Cosmic Distribution of Chemical Complexity
Louis Allamandola, Lead Co-Investigator
Co-Investigators: Scott Sandford, Andrew Mattioda, Murthy Gudipati
Ames Postdocs: Joseph Roser, Nathan Bramall, Michel Nuevo, Christiaan Boersma
Collaborators: Max Bernstein, Els Peeters, Jan Cami, Jamie Elsila Cook, Jason Dworkin
OBJECTIVE 1: Trace spectroscopically the cosmic evolution of organic molecules from the interstellar medium to protoplanetary disks, planetesimals and finally onto habitable bodies. Characterize the constructive and destructive effects of radiation on molecular complexity with specific reference to the formation and distribution of complex carbon molecules and the synthesis of complex molecules from simple precursors during irradiation of ices. Develop the capability to detect complex organics on planetary bodies.
Objectives, Expected Significance,
and Extending the State of Knowledge
In this investigation we address Objective 1 by tracing the development and distribution of chemical complexity from the appearance of the first complex organics a few billion years after the Big Bang to the Solar System by a combination of laboratory work and observations that includes mining our world-unique spectral database as well as measuring new spectra to interpret deep astronomical observations. We will study chemical reactions under low temperature and high radiation conditions to understand the formation of biogenic molecules such as purines and pyrimidines under space conditions. We will determine the UV induced photoluminescent properties of organic, mineral, and biotic materials to develop new, astrobiologically focused instrumentation for rovers and landers deployable on solar system bodies such as the Moon and Mars.
Carbon forms in stars and is ejected at the end of the star's life. Organic compounds arise as a by-product of this stellar death. Following ejection, carbon-containing material, including polycyclic aromatic hydrocarbons (PAHs) disperses into the surrounding diffuse interstellar medium (ISM), where it is modified by different physical and chemical processes. Eventually, much of this becomes concentrated in dense molecular clouds in which, as described in the next section, new stars and planetary systems form. In these dense molecular clouds molecules are modified and new organic compounds form, some of which are of keen biogenic interest, due to their similarity to biological molecules (e.g. Dworkin et al. 2001, Bernstein et al. 1999, Ehrenfreund and Charnley 2000). This process is clearly illustrated in Figure 1. Thus to study the PAHs that are widespread throughout deep space and follow the chemical evolution that occurs in dense clouds is to track the distribution and evolution of the basic carbon bearing molecules from which planetary systems are made. Through lab spectra we can interpret astronomical observations and identify materials from which planets and perhaps life itself were constructed, and through realistic lab simulations we can understand the growth in chemical complexity as the simple molecules that condense to form cosmic ice grains are photolytically transformed to larger and multifaceted molecules.
The production of prebiotic molecules in the interstellar medium is of little consequence to the origin of, and search for, life unless they can be delivered intact to planets. This requires that they survive the transition from the dense cloud to protostellar nebula and subsequently are incorporated into planetesimals, followed by delivery to a planetary surface (See Figure 1). Meteorites and IDPs bring ~1x106 kg/yr of organics to Earth (Love and Brownlee 1993), and probably provided orders of magnitude more material to the early Earth (Chyba and Sagan 1992). This suggests that this "exogenous" delivery helped to make the early Earth (and other planets) habitable by contributing to our prebiotic chemistry (Oro et al. 1990; Chyba and McDonald 1995; Thomas et al. 1997). This work will provide the laboratory data to needed to interpret telescopic observations of planet forming disks and carrying out experiments relevant for the chemical and physical processes that occur in ices needed to model these disks as outlined in our research section "Disks and the Origins of Planetary Systems."
The early Earth swept up vast quantities of extraterrestrial organic matter, some from the interstellar medium, some from the protoplanetary disk. These exogenous molecules include many of prebiotic importance, including modified polycyclic aromatic hydrocarbons, or PAHs, which contain functional side groups similar to those found in living systems and amino acids (Cronin and Chang 1993). Perhaps exogenous materials did more than simply deliver carbon as a starting ingredient for the primordial soup. Rather, the specific molecules might matter, having properties that were of relevance to the rise of life on Earth. We outline below a program that addresses Objective 1 by exploring all three stages in the sweeping development of chemical complexity prior to the formation of habitable environments: PAH production and distribution, complex molecule production in cosmic ices, and the surface and subsurface distribution of organics on mineral and icy terrain in the Solar System.
Investigation 1.1We will utilize our collection of astrophysically relevant PAH spectra to interpret telescopic observations across deep space. NASA's Spitzer Space Telescope Legacy programs have shown that PAHs are the most common and abundant of the known polyatomic molecules in the Universe (Smith et al. 2007, Draine et al. 2007a,b). Their infrared (IR) signature is associated with hundreds of different objects in our Galaxy, the Milky Way, and in like numbers of other galaxies. This aspect of our program will provide the tools and approach needed to revolutionize our understanding of how this important carbon reservoir is distributed and processed across the Universe.
Investigation 1.2Laboratory studies of ice spectroscopy and chemistry will be conducted to identify and describe the chemistry of the pre-biotic organic compounds within the ices in which they form as well as the modifications of PAHs within those ices in the cold environments in space. This will lead to a better understanding of the origin, distribution, and significance of these molecules in meteorites and on other possible habitable worlds, and will help in the interpretation of potential biomarkers.
We will measure the UV-visible pumped luminescence of a variety of abiotic and biotic materials under conditions that simulate Solar System environments. The purpose is to provide the fundamental data necessary to develop instrumentation to search for and characterize the types of extraterrestrial materials delivered to the surfaces of habitable objects.
Being pioneers in the analysis of mid-IR spectra of interstellar material, we participated in astronomical observations and developed laboratory techniques for comparison, thus we are well suited to study extraterrestrial and prebiotic organic compounds. Likewise, we have identified interstellar ice constituents, established the ubiquity of PAHs in space (Allamandola et al. 1989, 1999), and constrained the nature of the refractory organic component of the diffuse ISM (Sandford et al. 1995; Pendleton and Allamandola, 2002), and are now involved in analysis of Stardust samples of cometary dust (Sandford et al. 2006).
Technical Approach and Methodology
Distribution of Cosmic Chemical Complexity Across the Universe (Investigation 1.1)This investigation will utilize our extensive infrared spectral database to characterize and understand the effects of stellar radiation on the formation and distribution of polycyclic aromatic hydrocarbons (PAHs). As stated above, PAHs are considered to be the most plentiful and widespread class of organic compounds in the universe (Smith et al. 2007, Draine et al. 2007 a,b), with their infrared (IR) signature being associated with hundreds of different objects in our Galaxy, the Milky Way, and in similar numbers of other galaxies. Some of these galaxies are at cosmological distances, having redshifts as high as 5 (Wiklind et al. 2007). This recent and surprising result shows that carbon chemistry was well underway only a few billion years after the Big Bang. Closer to home, the PAH spectral features vary from one object to the next in the Milky Way and spatially from one place to the other in extended objects (Peeters et al. 2004). The overall PAH IR signature is now used to trace star formation (and, by inference, planet formation) in distant galaxies. In closer objects, for which spectroscopic details can be resolved, the spectra reveal details about the specific PAH molecules present and conditions within the emission zones. For example, in a paper by Geers et al. (2006), the variation in PAH emissions for a wide variety of pre-main sequence stars with disks were investigated (see Figure 2) as part of the Spitzer Legacy program "From Molecular Clouds to Planet Forming Disks" (Evans et al. 2003). Using our unique spectral dataset, we can deconvolve the observed variations and thus gain a deeper understanding of the molecular processes occurring in the planet forming disks. The molecular information gained will directly impact the planet forming models discussed in our research section "Disks and the Origins of Planetary Systems".
Understanding the formation and evolution of astronomical PAHs are central to astrobiology because:
• PAHs are the single largest source of accessible prebiotic organic material available in the early Solar System as well as in star and planet forming regions on galactic scales.
• PAHs are more abundant than all other known interstellar polyatomic molecules combined.
• PAHs account for 10 to 30% of cosmic carbon.
• PAH IR features trace the first appearance of carbon in galaxies on cosmological timescales.
• PAHs are found in meteorites and extraterrestrial interplanetary dust particles.
The foundation on which this aspect of our investigation rests is the extensive, one-of-a-kind collection of PAH infrared spectra we have created at NASA Ames. This spectral collection, the product of over 15 years of dedicated research, consists of more than 800 laboratory and computational IR spectra of individual PAHs in their neutral, cation, and anion forms. Prior to 2000, less than 70 of these spectra were applied to the handful of astronomical spectra then available, and only in a piecemeal fashion. Furthermore, the IR spectroscopic properties of PAHs were limited to species containing 50 or fewer carbon atoms. Although complete analysis of the spectral details and variations was not possible because of these limitations, these comparisons formed the foundation of today's interstellar PAH model because they showed PAHs can reproduce the pattern of overall band positions. In 2003, NASA launched the Spitzer Space Telescope. Thanks to the unprecedented sensitivity of this orbiting infrared observatory, hundreds of high quality spectra of different objects in the Milky Way and the Universe are now available and, thanks to our sustained laboratory and theoretical investigations, our spectral collection has expanded tenfold and includes species containing over 100 carbon atoms. Thus, we are uniquely poised to interpret the Spitzer spectra in ways never before possible, nor possible elsewhere.
Here we will use our spectral collection to analyze the details in the IR emission spectra from different groups of objects that trace the evolution of molecular carbon through space and time. The objects chosen and information deduced will be guided by astrobiology, not astronomy, with the intent to understand the distribution of carbon on the grand scale. One grouping involves galaxies. Distant galaxies as well as galaxies close to the Milky Way will be analyzed grouping them as a function of galactic classification by energy (i.e., Ultra luminous IR, star forming, Seyfert, etc.), morphology (i.e., spiral, elliptical, etc.) and properties such as metallicity (the ratio of heavy elements to hydrogen) and hardness of the UV radiation field.
Another grouping involves different objects in the Milky Way, which span stellar lifetimes. We will trace the stepwise changes of the PAH population from where they are formed in circumstellar shells, when stars are at the end of their life cycle, through all intermediate stages leading to their incorporation into star and planet forming nebula. This analysis will provide the observational details about the PAHs that are raining in on planet forming disks. This information will then be incorporated into the planet forming models discussed in our research section "Disks and the Origins of Planetary Systems".
We can perform detailed spectral analyses of these individual objects because they are closer and brighter than the galaxies. An example of a close up view of the PAH emission in a galactic object is shown in Figure 3.
This will provide information about PAH sizes, geometries, charge state, nitrogen incorporation, and extent of hydrogenation in each type of object and, for extended objects, this same information as a function of local radiation field and density. Specifically, we will characterize the PAH population in several late type stars (where PAHs are formed), in the planetary nebula phase (where PAHs becomes exposed to highly energetic radiation and are injected into the diffuse interstellar medium, ISM), in the diffuse ISM (where they are modified by the ambient radiation field), at the edges of dark molecular clouds (where ISM densities increase), and on the surfaces of planet forming disks deep within molecular clouds (where the radiation from the forming star excites the PAHs raining onto the surface of these disks).
In order to perform these analyses and derive maximum benefit from our PAH spectral collection, the complete set of experimental and theoretical spectra for each PAH (charge state, band frequencies and intensities, molecular formula, symmetry, and structure) must be put in a uniform, machine-readable format. We have started to develop algorithms which allow subsets of the PAHs to be selected according to characteristics such as charge, number of C, N, and H nuclei, symmetry, molecular weight, etc. These algorithms allow the user to combine the spectra of the selected PAHs according to user-selected weights into a 'co-added' PAH spectrum to compare with observational spectra. This permits one to narrow down the key characteristics of the astronomical PAH population. This work is also underway. Figure 4 shows a co-add of the spectra of a subset of large PAHs in the database with 78 to 130 carbon atoms, compared to the average spectra of small PAHs previously available and the spectra of the objects IRAS 23133+6050 and the Red Rectangle measured with the ISO satellite. The close agreement between the summed spectra from our database with the observations shows that we are poised, as never before, to truly analyze and understand the widespread astronomical PAH emission spectra. Figure 5 shows that PAH anions are an important part of the astronomical PAH population.
We are also creating the tools to allow one to import an astronomical spectrum, say from the Spitzer archive, and fit the spectrum using the PAH spectral database. Once the troubleshooting phase is over and all of these have been developed into robust and reliable tools, we will make the PAH spectral database and analytical tools available to the public (See Figure 6).
In addition to analyzing the mid-IR astronomical PAH features as described above, we plan to work on two other important PAH related activities. Two new programs, NASA's SOFIA and the NASA/ESA Herschel Satellite will pioneer the Far Infrared (FIR), the region between roughly 20 to 200 µm. These missions will provide the first complete FIR spectra of many of the same PAH objects explored with Spitzer. Since PAHs also have strong FIR bands, the astronomical objects exhibiting the mid-IR PAH bands will also have FIR PAH bands. In anticipation of the challenges that interpreting these new astronomical data will bring, we have pushed our laboratory and theoretical PAH spectroscopy studies into the Far-IR. In Figure 7, we show the FAR-IR spectra of the three PAHs: coronene, ovalene, and dicoronylene. Because the PAH FIR bands originate in the motions in which the entire molecule vibrates like a taught drumhead, they reflect the individual PAH's size and shape. As a result, unlike PAH mid-IR transitions, which pile up at similar frequencies (e.g. Figure 4), as shown in Figure 7, PAH FIR spectra are quite different from one PAH to the next. Thus, part of this investigation will be to apply our growing PAH FIR spectral database to the pioneering data obtained by these two NASA initiatives. Because PAHs are the largest source of accessible, prebiotic organic material available in early solar systems as well as planet forming regions the astrobiological implications of these observations are immense. These FIR spectra will eventually become part of the PAH IR spectral database. Apart from the obvious direct support for NASA's Spitzer Space Telescope, SOFIA, and Hershel programs, the database and web tools will play an important role in analyzing the information gleaned by the IR instruments on NASA's upcoming Flagship Observatory, the James Webb Space Telescope (JWST).
This investigation also includes a new experimental component focused on measuring the spectroscopic properties of neutral and charged PAH clusters. Observational evidence is mounting that PAH clusters are responsible for the reddish emission that is spatially associated with the PAHs responsible for the astronomical mid-IR emission bands (Rhee et al. 2007, Berne et al. 2008). This evidence is known as the 'Extended Red Emission' (ERE), a dull red glow extending from about 540 to 900 nm, which is associated with a wide variety of different interstellar environments. The ERE is a highly efficient photoluminescent process (Gordon et al. 1998, Szomoru and Guhathakurta, 1998). High spatial resolution Hubble images of the ERE in the reflection nebula NGC 7023 (a well known PAH IR emission object), and a thorough review of the ERE observations, has been given by Witt et al. (2006). Rhee et al (2007) have suggested that the peculiar emission spectra and unique properties of charged PAH dimers satisfy all the observational constraints, a suggestion that has been supported by recent observations made by Berne et al. (2008).
The picture that is emerging from these and other observations is one in which the PAHs that produce the mid-IR bands are at the molecular end of a much larger carbon reservoir. This reservoir is thought to have three components: clusters made up of two to a few PAHs, larger groupings of aromatic moieties cross-linked with aliphatic chains and, finally, macroscopic amorphous carbon particles. The clusters are of particular relevance here as, unlike the larger particles and cross-linked species, they have characteristic spectroscopic features making them accessible to observational study.
Our group is involved in two aspects of ERE studies. First, our laboratory work is dedicated to measuring the spectra of individual, mass selected charged PAH clusters spanning the UV through the IR. As with the case for PAHs before, there are no spectra of isolated, charged PAH clusters available with which to interpret observations. Our experimental program will provide this critical information. Several people in our group are Co-Is on a small satellite proposal that falls under the Mission of Opportunity category of NASA's Small Explorer Program. Called the 'ERE Mapper', this spacecraft will image the ERE across 50% of the sky. While this mission has not yet been formally approved, it illustrates this team's focus on tracing the distribution and forms of molecular carbon across the Galaxy and through deep space.
Development of Cosmic Chemical Complexity (Investigation 1.2)While Investigation 1.1 studies the formation and distribution of PAHs, other questions remain. How are non-aromatic, organic molecules formed? What cosmic processes modify the ever-present PAHs? Investigation 1.2 addresses these issues by examining the chemical synthesis as well as the chemical modifications, which occur during the irradiation of ices in interstellar, nebular and planetary systems.
New stars and planetary systems form in dense interstellar molecular clouds; these clouds are made by the concentration of material from the diffuse interstellar medium. The concentration of material in dense clouds is sufficiently large that they produce large opacities. The screening of diffuse starlight from the insides of these clouds allows their interiors to cool to temperatures as low as 10 K. Despite the low temperatures in these clouds, significant chemistry is largely driven by energy associated with cosmic rays and traces of high-energy photons. These can ionize individual gas phase molecules and lead to a complex network of gas phase ion-molecule reactions (e.g., Herbst, 1987; Millar et al., 1995). However, under typical cold dense cloud conditions, most molecular species are efficiently condensed out onto grains. This process in itself can lead to additional gas-grain chemistry (Charnley et al., 1992; Hasegawa et al., 1992; Tielens, 1997). Additional chemistry can be driven by irradiation of the resulting ice mantles by energetic particles and photons (for example, Allamandola et al., 1987; Bernstein et al., 1995; Gerakines et al., 2000). Laboratory studies, many of them done in the Astrochemistry Laboratory at Ames, of the radiation chemistry of ices having interstellar compositions have demonstrated that such processes can yield a wide variety of complex organic compounds, many of which are of astrobiological interest. For example, the radiation processing of typical interstellar ices is expected to produce amino acids (e.g., Bernstein et al., 2002) and their precursors (e.g., Bernstein et al. 1995; Cottin et al., 2001), and amphiphiles, molecules that spontaneously self-organize in liquid water to form vesicles (Dworkin et al., 2001).
The non-aromatic population of materials produced by the irradiation of icy grain mantles has attained additional importance in light of the organics that have been returned from Comet 81P/Wild 2 by the Stardust mission and studied in terrestrial laboratories. The population of organic materials in the returned samples was found to be very complex. Some of the returned organics looked qualitatively similar to the largely insoluble, very aromatic-rich organics seen in primitive meteorites (Sandford et al., 2006). However, a relatively labile, largely non-aromatic component, not seen in previous primitive extraterrestrial samples was also seen in many particles (Sandford et al., 2006; Cody et al., 2008) (see Figure 8). The exact nature of this additional organic component is not yet fully understood, but its chemical bonding is suggestive of the sorts of materials seen in our laboratory ice simulations. For example, some of the material looks similar to what is produced by complex formaldehyde polymerization in the presence of other molecular species (Schutte et al., 1993a,b).
As mentioned in the previous section, PAHs are major carriers of cosmic carbon and, because of their stability, they are found in nearly all astrophysical environments. Thus, PAHs are expected to be a major component of the materials from which dense molecular clouds form, and in cold dense clouds would be expected to be efficiently condensed out of the gas phase into icy grain mantles. However, PAHs are more difficult to detect in dense clouds than they are in more energetic environments. This is because gas phase PAHs can be 'lit up' by the absorption of UV photons resulting in the characteristic infrared emissions, whereas PAHs frozen onto grains cannot. PAHs frozen onto grains can only be detected in dense clouds through the relatively weak absorptions bands they produce in the spectra of background IR sources. However, despite these difficulties, a number of absorption bands attributed to cold PAHs in dense clouds have been reported (Smith et al., 1989; Sellgren et al. 1995; Brooke et al., 1999; Chiar et al. 2000; Bregman et al., 2000).
Laboratory IR spectra of neutral PAHs and their ions have received some attention by our group (Sandford et al., 2004; Bernstein et al., 2005a,b, 2007) and these studies enabled more detailed interpretation of the relevant telescopic data. However, the chemical consequences of PAHs and related species being present in interstellar ices have yet to be fully explored. Limited work done at the Astrochemistry Laboratory at Ames (some done in collaboration with the Goddard laboratory group of Marla Moore) has demonstrated that the UV photo-processing of PAHs in interstellar ices can yield a wide variety of functionalized PAHs (Bernstein et al., 2002, 2003). In H2O-rich ices the principle reactions are the additional of peripheral H atoms (to make Hn-PAHs; Bernstein et al., 1999) and the addition of peripheral O atoms to make aromatic alcohols, ethers, and ketones (Bernstein et al., 1999, 2001). Many of these species are of astrobiological interest. For example, aromatic ketones (quinones) are common molecules found in living systems on Earth, where they play a variety of important biochemical roles. It should be noted that these studies have been almost entirely restricted to 'classical' PAHs consisting only of C and H in six-membered rings. Aromatic molecules containing skeletal heteroatoms, side groups, and five-membered rings have yet to be studied.
Radiation processing of PAHs in interstellar and protostellar environments. As part of our work for the next cycle of the NAI, we propose to carry out a series of comprehensive laboratory studies designed to better constrain the presence of PAHs and related species in cold, dense interstellar/ protostellar environments and better understand the astrochemical and astrobiological consequences of their radiation processing. The information gained from this study will then be fed into the planet forming models discussed in our research section "Disks and the Origins of Planetary Systems".
The four specific tasks for this effort are:
Task 1.2A. The photochemical evolution of interstellar and Solar System ices containing PAHs – Only limited laboratory work has been done on the photochemistry of PAHs in ices. This work has largely focused on establishing the basic nature of the addition of chemical groups to peripheral sites on the PAHs. The previous work has been largely limited to the study of a few specific PAHs in H2O-rich ices and pure ices of a few other compounds (NH3, CH3OH, CO2, etc.) (Bernstein et al., 2001, 2002, 2003). These ice compositions were kept simple to facilitate tracing individual addition reactions. Very little work has been done, however, to study the chemistry that occurs when PAHs are irradiated in astrophysically relevant ices, i.e., in ices that contain H2O, NH3, CH3OH, CO2, CO, etc. in mixtures having relative abundances typical of dense interstellar clouds. As one of our tasks, we propose to study the photochemistry of a wider variety of PAHs in more astrophysically relevant ice mixtures.
In addition, the previous work has been devoted almost exclusively to classical PAHs, i.e., aromatic systems containing only C and H in six membered rings. However, astronomical spectra indicate that a wider variety of aromatic species probably exist in space (Allamandola et al., 1989). These include PAH structures with extra H on some of their peripheral rings (Bernstein et al., 1996; Sloan et al., 1997) and aromatic species that contain nitrogen in their ring structures (Peeters et al., 2002; Mattioda et al., 2003; Hudgins et al. 2005). Some of these nitrogenated species are of particular interest because they are found in meteorites (see Pizzarello et al., 2006 for a recent review), and because aromatic, N-containing heterocycles play key roles in terrestrial biochemistry. Thus, as another of our research tasks, we plan to study the photochemistry of N-containing aromatic molecules in ices. As a preliminary test of the viability of this work, we have already done a limited number of experiments on the photolysis of pyrimidine (C4H4N2) in H2O ices. This work has already demonstrated that new species are formed, including, tentative evidence for the production of uracil (see Figure 9). This raises the exciting possibility that such ice photochemistry could yield the pyrimidine nucleobases, cytosine, thymine, and uracil.
Task 1.2B. Connecting the results from laboratory ice simulations to comet observations – One of the exciting discoveries resulting from studies of the Wild 2 cometary samples returned by the Stardust mission is the presence of very heterogeneous, non-aromatic, 'primitive' organics whose composition and structure are suggestive of the products seen in the organic residues made during ice irradiation experiments (Sandford et al., 2006). For example, one component of the Wild 2 organics looks much like the materials produced when H2CO polymerization reactions occur in ices of mixed composition (Figure 5). This component of the Wild 2 organic population is unique and has not been seen in any abundance in IDPs and primitive meteorites. We plan to explore further the possible connection of these cometary organics with ice photochemistry by (i) studying our ice residues using many of the same techniques used to study the Stardust samples, and (ii) identifying key products and intermediates of this chemistry that might be directly identified in cometary comae using sub-mm and radio observations. Scott Sandford led the Stardust Organics Preliminary Examination Team and is in an ideal position to make detailed comparisons of our lab residues and Stardust samples. He is already arranging collaborations with other members of the Stardust Organics Preliminary Examination Team to analyze residues, made in our laboratory, with a host of analytical techniques (XANES, STXM, IR, etc.). These activities will be highly leveraged by other resources. Stefanie Milam, postdoctoral research associate, will identify products and intermediates that might be amenable to telescopic detection. Milam's dissertation addressed the sub-mm detection of molecules in cometary comae and the interstellar medium.
Finally, the results of this investigation will be instrumental in guiding a new NASA mission. The Comet Coma Rendezvous Sample Return Mission (CCRSR), for which Scott Sandford serves as P.I., has been selected as a concept study for a possible Discovery or Scout mission in the near future. This mission represents the next step in our knowledge of comets and is focused on learning more about cometary organics. The laboratory work discussed here would be invaluable in the final design of the CCRSR mission.
Task 1.2C. Measuring the spectroscopic properties of PAHs in ices from the UV through the IR - Members of the Astrochemistry Laboratory are world experts at taking the spectra of PAHs and related compounds. This includes UV, visible, and IR spectra taken of PAHs in the gas phase, PAHs isolated in inert matrices (like Ne and Ar), and PAHs frozen into a side variety of mixed molecular ice matrices. The combined work in these databases has played an important role in the identification of PAHs in space and allows the spectra of these molecules in space to be used to place constraints on the astrophysical conditions in which these molecules are found. We plan to continue this work, with an emphasis on obtaining spectra that lay the groundwork for the interpretation of measurements that will one day be taken by remote and in situ landers and rover sent to various bodies in the Solar System, one aspect of which is described in Investigation 1.3.
Task 1.2D. Measuring the condensation/ sublimation properties of mixed molecular ices – Considerable work has been done in the Astrochemistry Laboratory to measure the surface binding energies and sublimation/ condensation behaviors of mixed molecular ices (Sandford and Allamandola, 1990, 1993a,b, Sandford et al., 2008). These studies show that the true sublimation/ condensation behavior of mixed molecular ices can be quite complex. The behavior of ices in the solar nebula must have been considerably more complex than described by the simple concept of a 'snowline' outside of which volatiles all existed in ices and inside of which they were all found in the gas phase. The data and knowledge about these processes will then be available for incorporation into the solar nebula models discussed in our research section "Disks and the Origins of Planetary Systems".
Chemical Complexity in the Solar System (Investigation 1.3)As discussed above, PAHs, as well as other smaller organic molecules are formed in the interstellar medium and become incorporated into ice on dust grains as well as in comets, planetesimials, where their chemical complexity increases resulting in the formation of astrobiologically significant molecules. In turn, tens of tons of this extraterrestrial organic and mineral matter fall on the Earth, Moon and planets as interplanetary dust particles (IDPs) and meteorites every few months (Love and Brownlee 1993). These extraterrestrial organics include many polycyclic aromatic hydrocarbons (PAHs) as well as complex organic refractory materials (Allamandola et al. 1987; Clemmett et al. 1993). Such materials, containing prebiotic molecules, may also harbor biogenic molecules and perhaps even biomarkers (Dworkin et al. 2001; Bernstein et al. 2002). Much of this material is highly luminescent and as such is particularly sensitive to probing by the radiation induced luminescent (RIL) techniques described.
Although we know that these complex organic molecules exist on planetary bodies in our solar system, such as Mars, they are not easy to find. One reason for this are the harsh conditions on Mar's surface which oxidizes and destroys organic molecules exposed on the surface for a period of time, another reason is the large area that must be searched in order to identify any signs of chemical complexity. What is needed in order to search the solar system for complex organic molecules is a method to screen quickly and effectively large areas of a planetary surface, as well as the subsurface.
The old saying, "One person's trash is another person's treasure" clearly applies to this investigation. During the past 15 years there has been a search for complex organic molecules on Earth, but as hazardous wastes (see Kram et al. 2001 for a short review). PAHs, as well as other large organic molecules, are found in petroleum products and are also formed during the combustion of organic compounds. Given their similarities to biological molecules, these compounds are known or suspected carcinogens (Liberman et al. 1998). Thus the environmental field has been actively searching for a rapid and efficient means to identify these compounds in our earthly environment. The most efficient screening tools developed to date rely on the luminescence of these organic materials when they are exposed to UV light (i.e. radiation induced luminescence or RIL). In addition to aiding in the identification of organic pollutants, UV-pumped luminescence has been used by musea to showcase mineral collections, forensic scientists to classify (bio) evidence, and electrical engineers to develop miniature electro-optical devices. This technology is now sufficiently mature to enable astrobiologically driven applications of UV pumped luminescence to space exploration.
However, as quickly discovered in the environmental field, the luminescence of a wide variety of materials can lead to misidentifications (false positives) when one tries to identify particular molecular compounds. What is needed to avoid such errors is a database of the short and long-lived luminescence spectra of a wide variety of organic molecules, biological colonies, minerals and rock matrices relevant to astrobiological concerns. Therefore we plan to measure the radiation induced luminescence spectra, and the time dependence of these spectra, of a wide variety of organic, biological and mineral samples relevant to the Solar System. These measurements will be made under simulated extraterrestrial conditions.
The main objective of this work is to create a database of radiation induced luminescence spectra of many astrobiologically relevant materials. This collection of spectra will lay the foundation to develop new, low-weight, compact instruments that will ultimately be capable of remotely surveying the distribution and nature of organic and inorganic material over hundreds of square meters surrounding a lander or rover. When placed on a spacecraft, such instruments would be capable of surveying (mapping) the distribution of organic and mineral matter on or slightly below the surface of normal and icy terrain over areas of at least several hundred square meters. These devices can also be coupled with abrasion, penetration and excavation tools extending measurement capabilities into subsurface areas at the site. Indeed, such devices are already widely used to conduct environmental field investigations by both commercial vendors and the military (Liberman 1998, McMahom 2005). RIL can also provide valuable sample vetting capability for future sample return missions.
With the added advantages of time resolution, one has the core working principle of a very sensitive new probe that can discriminate between fluorescence and phosphorescence. Taken together, the time dependence, the excitation wavelengths and the emission spectrum, put very tight constraints on the nature of the luminescent materials in a particular location. In favorable cases where the complexity of the materials is not too high, specific molecules can be identified.
Early Raman studies of bona-fide IDPs showed that many exhibit strong, broad, red luminescence, even when pumped by the Argon ion (Ar+) laser line at 5145 Å (Allamandola, Sandford, Wopenka, 1987) and many of the recently returned Stardust comet dust samples are reported to be luminescent (Sandford et al. 2006). Furthermore, as shown in Figure 10, the complex organic residues produced by the photolysis of simple interstellar and precometary ice analogs under realistic, simulated interstellar conditions are also highly luminescent. PAHs, the complex organic molecules identified in meteorites and IDPs, are well known to be highly luminescent. PAH clusters are also very luminescent and may be responsible for the extended red emission (ERE), which is widespread throughout the galaxy (Rhee, et al. 2007).
Figure 11 displays the auto-fluorescence emission spectra of a diverse set of microprobes in addition to that of the amino acid tryptophan. The emissions are mainly due to protein fluorescence, which is dominated by the tryptophan.
Examples of Lander/Rover Deployable RIL Instruments. "Point and Shoot" RIL Surveyor - A very simple approach would be to include a UV flash lamp on the rover or lander. As illustrated in Figure 12, the cameras could take 'UV flash' photographs of the nearby surroundings during periods of darkness. These photographs could then be overlayed with the photos taken when the area is sunlit. Depending on the strength of the lamp, areas of several hundred to several thousand square meters surrounding the spacecraft could be surveyed for luminescent materials rapidly in this way. Such a RIL survey can greatly guide site selection for those instruments that perform more in-depth analyses. More information could also be obtained by simply filtering the flash lamp. For example, flashes of light from 100 to 200, 200 to 300, and 300 to 400 nm would induce a different response from most materials and this, in turn, reveals much about the bonding nature of emitting species.
RIF Capabilities on Penetrometers, Abrasion, and Excavation Tools - During the 1990's the Tri-Services (Army, Navy and Air Forces), under the SCAPS (Site Characterization and Penetrometer System) program combined two well-established techniques, cone penetrometer and fluorescence, into a single instrument that could produce in-situ soil type and engineering properties in conjunction with the detection of subsurface organic compounds. As the penetrometer probe is advanced it can provide continuous detailed delineation of subsurface data/soils. The probe also carries a laser-induced fluorescence instrument, which, as the probe advances, detects hydrocarbons in real time from their fluorescent response to excitation by the excitation light hitting the soil. This fluorescent signal is collected by the probe and returned to a spectrometer. This data is then combined to produce a three dimensional subsurface picture of the soil type and organic distribution (Kram et al. 2001, Davis et al. 1997). Such an instrument would be beneficial in determining if microbes, or anything other fluorescent organics, are associated with a particular sub-surface soil layer on Mars or any other planetary body. Several commercial companies now use this technology to identify hazardous organic wastes in the subsurface soils. Andrew Mattioda has experience with the SCAPS program and its technologies. Because this is a mature technology it will only need to be modified and qualified for use in space.
Samples to be analyzed. Measurements of the radiation induced luminescence (RIL) spectra and the time dependence of these luminescence spectra for the materials listed below will be carried out using a tunable broadband UV excitation filtered to 10 to 20 nm widths through a monochromator. We will also use short-pulse radiation (10 ns) from a Nd-YAG laser to provide monochromatic excitation at 266 and 355 nm and carry out time dependent studies.
The following materials are all available at Ames in the Exobiology Laboratory and the Astrochemistry Laboratory:
• Astrobiologically significant PAHs and PAH clusters available in the Astrochemistry Laboratory
• Organic residues produced in the cosmic ice analog irradiation experiments
• Samples of different carbonaceous chondrites
• Cyclic amino acids, such as tyrosine and tryptophan
• Lipids common to microbial membranes available in the Exobiology Biomarker Laboratory
• Freeze-dried cultures of various archaea (thermophilic methanogens and sulfur-oxidizers, and extreme halophiles)
• Bacteria (purple and green anoxygenic photosynthetic bacteria, cyanobacteria, methanotrophs, manganese oxidizers)
• Endoevaporitic microbial communities found within gypsum crusts
• Soil, rock and mineralogical samples characteristic of the Martian environment
Once collected, the spectra will be assembled in to a searchable spectral database.
AMES TEAM RESEARCH
Early Habitable Environments and the Evolution of Complexity
---David J. Des Marais, Principal Investigator
Cosmic Distribution of Chemical Complexity
---Lou Allamandola, Lead Co-Investigator
Disks and the Origins of Planetary Systems
---Sanford Davis, Lead Co-Investigator
Mineralogical Traces of Early Habitable Environments
---Tori Hoehler, Lead Co-Investigator
Origins of Functional Proteins and the Early Evolution of Metabolism
---Andrew Pohorille, Lead Co-Investigator