How to Use State-of-the-Art Spectroscopy to Trace Back the History of the Solar System
By Doug Banda
Fast forward a couple stellar generations after the Big Bang--there exists a collapsing cloud of stardust that will one day harbor our planet Earth. As the cloud condenses over millions of years, countless chemical reactions will eventually lead to the birth of the Solar System. At this point in time the cloud is primarily hydrogen, helium and lithium. Eventually, carbon, oxygen and nitrogen will travel in from distant exploding stars, and the molecular precursors to life will begin to form.
But what reactions had to occur for those molecular precursors to exist? And how did they eventually lead to biological building blocks such as nucleic acids and amino acids? Can we still observe this kind of chemistry in space today? If so, what can it tell us about the history of the Solar System?
Dr. Kyle Crabtree in the Department of Chemistry seeks to answer some of these questions using spectroscopic techniques to model chemical reactions that may occur in space. "How does one model things in space?" asks Crabtree. "As a field, we start by looking at some interstellar cloud or gas spectroscopically and measure how much of various molecules are present. This cloud probably started with mostly atoms, maybe some molecular hydrogen--how do we get from that to this snapshot?"
Based on what is known for a given environment in space, Crabtree explains that scientists next have to imagine all the possible chemical reactions that could take place to make up that environment. He illustrates an example by telling that "a cosmic ray can ionize H2 to make H2+, H2+ can then collide with H2 to make H3+ and a hydrogen atom, and so on."
To accurately model these systems, scientists need to imagine potential chemical reactions and know the rates of those reactions at the temperatures in the environments in which they are found. This can be anywhere from ten kelvin in the deepest coldest parts of space all the way up to several hundred kelvin in the area surrounding a newly forming star. There are also a wide range of external energy sources to consider, including cosmic rays, x-rays and ultraviolet radiation.
"What we try to do, considering the conditions of a particular environment, is make a physical model," explains Crabtree. "Given the temperatures and radiation field of that physical model, we try to simulate the chemistry and play it forward in time. In order to do that, you need to know the rates of the chemical reactions that go into the model."
Dr. Crabtree and his lab use a unique combination of molecular beam methods with laser spectroscopy and cutting edge chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. CP-FTMW spectroscopy is a rotational spectroscopy technique that has the capability of scanning a large chunk of the microwave spectrum all at once with high spectral resolution. This is a powerful tool in the field of astrochemistry, where most of the known molecules in various interstellar environments have been detected by their rotational emissions. Using CP-FTMW scientists can initiate chemistry in a molecular beam and monitor the formation of molecules detected in space as a function of time.
Moreover, the Crabtree group is building a large instrument for studying the rates of various chemical reactions at temperatures anywhere from 20 K to 100 K. This is important in the field of astrochemistry because, although there are models that describe thousands of chemical reactions, Crabtree notes, "the challenge is that only a small fraction of those chemical reactions have actually been studied in the lab, and only a fraction of those studies have been done at low temperatures."
Supported by these instruments, Dr. Crabtree and his lab aim to produce a more predictive model of the chemistry occurring in space so that researchers can more accurately describe molecular abundances observed by telescopes, such as the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescopes in the Atacama desert of northern Chile. This research may aid in the prediction of other molecules that exist in interstellar environments that have yet to be detected.