by John Rennie and
from QuantaMagazine Website
Illustration of asteroids in space
that are interconnected into a structure
like that of the amino acid glycine.
A new radiation-based mechanism
adds to the ways that amino acids
could have been made in space
and brought to the young Earth...
Last spring, scientists revealed that the chemical composition of the asteroid includes 10 amino acids, the building blocks of proteins.
The discovery added to
the evidence that the primordial soup from which life on Earth arose
may have been seasoned with amino acids from pieces of asteroids.
What non-biological mechanism could have put them in meteorites and asteroids?
Their discovery makes it
seem even more likely that meteorites could have contributed to the
origin of life on Earth.
Seventy years ago, famous experiments by Stanley Miller and Harold Urey proved that an electrical discharge in a gaseous mixture of methane, ammonia and hydrogen (which at the time was incorrectly thought to mimic Earth's early atmosphere) was all it took to make a mixture of organic compounds that included amino acids.
Later laboratory work
suggested that amino acids could also potentially form in sediments
near hydrothermal vents on the seafloor, and a
discovery in 2018
confirmed that this does sometimes occur.
Both were carbonaceous chondrites, a rare class of meteorites resembling Ryugu that scientists think accreted from smaller icy bodies after the solar system first formed.
Both also contained small
but significant amounts of amino acids, although scientists couldn't
rule out the possibility that the amino acids were contaminants or
byproducts of their impact.
Experimental work suggested that ultraviolet radiation from supernovas could have been strong enough to do it.
Collisions between the dust bodies could also have heated them enough to produce a similar effect.
was visited and sampled by
the Hayabusa2 space probe two years ago.
Now a team of researchers at Yokohama National University in Japan led by the chemists Yoko Kebukawa and Kensei Kobayashi have shown that gamma rays could also have produced the amino acids in chondrites.
In their new work, they showed that gamma rays from radioactive elements in the chondrites - most probably aluminum-26 - could convert the carbon, nitrogen and oxygen compounds into amino acids.
Of course, gamma rays can destroy organic compounds as easily as it can make them.
But in the Japanese team's experiments,
...Kebukawa said, so the gamma rays produced more amino acids than they destroyed.
From the rates of production observed in their experiments, the researchers calculated very roughly that gamma rays could have raised the concentration of amino acids in a carbonaceous chondrite asteroid to the levels seen in the Murchison meteorite in as little as 1,000 years or as many as 100,000.
Since gamma rays, unlike ultraviolet light, can penetrate deep into the interior of an asteroid or meteorite, this mechanism could have extra relevance to origin-of-life scenarios.
If meteorites are big enough,
called carbonaceous chondrites,
such as the one at left,
accreted from smaller icy bodies
that contained mixtures
of compounds rich in
carbon, nitrogen and oxygen.
Their conglomerated structure
is visible in a magnified cross section.
Laurence Garvie/ Buseck Center for Meteorite Studies,
Arizona State University
One requirement of the new mechanism is that small amounts of liquid water must be present to support the reactions.
That might seem like a significant limitation:
But carbonaceous chondrite meteorites are full of minerals such as hydrated silicates and carbonates that only form in the presence of water, she explained, and tiny amounts of water have even been found trapped inside some of the mineral grains in chondrites.
From such mineralogical evidence, said Vassilissa Vinogradoff, an astrochemist at Aix-Marseille University in France, scientists know that young asteroids held significant amounts of liquid water.
Sandford notes that in experiments he and other researchers have conducted, irradiation of icy mixtures like those in the primordial interstellar molecular clouds can give rise to thousands of compounds relevant to life, including sugars and nucleobases,
Vinogradoff echoed that view and said that the diversity of organic compounds that can be present in meteorites is now known to be vast.
Why, for example, does terrestrial life use only 20 of the scores of amino acids that can be produced, and why does it almost exclusively use the "left-handed" structures of those molecules when the mirror-image "right-handed" structures naturally form in equal abundance?
Those may be the mysteries that dominate chemical studies of life's earliest origins in the future...