Geronimo L. Villanueva

Strong Release of Methane on Mars in Northern Summer 2003

Living systems produce more than 90% of Earths atmospheric methane; the balance is of geochemical origin. On Mars, methane could be a signature of either origin. Using high-dispersion infrared spectrometers at three ground-based telescopes, we measured methane and water vapor simultaneously on Mars over several longitude intervals in northern early and late summer in 2003 and near the vernal equinox in 2006. When present, methane occurred in extended plumes, and the maxima of latitudinal profiles imply that the methane was released from discrete regions. In northern midsummer, the principal plume contained ∼19,000 metric tons of methane, and the estimated source strength (≥0.6 kilogram per second) was comparable to that of the massive hydrocarbon seep at Coal Oil Point in Santa Barbara, California.

A SENSITIVE SEARCH FOR DEUTERATED WATER IN COMET 8P/TUTTLE*

We report a sensitive search for deuterated water (HDO) in comet 8P/Tuttle using high-resolution spectroscopy at infrared (IR) wavelengths. The deuterium enrichment of cometary water is one of the most important cosmogonic indicators in comets. The ratio preserves information about the conditions under which comet material formed, and tests the possible contribution of comets in delivering water for Earths oceans. Water (H2O) and HDO were sampled in comet 8P/Tuttle from 2008 January 27 to 2008 February 3 using the new IR spectrometer (Cryogenic Infrared Echelle Spectrograph) at the 8.2 m Antu telescope of the Very Large Telescope Observatory atop Cerro Paranal, Chile. Twenty-three lines of HDO were sampled near 3.7 μm, leading to a production rate of 4.73 ± 1.68 × 1025 s–1. Combining this value with the H2O production rate of 5790 ± 250 × 1025 s–1 provides a formal value of = 4.09 ± 1.45 × 10–4 in comet 8P/Tuttle. This value is larger by a factor of 2.62 ± 0.93 than Vienna Standard Mean Ocean Water, and is comparable to enrichment factors measured for three other Oort cloud comets. The technique described here provides unprecedented sensitivities, ultimately permitting us to routinely measure this prime cosmogonic indicator, even in comets having relatively modest gas production rate like 8P/Tuttle.

Detection of Formaldehyde Emission in Comet C/2002 T7 (LINEAR) at Infrared Wavelengths: Line-by-Line Validation of Modeled Fluorescent Intensities

Formaldehyde (H2CO) was observed in comet C/2002 T7 (LINEAR) with spectral resolving power k/� k � 2.5 ; 10 4 using the Cryogenic Echelle Spectrometer (CSHELL) at the NASA Infrared Telescope Facility, on UT 2004 May 5, 7, and 9. The observations, which sampled emission in the � 1 and � 5 rovibrational bands between 3.53 and 3.62 � m, represent the first spectrally resolved detection, at infrared wavelengths, of monomeric H2CO spanning a range of rotational energies. A comparison of measured line intensities with an existing fluorescence model permitted extraction of rotational temperatures and production rates. Two complementary approaches were used: (1)acorrelationanalysisthatprovidedadirectglobalcomparisonoftheobservedcometaryemissionswiththemodel and (2) an excitation analysis that provided a robust line-by-line comparison. Our results validate the fluorescence model. The overall correlation coefficient was near or above 0.9 in our two principal grating settings. The excitation analysis provided accurate measures of rotational excitation (rotational temperature) on all three dates, with retrieved values of Trot clustering near 100 K. Through simultaneous measurement of OH prompt emission, which we use as a proxy for H2O, we obtained native production rates and mixing ratios for H2CO. The native production of H2CO varied from day to day, but its abundance relative to H2O, Xnative, remained approximately constant within the errors, which may suggest an overall homogeneous composition of the nucleus. We measured a mean mixing ratio Xnative = (0.79 � 0.09) ; 10 � 2 for the three dates. Subject headingg astrochemistry — comets: individual (C/2002 T7 (LINEAR)) — infrared: solar system Online material: color figures

Temporal and Spatial Aspects of Gas Release During the 2010 Apparition of Comet 103P/Hartley 2

We report measurements of eight primary volatiles (H_(2)O, HCN, CH_4, C_(2)H_6, CH_(3)OH, C_(2)H_2, H_(2)CO, and NH_3) and two product species (OH and NH_2) in comet 103P/Hartley 2 using high-dispersion infrared spectroscopy. We quantified the long- and short-term behavior of volatile release over a three-month interval that encompassed the comets close approach to Earth, its perihelion passage, and flyby of the comet by the Deep Impact spacecraft during the EPOXI mission. We present production rates for individual species, their mixing ratios relative to water, and their spatial distributions in the coma on multiple dates. The production rates for water, ethane, HCN, and methanol vary in a manner consistent with independent measures of nucleus rotation, but mixing ratios for HCN, C_(2)H_6, and CH_(3)OH are independent of rotational phase. Our results demonstrate that the ensemble average composition of gas released from the nucleus is well defined and relatively constant over the three-month interval (September 18 through December 17). If individual vents vary in composition, enough diverse vents must be active simultaneously to approximate (in sum) the bulk composition of the nucleus. The released primary volatiles exhibit diverse spatial properties which favor the presence of separate polar and apolar ice phases in the nucleus, establish dust and gas release from icy clumps (and from the nucleus), and provide insights into the driver for the cyanogen (CN) polar jet. The spatial distributions of C_(2)H_6 and HCN along the near-polar jet (UT October 19.5) and nearly orthogonal to it (UT October 22.5) are discussed relative to the origin of CN. The ortho-para ratio (OPR) of water was 2.85 ± 0.20; the lower bound (2.65) defines T_spin > 32 K. These values are consistent with results returned from the Infrared Space Observatory in 1997.

FIRST DETECTION OF NEAR-INFRARED LINE EMISSION FROM ORGANICS IN YOUNG CIRCUMSTELLAR DISKS*

We present 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 C_2H_2 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 H_2O, OH, HCN, and C_2H_2. We also present stringent upper limits for two other molecules thought to be abundant in the inner disk, CH_4 and NH_3. 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).

Non-Detection Of L-Band Line Emission From The Exoplanet HD189733B

We attempt to confirm bright non-local thermodynamic equilibrium (non-LTE) emission from the exoplanet HD 189733b at 3.25 μm, as recently reported by Swain et al. based on observations at low spectral resolving power (λ/δλ ≈ 30). Non-LTE emission lines from gas in an exoplanet atmosphere will not be significantly broadened by collisions, so the measured emission intensity per resolution element must be substantially brighter when observed at high spectral resolving power. We observed the planet before, during, and after a secondary eclipse event at a resolving power λ/δλ = 27, 000 using the NIRSPEC spectrometer on the Keck II telescope. Our spectra cover a spectral window near the peak found by Swain et al., and we compare emission cases that could account for the magnitude and wavelength dependence of the Swain et al. result with our final spectral residuals. To model the expected line emission, we use a general non-equilibrium formulation to synthesize emission features from all plausible molecules that emit in this spectral region. In every case, we detect no line emission to a high degree of confidence. After considering possible explanations for the Swain et al. results and the disparity with our own data, we conclude that an astrophysical source for the putative non-LTE emission is unlikely. We note that the wavelength dependence of the signal seen by Swain et al. closely matches the 2ν_2 band of water vapor at 300 K, and we suggest that an imperfect correction for telluric water is the source of the feature claimed by Swain et al.

THE VOLATILE COMPOSITION OF THE SPLIT ECLIPTIC COMET 73P/SCHWASSMANN-WACHMANN 3: A COMPARISON OF FRAGMENTS C AND B

The composition of fragments C and B of the Jupiter-family comet 73P/Schwassmann-Wachmann 3 (SW3) was investigated in early April of 2006 at IR wavelengths using high-dispersion echelle spectroscopy. Both fragments were depleted in ethane, and C was depleted in most forms of volatile carbon. In particular, fragment C shows a severe depletion of CH3OH but a “normal” abundance of HCN (which has a similar volatility). Thermal processing is a possible explanation, but since fragment B is perhaps sublimating fresher material because of the frequent outbursts and fragmentation, the observed depletions might have cosmogonic implications. The chemistry of the volatile ices in SW3, like in the Oort Cloud comet C/1999 S4 (LINEAR), may be associated with sublimation of icy mantles from precometary grains followed by subsequent gas-phase chemistry and recondensation. Subject headings: comets: individual (73P/SW3) — Kuiper Belt — solar system: formation

CHEMICAL COMPOSITION OF COMET C/2007 N3 (LULIN): ANOTHER ‘‘ATYPICAL’’ COMET

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 ,C H 3OH, H2CO, CH4 ,H CN, C 2H2, NH3, CO) and two product species (OH ∗ and NH2). We also report 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.

Depleted Carbon Monoxide in Fragment C of the Jupiter-Family Comet 73P/Schwassmann-Wachmann 3

Carbon monoxide emission was targeted in fragment C of the recently split Jupiter-family comet 73P/Schwassmann-Wachmann 3 during its 2006 apparition, using the Cryogenic Echelle Spectrograph (CSHELL) at the NASA IRTF on Mauna Kea, Hawaii. Simultaneous sounding with H2O near 4.65 mm revealed highly depleted CO, consistent with a mixing ratio of 0.5% 0.13%. Along with depleted CH3OH but nearly normal HCN, this may indicate that this comet formed in the inner giant planets’ region or, alternatively, that it formed relatively late, after significant clearing of the protosolar nebula. Subject headings: comets: general — comets: individual (73P/Schwassmann-Wachmann 3) — molecular processes — solar system: formation Online material: color figure

Discovery of OH in Circumstellar Disks Around Young Intermediate-Mass Stars

We detect emission from multiple low-excitation rovibrational transitions of OH from the two Herbig Ae stars AB Aurigae and MWC 758 in the 3.0-3.7 μm wavelength range (L band), using the NIRSPEC instrument on Keck II. The inner radius for the emitting region in both stars is close to 1 AU. We compare an optically thin LTE model and a thin-wedge fluorescence model, finding rotational temperatures of 650-800 K and OH abundances of 10^42-10^45 molecules for the two stars. Comparisons with current chemical models support the fluorescence excitation model for AB Aurigae and possibly MWC 758, but further observations and detailed modeling are necessary to improve constraints on OH emission in different disk environments.