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2014 Annual Science Report

NASA Ames Research Center Reporting  |  SEP 2013 – DEC 2014

Cosmic Distribution of Chemical Complexity

Project Summary

This project explores the connections between chemistry in space and the origin of life. It is comprised of three tightly interwoven tasks. We track the formation and evolution of chemical complexity in space starting with simple carbon-rich molecules such as formaldehyde and acetylene. We then move on to more complex species including amino acids, nucleic acids and polycyclic aromatic hydrocarbons. The work focuses on carbon-rich species that are interesting from a biogenic perspective and on understanding their possible roles in the origin of life on habitable worlds. We do this by measuring the spectra and chemistry of analog materials in the laboratory, by remote sensing with small spacecraft, and by analysis of extraterrestrial samples returned by spacecraft or that fall to Earth as meteorites. We then use these results to interpret astronomical observations made with ground-based and orbiting telescopes.

4 Institutions
3 Teams
10 Publications
1 Field Site
Field Sites

Project Progress

Over the course of the past year the Astrochemistry Laboratory has continued to study the chemistry that happens in space and the possibility that the organic products of this chemistry may be incorporated into forming planets and play a role in any subsequent origin of life. This work has largely centered around the study of (i) radiation-induced chemistry that happens in astrophysical mixed-molecular ices of the types found in interstellar clouds and protostellar disks, and (ii) polycyclic aromatic hydrocarbons (PAHs), which are one of the main forms of molecular carbon in the universe.

Over the past year, the study of ice irradiation chemistry has focused on two, largely unrelated, issues. First, we have carried out numerous simulations to study the radiation-induced chemistry that can occur in Pluto ice analogs when they are dosed with UV photons or electrons. Ices on Pluto are dominated by N2, but also contain small amounts of CO, CO2, and CH4. This work has demonstrated that irradiation of these ices leads to the formation of more complex organics that contain carboxylic acids, nitriles, and urea (Materese et al. 2014). These results are of interest in their own right, but are also of interest because the infrared spectra obtained form these samples will be used to help interpret data returned from Pluto by the New Horizons spacecraft when it flies by Pluto later this year.

The other main task associated with ices has been the completion of our work on studying the formation of the pyrimidine-based nucleobases and other heterocyclic molecules. We have now demonstrated that all three of the pyrimidine-based nucleobases – uracil, cytosine, and thymine – can be made when pyrimidine is irradiation in mixed-molecular ices.

Figure 1.
Figure 1 - Laboratory experiments have demonstrated that the photolysis of pyrimidine in mixed molecular ices can result in the formation of all three of the pyrimidine-based nucleobases - uracil, cytosine, and thymine.

Interestingly, uracil and cytosine are fairly easy to make, while the efficiency of production of thymine is much smaller. It has been posited that the origin and evolution of life may have passed through an RNA World stage that preceded the use of DNA. Our results may provide an explanation of why this might have occurred – uracil may have been much more abundantly available in the early stages of Earth’s history than thymine. In related work, we have also recently shown that ice photochemistry can result in the O or N substitution of the skeletal C atoms in PAHs, i.e., PAHs irradiated in ices can be converted into O- and N-heterocycles (Materese et al. 2015). This process may be responsible for making molecules like pyrimidine from which the nucleobases are formed.

This past year’s work on PAHs has focused on two areas, (i) the interactions of these molecules with water ice as well as their photo-induced chemical reactions and (ii) interpreting Spitzer observations using the PAH database. Regarding (i), we recently published papers describing the impact PAH:water ice concentrations have on the resulting photo-chemistry (Cuylle et al. 2014, Cook et al. 2014). We have also been exploring the interaction of aromatic molecules, such as PAHs, with mineral substrates as well as any UV induced catalytic effects. The initial portion of this work was published in Langmuir ( Elesaessar et al. 2014).

Figure 2. Our publication on OREOCube was awarded the cover of the prestigious American Chemical Society’s Langmuir. Cover image by Gabriel Alvarado-Marín, SETI Institute (ISS, Earth, Sun photo credit: NASA). Molecular graphics by Andreas Elsaesser (visualization software: VESTA and Jmol). The artwork illustrates the effect of solar radiation on the anthraquinone anthrarufin while in contact with a hematite substrate and in a CO2 atmosphere. This and similar experiments will be performed outside the International Space Station in a project called OREOcube: Organics Exposure in Orbit. The photostability of biogenic molecules when in contact with inorganic surfaces is of great interest for space exploration and the search for life on other planets, such as Mars

This article was awarded the cover of the prestigious journal and was selected as the ACS (American Chemical Society) Editor’s Choice article for the month in which it was published. Concerning (ii), we released a significantly updated version of the NASA Ames PAH IR Spectroscopic Database that includes very large PAHs and the ability for users to import their astronomical spectra and perform least squares fitting analysis (Boersma et al. 2014a), and quantified the traditional qualitative proxy used to evaluate PAH charge distribution in the various astronomical emitting regions (Boersma et al. 2014b).

Figure 3.
Figure 3. The 11.2 versus 8.6 μm PAH band strengths plot from Boersma et al. (2014b) calibrated to PAH ionization fraction (fi). PAH ionization fraction is quantitatively and directly tied to the radiation field, temperature and electron density in the emitting region. This is the first time the PAH features are used as a quantitative probe of conditions in the emitting zones.