Some
crucial ingredients for life on Earth may have formed in interstellar space,
rather than on the planet's surface.
A new
computer model indicates clouds of adenine molecules, a basic component of DNA,
can form and survive the harsh conditions of space, and possibly sprinkle onto
planets as the stars they orbit travel through a galaxy.
"There
may be only a few molecules of adenine per square foot of space, but over
millions of years, enough could have accumulated to help make
way for life," said study co-author Rainer Glaser, a molecular chemist
at the University of Missouri-Columbia.
Glaser and
his team's findings are detailed in a recent issue of the journal Astrobiology.
Spacey
chemistry
Adenine is
one of four "letters" of DNA's alphabet used to store an organism's
genetic code. Glaser said the idea that large, two-ringed organic molecules
like adenine formed in space may seem outrageous, but current evidence leaves
the possibility wide open.
"You
can find large molecules in
meteorites, including adenine," Glaser said. "We know that
adenine can be made elsewhere in the solar system, so why should one consider
it impossible to make the building blocks somewhere in interstellar dust?"
Using
computer simulations of the cold vacuum of space, Glaser and his colleagues
found that hydrogen cyanide (HCN) gas can build adenine. Like pieces in a set
of tinker toys, hydrogen cyanide serves as adenine's building blocks; the small
molecules bond together into chains and, with a little wiggling, eventually
assemble into rings.
Although
adenine's first ring needs a tiny energy boost from starlight to form, Glaser
said the second ring of the molecule self-assembles without any outside help.
"When
you want to have a reaction, you usually need to heat it up," Glaser said.
"It's remarkable to find a reaction that doesn't require activation
energy. If you do this reaction in space, this is a huge advantage because it
takes a long time for a molecule to be hit by a piece of light."
Seasoned
for life?
Glaser said
adenine's ringed shape helps it absorb and release any excess energy without
breaking apart, making it stable enough to form concentrated clouds that
planets can drift through.
While
getting adenine safely onto a rocky planet's surface is a less developed idea,
Glaser said many chemists have barely toyed with the notion that life's basic
ingredients formed off of the planet's surface.
"We're
at a very early stage of anybody even thinking about these things," he
said. "The discussion of life's origin has been highly focused on the idea
of a warm pool of liquid on the planet's surface." But Glaser said recent discoveries
of planets around distant stars is changing that focus.
"Chemistry
in space isn't the chemistry most of us are trained for," Glaser said.
"We should take a much bigger approach: Where are all the chemicals in the
galaxy and its solar systems, and what can you do with them?"
Antonio
Lazcano, an evolutionary biologist at the National Autonomous University of
Mexico who has studied life origins for the past 30 years, said Glaser and his
colleagues' work is compelling.
"We
already know hydrogen cyanide is abundant in interstellar clouds, and it's been
suggested that comets can bring some of that material onto planets,"
Lazcano said. For Glaser and his team's idea to be widely supported, however,
adenine needs to be detected in the deep space clouds, Lazcano said.
"The
likelihood of detection is very small, but it's still possible," he said.
"If astronomers can better eliminate background noise, I think we'll have
equipment sensitive enough to detect adenine dust clouds."
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