2012 Annual Science Report
NASA Goddard Space Flight Center Reporting | SEP 2011 – AUG 2012
Remote Sensing of Organic Volatiles in Planetary and Cometary Atmospheres
In the last year, we have greatly advanced our capabilities to model spectra of cometary and planetary atmospheres (Villanueva et al. 2012a, 2012b). Using these newly developed analytical methods, we derived the most comprehensive search for biomarkers on Mars (Villanueva et al. 2012, submitted) from our extensive database of high-quality Mars spectra. Furthermore, we retrieved molecular abundances of several comets (Villanueva et al. 2012c, Gibb et al. 2012, Paganini et al. 2012a/b), and of several young circumstellar disks (Mandell et al. 2012). These great advancements have allowed us to understand the infrared spectrum of planetary bodies and their composition with unprecedented precision.
Using the newly developed analytical methods presented in our last report and, we derived the most comprehensive search for biomarkers on Mars from our extensive database of high-quality Mars spectra acquired since 2006. We recently submitted a detailed manuscript to the Journal Icarus presenting our latest results. In this paper, we resent a comprehensive search for trace species on Mars, targeting multiple volatile organic species (CH4, CH3OH, H2CO, C2H6, C2H2, C2H4), hydroperoxyl (HO2), several nitrogen compounds (N2O, NH3 HCN), and two chlorine species (HCl, CH3Cl) through their rovibrational spectra in the 2.7-3.7 μm spectral region.
The data were acquired over a period of 4 years (2006-2010) using powerful infrared high-resolution spectrometers (CRIRES, NIRSPEC, CSHELL) at high-altitude observatories (VLT, Keck-2, NASA-IRTF), and span a broad range of seasons, Doppler shifts and spatial coverage. We present results from a selection of high-quality spectra obtained on four separate dates, representing a fraction of our search space. For most of these species we derived the most stringent upper limits ever obtained, and because the targeted gases have substantially different resident lifetimes in the Martian atmosphere (from hours to centuries), our measurements not only test for current release but also provide stringent limits on the quiescent levels. In particular, we sampled the same regions where plumes of methane have been recently observed (e.g., Syrtis Major and Valles Marineris), allowing us to test for seasonal and temporal variability.
Villanueva, G. L., Mumma, M. J., Novak, R. E., Radeva, Y. L., Käufl, H. U., Tokunaga, A., Encrenaz, T. & Hartogh, P., A sensitive search for organics (CH4, CH3OH, H2CO, C2H6, C2H2, C2H4), hydroperoxyl (HO2), nitrogen compounds (N2O, NH3, HCN) and chlorine species (HCl, CH3Cl) on Mars using ground-based high-resolution infrared spectroscopy. Icarus. Submitted.
Water (H2O and HDO)
We recently developed a modern methodology to retrieve water (H2O) and deuterated water (HDO) in planetary and cometary atmospheres, and constructed an accurate spectral database that combines theoretical and empirical results. Based on a greatly expanded set of spectroscopic parameters, we built a full non-resonance cascade fluorescence model and computed fluorescence efficiencies for H2O (500 million lines) and HDO (700 million lines). The new line list was also integrated into an advanced terrestrial radiative transfer code (LBLRTM) and adapted to the CO2 rich atmosphere of Mars, for which we adopted the complex Robert–Bonamy formalism for line shapes. We then retrieved water and D/H in the atmospheres of Mars, comet C/2007 W1 (Boattini), and Earth by applying the new formalism to spectra obtained with the high resolution spectrograph NIRSPEC/Keck II atop Mauna Kea (Hawaii). The new model accurately describes the complex morphology of the water bands and greatly increases the accuracy of the retrieved abundances (and the D/H ratio in water) with respect to previously available models. The new model provides improved agreement of predicted and measured intensities for many H2O lines already identified in comets, and it identifies several unassigned cometary emission lines as new emission lines of H2O. The improved spectral accuracy permits retrieval of more accurate rotational temperatures and production rates for cometary water.
Villanueva, G. L., Mumma, M. J., Bonev, B.P., Novak, R.E., Barber, R.J. & DiSanti, M.A., Water in Planetary and Cometary Amospheres: H2O/HDO Transmittance and Fluorescence Models. Journal of Quantitative Spectroscopy and Radiative Transfer (J. Quant. Spectrosc. Radiat. Transfer). Vol 113, Issue 3, pp. 202-220, 2012.
Methanol (CH3OH) radiates efficiently at infrared wavelengths, dominating the C-H stretching region in comets, yet inadequate quantum mechanical models have imposed limits on the practical use of its emission spectra. Accordingly, we constructed a new line-by-line model for the ν3 fundamental band of methanol at 2844 cm-1 (3.52 μm), and applied it to interpret cometary fluorescence spectra. The new model permits accurate synthesis of line-by-line spectra for a wide range of rotational temperatures, ranging from 10K to more than 300K. We validated the model by comparing simulations of CH3OH fluorescent emission with measured spectra of three comets (C/2001 A2 LINEAR, C/2004 Q2 Machholz and 8P/Tuttle) acquired with high-resolution infrared spectrometers at high altitude sites. The new model accurately describes the complex emission spectrum of the ν3 band, providing distinct rotational temperatures and production rates at greatly improved confidence levels compared with results derived from earlier fluorescence models. The new model reconciles production rates measured at infrared and radio wavelengths in C/2001 A2 (LINEAR). Methanol can now be quantified with unprecedented precision and accuracy in astrophysical sources through high dispersion spectroscopy at infrared wavelengths.
Villanueva, G. L., DiSanti, M. A., Mumma, M. J. & Xu, L.-H. A Quantum Band Model of the ν3 Fundamental of Methanol (CH3OH) and its Application to Fluorescence Spectra of Comets. The Astrophysical Journal (Astrophys. J.). Vol 747, Issue 1, Article 37, 2012.
Comet 2009/P1 Garradd
We observed comet C/2009 P1 (Garradd) on UT 2011 September 8th and 9th at a large heliocentric distance of 2.1 AU upon its entry to the inner Solar System. The observations were performed using high-resolution infrared spectrometers (NIRSPEC at Keck II and CSHELL at IRTF), allowing us to obtain strong detections of H2O, CO, CH4 and HCN and sensitive upper-limits for C2H6, C2H2, NH3 and HC3N. We oriented the slit at 45° from the projected Sun-comet vector and obtained spatial profiles of H2O, CH4, and HCN that revealed notable differences among these species. In particular, we observed a strong excess of water in the projected sunward direction, probably due to a solar-activated jet releasing water–rich icy grains. We also investigated the composition of the comet at 2.4 AU and 2.0 AU from the Sun using CRIRES at VLT, deriving an unusual CO rich composition (Paganini et al. 2012).
Villanueva, G. L., Mumma, M. J., DiSanti, M. A., Bonev, B. P., Paganini, L. & Blake, G. A., A Multi-Instrument Study of Comet C/2009 P1 (Garradd). Icarus, 220, pp. 291-295, 2012.
Paganini, L., Mumma, M. J., Villanueva, G.L., DiSanti, M. A., Bonev, B. P., Lippi, M. & Böhnhardt, H., The Chemical Composition of CO-rich Comet C/2009 P1 (Garradd) at Rh = 2.4 and 2.0 AU before Perihelion. The Astrophysical Journal Letters (Astrophys. J. Lett.). Vol 748(1), p. L13, 2012.
Comet C/2007 N3 Lulin
We measured the volatile chemical composition of comet C/2007 N3 (Lulin) on three dates from 2009 January 30 to February 1 using NIRSPEC, the high-resolution (λ/Δλ ≈ 25,000), long-slit echelle spectrograph at Keck 2. We sampled nine primary (parent) volatile species (H2O, C2H6, CH3OH, H2CO, CH4, HCN, C2H2, NH3, CO) and two product species (OH∗ and NH2). We also reported upper limits for HDO and CH3D. C/2007 N3 (Lulin) displayed an unusual composition when compared to other comets. Based on comets measured to date, CH4 and C2H6 exhibited “normal” abundances relative to water, CO and HCN were only moderately depleted, C2H2 and H2CO were more severely depleted, and CH3OH was significantly enriched. Comet C/2007 N3 (Lulin) is another important and unusual addition to the growing population of comets with measured parent volatile compositions, illustrating that these studies have not yet reached the level where new observations simply add another sample to a population with well-established statistics.
Gibb, E. L., Boncho, B. P., Villanueva, G. L., DiSanti, M. A., Mumma, Sudholt, E. & Radeva, Y. L., Chemical Composition of Comet C/2007 N3 (Lulin): Another “atypical” comet. The Astrophysical Journal (Astrophys. J.). Vol 750(2), Article 102, pp. 14, 2012.
Comet 10P/Tempel 2
Comet 10P/Tempel 2 was observed after its perihelion passage in 2010. In our recent Icarus paper, we presented spectral and spatial information of major volatile species using high- dispersion infrared spectroscopy. On UT 2010 July 26, our observations revealed a total water production rate of (1.93 ± 0.2) × 1028 molecules s−1, and abundances of seven trace gases relative to water, HCN (0.13%), C2H6 (0.39%), CH3OH (1.89%), NH3 (0.76%), and (3σ) upper limits for NH2 (<0.07%), C2H2 (<0.06%), and H2CO (<0.24%). Detailed analysis of water emission lines intensities resulted in a rotational temperature of 35 ± 3 K and an ortho-to-para ratio of 2.95 ± 0.25, corresponding to a spin temperature Tspin > 34 K. In particular, our campaign was contemporaneous with a jet-like feature observed at optical and visual wavelengths in 2010 mid-July. Spatial profiles of primary volatiles display strong enhancements toward the jet direction, which favors the idea of direct sublimation in response to local insolation. The relative low abundance of organic volatiles, together with the estimated spin temperature in comet 10P/Tempel 2, suggests that its cometary ices might have agglomerated in the proto-planetary disk, relatively close to the Sun, where thermal processing occurred. However, evidence of efficient hydrogenation, as implied by the high abundance ratios of C2H6/C2H2 and CH3OH/H2CO, may indicate processing in a colder region. Observations in UT 2010 September did not reveal any apparent spectral features, but allowed us to retrieve sensitive upper limits for water production in comet 10P.
Paganini, L., Mumma, M. J., Bonev, B. P., Villanueva, G. L., DiSanti, M. A., Keane, J. V. & Meech, K. J., The formation heritage of Jupiter Family Comet 10P/Tempel 2 as revealed by infrared spectroscopy. Icarus. Vol 218(1), pp. 644-653, 2012.
Young Circumstellar Disks
In our recent Astrophysical Journal paper, we presented an analysis of high-resolution spectroscopy of several bright T Tauri stars using the CRIRES spectrograph on the Very Large Telescope and NIRSPEC spectrograph on the Keck Telescope, revealing the first detections of emission from HCN and C2H2 in circumstellar disks at near-infrared wavelengths. Using advanced data reduction techniques, we achieve a dynamic range with respect to the disk continuum of ∼500 at 3 μm, revealing multiple emission features of H2O, OH, HCN, and C2H2. We also present stringent upper limits for two other molecules thought to be abundant in the inner disk, CH4 and NH3. Line profiles for the different detected molecules are broad but centrally peaked in most cases, even for disks with previously determined inclinations of greater than 20◦, suggesting that the emission has both a Keplerian and non-Keplerian component as observed previously for CO emission. We apply two different modeling strategies to constrain the molecular abundances and temperatures: we use a simplified single-temperature local thermal equilibrium (LTE) slab model with a Gaussian line profile to make line identifications and determine a best-fit temperature and initial abundance ratios, and we compare these values with constraints derived from a detailed disk radiative transfer model assuming LTE excitation but utilizing a realistic temperature and density structure. Abundance ratios from both sets of models are consistent with each other and consistent with expected values from theoretical chemical models, and analysis of the line shapes suggests that the molecular emission originates from within a narrow region in the inner disk (R < 1 AU).
Mandell, A., Bast, J., van Dishoeck, E., Blake, G.A., Salyk C., Mumma M.J., Villanueva, G. L., First Detection of Near-infrared Line Emission from Organics in Young Circumstellar Disks. The Astrophysical Journal (Astrophys. J.). Vol 747(2), p. 92, 2012.
PROJECT MEMBERS:Geronimo Villanueva
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Indirect and direct astronomical observations of extrasolar habitable planets.
Outer Solar System exploration
Sources of prebiotic materials and catalysts
Origins and evolution of functional biomolecules
Biosignatures to be sought in Solar System materials
Biosignatures to be sought in nearby planetary systems