Comet crash could yield life’s building blocks

Biologists, chemists and physicists continue to puzzle over how the early conditions of our planet could have created the crucial molecules that gave rise to you and me. In a new study that may only add to the controversy over life’s origins, researchers at the Lawrence Livermore National Laboratory in California have used computer modeling to show that a comet’s impact on early Earth could have formed amino acids, the molecular building blocks of proteins and an essential starting point for life.

Physical chemist Nir Goldman and his colleagues ran simulations showing that a glancing, angled blow to the Earth’s surface could have produced low enough temperatures and pressures that the comet’s molecular precursors to amino acids—including carbon dioxide, ammonia and methanol—could have survived.

However, many researchers believe that comet impacts aren’t likely to produce amino acids, said biophysicist André Brack of the Center of Molecular Biophysics in Orléans, France. He says that the impact would deliver so much heat that everything, including those critical precursor deposits deep in the ice of comets, would be destroyed.

But in Goldman’s model, those simple molecules from the angled comet strike did make it safely to Earth, where they confronted a second hurdle: without the right chemical conditions, they couldn’t build more complex molecules. On its own, the early atmosphere didn’t provide the right conditions for spontaneous synthesis of these molecules, says Goldman. But his simulations show that the shock wave produced by a comet’s impact could have created just the right environment, one with lots of spare electrons waiting to form new bonds.

Under those conditions, carbon-nitrogen bonds—the links that form the backbone of amino acids and longer proteins—could have formed. After long chains of these bonds are created, the model shows, the rapid cooling that occurs after a comet’s impact could have broken them up into smaller compounds resembling glycine, the simplest amino acid.

While the study appeared in the well-regarded journal Nature Chemistry in November, some researchers remain skeptical of work that uses computer modeling to make assertions about chemical reality without conducting physical experiments. Sandra Pizzarello, a biochemist at Arizona State University in Tempe, emphasizes that “the impact [of the work] is zero” without physical experiments, and Brack calls the simulations “just a piece of fun.”

But according to Goldman, the right follow-up studies are not outside the realm of possibility. He says laboratory researchers have the tools to mimic a comet impact and to recover any biological precursors formed by a man-made shock. Astrophysicist Mark Price at the University of Kent has already initiated such research, blasting ice with pellets and recreating the high pressures that Goldman’s modeled impact would have caused. Though Price still hasn’t confirmed that the crystals of organic material he found were created by the blasts alone, his results may take us one step closer to understanding the beginning of life on Earth.