Somecrucial ingredients for life on Earth may have formed in interstellar space,rather than on the planet's surface.
A newcomputer model indicates clouds of adenine molecules, a basic component of DNA,can form and survive the harsh conditions of space, and possibly sprinkle ontoplanets as the stars they orbit travel through a galaxy.
"Theremay be only a few molecules of adenine per square foot of space, but overmillions of years, enough could have accumulated to help makeway for life," said study co-author Rainer Glaser, a molecular chemistat the University of Missouri-Columbia.
Glaser andhis team's findings are detailed in a recent issue of the journal Astrobiology.
Spaceychemistry
Adenine isone of four "letters" of DNA's alphabet used to store an organism'sgenetic code. Glaser said the idea that large, two-ringed organic moleculeslike adenine formed in space may seem outrageous, but current evidence leavesthe possibility wide open.
"Youcan find large molecules inmeteorites, including adenine," Glaser said. "We know thatadenine can be made elsewhere in the solar system, so why should one considerit impossible to make the building blocks somewhere in interstellar dust?"
Usingcomputer simulations of the cold vacuum of space, Glaser and his colleaguesfound that hydrogen cyanide (HCN) gas can build adenine. Like pieces in a setof tinker toys, hydrogen cyanide serves as adenine's building blocks; the smallmolecules bond together into chains and, with a little wiggling, eventuallyassemble into rings.
Althoughadenine's first ring needs a tiny energy boost from starlight to form, Glasersaid the second ring of the molecule self-assembles without any outside help.
"Whenyou 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 activationenergy. If you do this reaction in space, this is a huge advantage because ittakes a long time for a molecule to be hit by a piece of light."
Seasonedfor life?
Glaser saidadenine's ringed shape helps it absorb and release any excess energy withoutbreaking apart, making it stable enough to form concentrated clouds thatplanets can drift through.
Whilegetting 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 basicingredients formed off of the planet's surface.
"We'reat a very early stage of anybody even thinking about these things," hesaid. "The discussion of life's origin has been highly focused on the ideaof a warm pool of liquid on the planet's surface." But Glaser said recent discoveriesof planets around distant stars is changing that focus.
"Chemistryin 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 thegalaxy and its solar systems, and what can you do with them?"
AntonioLazcano, an evolutionary biologist at the National Autonomous University ofMexico who has studied life origins for the past 30 years, said Glaser and hiscolleagues' work is compelling.
"Wealready know hydrogen cyanide is abundant in interstellar clouds, and it's beensuggested 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.
"Thelikelihood of detection is very small, but it's still possible," he said."If astronomers can better eliminate background noise, I think we'll haveequipment sensitive enough to detect adenine dust clouds."
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