No one knows how life began on Earth. But for it to happen, some simple chemical building blocks would have been needed. Many scientists argue the Earth’s violent past, with its massive volcanoes and regular meteor impacts, played a role in making these building blocks.
But some of the simple molecules that could have served as building blocks are found on comets, too. Mark Price, a space scientist at the University of Kent, suspected that on these icy bodies could be involved in making these chemicals.
The modern era of the origins of life research began 60 years ago at the University of Chicago. In 1953, chemists Stanley Miller and Harold Urey simulated a lightning strike in the atmosphere of the early Earth. They found that passing a spark through a mixture of methane, ammonia, hydrogen, and water made small amounts of amino acids. Amino acids are among the fundamental building blocks of life and assumed to be necessary for life to begin.
Then in 1963, two chemists from the Space Science Laboratory in California suggested that meteorite impacts on Earth could also provide enough energy to make amino acids. This process, called shock synthesis, occurs when chemicals react at the incredibly high temperatures and pressures created during a collision.
Carl Sagan, the famous astronomer, experimented with shock synthesis by bursting hot pressurised tubes containing gases that resembled early Earth’s atmosphere. His process also created amino acids, in surprisingly large amounts.
Price’s idea, reported in the journal Nature Geoscience, was not too different from Sagan’s. But creating a comet impact in a lab is not easy. Price used a gun to fire tiny bullets at 7,150 metres per second, more than 20 times the speed of sound, at blocks of ice. To model a comet these ice blocks were made out of methanol, ammonia, and carbon dioxide. Right after the bullet hits the ice block, heat is used to evaporate chemicals that did not react, leaving behind what Price hoped would be amino acids.
And indeed they were. Analysis revealed them to be glycine, alanine, and four other amino acids that are not used by life on Earth. Glycine made up most of the mixture.
The range of amino acids produced can be used to work out how they would have been formed. Because they had fewer carbon atoms than most amino acids, Price thinks they were made through a similar process as the Miller-Urey experiment. The impact of the pellet creates intense pressure, up to 500,000 times higher than atmospheric pressure. In these conditions, oxygen can add to methanol to create the carbonyl compounds needed to make amino acids.
If comet impacts can make amino acids too, these results add to the growing body of evidence that some of life’s building blocks may have been abundant in the Solar System before life began. But the steps involved in using them to make life still remain a matter of hot debate.