Proceedings of a Workshop
Astrochemistry: Discoveries to Inform the Chemical Sciences and Engineering Communities
Proceedings of a Workshop—in Brief
Throughout much of human history, space was thought to be a void in which only ions or radicals existed. It was only in the last half of the 20th century that scientists began to discover the existence of molecules, such as ammonia, in space. Discovery has accelerated in the last decade with the installation of new facilities and cutting-edge advances in spectroscopic analysis. These exciting discoveries in astrochemistry—a multidisciplinary field that focuses on the chemical composition of and processes in astrophysical and planetary environments—have potential applications to the general chemistry and chemical-engineering communities. Accordingly, the Chemical Sciences Roundtable of the National Academies of Sciences, Engineering, and Medicine held a workshop on November 8–9, 2018, to (1) explore the chemistry of space—its novel chemicals and reaction mechanisms, (2) discuss information from remote sensing through spectroscopy, and (3) consider discoveries from spacecraft missions in the solar system and laboratory studies of extraterrestrial samples.1 The ultimate goals of the workshop were to bring the various communities together to explore how discoveries in astrochemistry might provide insights or opportunities for the general chemistry and chemical-engineering communities and to promote understanding in the chemistry and chemical-engineering communities of how they might be able to help the astrochemistry community approach its challenges. This brief proceedings summarizes the presentations and discussions that took place during the workshop. The workshop videos and presentations are available online.2
KEYNOTE ADDRESS: OUR ASTROCHEMICAL ORIGINS
Paola Caselli, director of the Center for Astrochemical Studies of the Max Planck Institute for Extraterrestrial Physics, opened the workshop by highlighting some important findings in astrochemistry. She noted that complex organic molecules have been around since the dawn of our solar system, and meteorites that have bombarded our planet since its origin are rich in organic compounds and might have brought the important building blocks of life. In fact, some meteorites have been found to contain more than 200 amino acids, fatty acids, and the nucleobases adenine, guanine, and uracil. Caselli described dark clouds in which the temperature is so low (6 K) that molecules freeze out onto dust particles and form thick icy mantles, which are composed primarily of water, carbon monoxide, and other molecules and represent the precursors of planets.
Astrochemistry fundamentally starts with the formation of the ion H3+, the most important ion in interstellar chemistry, Caselli said. Hydrogen (H2) forms on the surface of dust particles and is ionized by cosmic rays to H2+, which then reacts quickly with H2 to form H3+. From that point, water and hydrocarbons can be formed. Key to those reactions are the dust particles, their icy mantles, and cosmic rays, which can heat the ice, break bonds, create ultraviolet (UV) fields, and inject energy; all these actions can result in interesting chemistry.
Caselli stated that studying the abundance of various elements, ions, and molecules in space helps us to understand Earth’s geochemistry. She used the deuterium:hydrogen ratio in Earth’s oceans as one example. Scientists have examined that ratio in various comets and found that one has the same ratio as that in Earth’s oceans. One could imagine that the deuterium:hydrogen ratio in Earth’s oceans resulted from the early bombardment of Earth by meteor-
1 See https://www8.nationalacademies.org/pa/projectview.aspx?key=51363 for more information on the workshop.
ites and is a relic of the early solar system. It is interesting to note that the deuterium:hydrogen ratio in organic compounds in space is higher than (or different from) that in water.
High-resolution spectroscopy has revealed the abundance of complex organic molecules in space; many have been identified, and others remain a mystery.3 The abundances of complex organic molecules appears to be similar in comets and star-forming regions. Caselli noted that different families of molecules are found in different areas of the prestellar core, depending on their sensitivities to various environmental conditions. For example, oxygen-containing molecules avoid the core center, whereas hydrogen cyanide and nitrogen-containing molecules are found near the core center. However, it remains to be explained why some organic compounds are found in different regions and especially why nitrogen-bearing species do not present the same magnitude of freeze-out as carbon-bearing and oxygen-bearing molecules.
Caselli closed her presentation by introducing the topic of protoplanetary disks (see Figure 1), an active field of research in which astrochemistry plays an important role. Research has focused on how protoplanetary disks form from rotating and contracting magnetized clouds. A problem has been that the dynamic models have failed to show how the clouds can flatten to form the disks, given the presence of the magnetic flux. However, Caselli’s research group recently found that removing the very small dust grains (freezing them out on top of larger dust grains) enables the disks to form. Caselli concluded that although much progress has been made, there is still much to be discovered to link the various phases in the process of star and planet formation with the ultimate goal of understanding our astrochemical origins.
SESSION 1: THE CHEMISTRY OF SPACE: NOVEL CHEMICALS AND REACTION MECHANISMS
Eric Herbst, Commonwealth Professor of Chemistry at the University of Virginia, provided a few introductory remarks to begin Session 1. He reviewed the types of molecules that have been identified in space—simple species, carbon chains, complex organic molecules (COMs), fullerenes, and polycyclic aromatic hydrocarbons (PAHs)—and stated that COMs are not complex by most chemical standards but were so named in astrochemistry because they are more complex than such simple species as hydrogen and carbon monoxide. He emphasized that the various environments in space have challenged and changed our thinking on low-temperature reaction mechanisms given the species identified in them. The presentations that followed elaborated on those points.
Detection of Exotic Species in Space and in the Laboratory
Michael McCarthy, associate director of the Atomic and Molecular Physics Division of the Harvard–Smithsonian Center for Astrophysics, began by reminding the audience that scientists in the early 20th century were skeptical that molecules even existed in space. They could rationalize how molecules were destroyed but not how they could be formed in space. However, molecules are ubiquitous in space; they have been detected at every stage of stellar evolution and in regions and situations that might seem inhospitable to formation and survival of chemical bonds. Those discoveries have been facilitated by better astronomical facilities and laboratory spectroscopy. To make greater progress, however, McCarthy stated that scientists need improved astronomical measurements, sensitive laboratory techniques that can detect transient species, theoretical calculations for predicting spectra, exploratory chemistry that can elucidate reaction pathways, instrumentation and technology development, and data management and new analytic tools that can handle the massive amounts of data that are being collected.
McCarthy used several examples to illustrate how species in space have been detected. Spectral fingerprints generated in the laboratory are compared with astronomical measurements to provide compelling evidence of the existence of various species. The laboratory spectra are obtained by generating transient species with, for example, an electric discharge source and then cooling the system to extremely
3 In astrochemistry, complex organic molecules are often defined as molecules with greater than five atoms.
low temperatures. Those approaches have allowed the detection of such interstellar species as CC34S, HC5O radical, H2NCO+, and fullerenes (C60). McCarthy noted that the search for anions in space was frustrated by several misconceptions or mistakes, such as focusing on small anions. Today, however, six molecular anions—CCH-, CN-, C3N-, C4H-, C6H-, and C8H-—have been detected on the basis of laboratory data; some have been shown to be relatively abundant in space. Metal-bearing molecules, such as MgNC, have been identified on the basis of laboratory study, and McCarthy stated that more metal-bearing molecules will likely be found with powerful facilities, such as the Atacama Large Millimeter/Submillimeter Array (ALMA). The most exciting recent discovery might be that of the polar aromatic molecule, benzonitrile—exciting primarily because it establishes that aromatic chemistry occurs under the inhospitable conditions of space and because formation of the first aromatic ring is the rate-limiting step in PAH formation, which means that much larger organic compounds are possible.
McCarthy concluded that interstellar space consists of a rich broth of familiar and exotic molecules whose existence in space can be proved with great confidence on the basis of precise laboratory measurements. “Never underestimate nature’s ability to synthesize complex organic molecules in what appear to be inhospitable conditions; exciting, perhaps unexpected, discoveries may be in store.”
Observations of Organic Molecules in Disks and Young Stars
Catherine Walsh, University Academic Fellow of the School of Physics and Astronomy of the University of Leeds, reviewed the evolution of dense clouds to planetary systems (see Figure 2) and stated that the “holy grail” of astrochemistry is the detection of complex organic molecules in protoplanetary disks. Such a discovery would connect the complexity that we see in early evolutionary stages with what we see in our own solar system. Comets provide important clues because they are frozen relics of planet-building material that probably brought water and complex organic molecules to Earth. The Rosetta mission that visited comet 67P/C-G provided evidence that comets are rich in organic molecules; most important was the discovery of the amino acid glycine.
Walsh then described the chemical composition and structure of a protoplanetary disk, which is composed of two phases—a gas phase dominated by hydrogen and an ice phase dominated by water. A rich molecular layer enshrouds a frozen midplane; the surface is the most irradiated region and is where molecules can be split by photodissociation or ionization into atoms or ions. Mapping the molecular structure of disks provides insight into the material available for building planetary or cometary systems; the relative ratios of dominant volatile chemicals will dictate the compositions of planetary atmospheres. Walsh stated that the chemical structure of protoplanetary disks is set by the complex interplay between gas-phase chemistry, surface chemistry, radiation, and dynamics.
ALMA is an important tool because it provides the high spatial resolution and sensitivity needed to detect the larger molecules in protoplanetary disks, Walsh said. Using ALMA, scientists have been able to detect methyl cyanide, methanol, and formic acid in protoplanetary disks. Walsh noted that modeling has helped to decipher some of the origins of the molecules. She closed, however, by stating that the gas-phase origin of organic molecules in protoplanetary disks is still uncertain and posed the following questions to investigate: Does the measured gas-phase composition reflect the ice composition? Do all “large” molecules fragment on photodesorption? Does processing by x-rays or cosmic rays also lead to fragmentation? How important is chemical desorption? Are gas-phase routes missing from the models?
PAHs and Fullerenes: Complex Carbon Species Throughout the Universe
Els Peeters, associate professor in the Department of Physics and Astronomy of the University of Western Ontario, described the detection of PAHs and fullerenes in space, their importance and formation, and differences between interstellar and circumstellar environments. Strong infrared (IR) emission features have provided evidence on the widespread abundance of PAHs and fullerenes in space. They are important for several reasons: they are a dominant heating source in the interstellar medium and thus set environmental temperature, they provide a large surface area on which various chemical reactions can occur, and they can be ionized and thus influence the charge balance and change the abundances of elements and molecules.
Peeters noted that because the available spectroscopic data cannot distinguish between different molecules, there are two hypotheses about the number of different PAHs in space. One is that there are millions, and the other is that only the most stable PAHs (perhaps only 10) survive. What is clear is that the PAH emission-band profiles depend on object type, environment (interstellar vs circumstellar), location within the medium, and temperature; and it is postulated that these factors determine the aliphatic:aromatic ratio in the species. Peeters stated that researchers have found that fullerene abundance is much higher closer to a star and that PAH abundance is much higher farther from a star, and these observations have led to the hypothesis that UV processing is needed if PAHs are to form and be modified to fullerenes in the interstellar medium. Laboratory experiments support that hypothesis, but mechanisms of PAH evolution in circumstellar environments are unclear.
One challenge today is that the data have low spectral resolution and that researchers therefore cannot define PAH molecular structures or features, Peeters said. However, she emphasized that the James Webb Space Telescope (JWST) will revolutionize PAH–fullerene research. It will simultaneously provide long-wavelength coverage, medium spectral resolution, high spatial resolution, and high sensitivity. But she noted that optimizing the output of the JWST will require laboratory and theoretical studies of PAHs and fullerenes for interpreting the data.
Production of Interstellar Molecules by Proton Bombardment of Ices
Reggie Hudson, lead scientist of the Cosmic Ice Laboratory of the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center, concluded the first session by describing the unique chemistry that results from proton bombardment of ices. To study interstellar ices, scientists have to use remote sensing—infrared-optimized telescopes at high altitudes on Earth (such as the Infrared Telescope Facility, IRTF), flown at high altitudes on aircraft (such as the Stratospheric Observatory for Infrared Astronomy, SOFIA), or launched into space (such as the Spitzer Space Telescope). Hudson reminded the audience that the interstellar medium contains dust grains that have a siliceous core with an ice coating. UV photons and cosmic rays (H+ and He+) strike the dust grains and enable the chemical reactions that occur. He noted that his research group studies this system by growing ice on a substrate in a vacuum chamber, irradiating the sample with cosmic rays, and collecting the IR spectra; the ionizing radiation creates secondary electrons that make the chemical products.
Hudson next described various chemical products that have been predicted on the basis of laboratory experiments and later discovered in interstellar ices. An early laboratory experiment showed that carbon monoxide could be converted to methanol by using radiation chemistry. Researchers then predicted the existence of ethylene glycol in 2000, which was confirmed a few years later. In 2004, researchers predicted the isomerization of methyl cyanide to the highly unstable ketenimine, which was discovered in 2006. Hudson emphasized that what might be considered difficult or unusual chemistry on Earth might be possible under the conditions of space and that several unexpected products have been discovered. One important product is ketene; it is considered a gateway molecule because so many other products—such as acetone, acetic acid, methyl acetate, acetamide, and more complex molecules—can be made from it. As a last example, Hudson described the formation of the chiral molecule propylene oxide. He concluded that no one originally thought that any reactions in ices could be discovered by remote sensing, but a wide array of chemical reactions in interstellar ices—oxidation, reduction, decomposition, elimination, isomerization, acid–base reactions, hydration, hydrolysis, and dimerization—have been reported over the last few decades.
Workshop participants raised several interesting questions during the panel discussion. McCarthy mentioned during his presentation that the current rate of discovery was about four species per year, and a workshop participant asked whether current date-science tools and methods could expedite discoveries. McCarthy responded that complicated chemical mixtures are formed in the laboratory simulations and that the ultimate goal is their real-time analysis. He posited that researchers should be able to use machine-learning algorithms or methods to help to analyze rotational spectroscopy and other large datasets. McCarthy also noted that increasing the rate of discovery also depends on funding and collaboration—the need to connect astronomy to laboratory experiments to calculations.
Another workshop participant noted that the discussion thus far had centered on carbon chemistry and asked, What about silicon chemistry, given the silica core of the dust particles? McCarthy responded that a few silicon
molecules have been found close to central stars; however, Caselli stated that because silica tends to be restricted to the particle core it probably does not actively participate in the observed chemistry. And although there is some “peeling off” of the silicates from the particles, scientists do not see an equivalent amount of silicates compared to the organic carbonaceous material. Hudson added that it is difficult to work with silane in the laboratory, given its reactivity.
A participant asked about the gaps in the technology for identifying complex molecules. Peeters reiterated that the biggest problem has been the low spectral and spatial resolution of the data but noted that this problem should be alleviated by use of the JWST. Caselli stated that laboratory experiments that expose PAHs found on Earth to the various conditions in space could provide useful information. McCarthy added that PAH chemistry is combustion chemistry and that there is an opportunity for astronomers and combustion chemists to collaborate.
Herbst closed the session by asking Walsh about the largest molecules that can be seen in the protoplanetary disks. Walsh stated that molecules bigger than, for example, methanol or methyl cyanide have very low abundance, which makes them difficult to detect. A further problem is that their spectral lines overlap with those of other molecules. She concluded that it is difficult to see molecules of more than six or seven atoms in the protoplanetary disks. However, Herbst countered that researchers have detected PAHs. Walsh agreed that PAHs have been detected but noted that it is their spectral features that have been identified rather than specific PAHs. She emphasized that the primary problem is the small volume of the materials that scientists are trying to detect.
SESSION 2: INFORMATION FROM REMOTE SENSING THROUGH SPECTROSCOPY
Susanna Widicus Weaver, associate professor and director of graduate studies in the Department of Chemistry of Emory University, introduced Session 2 by first providing a perspective on the physical environments in the interstellar medium (see Table 1). She stated that a goal of astrochemistry is to understand how the chemistry changes as the physical conditions change through each step of the star- and planet-formation process. That understanding comes from remote observations. Widicus Weaver reminded the audience that visible spectroscopy provides information on electronic or atomic transitions and that IR spectroscopy provides information on molecular vibrations (stretches or bends), which helps to identify molecular functional groups but does not give the specific details needed to determine exact identity. However, rotational transitions in the radio, microwave, millimeter, and submillimeter frequencies are high-resolution fingerprints that are specific to molecular structure. It is on the basis of rotational transitions that most molecules in space have been positively identified.
Table 1 Physical Environment of Various Interstellar Regions with Earth as a Reference Point
|Environment||Molecular Density, molecules/cm3||Temperature, K|
|Earth at sea level||~1019||~300|
|Diffuse interstellar medium||~10||~50|
|Dense interstellar clouds||~103||~10|
Widicus Weaver stated that observatories today are providing complex spectral data to analyze, but the challenge is to assign and analyze all the information. The field needs fundamental laboratory studies that measure molecules of interest and tools that can rapidly compare the laboratory spectra with observational spectra to sort out the complex mixture chemistry. Regardless of the challenges, such observatories as ALMA are providing data that can be used to understand the varied chemistry of the many regions in space, and the presentations in this session illuminated where we are in our understanding.
Carbon in the Solar System
Ted Bergin, professor in and chair of the Department of Astronomy at the University of Michigan, stated that a new age has arrived with the detection of planets that orbit stars other than our own sun (exoplanets). Discoveries have prompted such questions as these: Is a planet’s chemical composition that scientists observe today related to its chemical composition at birth? If so, what does that imply? Is an Earth-like composition a preordained outcome? Although scientists can detect major and trace species in our solar system, only major species in the atmospheres of exoplanets can be detected today. One approach to making a connection between our solar system and exoplanets is to study the bulk composition of, for example, carbon.
Bergin noted that most of the carbon on Earth is in the core and that there is a substantial subsurface reservoir; only a very small percentage is in the biosphere. In the interstellar medium, 50 percent of carbon is in a refrac-
tory form as solid particles (the seeds of terrestrial worlds), and 50 percent is in carbon monoxide. So the question is, How did Earth evolve as a “silicate rock.” Bergin stated that by studying comets and meteorites one can deduce that something happens to interstellar carbonaceous grains in the solar system and that something happens during planet formation to cause massive carbon loss. He explained that some carbon loss could be caused by sublimation of carbon from the grains, and if carbon were present in the magma ocean phase of Earth formation, it would migrate with iron to the core. The key message is that carbon on the surface of a terrestrial world is not preordained; rather, the distribution is likely determined by a stochastic process that could have various outcomes, Bergin said. He concluded by noting that a better understanding of Earth’s formation can lead to predictive models of terrestrial-world formation that can be used to analyze observations of planets in other solar systems—observations soon to be enhanced by the use of the JWST.
Chemistry in Disks of Gas and Dust
Ilse Cleeves, assistant professor of astronomy and chemistry at the University of Virginia, focused on observations of various molecules in disks. She noted that ALMA has been immensely valuable in this field because it has the sensitivity needed to detect many molecules in disks that have previously been undetectable and provides great spatial resolution. A key goal of her work is to understand how the disk components are incorporated into the forming planets. Another goal is to understand how the physical properties of the disks affect planet formation. Cleeves described the disk chemical composition (see Figure 3) and noted that various physical factors can change the composition of protoplanetary disks. For example, winds can remove light species, and transport through a disk can cause mixing of materials between its inner and outer parts.
Cleeves described several methods for converting an image to a distribution of molecules. One is direct extraction, which is best used when little information on the host star or disk is available; it requires assumptions of temperature, optical depth, and distance, which might be too simplistic. A second is forward modeling with parametric models. That approach also requires assumptions of temperature and density but does not require complex chemical modeling; emissions can be calculated by solving radiative-transfer equations or by using publicly available radiative-transfer codes. A third method is forward modeling with detailed models, which is best used when much is known about a source, such as the star’s properties, gas and dust densities, and reaction networks. That approach is especially useful in constraining spatial-abundance maps, assessing what reactions are important, and determining the physical drivers of the chemistry observed.
Cleeves listed the molecules that have been detected in protoplanetary disks and stated that disks have not been well studied chemically because their low mass makes study difficult. She concluded, however, that observations of C2H are providing valuable information on carbon:oxygen ratios in disks. Observations of C2H in disks that varied in age suggest that the carbon:oxygen ratio evolves spatially and with age and that a planet’s carbon:oxygen ratio might be determined by whether it forms “early” or “late.”
Sarah Hörst, assistant professor in the Department of Earth and Planetary Sciences of Johns Hopkins University, be-
gan by reminding the audience of a key question to answer: What is the role of planetary atmospheres in the origin, evolution, and detection of life? She stated that planetary atmospheres determine the chemical composition of atmospheric gases, whether particles are present and their properties, how radiation moves through the atmosphere, and atmospheric and surface temperatures. She emphasized, one needs to know what the atmosphere contains, and remote sensing helps to provide that information. She used Titan, the largest moon of Saturn, to illustrate how planetary atmospheres have been probed.
Hörst explained that there was suggestive evidence that Titan has an atmosphere as early as 1907, but it was not until the 1940s that conclusive evidence was obtained. Not much, however, was known beyond the presence of methane. In the 1980s, the Voyager spacecraft, which carried an IR spectrometer, provided a wealth of information on Titan’s atmosphere. It showed that several organic molecules were present in Titan’s atmosphere, such as ethane, propane, hydrogen cyanide, and acetylene. Before spacecraft returned to Titan, Earth-based remote sensing provided more information on Titan’s atmosphere; it showed clouds and benzene in Titan’s atmosphere. Discoveries provided by Voyager also helped to direct research on Earth. For example, Voyager showed that carbon dioxide was present, and scientists concluded then that carbon monoxide must also be present and soon proved it. That example illustrates a common story: once scientists know what to look for, it is often easy to find. But it sometimes is not obvious what should be investigated.
Important new discoveries were made with the Cassini-Huygens mission that arrived at Saturn in 2004 and carried various spectrometers that could probe the atmosphere. For the first time, composition and temperature in the entire atmosphere could be measured, and the data collected helped to explain the chemistry observed and revealed much that could not be seen with remote sensing. For example, atmospheric models based on Voyager data predicted that Titan’s ionosphere would be chemically complex but did not predict the substantial abundances at higher masses that were revealed by mass spectral data from Cassini. Those data indicate that researchers underestimated the role of nitrogen in the chemistry of Titan’s atmosphere. The Cassini data showed that heavy ions exist at high altitudes in Titan’s atmosphere, and this discovery has dramatically changed our understanding of aerosol formation. The Cassini mission also showed that O+ flows into Titan’s atmosphere; this finally explained why carbon monoxide was present in the atmosphere. Hörst emphasized that scientists would never have been able to explain that conundrum without the data from Cassini and that the data led to predictions of various organic compounds, such as amino acids, in Titan’s atmosphere. However, she noted that while Cassini was orbiting Saturn, ALMA revealed five new molecules: C17O, H13C15N, DCN, C2H3CN, and C2H5CN. Hörst concluded by emphasizing the important synergies between observations, models, laboratory experiments, and spacecraft missions that can reveal things that scientists could not find otherwise.
Jonathan Fortney, professor and director of the Other Worlds Laboratory in the Department of Astronomy and Astrophysics of the University of California, Santa Cruz, discussed planetary characteristics and evolution. He stated that although much detailed information about our solar system is being generated, few details of specific exoplanets can be obtained. Many are seen only as points of light or shadows, so scientists have to look at broad trends among planets or systems. One observation is that the vast majority of exoplanets are much hotter than the planets in our solar system and have short orbital periods (less than 100 days). Fortney stated that planets can be categorized according to the origin of their atmospheres. The atmospheres of primary planets—such as Jupiter, Saturn, Uranus, and Neptune—originated from the protoplanetary disk and are composed primarily of hydrogen and helium. The atmospheres of secondary planets—such as Venus, Earth, Mars, and Titan—were outgassed from the planets’ interiors.
Fortney stated that planets evolve, typically forming hot and then cooling. Rocky planets, such as Earth, cool relatively rapidly (over 10 million years), and giant planets, such as Jupiter, cool much more slowly; some that have short orbital periods will never cool. The temperature will dictate the chemistry and determine what exists in solid, liquid, and gas phases. Fortney noted that planetary evolution can diverge on the basis of UV-driven photochemistry, and this might explain why Venus and Earth are so different even though they formed in a similar manner. He explained that planets appear to be diverging into two populations: ones with a thin-skinned hydrogen–helium atmosphere and ones with no hydrogen or helium in their atmospheres. The distinction appears to grow as the system ages.
Fortney concluded his presentation by stating that laboratory work is needed to advance the field and listed the following major needs: molecular line lists of opacities for diverse gases at high spectral resolution, extended databases on collision-induced absorption and dimer opacities, laboratory studies of haze and condensate formation and optical properties, substantially expanded databases on chemical reaction rates, and measurements of gas photoabsorption cross sections and vapor pressures. He reiterated the excitement about the launch of the JWST; scientists will have an opportunity to explore interesting systems of exoplanets and see how similar planets have diverged over their lifetimes.
The panel debated the feasibility of a telescope on the moon. Stefanie Milam stated that one problem is that because the moon orbits Earth, an infrared telescope on the backside of the moon would suffer from the warming effects of the
sun. The use of a radio telescope would also be inhibited by the sun for some periods, and there would not be much to gain by having a radio telescope on the moon inasmuch as Earth radio telescopes, such as ALMA, can already “see” through our atmosphere. Fortney added that getting a telescope to the moon would also be difficult.
A workshop participant asked Hörst whether the species revealed through mass spectroscopy form in situ or are being deposited in the atmosphere. Hörst responded that much of the chemistry of Titan occurs in situ, but there are questions about the role of external inputs, such as the oxygen ions that she mentioned in her presentation. The origin of the species observed on Saturn is a bigger question, and scientists have concluded that some species come from the rings; however, some species might have come from contamination of the instrument.
A final question concerned the development of models that might never be validated. Specifically, if people build models that simulate atmospheres of planets, they will ultimately find it difficult to test their hypotheses because with few exceptions they cannot go to those places. Fortney stated that remote sensing has provided information that has allowed scientists to draw inferences that have been proven to be correct. Furthermore, if different research groups develop models, and the models arrive at the same answer or converge, that will likely indicate that the scientists are on the right track.
SESSION 3: DISCOVERIES FROM SPACECRAFT MISSIONS IN THE SOLAR SYSTEM AND FROM LABORATORY STUDIES OF EXTRATERRESTRIAL SAMPLES
Stefanie Milam, a research physical scientist in the Astrochemistry Laboratory of the NASA Goddard Space Flight Center, introduced the third session by emphasizing the importance of planetary missions and analysis of extraterrestrial samples. Those activities provide insight into interstellar chemistry and the formation of planetary systems. They provide data to validate inferences drawn from remote observations and help to answer such basic questions as whether all planetary systems are similar. She highlighted the valuable information that has been obtained from missions to comets and concluded that the presentations in this session will illuminate the fascinating discoveries that have been based on planetary exploration.
Analysis of Organic Compounds in Solar System Materials
Jamie Elsila, an astrochemist in the Astrobiology Analytical Laboratory of the NASA Goddard Space Flight Center, is trying to answer these questions: What were the origin and distribution of complex organic molecules in the early solar system? What can organic compounds reveal about the history of solar system materials? She stated that analyses of solar system samples reveal complex mixtures of organic compounds, which retain signatures of early solar system chemistry and processing histories of parent bodies. The samples are a snapshot of the chemistry of the solar system when it formed, but one has to recognize that they have also undergone processing in the last 4.5 billion years.
Typical analyses of interest include chemical distribution, absolute and relative abundances, isotopic ratios, and chirality. For example, life on Earth evolved to use L-amino acids, and one might expect amino acids in an abiotic source, such as a meteorite, to be a racemic mixture; however, meteorites typically have excesses of L-amino acids, and this raises the question of whether the delivery of these chiral molecules played a role in the development of life on Earth. Elsila stated that her group focuses on soluble organic compounds and that development of methods for studying extraterrestrial samples is a long process. Method development must be customized in light of interferences, low abundances, complex mixtures, minimal contamination, and minimal bias and isotopic fractionation. Specifically, one has to remove species that affect separation or derivatization, maximize sensitivity in exchange for precision, understand all possible isomers and enantiomers, maximize separation in exchange for analysis time, understand contamination sources, and understand how technique affects detection of species and isotopes. Contamination is a huge concern, so laboratory glassware is heated to high temperatures for extended periods, reagents are purified, and blanks and standards are used extensively. Elsila stated that a given sample is typically more precious than the researchers’ time and that method development can take years.
Primitive solar system materials that are available for laboratory analysis include meteorites, lunar samples, and cometary material. Elsila highlighted that the advantages of meteorites are that they are delivered to Earth and that they come from an array of parent bodies in the solar system and provide diverse samples of different environments. She stated that her group focuses on the analysis of meteorites that are carbonaceous chondrites, which make up about 4 percent of meteorite falls and are rich in organic compounds. Those meteorites can be grouped on the basis of mineral and petrographic analyses, and researchers have found that the groups vary widely in their amino acid content. For example, distributions of five-carbon amino acid isomers vary profoundly. Elsila stated that the relative distributions can provide information on formation mechanisms, formation environments, and processing histories. Although meteorites provide valuable information, she noted that sample returns are extremely important because the laboratory analysis that can be conducted on Earth is far more sophisticated than what can be accomplished remotely: complex laboratory instruments cannot be miniaturized and flown on spacecraft. She also emphasized the importance of careful curation and storage of samples because future analyses will be able to make discoveries not possible with
Elsila concluded by listing a few challenges in the analysis of extraterrestrial samples. As indicated, method development is difficult because astrochemists deal with complex samples that do not have suitable terrestrial analogues. The cost and maintenance of laboratory facilities can be prohibitive. There are issues with sample curation, contamination, and availability. Finally, the frequency and length of sample-return missions are problematic.
Preservation of Organic Compounds on Mars and the Search for Chemical Biosignatures
Daniel Glavin, associate director for strategic science in the NASA Solar System Exploration Division, began by providing a brief background on discoveries on Mars. He stated that scientists were skeptical about finding organic compounds on Mars given the ionizing-radiation environment. The Viking mission in the 1970s found chloromethanes, but scientists concluded that those compounds probably resulted from contamination and discounted their origin as Martian. However, recent reanalysis of Viking gas-chromatography–mass spectral data suggests the presence of chlorobenzene on Mars. In the 1990s, the discovery of a Martian meteorite that had worm-like fossil structures began a debate about past life on Mars. What was especially interesting was the discovery of PAHs in Martian meteorites, most likely of abiotic origin. In 2008, the Phoenix Lander found the perchlorate anion in Martian soil. Scientists concluded that chlorinated hydrocarbons were being formed by pyrolysis of perchlorate with organic compounds.
Glavin stated that the Curiosity rover, which landed in the Gale Crater on Mars (see Figure 4), has provided valuable information. Scientists believe that the Gale Crater is a lakebed and that sampling of this area could provide a history of Mars—its transformation from a warm wet environment to a cold dry one. Glavin said that the Curiosity rover contains a complex suite of instruments, including a quadrupole mass spectrometer and gas chromatograph. The soil sample is heated to over 900°C, some gas is analyzed directly by the mass spectrometer to provide information on the bulk chemistry, and some gas is diverted to the gas chromatograph so that the components can be separated and then identified by the mass spectrometer. Glavin stated that one exciting discovery was the substantial amount of carbon in the samples (200–2,400 ppm); this concentration is more than what would be predicted by the study of meteorites and is similar to that in the seafloor sediments of the South Pacific. Chlorobenzene was also found in the samples, and this discovery spurred the reanalysis of the Viking data. Although scientists attributed the chlorobenzene to the reaction of perchlorate with organic compounds, the question became, What are the “native” organic materials? Glavin noted that other exciting discoveries were the detection of such sulfur compounds as methanethiol, dimethyl sulfide, and thiophenes. He emphasized that the challenge is to determine whether the material has a biotic or abiotic origin. Ultimately, was there ever life on Mars?
Glavin concluded by noting that a complicating factor for the detection of organic compounds on Mars is cosmic radiation, which can penetrate the surface and greatly affect preservation of biosignatures, such as amino acids. One strategy is to drill near scarps where there is substantial wind erosion and the subsurface material is exposed. However, advanced laboratory techniques and instrumentation will likely be needed to detect biosignatures, and this will require a sample-return mission and analysis on Earth.
Searching for Habitability at the Extremes
Amanda Stockton, assistant professor at the Georgia Institute of Technology, built on the presentations by Glavin and Hörst. She asked where besides Mars and Titan scientists should be looking for life and noted the icy moons of Jupiter (Europa) and Saturn (Enceladus). Europa and Enceladus have an ice crust beneath which is a liquid-water ocean that sits on a rocky core (see Figure 5). Both are expected to have alkaline hydrothermal vent systems, which are excellent environments for extremophiles—these have been implicated in some models of the emergence of life on Earth. She stated that we cannot yet define fully the conditions necessary for habitability, but it is known that all life is based on organic chemistry. Furthermore, data from the Cassini mission that flew through plumes from massive geyser systems show that organic molecules are present in the environment on Enceladus.
Because missions in the solar system
are expensive, scientists use locations on Earth that are terrestrial analogues to advance the understanding of extreme spatial environments that might be habitable, Stockton said. For example, a reasonable analogue of Mars is the Atacama Desert, and glaciers and subglacial lakes are reasonable analogues of the icy moons. Stockton stated that scientists also use the terrestrial analogues to plan sample collection so that biosignatures will not be missed. She described work that was conducted in the Atacama Desert and emphasized the importance of both depth sampling, given the surface sterilization caused by UV radiation, and the taking of multiple samples, given the expected low biomass.
Stockton described her work on Icelandic lava fields to illustrate how field explorations are used to determine the best ways to collect samples for planetary missions. She noted some critical questions, such as, How do you select the landing site? How do you analyze a sample? Are geochemical measurements adequate for predicting habitability? What are the critical factors to consider for sample collection? Stockton stated that her group uses ATP and DNA as proxies for biomarkers in its fieldwork but noted that she is not suggesting that these are the biomarkers to use in space exploration. She said that the primary focus has been on exploring how many samples and what spatial distances are required to represent an area fully. The group’s initial fieldwork on Dyngjusandur (an alluvial plain in Iceland) where it used a sampling strategy based on triangular grids indicated that at least three samples at 10-cm spacing would need to be collected. Expanding the fieldwork to Holuhruan (a volcanic cinder cone in Iceland) and including additional measurements indicated that a machine-learning algorithm might be used to guide sampling. Stockton concluded by stating the hope that the fieldwork in Iceland and possibly future fieldwork in the Atacama Desert will help to guide fruitful exploration on Mars.
Pluto and Other Icy Worlds: Nature’s Chemical Laboratories
Michael Wong, a research associate at the University of Washington, described the exciting discoveries that the New Horizon spacecraft made possible when it flew past Pluto. It provided a sharp image of Pluto (see Figure 6), a planet that has peaks capped by methane snow, has a giant glacier of nitrogen ice, and is surrounded by a brilliant blue organic haze. Wong explained that an unexpected observation was an underabundance of acetylene, ethylene, and ethane at low altitudes—lower than what scientists had predicted. To understand that finding, he said, one must recognize what is happening with the solid particles in the haze. Scientists deduced that the solid particles in Pluto’s atmosphere are fractal aggregates that grow, age, and harden as they descend in the atmosphere and that there is a point at which the conditions are optimal for gas molecules not only to adhere to the haze particles but to be absorbed by them. The absorption explains the observed underabundance of the hydrocarbons.
Wong noted that there are still many data to mine from the New Horizons flyby of 3 years ago. Scientists are still probing the properties of the haze particles. He stated that the haze particles in Pluto’s atmosphere might be extremely important in controlling its thermal profile. The original modeling indicated a temperature profile different from what was detected: Pluto’s atmosphere is much colder than expected. The hypothesis is that the haze is effective in radiating thermal energy from collisions back to space; this could be tested by the JWST. Wong emphasized that the
chemical and chemical-engineering communities can contribute to the study of Pluto by conducting laboratory studies that, for example, measure sticking coefficients and the saturation vapor pressure of gases at Pluto’s low temperatures and pressures. Such laboratory studies would bolster confidence that the conclusions about Pluto are correct or could teach us new physics and chemistry relevant to the atmospheres of outer solar system bodies.
Wong turned his attention to the ocean worlds—Europa and Enceladus—and the possibility of life in the oceans that lie beneath the ice crusts. He mentioned that NASA will be sending a mission to Europa that will assess its habitability. He highlighted the valuable information that was collected by the Cassini mission on the composition of the plumes from Enceladus—information that supports the likelihood of continuing hydrothermal activity and the presence of macromolecular organic compounds. He closed by saying that the ocean worlds are nature’s laboratory for organic synthesis at the water–rock interfaces and that a mission should return to Enceladus with higher-resolution mass spectrometers to search for biosignatures.
Several questions posed by workshop participants centered on sample analysis. One participant asked about advances in developing highly sensitive analytic techniques, such as position-specific isotopic analysis. Elsila acknowledged that some groups are developing that technique because it has the potential to predict the origins of the material. Glavin commented that the discussion indicated the need to archive samples for decades because new techniques that will have much greater sensitivities than current ones will eventually be developed. Another participant asked how samples are archived to maintain their native environment. Elsila stated that most samples are stored under nitrogen to eliminate moisture and prevent oxidation. Glavin added that some samples—for example, from comets—will need to be hermetically sealed to avoid the loss of volatile substances. Elsila noted that NASA will be re-examining some Apollo samples to answer questions about sample curation and the best curation methods. She added that it is also important to archive materials from the various construction phases of a spacecraft because those samples can help to answer questions about possible sample contamination.
Several speakers noted the importance of sample-return missions, but a workshop participant asked about the prospects of improving in situ measurements to the level achievable in a laboratory, given all the issues with contamination and the difficulties of retrieving samples. Stockton stated that a primary focus in her laboratory is on developing instrumentation for space missions and noted that microfluidic technology, particularly microfluidic capillary electrophoresis with laser-induced fluorescence detection, is showing great promise. Elsila commented, however, that sample return is still important, given that one can store samples for many years and be able to reanalyze samples with future technology. Glavin added that the technology that has been used and is being used for in situ measurements heats the sample and might thus kill the “life” for which we are searching.
A workshop participant asked the panel to explain how chemists and chemical engineers could assist or collaborate in research described in this workshop. Wong stated that a better understanding of the catalytic properties of metal-bearing minerals and hydrothermal environments in creating organic compounds relevant to life would be valuable. He added that better communication technology is needed; it takes 18 months to get data from the farthest section of the solar system to Earth. Milam emphasized the need to develop better methods for analyzing all the data that are returning to Earth. Stockton agreed that collaborations with data scientists are needed to manage and mine the massive datasets. She said that batteries and materials that can survive extreme environments are needed. A participant added that radiation-hardened instrumentation that can withstand space travel is also needed. Elsila emphasized the need for methods for analyzing complex organic mixtures in trace concentrations in extremely small samples. Milam noted that the challenge is to build instrumentation to detect things that we do not know exist; how do scientists broaden the scope of detection in this universe of unknowns?
As a final query, the speakers were asked how they would inspire undergraduates to pursue a career in astrochemistry? Stockton said that we are trying to answer some of the most important questions that humans have been asking since we became human: What is the origin of life, and how does it evolve? Are we alone in the universe? Glavin concluded that if those questions do not excite undergraduates, nothing will!
DISCLAIMER: This Proceedings of a Workshop—in Brief was prepared by Ellen Mantus as a factual summary of what occurred in the workshop. The committee’s role was limited to planning the workshop. The statements recorded here are those of the individual workshop participants and do not necessarily represent the views of all participants, the planning committee, the Chemical Sciences Roundtable, or the National Academies. To ensure that this proceedings meets institutional standards of quality and objectivity, it was reviewed in draft form by William Latter, NASA; Stefanie Milam, NASA; and Susanna Widicus Weaver, Emory University. The review comments and draft manuscript remain confidential to protect the integrity of the process.
The members of the planning committee were Michael J. Fuller, Chevron Energy Technology Company; Franz M. Geiger, Northwestern University; Eric Herbst, University of Virginia; Stefanie Milam, NASA Goddard Flight Center; and Susanna Widicus Weaver, Emory University.
ABOUT THE CHEMICAL SCIENCES ROUNDTABLE
The Chemical Sciences Roundtable provides a neutral forum to advance the understanding of issues in the chemical sciences and technologies that affect government, industry, academic, national laboratory, and nonprofit sectors and the interactions among them and to furnish a vehicle for education, the exchange of information, and the discussion of issues and trends that affect the chemical sciences. The Roundtable accomplishes its objectives by holding annual meetings of its members and by organizing workshops on relevant important topics the published proceedings of which are made broadly available throughout the chemical-sciences community.
The members of the Chemical Sciences Roundtable are Linda Broadbelt, Northwestern University (Co-Chair); Mark E. Jones (Co-Chair), The Dow Chemical Company; Tina Bahadori, US Environmental Protection Agency; Brian Baynes, MODO Global Technologies; Michael R. Berman, Air Force Office of Scientific Research; Carol Bessel, National Science Foundation; Martin Burke, University of Illinois at Urbana-Champaign; Michelle Chang, University of California, Berkeley; Miles Fabian, National Institutes of Health; Michael J. Fuller, Chevron Energy Technology Company; Laura Gagliardi, University of Minnesota; Bruce Garrett, US Department of Energy; Franz Geiger, Northwestern University; Carlos Gonzalez, National Institute of Standards and Technology; Malika Jeffries-El, Boston University; Jack Kaye, NASA; Mary Kirchhoff, American Chemical Society; Robert E. Maleczka, Jr., Michigan State University; David Myers, GCP Applied Technologies; Timothy Patten, National Science Foundation; Nicola Pohl, Indiana University Bloomington; Ashutosh Rao, US Food and Drug Administration; Leah Rubin Shen, legislative assistant, office of Senator Chris Coons; and Jake Yeston, American Association for the Advancement of Science.
This activity was supported by the US Department of Energy under Grant DE-FG02-07ER15872, the National Institutes of Health under Contract HHSN26300024, and the National Science Foundation under Grant CHE-1546732. Any opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of any organization or agency that provided support for the project.
Suggested citation: National Academies of Sciences, Engineering, and Medicine. 2019. Astrochemistry: Discoveries to Inform the Chemical Sciences and Engineering Communities: Proceedings of a Workshop—in Brief. Washington, DC: The National Academies Press. doi: https://doi.org/10.17226/25541.
Division on Earth and Life Studies
Copyright 2019 by the National Academy of Sciences. All rights reserved.