About a hundred different types of amino acids have been found in meteorites, but only a dozen of the 20 essential for life have been found. Biological amino acids also have a feature that gives them away: they all have a “left-handed” structure, while abiotic processes create left-handed and right-handed molecules equally. Several meteorites found on Earth have an excess of left-handed amino acids, Dworkin says, the only non-biological system ever observed with such an imbalance.
For this experiment, the team tested the theory that amino acids were first created in interstellar molecular clouds and then made their way to Earth inside asteroids. They set out to recreate the conditions these molecules were subjected to at each stage of their journey. If this process produced the same set of amino acids – in the same ratios – as those found in recovered meteorites, this would help confirm the theory.
The researchers started by creating the most common molecular ices found in interstellar clouds—water, carbon dioxide, methanol, and ammonia—in a vacuum chamber. They then bombarded the ice with a beam of high-energy protons, simulating collisions with cosmic rays in deep space. The ice broke apart and reassembled into larger molecules, eventually forming a sticky residue visible to the naked eye: chunks of amino acids.
They then modeled the interior of asteroids, which contain liquid water and can be surprisingly hot: between 50 and 300 degrees Celsius. They immersed the remains in water at 50 and 125 degrees Celsius for different times. This increased the levels of some amino acids but not others. The amount of glycine and serine, for example, doubled. The content of alanine remained the same. But their relative levels remained constant before and after the chunks were immersed in the asteroid simulation—there was always more glycine than serine, and more serine than alanine.
This trend is noteworthy, Qasim says, because it shows that conditions in the interstellar cloud had a strong effect on the composition of amino acids inside the asteroid. But in the end, their experiment ran into the same problem as other laboratory studies: the distribution of amino acids still did not match that found in real meteorites. The most notable difference was the excess beta-alanine versus alpha-alanine in their lab samples. (In meteorites, this usually happens the other way around.) If there is a recipe for creating the precursors of life, they haven’t found it.
That’s probably because their recipe was too simple, Qasim says: “Next experiments should be more complex – we need to add more minerals and consider more suitable asteroid parameters and conditions.”
But there is another possibility. It’s possible that the meteorite samples they used for comparison are contaminated. As the meteorites fell, they may have been altered by their interactions with the Earth’s atmosphere and biology, as well as centuries of geological activity that melted, subducted, and reworked the planet’s surface.
One way to test this is to use a pristine sample as a starting point: this September, NASA’s OSIRIS-REx mission will bring home what looks like a 200-gram chunk of the asteroid Bennu. (That’s 40 times more than the last pristine space rock sample we received.) A quarter of the sample will be analyzed for amino acids, which will help pinpoint the source of the discrepancy between laboratory tests and meteorites. It can also detect what other fragile materials are present in asteroids, but it won’t be able to survive the journey to our planet without the protection of a spacecraft. This information will help Kashima’s team improve their recipe.