Allamandola’s solution was to create an apparatus that could replicate the exotic cold depths of space—in essence, an extraterrestrial version of the Miller-Urey experiment. With colleague Fred Baas, he installed equipment to chill a shoebox-size chamber to within several degrees of absolute zero and depressurize it to a near vacuum. Then he set up a plasma lamp to fire beams of ultraviolet light at the chamber, much like the radiation present in planet- and star-forming regions of dust clouds. Finally, in true Miller-Urey fashion, he threw in a gaseous mixture of simple molecules, mimicking what was then known about the composition of interstellar clouds, and watched the results.
Allamandola’s simulations, carried out first at Leiden and now at NASA’s Ames Research Center, revealed not only that some chemical reactions really do occur at extremely low temperatures, but also that these reactions produce other reactive chemicals, thereby providing the spark for more molecular hookups. Ultraviolet radiation spices things up as well: It heats the grains and breaks up some of the molecules into reactive fragments, which in turn bond with other fragments to form new kinds of molecules.
Once again, nature proved extremely adept at brewing complex molecules. In current versions of Allamandola’s experiment, the resulting icy mixtures contain dozens of prebiotic molecules, among them the same amino acids that Miller and Urey found. In fact, Allamandola’s nebula-in-a-box has yielded an even richer chemical palette. He has manufactured intricate molecular rings containing carbon, nitrogen, and hydrogen; fatty-acid-like molecules that look and behave like the membranes protecting living cells; and nucleic acids or nucleotides, the primary components of RNA and DNA.
Creating molecules in the lab does not prove that the same molecules exist on dust grains in distant nebulas, but so far Allamandola’s technique has an impressive track record. By 1990 he had published a list of simple compounds his group at Ames had created in simulations. By 2000 radio astronomers had found almost all of them in various dust clouds throughout our galaxy, suggesting that the interplay between ice and gas may be one of the most important mechanisms for synthesizing the precursors of life.
Still, Allamandola’s research could not explain how compounds moved from the far reaches of space to the surface of Earth, where life actually took hold (though speculation abounds). Addressing this question meant bridging the gap between diffuse interstellar clouds and the condensed objects that ultimately emerge from them. When dense regions of a cloud collapse, the massive inner part becomes a star while the rest forms a swirling disk of gas and dust that may give rise to planets. (We now know that many, perhaps most, stars produce such planetary systems.) As large planets come together, the process involves such heat and pressure that all traces of preexisting organic matter are destroyed. Not all material in the disk gets treated so brutally, however. Some of it remains nearly intact in comets and asteroids, smaller conglomerations of ice and rock. When bits of these objects struck Earth as meteorites, they could have delivered organic molecules back onto its surface.
Convincing evidence that meteorites could be rich sources of organic molecules came in 1969, when a 200-pound meteorite hurtled to the ground in Murchison, Australia. Analysis indicates that the rock contains millions of organic compounds, including amino acids that could not have come from terrestrial contamination. Two years ago, Zita Martins from Leiden showed that the meteorite contains nucleobases. David Deamer of the University of California, Santa Cruz, even found fatty-acid-like molecules similar to those Allamandola created in the lab. Other meteorites—including Murray, which landed in Kentucky in 1950, and Allende, which made landfall in Mexico in 1969—have been shown to contain similar organic compounds.
Meteorites carrying the same complex chemicals have been striking Earth since it formed 4.5 billion years ago. “The things we see landing on Earth now are probably representative of what was landing on us back during Earth’s infancy,” says NASA’s Sandford, who has traveled the world searching for samples from on high. In 1984 he found a rock from Mars, and in 1989 a piece of the moon, all right here on Earth. During a six-week tour of Antarctica, he slept in a tent under the midnight sun and rode a snowmobile to ice fields littered with meteorites by day.
Gradually, painfully, through some four decades of effort, Sandford and the other scientists have teased out different strands of the story of prebiotic chemistry. Carbon, hydrogen, oxygen, and other atoms knock about in nebulas, sometimes freely and sometimes bound up with ice and dust. They arrange themselves into elaborate molecular structures. Meteorites abound with organic compounds, which rain down on any nearby planets.
Helping to weave all those strands into a single, elegant narrative is an Emory University astrochemist with a providential name: Susanna Widicus Weaver. Through a series of models and experiments, she has demonstrated that ultraviolet radiation can break chemical bonds and split molecules into highly reactive fragments called radicals. It is difficult for radicals to do much at –440ºF, but when the temperature warms even slightly (as when a star begins to form), the radicals merge to form larger molecules. “You can take methanol [CH3OH], break it apart, and make several types of radicals, and then those can all find each other,” Weaver says. “In just two or three steps on the grain surface, you can go from a simple mixture to something a lot more complex, like methyl formate [HCOOCH3].” In a major 2008 paper, Weaver predicted an abundance of such radicals in dust clouds (pdf). A thorough search of interstellar ice grains by infrared astronomers should determine whether radicals indeed play a primary role in constructing prebiotic molecules. If they do, astrochemists in the lab could see what other complex combinations result from these radicals and then search for those molecules in space.
Weaver’s models also demonstrate that once the temperature in the dust cloud reaches about –280ºF, most of the molecules evaporate from the ice on dust grains and enter a gas phase, allowing them to react a lot more quickly and to form complex molecules. Molecular players might include acetone (the stuff in nail polish remover), methyl formate, and ethylene glycol (antifreeze), she notes. That explains why radio astronomers have found more complex molecules in the warmer, more active star-birthing regions of dust clouds than in the colder, darker areas.