Nearly a hundred different types of amino acids have been observed in meteorites, but only a dozen of the 20 essential for life have been found. Biological amino acids also have a peculiarity that betrays them: they all have a “left-handed” structure, while abiotic processes create left- and right-handed molecules in equal measure. Several meteorites discovered on Earth have an excess of left-handed amino acids, says Dworkin — the only non-biological system ever observed with this imbalance.
For this experiment, the team tested the theory that amino acids were first made in interstellar molecular clouds and then traveled to Earth in asteroids. They decided to recreate the conditions these molecules would have been exposed to at each stage of their journey. If this process produced the same assortment of amino acids — in the same proportions — as those found in recovered meteorites, it would help validate the theory.
The researchers started by making the most common molecular ices 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, mimicking cosmic-ray collisions in deep space. The ices broke apart and reassembled into larger molecules, eventually forming a muck visible to the naked eye: chunks of amino acids.
They then simulated the interior of asteroids, which contain liquid water and can be surprisingly hot: between 50 and 300 degrees Celsius. They immersed the residue in water at 50 and 125 degrees Celsius for different periods of time. This increased the levels of some amino acids, but not others. For example, the amount of glycine and serine doubled. The alanine content remained the same. But their relative levels remained consistent before and after the chunks of the asteroid simulation crashed — there was always more glycine than serine and more serine than alanine.
This trend is remarkable, says Qasim, because it shows that conditions in the interstellar cloud would have strongly influenced the composition of amino acids in the asteroid. But in the end, their experiment ran into the same problem as other lab studies: the distribution of amino acids still didn’t match that in real meteorites. The most notable difference was the excess of beta-alanine over alpha-alanine in their lab samples. (With meteorites, this usually happens the other way around.) If there was a recipe for creating the precursors of life, they wouldn’t have found it.
That’s probably because their recipe was too simple, says Qasim: “The next experiments need to be more complicated — we need to add more minerals and consider more relevant asteroid parameters and conditions.”
But there is another possibility. Perhaps the meteorite samples they used for comparison are contaminated. When the meteorites crashed, they could have been altered by their interactions with Earth’s atmosphere and biology, as well as centuries of geologic activity that has melted, disappeared and recycled the planet’s surface.
One way to test this is to use a pristine sample as a starting point: In September, NASA’s OSIRIS-REx mission will bring home something like a 200-gram chunk of the asteroid Bennu. (That’s 40 times larger than the last sample we got of untouched space rock.) A quarter of the sample will be analyzed for amino acids, which will help pinpoint the source of discrepancies between lab studies and meteorites. It could also discover what other fragile materials are present in asteroids but cannot survive the journey to our planet without the protection of a spacecraft. That information would help Qasim’s team perfect their recipe.