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Project Progress

Theme I Progress: We have continued observational programs designed to explore the chemical composition of comets and establishing their potential for delivering pre-biotic organic materials and water to the young Earth and other planets. State of the art, international facilities are being employed to conduct multiwavelength, simultaneous, studies of comets in order to gain more accurate abundances, distributions, temperatures, and other physical parameters of various cometary species. This effort is accomplished with collaborations from the National Radio Astronomy Observatory (A. Remijan), the Joint Astronomy Center (I. Coulson), ASIAA (group of Prof. Y-J. Kuan), Observatoire de Paris (D. Bockelee-Morvan & N. Biver), and Caltech (M. Drahaus) utilizing the Arizona Radio Observatory’s 12m and Submillimeter Telescopes, the JCMT, NRAO GBT 100m, Herschel Space Observatory, SOFIA, APEX, ALMA, and the Onsala Space Observatory.

Data from cometary apparitions in 2010-2012 are currently being analyzed and prepared for publication. Comet C/2009 R1 (McNaught), a long-period comet, came a mere 0.4 AU from Earth in June 2010. The Jupiter family comet 103P/Hartley 2, approached < 0.1AU from Earth, was the rendezvous target for NASA’s Deep Impact Extended Investigation (the DIXI component of the EPOXI space mission), was observed, collectively, from 12 October 2010 to 5 November 2010. Two interns, Lillian Haynes and Maria Stenborg, were selected through the Astrobiology program to fully analyze and help interpret the results of McNaught under the supervision of Milam and DiSanti. These data suggest a normal organic abundance with respect to H₂O and a publication is in preparation to be submitted to ApJ this fall, Figure 1. The results of Hartley 2 are also fully analyzed from 4 facilities with nearly simultaneous observations. Two publications are in preparation and will be finalized during the 10 day visit (Nov. 2012) to Taipei, Taiwan on behalf of the National Taiwan Normal University and collaborator Y.-J. Kuan. A significant effort was in place to observe comet C/2010 X1 (Elenin), though the untimely disintegration of this target only provided us with upper limits on a few simple species. 45P/Honda-Mrkos-Pajdusakova is a JFC that reached only 0.06AU from Earth. Sadly, this close encounter did not produce the anticipated molecular flux, though a few detections were made and have been published (Coulson et al. 2011, IAU Circ, 9237). More recently, observations have been conducted towards comet C/2009 P1 (Garradd), verifying significant activity at large heliocentric distances. This provoked a long term monitoring campaign for this target at multiple facilities including the JCMT, ARO 12m and SMT, GBT, and in collaboration with high resolution infrared measurements being conducted by Mumma, DiSanti, Villanueva, and Bonev. Observations of this target, and its early activity, prompted the activation of the Herschel OT2 proposal and additional observations of H₂O⁺ at optical wavelengths (PI: Cordiner) with NOT. The molecular abundances in these objects will be compared to those observed in other comets, supporting a long-term effort of building a comet taxonomy based on composition. Previous work (Mumma et al. 2003; DiSanti & Mumma 2008) has revealed a range of abundances of parent species (from “organics-poor” to “organics-rich”) with respect to water among comets, however the statistics are still poorly constrained and interpretations of the observed compositional diversity are uncertain. Additionally, other cosmogonic quantities, such as the ortho:para ratio and isotope ratios, were measured to probe the origin of these cometary organics. Results from these recent apparitions have been presented at the Asteroids, Comets, and Meterors Meeting and AbSciCon2012 and will be presented at the DPS 2012 meeting in October.

Cordiner and Charnley have incorporated a new anion chemistry into their coma chemistry model and performed a study aimed at comparing the predicted anion molecules and their neutral counterparts with the Giotto data obtained during the comet Halley campaign. This work was reported at the ACM meeting in Japan and a manuscript has been submitted to Meteoritics and Planetary Science.

We have been selected to guide the development of new technology to be implemented at the NRAO GBT focused on cometary science. This proposal has been submitted and is under review.

Milam and Mumma (with A. Remijan, G. Villanueva, and M. Cordiner) have submitted a science case to observe Garradd with ALMA and been selected for the Science Verification project for ephemeris objects. These observations are expected to be conducted in the near future.

Theme II Progress: Isotopologues provide important tests of processes and models pertaining to the origin and evolution of organics in Planetary Systems. A significant theoretical effort has been made to understand how low-temperature chemistry, either in the outer regions of nebula or in the presolar dense core, could fractionate organic molecules, and to so identify possible observational and experimental signatures (Charnley & Rodgers 2008, 2009; Rodgers & Charnley 2008).

Observational programs designed to test current theories of the origins of isotopically fractionated meteorite (and cometary) materials have been recently completed. New chemical models by our group have suggested that in the cold dense cores of star forming regions, significant isotope enrichment can occur for nitrogen, under the influence of ortho and para H₂, and even vary between molecular species (Wirstrom et al. 2012).

Observations have been conducted at millimeter and submillimeter wavelengths of HCN and HNC isotopologues, employing various facilities in order to both spatially and spectrally, resolve emission from these cores. Spectra were obtained at high resolution (0.08 km/s) in order to resolve dynamic properties of each source as well as to resolve hyperfine structure present in certain isotopologues. Most of the previous work on nitrogen and carbon isotopes in dense cores has applied a double isotope method to their analysis to overcome large opacity effects, requiring assumptions for one of the isotopes as either a measurement of a different species known to trace different regions (i.e. ¹²CO and ¹³CO) or using the local ISM values. These methodologies may provide false abundances for various species and can introduce errors to further analysis and applications to chemical models. Thus, we directly measured all isotopologues for a given species and employed a direct analytical method (when possible) for determining abundances and thus the true isotope ratios. Multiple transitions were observed to further constrain the abundances and ratios. Our data suggests significant fractionation in the nitrile species compared to other N-bearing molecules towards the same objects (Milam and Charnley, submitted). These findings support our latest model and are guiding further observations to test the current theory. This work has been presented at the annual LPSC meeting, the University of Maryland, and the Carnegie Institute of Washington.

Charnley co-authored a paper with members of the GCA Organics Analysis Laboratory led by Jamie Elsila (Elsila et al. 2012). This article reported measurements of carbon, nitrogen and hydrogen isotopic fractionation in meteoritic amino acids and attempted to use these data to identify probable formation mechanisms and place of origin.

In regards to the origins of primitive materials, a workshop at the Lorentz Center in Leiden, NL was organized for 5-9 December 2011 with partial support by the Goddard Center for Astrobiology (see Figure 2). This very interdisciplinary workshop, with a key science goal, was highly successful and promoted new insights for every field. The participants seemed inspired and some have found new collaborations in other disciplines for observing proposals, visiting speakers, sharing data sets, and advisement. Many of them also commented on the highly successful meeting including organization by the Lorentz Center and its staff, the scientific quality and interdisciplinary nature of the workshop, and look forward to future meetings on this topic.

Additionally, a session at the Astrobiology Science Conference was organized by Milam (with E. Wirstrom and A. Remijan) titled, “Extraterrestial Biomolecules in the New Age of Astronomical Instrumentation”. This session demonstrated the capabilities of new technologies in radio astronomy and how they can help decipher the presence, formation, and distribution of complex organics throughout the interstellar medium and how they may be distributed to primitive materials. We had 7 talks and 15 posters.

Cordiner and Charnley (2012) continued their work on the interstellar chemistry of organic anions with a theoretical study of dense cloud chemistry in which the anion component interacts with the population of neutral and negatively-charged `classical’ dust grains. They found that better agreement with observations could be had compared to previous dust-free models. This work also predicted that large oxygen-containing carbon chains – C_nO (n=4-7) – could be present in dark clouds, however a subsequent search in TMC-1 with the GBT failed to detect any of the expected emission. A paper reporting these upper limits, as well as those for some other previously-undetected organic molecules, is in preparation.

Cordiner was part of an international team that made observations to test the recent claim that the simple organic molecule, H₂CCC, is the carrier of some of the unidentified Diffuse Interstellar Bands (DIBs). Liszt et al. (2012) found that the measured abundance of this molecule is three orders of magnitude less than that required for it to be a DIB carrier, thus negating the claim.

The formation mechanism for interstellar water ice is still hotly debated. Recent surface chemistry experiments have shown that the hydrogenation of molecular oxygen on interstellar dust grains is a plausible formation mechanism, via hydrogen peroxide (H₂O₂), for the production of water (H₂O) ice mantles in the dense interstellar medium. Theoretical chemistry models also predict the formation of a significant abundance of H₂O₂ ice in grain mantles by this route. Charnley was part of an international team that compared laboratory IR spectra of H₂O₂-H₂O mixtures with IR spectra of protostars and molecular clouds. Smith et al. (2011) found that typically H₂O₂ can be no more than 9% of the ice mantles relative to water. This is a strong constraint on parameters for surface chemistry experiments and dense cloud chemistry models.

A Review chapter for the Protostars and Planets VI meeting next summer has been submitted on “Quantifying the Interstellar Contribution to Primitive Solar System Organic Material”, by Wirstrom, Charnley, Milam, Elsila, Alexander, Nittler, Kuan, and Marty. The focus of this Chapter will be on the ISM-Solar System connection. Nebular/disk chemistry undoubtedly played a pivotal role in producing many of the chemical characteristics found in meteorites and comets. However, the present difficulties in observing large organic molecules, and measuring isotopic ratios, in protoplanetary disks means that the connection between disk chemistry and primitive materials cannot be as well-constrained as that involving interstellar chemistry. Thus, we will focus directly on the ISM-Solar System connection. This approach will allow us to identify where the ISM connection is untenable and disk processes are necessary.

Charnley and Milam will be writing a review article for Chemical Reviews Special Issue on Astrochemistry (Eds. Prof. Eric Herbst and Prof. John Yates) this winter temporarily titled, “Isotopes in Astrochemistry”. This review will cover all aspects of isotope measurements in all fields of Astrochemistry and will review (1) the contribution of stellar nucleosynthesis to Galactic chemical evolution, (2) observed isotopic fractionation in interstellar clouds and protoplanetary disks, (3) isotopes in comets, (4) isotopic anomalies in meteorites and IDPs (including the Stardust samples).

Theme III Progress: S. Milam has recently been selected to receive funding through the Goddard Science Innovation Fund to develop an experiment designed to simulate interstellar/cometary/planetary ices and detect trace species. This experiment employs the same techniques used for remote observations will help constrain the chemical complexity of ices, the amount of processing that occurs, and interpret past and present data from missions that observe ice features. To date, over 150 molecular species have been confirmed in space, primarily by their rotational spectra (at millimeter/submillimeter wavelengths – mm/sub-mm) which yield an unambiguous identification. Many of the known interstellar organic molecules cannot be explained by gas-phase chemistry. It is now presumed that they are produced by surface reactions of the simple ices and/or grains observed and released into the gas phase by sublimation, sputtering, etc. Additionally, the chemical complexity found in meteorites and samples returned from comets far surpasses that of the remote detections for the interstellar medium (ISM), comets, and planetary atmospheres. Laboratory simulations of interstellar/cometary ices have found, from the analysis of the remnant residue of the warmed laboratory sample, that such molecules are readily formed; however, it has yet to be determined if they are formed during the warm phase or within the ice during processing. Most analysis of the ice during processing reveals molecular changes, though the exact quantities and species formed are highly uncertain with current techniques due to overwhelming features of simple ices. Remote sensing with high resolution spectroscopy is currently the only method to detect trace species in the ISM and the primary method for comets and icy bodies in the Solar System due to limitations of sample return. Current ice experimental techniques address certain chemical processes that occur in ices where the IR spectra can conclusively identify only a limited set of species in the solid phase, and/or analysis of residues can be conducted. These experiments are not directly comparable to techniques currently employed for the remote detection of new/complex/trace species. Thus, this new, multidisciplinary experiment will pursue laboratory studies of interstellar/cometary/atmospheric ices to directly compare trace gas-phase species that are formed when exposed to thermal or radiation processing. The experimental objectives are to simulate interstellar/cometary/atmospheric chemistry and unambiguously detect all products in both the solid and gas phase with precise abundances as well as determine reaction pathways and lifetimes of newly formed species. The preliminary experiment has been set-up in the laboratory of Susanna Widicus Weaver (Emory University) to utilize her submillimeter spectrometer (see Figure 3).

To date, we have designed (Figure 3), assembled the experiment – from ALL borrowed components, and conducted the preliminary studies of water ice. We have also tested the thermal control of our system by monitoring water vapor in the chamber through the sublimation point (Figure 4). Currently, we are optimizing the radiation conditions, with a new UV source, and experimental setup and will measure water and OH, the main photoproduct of H₂O, during exposure.