Earth is the only planet in the Universe known to host life, but even that was desolate in the beginning. About 4 billion years ago, something happened that gave our barren rock the building blocks for life.
Amino acids, for example, had to exist before the Earth could have proteins, a vital component of all life forms. The origins of Earth’s amino acids remain murky, but some scientists suspect that these organic compounds were delivered from space by meteorites.
In a new study, researchers reveal details about how this might have happened, supporting the idea that meteorites helped establish life on Earth.
The study shows how a certain class of meteorites called chondrites could produce their own amino acids thanks to reactions driven by gamma rays from the meteorites themselves.
Meteorites are pieces of ancient debris left over from the Solar System’s infancy that crash into a planet or moon. Different types of meteorites have different materials.
Chondrites are stony meteorites embedded with mysterious spheres known as chondrules. Made primarily of silicate minerals, chondrules are among the oldest objects in the Solar System.
Meteorites have bombarded Earth since the beginning, and some of the earliest bombardments may have included carbonaceous chondrites, a relatively rare subcategory of chondrites that contain significant amounts of water and small molecules, including amino acids.
These meteorites could have given Earth vital ingredients for life, but how did these ingredients get to a meteorite in the first place?
We’re not sure yet, but the new study sheds light on how chondrites (or their parent bodies) are at least theoretically capable of synthesizing these compounds.
Led by cosmochemist Yoko Kebukawa of Yokohama National University in Japan, the researchers sought to resolve questions from previous laboratory experiments investigating the possible formation of amino acids in carbonaceous chondrites.
These experiments showed that simple molecules like ammonia and formaldehyde could generate amino acids, but only in the presence of heat and liquid water. In the new study, the researchers examine a possible source of the meteorite’s heat: gamma rays.
The earliest carbonaceous chondrites are known to contain aluminum-26, a radioactive element that can release gamma radiation as it decays. Kebukawa and his colleagues decided to test whether this could provide the heat needed to form amino acids.
The researchers dissolved ammonia and formaldehyde in water, sealed the resulting solution inside glass tubes, and then exposed the tubes to high-energy gamma rays from decaying cobalt-60.
As the dose of gamma radiation increased, so did the production of α-amino acids such as alanine, glycine, α-aminobutyric acid, and glutamic acid, along with β-amino acids such as β- alanine and β-aminoisobutyric acid.
The researchers note that these amino acids could help explain the presence of these amino acids in carbonaceous chondrites that have fallen to Earth, such as Australia’s famous Murchison meteorite.
Loaded with “presolar” (ie older than the Sun) silicon carbide particles, the Murchison meteorite burst into the sky over Murchison, Victoria on September 28, 1969. It was a widely observed event ; then people collected a fund of fragments from the area. Since then it has become one of the most studied space rocks in history.
Among many interesting finds, the Murchison meteorite was full of amino acids. Scientists have so far identified more than 70 amino acids from the meteorite, only 19 of which are known from Earth, according to Museums Victoria.
This has sparked widespread fascination, suggesting that life on Earth’s basic chemical building blocks can easily form elsewhere as well.
A meteor with the outside fused in the heat of hitting the Earth’s atmosphere and its interior more granulated in organics. (Steve Jurvetson/Flickr/CC BY 2.0)
In the new study, Kebukawa and his colleagues investigated how amino acids might arise in a meteorite like this, and how long it might take.
Based on their results, plus the expected dose of gamma radiation from decaying aluminum-26 in meteorites, they estimate that it would take between 1,000 and 100,000 years for this process to generate the amount of alanine and β-alanine found in the meteorite Murchison.
Although we still have a lot to learn about abiogenesis, or the spontaneous generation of life, researchers say this study shows how reactions triggered by gamma rays can produce amino acids in a meteorite, which may contribute to the origin of life on Earth.
The study was published in ACS Central Science.