New Clues to Formation of Hydrocarbons in Space

By Thomas HornigoldMar 7 2018

It’s well understood by physicists that stars are responsible for building most of the abundance of elements heavier than helium, through nuclear fusion. Recent observations from LIGO and optical counterparts of neutron star collisions indicate that these processes, and not supernovae, may be responsible for constructing certain elements such as gold and platinum.

But another considerable mystery is how complex chemistry occurs in outer space - how do these elemental building blocks meet and form complex hydrocarbons, like those that seem to be required for carbon-based life?

Now, new experiments have looked at conditions similar to those in outer space to assess the feasibility of processes to form a complex hydrocarbon molecule, pyrene. The study was an international collaboration between experimentalists at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), astronomers from the University of Hawaii at Manoa, and theoretical chemists to understand the chemical reactions at Florida International University. The study was published March 5, in Nature Astronomy.

Hydrocarbons can include a vast array of different chemical compounds, due to the rich chemistry made possible by the hydrogenic bonding; molecular rings can form to provide a huge range of substances, many of which are used by biological organisms. Examples that have been detected in space include benzene and naphthalene. Pyrene is a slightly more complex molecule, consisting of 16 carbon atoms and ten hydrogen atoms; you can view it as four rings of benzene fused together. These polycylic aromatic hydrocarbons (PAHs) are considered possible precursors for the origin of life, and they are associated with the melting pots that occur in new star and planet formation. In 2013, some hydrocarbon molecules were detected in the upper atmosphere of Saturn’s largest moon – Titan.

Ralf I. Kaiser, one of the study's lead authors and a chemistry professor at the University of Hawaii at Manoa, said, "When these hydrocarbons were first seen in space, people got very excited. There was the question of how they formed."

PAHs account for up to 20% of the carbon in our galaxy, but prior to these experiments, the process by which they formed in carbon-rich regions surrounding stars were not yet known. The experiments consist of synthesizing various precursor chemicals in the lab.

The ingredients were a complex hydrocarbon known as the 4-phenanthrenyl radical, which has a molecular structure that includes a sequence of three rings and acetylene (two carbon atoms and two hydrogen atoms).

Once these ingredients have been obtained, the gas mixture is injected into a microreactor at Berkley Lab’s Advanced Light Source (ALS). A beam of ultraviolet light – intended to simulate the light from the host star – ionizes the heated gas mixture. The scientists can then detect charged particles that arrive at the detector; the arrival times can be predicted by the calculations of the theoretical chemists to probe the individual chemical steps in the process. This allows the scientists to calculate the most likely means for the complex hydrocarbons to form. Alexander Mebel of Florida International University provided many of the theoretical chemistry calculations; his work showed how pyrene (a four-ringed molecular structure) could develop from a compound known as phenanthrene (a three-ringed structure).

"This is how we believe some of the first carbon-based structures evolved in the universe," said Musahid Ahmed, a scientist in Berkeley Lab's Chemical Sciences Division who joined other team members to perform experiments at ALS.

Starting off from simple gases, you can generate one-dimensional and two-dimensional structures, and pyrene could lead you to 2-D graphene. From there you can get to graphite, and the evolution of more complex chemistry begins.

Musahid Ahmed

Kaiser added, "Future studies could study how to create even larger chains of ringed molecules using the same technique, and to explore how to form graphene from pyrene chemistry."

There is huge potential for this study to continue and perhaps shed light on astrobiology, as well as astrochemistry. One potential future experiment at the University of Hawaii looks at mixing these precursor gases, in the presence of ice, and simulating cosmic radiation bombarding the mixture. Could even more complex molecules indicated in the origins of life be formed? Years after the Miller-Urey experiment that attempted to “recreate” life by recreating the conditions that were thought to predominate in the early Earth, we’re still trying to unlock the secrets of chemistry and physics that have allowed us to be here today.

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