Meteorites Bathed in Gamma Rays Produce More Amino Acids and Could Have Helped Life get Going on Earth

Our modern telescopes are more powerful than their predecessors, and our research is more focused than ever. We keep discovering new things about the Solar System and finding answers to long-standing questions. But one of the big questions we still don’t have an answer for is: ‘How did life on Earth begin?’

We won’t answer the question of life’s origins in one dramatic act. Instead, we’re chipping away at it, slowly piecing together an answer over generations. We know life has building blocks, molecules critical to life gaining a foothold here on Earth and, hopefully, elsewhere. Amino acids are some of those building blocks.

Amino acids play a critical role in life on Earth, though we don’t know exactly how they fit into a timeline of life’s origins. We’ve found them in space, which has led to speculation that they originated there and then found their way to Earth as biological building blocks.

A new study fills in part of the picture by showing that meteorites bathed in gamma rays produce more amino acids.

There are hundreds of amino acids, but only 22 appear in the genetic code. Glycine is one of the simplest amino acids in the genetic code, and scientists have found glycine in objects in space. They’ve seen it in comets, interstellar dust clouds, and meteorites that fell to Earth, which led to the idea that meteorites contributed to the appearance of life on Earth.

A new study shows that when meteorites are bathed in gamma rays, they produce more amino acids. The study is “Gamma-Ray-Induced Amino Acid Formation in Aqueous Small Bodies in the Early Solar System,” and it was published in ACS Central Science, the journal of the American Chemical Society. The lead author is Yoko Kebukawa, an associate professor in the Department of Chemistry at Yokohama National University.

When Earth formed, the Solar System was a much more chaotic place. Meteorites flew through space, slamming into things like particles in an accelerator. Many of them struck Earth. There are fewer of them now, though they still fall to Earth. When they do, people find some of them, and many have found their way to scientists’ labs. Over time, scientists have classified meteorites into different families.

There are three top-level categories of meteorites: stony meteorites, iron meteorites, and stony-iron meteorites which are a combination of both types. There are further classifications based on chemical compositions, isotopes, and mineralogy.

Carbonaceous chondrites (CCs) are a type of stony meteorite and are some of the most primitive. The name is a bit confusing. Scientists thought they contained carbon because of their dark and grey appearance, but they actually contain less carbon than other meteorites. However, CCs contain something more important than carbon: they’re known for containing water and other molecules, including amino acids.

That, along with their ancient age, makes them important because they hold clues to the early Solar System, back when Earth was settling down, and life was getting started.

There seems little doubt that carbonaceous chondrites contained amino acids back then and that they would’ve delivered them to the young Earth. But how did the amino acids form?

That’s the question behind the new study.

Artist’s concept of a meteorite entering Earth’s atmosphere. There were more meteorites striking Earth billions of years ago. Could they have delivered building blocks like amino acids to Earth, spurring on the appearance of life? Image Credit: University of Oxford

Lead author Kebukawa showed in previous research that reactions between simple molecules such as ammonia and formaldehyde could create macromolecules, including amino acids. But only in the presence of liquid water and only when there’s heat to drive reactions. We know carbonaceous chondrites contain water, but where did the heat come from?

It could’ve come from one of the two naturally-occurring isotopes of aluminum: 26Al.

26Al is cosmogenic, meaning it was created when cosmic rays bombarded meteor fragments. It was relatively abundant back when the Solar System was forming but has now decayed.

26Al is unstable and releases gamma rays as it decays. Scientists think the heat from that decay is responsible for the melting and differentiation of some asteroids after they formed in the early Solar System. But the heat could’ve also driven the production of amino acids in meteors.

Kebukawa and her colleagues tested this idea in their laboratory. They combined compounds like formaldehyde and ammonia, both common chemicals in space, and exposed them to gamma rays. Not gamma rays from 26Al but from more readily available 60Co (cobalt-60). 60Co is a synthetic radioisotope produced in nuclear reactors. It’s used in radiation therapy, sterilizing medical instruments, and other things.

Since it produces gamma rays as it decays, the researchers used 60Co as a proxy for primordial 26Al.

The experiment showed that the rays melted ice in carbonaceous chondrites when 26Al decayed and released gamma rays. In the presence of liquid water and the energy from the 26Al, simple chemicals like ammonia (NH3) and methanol (CH3OH) formed amino acids. Image Credit: Kebukawa et al. 2022.

The researchers found that the gamma rays increased the production of some amino acids as the gamma-ray exposure increased.

This figure from the study shows how gamma rays produced different amino acids. Gamma rays are measured in kGy, kilo-Grays (1 kGy. = 1,000 J/kg). The zero kGy rows on the bottom are the control group. The colour-coded legend explains how much of each amino acid was produced. Glycine, the simplest and most common amino acid, is shown in blue at the bottom of the legend. Image Credit: Kebukawa et al. 2022.

Amino acids are divided into four categories: alpha, beta, gamma, and delta. Alpha amino acids are the most essential amino acids because they’re used to synthesize proteins. Glycine (Gly), Alanine (Ala), Leucine (Leu), Serine (Ser), Asparagine (Asp), Isoleucine (Ile), and Glutamine (Glu) are all alpha amino acids produced in the experiment. The quantities of these essential amino acids rose in the irradiation solutions as the total gamma-ray dose increased.

What do these laboratory results mean in the real world?

The researchers took their results and calculated a plausible level of amino acid production in meteorites. They focused on a specific family of meteorites called CM chondrites, the most commonly recovered carbonaceous chondrite type. The M stands for the Mighei meteorite, and their calculations are for the parent body of all CM chondrites. Their analyses also take into account the decay of amino acids over time.

The team calculated the production of alpha-alanine and beta-alanine, a component of things like vitamin B5. They calculated the yields of amino acids in the liquid phase and the whole rock. Their work shows that it would’ve taken between 1,000 and 100,000 years to produce the amount of alanine and ?-alanine found in the Murchison meteorite, the most well-studied Mighei meteorite.

This figure from the study shows the calculated yields of alpha-alanine (green) and beta-alanine (pink) in the CM parent body. The top row shows (a) the Gamma-ray dose rate expected in the parent bodies of CM chondrites and calculated yields of alanine and beta-alanine per year (b) in the liquid phase, and (c) in the whole rock. The bottom row shows (d) the Total gamma-ray dose expected in the parent bodies of CM chondrites and calculated total yields of alanine and beta-alanine (e) in the liquid phase and (f) in the whole rock. Image Credit: Kebukawa et al. 2022.

In the paper’s conclusion, the researchers explain their results. “Our findings point to the possibility of gamma-ray-induced amino acid formation from ubiquitous, simple molecules such as formaldehyde and ammonia in the presence of water inside small bodies during the early stages of the formation of the Solar System.” Note the prudent use of the word “possibility.”

“The gamma-ray-induced production of amino acids could be a novel prebiotic amino acid formation pathway that could have contributed to the origins of life on early Earth, as building blocks of life were delivered through the fall of meteorites.”

The idea that meteorites could’ve brought amino acids to the Earth and helped spur on life is not new. But this study strengthens that idea and is another piece of the intricate puzzle that is life on Earth.