Tuesday, May 8, 2012
In a new study, scientists suggest that complex organic molecules--such as the amino acids that build proteins and the ringed bases that form nucleic acids--grow on the icy dust grains that lived in the infant solar system. All it takes are high-energy ultraviolet photons to provoke the rearrangement of chemical elements in the grains' frozen sheaths.
If making these organic ingredients happens this readily, then exoplanetary systems are probably seeded with the same fertile, organic pastures. "Anywhere you have ice and high-energy ultraviolet radiation, this process is going to take place. And those are both pretty common in the universe," says planetary scientist Dante Lauretta of the University of Arizona.
In the new work, reported online March 29 in Science, researchers simulated the young solar nebula, a swirling disk of gas and dust that surrounded the sun until planets began forming, about 4.5 billion years ago. Over a 1-million-year period, the team tracked the individual movements of 5,000 dust grains, tiny organic-toting particles covered in ices made from compounds such as water, carbon dioxide, methanol and ammonia.
"We wanted to know exactly what conditions those ice particles were seeing," says coauthor Fred Ciesla, a planetary scientist at the University of Chicago. "It's a turbulent environment, and every particle follows its own path."
Grains lofted above the disk's plane met warmer temperatures and high-energy ultraviolet photons--the catalysts needed to convert elements in the simple ices to more complex molecules. In these types of reactions, photons striking chemical bonds create what study coauthor Scott Sandford calls "unhappy radicals and ions"--species that are highly reactive and ready to recombine. As warming temperatures cause the ices to evaporate, those elements can find partners and form new molecules.
Even though it's relatively easy to create these rearranged molecules, scientists can't really predict which will form, because the chemical reactions don't follow familiar rules. "It's a bit like saying, 'I'm going to give you 10 kinds of Lego blocks, feel free to stack them in any combination you want,'" says Sandford, an astrophysicist at NASA's Ames Research Center in Mountain View, Calif.
But with enough photons slamming into enough dust grains in the early solar nebula, it's hard to avoid making complex molecules this way, Ciesla says.
Astrobiologists have identified such molecules as characters in the story of life's origins, and there's abundant evidence that they can survive in space. Scientists autopsying meteorites have found amino acids and nucleobases.
In the lab, researchers have shown how such compounds could be made astrochemically. By applying organic ices to tiny surfaces in a frigid vacuum, and then irradiating them, teams have produced an array of molecules, including one that spontaneously organizes itself into membranes, says Jason Dworkin, an astrobiologist at NASA's Goddard Space Flight Center in Greenbelt, Md.
Swirling around the young sun, organic-laden ice grains eventually clustered and clumped. The clumps grew into comets and asteroids that bore these molecules to Earth, depositing them in fiery collisions or lighting the infant skies with an organic-rich hailstorm. "I think it's well established that extraterrestrial compounds were delivered in this way," Dworkin says. It's not clear how much the space travelers contributed to the population of organic compounds on Earth, but their mode of delivery was certainly convenient. "If you want to build Lego castles, having Lego bricks falling out of the sky is not a bad idea," Sandford says.
Flinging rocks at Earth is not the only way to deposit organics. "I've never felt I had to look for an extraterrestrial source of amino acids to understand how amino acids could have arisen on this planet," says geochemist George Cody of the Carnegie Institution for Science in Washington, D.C. But lots of irradiated icy particles in the solar nebula, as Ciesla and Sandford have proposed, make a tantalizing natural reservoir, he says.
Scientists are still considering whether materials made on Earth helped supply organics. It seems likely, although any complex molecules would have needed to survive the planet's violent growth spasms, marked by magma oceans and extreme temperatures. Both extraterrestrial and homegrown processes probably played a role, Ciesla says. "These organics will be there to incorporate into planets as they form, or in later delivery, after the planets form, which could be interesting in terms of astrobiology."
Soon, scientists should have a better idea of the array of molecules that live on asteroids. In 2016, NASA will fire the OSIRIS-REx spacecraft at asteroid 1999 RQ36 (the subject of a renaming contest this fall) to scrape off some of its surface and return the samples to Earth in 2023, providing possible clues to the solar system's early years.
"Any organic molecule is going to be interesting for deciphering the history of the solar system," says Lauretta, the principal investigator for the mission. "But for tracing the origin of life, we really focus in on the building blocks."
Life's Legos growing nearby implies a high likelihood of such organic pastures in other planetary systems. "As far as the chemistry goes," Dworkin says, "this appears universal."
Ice in the sun A new simulation tracked the journey (white) of an icy grain in the newborn solar system over I million years. The grain moves from the outer reaches of the dusty protoplanetary disk (starting at pink diamond) toward the sun (reaching the blue circle). Along the way, the particle migrates vertically within the disk. As it bounces in and out of the disk's plane, the particle encounters ultraviolet photons--dramatically more at the fringes (red) than in the center (black). These photons can catalyze chemical reactions on the grain's surface.
Drake, Nadia. "Life's building blocks can grow close to home: complex organic chemicals may be common near stars." Science News 21 Apr. 2012: 5+. Environmental Studies and Policy. Web. 8 May 2012.
Gale Document Number: GALE|A288428356