Chemistry in the Universe
Molecules in Space: Astronomical Possibilities
by Beth Ashby Mitchell
March 14, 2016
From the beginning of time, human beings have stared into space in wonderment and tried to make sense of what they were seeing. Philosophers, mathematicians, and scientists studied the stars to determine the place of humans in the vast unknown, make sense of startling phenomena, learn the chemical and physical makeup of the universe, and explore in person what is out there.
The earliest detected molecules
In the second half of the 20th century, scientists began to learn about the cosmos at astronomical speeds. In late 1963, S. Weinreb, A. H. Barrett, and co-workers at MIT (Cambridge, MA), in an article titled “Radio Observations of OH in the Interstellar Medium”, stated
In this article, we wish to report the detection of 18-cm absorption lines of the hydroxyl (OH) radical in the radio absorption spectrum of Cassiopeia, thereby providing positive evidence for OH in the interstellar medium. (Nature DOI: 10.1038/200829a0)
Most astronomers at the time believed that the fragments OH, CN, CH, and CH+ were all that existed in space because “gas densities were so low in nebulae, and ultraviolet radiation so intense, that any normal molecules surviving would be too scarce to be detectable.”
When Charles H. Townes arrived at the University of California, Berkeley, in 1967, no additional chemical species had been discovered. It was speculated that hydrogen existed in interstellar clouds, but it had not been detected. Townes was determined to pursue astronomy in the infrared and radio wave regions of the electromagnetic spectrum.
In the fall of 1968, Townes and a dedicated team of engineers and grad students pointed a modest amplified 6-m (20-ft) antenna at the Hat Creek Radio Observatory at the dark, star-forming clouds of Sagittarius B2 and immediately found two ammonia spectral lines. They also determined that there were >1000 molecules/cm3 in these clouds. In 1969, Al Cheung, a grad student, and others in Towne’s team found water in Sagittarius B2; and at the end of the year, Cheung reported a water line in Orion.
The presence of complex molecules
Today almost 200 molecules have been identified. In September 2014, Arnaud Belloche at the Max Planck Institute for Radio Astronomy (Bonn, Germany) and coauthors at Cornell University (Ithaca, NY) and the University of Cologne (Germany) reported the first interstellar discovery of a branched carbon-based molecule, isopropyl cyanide (i-PrCN), in Sagittarius B2. (Molecules discovered earlier had only linear “backbones.”) The branched cyanides are noteworthy for the clues they provide about the formation of amino acids in the universe. This discovery suggests that biologically important molecules are formed early in process of star formation. (Science DOI: 10.1126/science.1256678)
Last April, Karin I. Öberg of the Harvard-Smithsonian Center for Astrophysics (Cambridge, MA) and an international team reported the presence of large amounts of acetonitrile (MeCN) in a protoplanetary disc surrounding a “young” star in the Taurus region, ≈455 light years away. (It is ≈1 million years old; the Sun is >4 billion.) This discovery adds evidence of the early formation of life-building molecules in space. (Nature DOI: 10.1038/nature14276)
Meanwhile, back on Earth
These molecules were detected via radio telescope arrays, most notably the Atacama Large Millimeter/submillimeter Array (see “ALMA Telescope”) in Chile, and orbiting spectrometers such as in the Cassini–Huygens orbiter. To elucidate the implications of these findings requires multidiscipline cooperation of astrochemists, astrophysicists, astrobiologists, and other scientists.
Many, if not most, of the astounding discoveries in the interstellar medium today are coming from ALMA (the Atacama Large Millimeter/submillimeter Array) located at 5000 m (16,400 ft) elevation in the northern Chilean mountains. ALMA comprises 66 12-m (40-ft) diameter telescopes spread across 6 km (10 mi) of cold, arid desert.
Sitting above most of Earth’s atmosphere, ALMA can detect 1-mm wavelengths of light. ALMA is the product of international collaboration; scientists from 119 countries have access to the array.
Here are a few recent examples in ACS publications:
Early last year, Vincenzo Barone, Malgorzata Biczysko, and Cristina Puzzarini at Italian research institutions in Pisa and Bologna showed the interplay of observations, models, and laboratory research by setting up a complex computational process to study oxiranes (ethylene oxide). The article is a detailed account of the process and results. The authors conclude that they can compute the physicochemical properties of biomolecules with an accuracy that rivals “the most sophisticated experimental techniques.” (Acc. Chem. Res. DOI: 10.1021/ar5003285)
Later in 2015, Ryan C. Fortenberry of Georgia Southern University (Statesboro) described the role of quantum chemistry in providing a full picture of anions in the interstellar medium (ISM). For a long time, astronomers did not believe that anions could exist in the ISM; but in 2006, C6H– was detected. Since then five other anions have been confirmed. It is still not clear, however, how they form or why.
Fortenberry describes the accuracy and efficiency of using quantum chemistry to study the known anions. For example, configuration interaction theory was used to analyze CH2CHO–. The electronic and geometric structures of the anion in its valence ground state and dipole-bound excited states gave insight into what the photoelectronic spectra should look like; whereas the experimental data had been most puzzling. (J. Phys. Chem. A DOI: 10.1021/acs.jpca.5b05056)
Nobel Prize recipient George A. Olah and his team at the University of Southern California (Los Angeles) are applying terrestrial chemistry to the study of extraterrestrial methanol. Their questions are not unlike Fortenberry’s: How and through what pathways did the observed hydrocarbons, their carbocations, and other products form in our solar system and far-flung galaxies? On the basis of the observed higher reactivity of methanol compared with methane, they propose a new pathway for the conversion of extraterrestrial methanol to hydrocarbons and their derivatives and offer a possible connection with methonium ion (CH5+)–based chemistry. (J. Am. Chem. Soc. DOI: 10.1021/jacs.6b00343)
The discoveries are continuing as tools become more precise and data become available more quickly. It is an exciting time to look to the stars, and astrochemists are key players in the exploration and explication. At the 2013 ACS National Meeting in Philadelphia, the ACS Physical Chemistry Division established a new Astrochemistry Subdivision. Stay tuned for more exciting news in this field.
Astrochemistry and Astrophysics
These two sciences are regularly confused and often described interchangeably. However, they are two distinct sciences. Michaël De Becker of the University of Liège (Wallonia, Belgium) offers helpful definitions of the two.
- Astrophysics is the science devoted to the study astronomical objects that populate the Universe, including their formation and evolution, through a dedicated application of physical concepts.
- Astrochemistry is the science devoted to the study of the chemical processes at work in astrophysical environments, including the interstellar medium, comets, and circumstellar and circumplanetary regions.