The Best American Science and Nature Writing 2011 (31 page)

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. 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 -440 degrees F, 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 [CH
₃
OH], 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 [HCOOCH
³
]." In a major 2008 paper, Weaver predicted an abundance of such radicals in dust clouds. 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 -280 degrees F, 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 starbirthing regions of dust clouds than in the colder, darker areas.

But then the story becomes less clear. Radio astronomers have yet to identify anything as complex as an amino acid, so astrochemists do not know exactly how complex these gaseous molecules can get. We know that meteorites contain amino acids and even nucleobases, but not whether they scooped up those molecules from dust clouds or created them later, on their interplanetary course. "We really don't know where the chemistry in the dust cloud stops and where the chemistry in meteorites starts up," Weaver says. She notes that the answer has tremendous implications for one of science's most fundamental questions: how common is life throughout the universe?

If meteorites create most of the direct chemical precursors of life, our solar system might be an unusual case. According to Weaver, the size of our sun, the region of the galaxy in which it formed, even how long it took for the planets to form—all these characteristics are different in other star systems and may influence the chemical inventory available to any Earthlike planets orbiting there. But if dust clouds can manufacture these molecules on their own, then life is probably prevalent throughout the universe. "No matter which dust cloud you look at, things look very similar chemically," Allamandola says.

A new generation of sensitive, high-resolution telescopes will help resolve the debate by probing both dust clouds and the proto-planetary disks from which asteroids, comets, and planets form. Remijan and his colleagues are salivating over the scientific potential of the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, a network of sixty-six radio dishes that will provide unprecedented resolution and sensitivity when it becomes fully operational in late 2012. And two space-based infrared observatories—the European Space Agency's Herschel Space Observatory, which has been gathering data since 2009, and NASA's James Webb Space Telescope, scheduled to launch in 2018—will allow astronomers to search for the infrared signatures of the specific icy radicals that Weaver's model predicts.

As much as astronomers love new instruments to play with, nothing beats seeing extraterrestrial chemistry in action. We cannot yet send probes to the Orion Nebula, but we can look for clues closer to home. Europa, a large satellite of Jupiter, is covered with a thick shell of ice crisscrossed with long brownish or pinkish fractures. Saturn's much smaller moon, Enceladus, features a network of icy volcanoes spewing ammonia, formaldehyde, and other organic molecules. These water-rich moons might have replicated the organic chemistry found within interstellar dust clouds. One clue involves the changing colors of the moons. "Some of the same processes that take place in deep space may occur in these icy bodies in the supercold outer regions of the solar system," Allamandola says.

 

Taking the big view, Remijan marvels at all he and his colleagues have achieved. Not so long ago, deep space seemed static and dull; now it looks like the possible breeding ground for a blueprint of life that might be shared all across the universe. Yet the greatest enigma remains untouched: How did a collection of organic molecules, whatever their origin, make the leap to life on Earth?

"The overall goal is to take this chemical inventory, mix it all together, and form a self-replicating molecule like RNA," Remijan says. He has personally helped detect eleven interstellar molecules with the Green Bank Telescope in West Virginia but recognizes that a full list of the organic chemicals out there is only a start. What scientists really need is a snapshot of the chemistry that was happening on Earth 4 billion years ago. That might actually be possible.

Titan, another of Saturn's moons, has a thick, methane-tinged atmosphere that is reminiscent of early Earth's. It even has pools of hydrocarbons on its surface, the only known bodies of liquid on any world other than our own. But Titan's -290 degree F surface temperature means that all of its chemistry moves in slow motion. Studies of Titan could therefore provide a peek at how complex prebiotic molecules came together on Earth, and potentially on many other worlds too.

In a new study, researchers from the United States and France conducted a new Miller-Urey–style experiment that mixed the organic molecules found in Titan's atmosphere. They ended up with all of the nucleobases that make up RNA and DNA. The study suggests the beginning of a new synthesis that reframes the old questions in a deeper and more meaningful way. It may not be a question of whether life's chemistry began in space or in meteorites or on the surface of a planet (or moon). All three environments may very well have lent a hand.

"When life was forming on Earth, it probably used a large variety of sources, some from the planet and some from the sky," Sandford says. "Life doesn't care about the 'Made in...' label on the molecules."

FROM
Scientific American

A
FEW YEARS AGO
the City Council of Monza, Italy, barred pet owners from keeping goldfish in curved fishbowls. The sponsors of the measure explained that it is cruel to keep a fish in a bowl because the curved sides give the fish a distorted view of reality. Aside from the measure's significance to the poor goldfish, the story raises an interesting philosophical question: How do we know that the reality we perceive is true? The goldfish is seeing a version of reality that is different from ours, but can we be sure it is any less real? For all we know, we too may spend our entire lives staring out at the world through a distorting lens.

In physics the question is not academic. Indeed, physicists and cosmologists are finding themselves in a similar predicament to the goldfish's. For decades we have strived to come up with an ultimate theory of everything—one complete and consistent set of fundamental laws of nature that explain every aspect of reality. It now appears that this quest may yield not a single theory but a family of interconnected theories, each describing its own version of reality, as if it viewed the universe through its own fishbowl.

This notion may be difficult for many people, including some working scientists, to accept. Most people believe that there is an objective reality out there and that our senses and our science directly convey information about the material world. Classical science is based on the belief that an external world exists whose properties are definite and independent of the observer who perceives them. In philosophy that belief is called realism.

Those who remember Timothy Leary and the 1960s, however, know of another possibility: one's concept of reality can depend on the mind of the perceiver. That viewpoint, with various subtle differences, goes by names such as antirealism, instrumentalism, or idealism. According to those doctrines, the world we know is constructed by the human mind employing sensory data as its raw material and is shaped by the interpretive structure of our brains. This viewpoint may be hard to accept, but it is not difficult to understand. There is no way to remove the observer—us—from our perception of the world.

The way physics has been going, realism is becoming difficult to defend. In classical physics—the physics of Newton, which so accurately describes our everyday experience—the interpretation of terms such as
object
and
position
is for the most part in harmony with our commonsense, "realistic" understanding of those concepts. As measuring devices, however, we are crude instruments. Physicists have found that everyday objects and the light we see them by are made from objects—such as electrons and photons—that we do not perceive directly. These objects are governed not by classical physics but by the laws of quantum theory.

The reality of quantum theory is a radical departure from that of classical physics. In the framework of quantum theory, particles have neither definite positions nor definite velocities unless and until an observer measures those quantities. In some cases, individual objects do not even have an independent existence but rather exist only as part of an ensemble of many. Quantum physics also has important implications for our concept of the past. In classical physics the past is assumed to exist as a definite series of events, but according to quantum physics, the past, like the future, is indefinite and exists only as a spectrum of possibilities. Even the universe as a whole has no single past or history. So quantum physics implies a different reality than that of classical physics—even though the latter is consistent with our intuition and still serves us well when we design things such as buildings and bridges.

These examples bring us to a conclusion that provides an important framework with which to interpret modern science. In our view, there is no picture- or theory-independent concept of reality. Instead we adopt a view that we call model-dependent realism: the idea that a physical theory or world picture is a model (generally of a mathematical nature) and a set of rules that connect the elements of the model to observations. According to model-dependent realism, it is pointless to ask whether a model is real, only whether it agrees with observation. If two models agree with observation, neither one can be considered more real than the other. A person can use whichever model is more convenient in the situation under consideration.