only a handful of other (mostly unreactive) molecules, he says. We found a way to do that for the first time, Rimmer notes.
What’s next?
These are experiments that are run for things like combustion engines, places where you have a lot of hydrogen and carbon and nitrogen, says Rimmer. The next steps will be to actually heat up some of this gunk and see if it converts in the way that our model predicts, he says.
But Prebiotic Chemistry Is Still Up To Chance
In chemistry, you want two molecules to react with each other, and not with a bunch of other molecules, says Rimmer. Picture it like a deck of 52 cards; there are two in the deck and you want them to be next to each other, he says.
That’s pretty likely in a deck of four cards, but it’s highly unlikely in a deck of a million cards, says Rimmer.
