Molecules of the interstellar medium must break the rules to make the stuff of space
The landscape could be the backdrop of a postapocalyptic film. It’s an environment of extremes, blasted by intense radiation, fierce winds and shock waves from violent explosions. Yet within this desolation, species persist. Not only are there ordinary, familiar faces, there is also, evidence suggests, a motley crew: galactic gangs that would make Mad Max cringe. Some are decked out in metal; others are radicals itching to react, amped up with positive and, new research shows, even negative charge.
These species are the molecules of space, the cosmic chemicals that dwell in the vacuous netherworld between stars. After decades of cataloging these chemical specimens, scientists are now bracing for a torrent of data that may lead to a better understanding of the reactions that create and destroy cosmic compounds.
Researchers are zooming in on the renegade reactive species, incorporating these players into models of the life cycles that govern space chemistry. The spiky, electrified characters may be major interstellar players in the formation of larger, more complex molecules — and could perhaps be the sparkling forerunners of life.
The chemical inhabitants of space are intimately linked to star formation and the greater cosmic cycle that gives rise to planetary systems. Scientists hope that the chemical exploration of the interstellar medium with its gas and bits of dust will reveal clues about the birth and evolution of galaxies, stars and planets. Added to this pursuit is the thrill of pushing the chemical envelope, probing an unmapped chemical frontier. It is an endeavor that will be aided by new telescopes, novel lab techniques and theory from astronomy and beyond.
“We’re trying to understand and attack fundamental chemistry principles,” says astrochemist Anthony Remijan of the National Radio Astronomy Observatory in Charlottesville, Va. “We’re taking the most fundamental chemical principles that we all know and love and seeing if they hold up in the extreme conditions of interstellar space.”
The harsh environments of space pose challenges for both the molecules that live there and for the scientists studying them. Space is, well, spacious, making it hard for compounds to connect. Temperatures are extreme and pressures can be exceedingly low. In space, some molecules tumble through desolate regions in the form of gas; others dwell and react on bits of icy dust — lifestyles seldom seen on Earth.
“On Earth, it’s always liquid phase, liquid phase, liquid phase,” says chemist Brooks Pate of the University of Virginia, also in Charlottesville. “But that’s the one thing you don’t get in space. It’s all gas phase and surface chemistry. This leads to a whole other type of chemistry that you don’t see in terrestrial conditions. It’s not like there are new physical laws. It’s the physical conditions where the reactions go on that are quite different.”
Terrestrial chemistry often happens in solution, where no molecule is ever alone. But within the interstellar medium, vast distances separate molecules — if two people were a proportional distance apart, one might be standing on the Earth and the other on the moon.
An emerging picture reveals how molecules in space use flashy charges to woo each other from a distance. Collisions with particles, ultraviolet starlight and cosmic rays, for example, can leave a molecule charged. Until recently, only neutral and positively charged species had been detected. Researchers thought radiation would quickly strip a molecule of the extra electrons that confer a negative charge. But now a handful of molecules with negative charges have been found.
Getting decked out with charges gives species an advantage, making them visible from afar or allowing them to tunnel through ice. Most earthly molecules are neutral. They don’t stand out. These chemical wallflowers need to be within a few nanometers of each other to interact, says Pate. But space’s highly reactive species can be drawn to each other from as far as a hundred nanometers away.
Although these fired-up species sound like cosmic vagrants, they do exist on Earth, Pate says, but in very small concentrations and often only on the way to becoming something else. In that regard, the chemistry of space is like watching Earth chemistry in slow motion. On Earth a molecule might exist for a nanosecond before colliding with another molecule. In space the time between collisions can be weeks to years. A molecule is more likely to encounter a partner if it is decorated with charges.
Only the strong survive
Evidence suggests that the cold, dark clouds that gather in the interstellar medium are the electrical tattoo parlors of the cosmos — hot spots for charge decoration. These clouds offer a refuge for the resilient few who survive the shocks and radiation of the interstellar medium.
The interstellar dust and gas is created locally — born of dying stars. For much of their lives, stars burn hydrogen into helium. After the hydrogen is consumed, helium converts to oxygen and carbon (if the star is massive enough, it keeps brewing and more elements are formed). Because there’s more oxygen in the galaxy than carbon, scientists had thought that this carbon wouldn’t be available for organic chemistry. It would just get snapped up by oxygen, forming carbon monoxide. But in the outflow of some stars, the oxygen-carbon ratio is tipped just enough that chains of carbons can form. The stellar ejecta of these extreme carbon stars throws carbon into the interstellar medium where it can feed the creation of carbon-rich molecular clouds.
Going from stellar outflow to the diffuse interstellar medium is a journey from cradle to grave for many molecules, says Scott Sandford of the NASA Ames Research Center in Moffett Field, Calif. Anything drifting in this interstellar wasteland experiences intense ultraviolet radiation, cosmic ray bombardment and shock waves that smash anything in their path. “The diffuse ISM isn’t about the production of things, it’s about winnowing,” says Sandford. “A lot of weaklings are weeded out.”
The bits of compounds shattered in the violent processes of the interstellar medium can gather into clouds along with resilient, extra-large molecules, such as polycyclic aromatic hydrocarbons or PAHs. On Earth, these compounds of fused six-carbon rings (picture chicken wire) are combustion-related pollutants, maligned for their carcinogenic nature. But in space, PAHs are emerging as chemical stars, molecules that may contain a good portion of the interstellar medium’s carbon.
It’s within the interstellar clouds that the chemistry picks up, says Sandford. “Starlight is blocked and suddenly molecules you make aren’t destroyed,” he says. “They can survive, and then you can get more compounds because there’s much more material in a smaller area.”
Many of the more complex molecular species are found in these cold, dark clouds. Since the hunt for interstellar molecules began in earnest in the 1960s, tuning in to the molecular symphony that pours from a region of dense clouds within the constellation Sagittarius has revealed a wealth of species. Many are several atoms strong. A note-by-note fingerprint — the spectra of energy emitted as a molecule twists and shouts — can be determined in the lab, and then that spectral signature can be detected in the sky, and vice versa. In April, an international team of astronomers viewing the Sagittarius region with the IRAM 30-meter telescope on Pico Veleta in Spain reported detecting ethyl formate, which helps give raspberries their fruity flavor here on Earth. And using the Robert C. Byrd Green Bank Telescope in West Virginia, Remijan and colleagues detected acetamide, one of two known interstellar molecules with a peptide bond, the same connection that links amino acids, the building blocks of proteins.
Evidence suggests that clouds are where a lot of the hard-core exotics get their stripes, says Eric Herbst of Ohio State University in Columbus. As the clouds warm, large neutral molecules can build up from small charged precursors, including perhaps the newly discovered negative species such as C6H-, a carbon chain anion, Herbst and colleagues reported last year in the Astrophysical Journal. Incorporating negatively charged species into models of the chemistry of dark clouds makes the models more accurate in predicting the observed abundance of certain compounds, the team reports.
A common way for these charged-up species to get their zing appears to be via protonated molecular hydrogen, H3+, says Pate. This molecule of three hydrogen atoms with a positive charge is one of the most abundant ions in the universe and forms when H2 is bombarded with cosmic rays that can penetrate dense clouds. Protonated molecular hydrogen is an acid, just itching to donate protons to other molecules, a gift that can help get reactions going by minimizing the energetic hump molecules must surmount in order to do their stuff.
“H3+ may be the greatest catalyst of the interstellar world,” says Pate.
Chains of reactions starting with H3+ can lead to a collection of complex species, notes Herbst, though many of the reactions are poorly understood. And when H3+ passes on a proton, it also confers reactivity. Pate is investigating how another abundant molecule, methanol, acquires reaction-spurring activities when it picks up extra protons.
PAHs may also be catalysts that spur the formation of protonated hydrogen, which can then enable more reactions, researchers from the University of Colorado at Boulder reported last fall in the Astrophysical Journal.
But just getting two molecules together doesn’t seal the deal. Gas-on-gas collisions can break bonds, and the molecules fall apart (the analytic technique mass spectrometry relies on this “collision-assisted dissociation”). Bonds can also form to yield a new species. But in space, molecules may be more noncommittal. Two gases can collide but, instead of bonding, form a complex in which they maintain their own identities.
Chemistry on ice
If gas-on-gas isn’t your thing, reactive species can also meet on the surface of — or even within — dust grains. The bits of soot and silicates that make up interstellar dust can develop mantles of ice several layers thick. The ice itself is usually frozen water, but other compounds, such as ammonia, methane and carbon dioxide may also be present. When temperatures inside a cloud plummet, gases can condense onto grains the way that ice builds up inside a freezer, says chemist Ralf I. Kaiser of NASA’s Astrobiology Institute at the University of Hawaii at Manoa.
“In classical high school chemistry, ices have no chemistry; chemistry is dead,” says Kaiser. But experiments by Kaiser and others suggest that energy from cosmic rays and from UV photons can penetrate these icy bits of dust, spurring reactions even in ice.
The mechanisms aren’t clear, but scientists think that these reactions might happen when photons excite the solid material, creating tunnels through which electrons can travel to meet species locked in the ice. Or gas-phase species can land on an ice grain and connect with molecules bound to the grain’s surface.
These incoming photons can also knock molecules from the ice right into the gas phase, says Louis Allamandola of the astrochemistry division of NASA-Ames. “The energy comes in, and it’s like hitting something with a hammer,” he says. “And the way of getting rid of some of that energy, since the grain is so small, is things pop off.”
Radicals — species that are reactive because they have unpaired electrons — and ions can form and build up in the ice or can pop off and help make bigger molecules.
How much complexity arises from gas-gas collisions versus ice-grain chemistry is still hotly debated, says Herbst. Work suggests that ice is where a lot of the action begins: Radicals and positively or negatively charged ions can be liberated from these ices if the temperature goes up, say from a nearby explosion or the gradual heating of a developing star. These molecules then may also pair up, though scientists are far from unraveling this chemical network.
Modeling becomes even more complicated as temperatures rise and as the chemical population and number of reactions increase. Theory, guesswork, lab experiments and comparisons with actual measurements from space mean constant adjustments, the addition of better numbers, and reruns of experiments and models. Fully understanding the cosmic chemical landscape will be aided by other fields, including combustion and atmospheric science (which are light-years ahead of astrochemistry in both theory and experiment).
A giant step forward will come when telescopes such as ALMA, the Atacama Large Millimeter/submillimeter Array, come online. A 66-telescope array in northern Chile, ALMA should be fully functioning by 2013. These telescopes will allow scientists to probe the distribution of molecular species at greater resolutions and envision where species are in relation to each other. It’s as if current technologies allowed scientists to identify lots of a molecule in Texas, while the arrays will make it possible to discern what’s happening in Dallas versus Houston.
“The real hope is that soon we’ll be able to have this spatial correlation,” says Pate, “where we can really say it looks like molecule A is being consumed and molecule B is being formed.”
Eventually, chemists may be able to predict how molecules evolve from one cosmic environment to another. For example, species associated with ice grains can be released when a shock wave rocks the firmament. Detecting the presence of such a molecule could help isolate and locate shocks through galactic time. “You can figure out lots of things,” says Herbst. “The temperature, the pressure, how fast a cloud is moving towards us or collapsing or moving away from us, or all of the above.”
Since many of these molecules get bottled in comets and meteorites and delivered to planet surfaces, understanding the cosmic chemistry set may even lead to a better understanding of the origin of life. Lab studies have generated uracil, a building block of RNA, from irradiated ices, Sandford and colleagues reported last year. In 2008, researchers found amino acetonitrile, a precursor of the simplest natural amino acid, in space.
The same ionizing radiation that gives many of space’s rocked-out radicals their pizzazz may also have lent life its spark. “It gets into the whole question of electricity — now you have electrical forces instead of chemical forces,” says Allamandola. “Once we’ve introduced electrons, I’m thinking sparks and Frankenstein.” He laughs. “It’s completely speculative, but it could play a role.”
Whether created in space and delivered to the planet, or cobbled together somewhere on the early Earth, space’s molecular species are whence we came. After all, all the carbon in the universe was created in space — we are stardust.
New techniques, telescopes and collaborations will plunge scientists into this final frontier — and challenge them to keep up with the data. When ALMA is operating, astronomers will be deluged by information. Improved lab techniques and broadband capabilities have also ramped things up.
“A year is a day now,” says Pate. “It’s going to force people to think differently about how you look at the data, analyze the data and extract the chemical information. I find it very exciting. When it doesn’t give me a headache, it excites me.”
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