► We sampled in Comet C/2007 W1 (Boattini) 11 molecular abundances, and retrieved three cosmogonic indicators. ► The abundance ratios of almost all trace volatiles are among the highest ever observed ...in a comet. ► The comet revealed a complex outgassing, with the polar species presenting very asymmetric spatial profiles. ► We interpret the emission profiles in terms of release from two distinct moieties of ice.
We measured the chemical composition of Comet C/2007 W1 (Boattini) using the long-slit echelle grating spectrograph at Keck-2 (NIRSPEC) on 2008 July 9 and 10. We sampled 11 volatile species (H
2O, OH
∗, C
2H
6, CH
3OH, H
2CO, CH
4, HCN, C
2H
2, NH
3, NH
2, and CO), and retrieved three important cosmogonic indicators: the ortho-para ratios of H
2O and CH
4, and an upper-limit for the D/H ratio in water. The abundance ratios of almost all trace volatiles (relative to water) are among the highest ever observed in a comet. The comet also revealed a complex outgassing pattern, with some volatiles (the polar species H
2O and CH
3OH) presenting very asymmetric spatial profiles (extended in the anti-sunward hemisphere), while others (e.g., C
2H
6 and HCN) showed particularly symmetric profiles. We present emission profiles measured along the Sun–comet line for all observed volatiles, and discuss different production scenarios needed to explain them. We interpret the emission profiles in terms of release from two distinct moieties of ice, the first being clumps of mixed ice and dust released from the nucleus into the sunward hemisphere. The second moiety considered is very small grains of nearly pure polar ice (water and methanol, without dark material or apolar volatiles). Such grains would sublimate only very slowly, and could be swept into the anti-sunward hemisphere by radiation pressure and solar-actuated non-gravitational jet forces, thus providing an extended source in the anti-sunward hemisphere.
We report measurements of eight primary volatiles (H2O, HCN, CH4, C2H6, CH3OH, C2H2, H2CO, and NH3) and two product species (OH and NH2) 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 comet's 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, C2H6, and CH3OH 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 C2H6 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.
We developed full cascade fluorescence models for NH3, HCN and HNC, and a new band model for the ν1 ro-vibrational band of HC3N. The models are based on ab-initio spectral databases containing ...millions of spectral lines and also include extremely precise spectral information contained in several high-resolution spectral databases. Using these new models we derive detailed cascade maps for these species, and obtain realistic fluorescence efficiencies applicable to high-resolution infrared spectra. The new models permit accurate synthesis of line-by-line spectra for a wide range of rotational temperatures. We validated the models by comparing simulated emissions of these nitrogen species with measured spectra of comet C/2007 W1 (Boattini) acquired with high-resolution infrared spectrometers at high altitude sites. The new models accurately describe the complex emission spectrum, providing distinct rotational temperatures and production rates at greatly improved accuracy compared with results derived from earlier fluorescence models. In addition, we made use of the completeness and scope of the new databases to investigate possible HCN↔HNC radiative isomerization mechanisms, obtaining estimates of conversion efficiencies under typical cometary conditions.
•We model fluorescence (non-LTE) spectra of HCN, HNC, NH3 and HC3N.•The model integrates self-consistently spectral information from 1.2 billion lines.•The model is applicable to the study of comets, exoplanets and proto-planetary disks.•The model predicts some HCN.•HNC radiative isomerization in comets.
Formaldehyde (H sub(2)CO) was observed in comet C/2002 T7 (LINEAR) with spectral resolving power l/ lambda 62.5 x 10 super(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 u sub(1) and u sub(2) rovibrational bands between 3.53 and 3.62 km, represent the first spectrally resolved detection, at infrared wavelengths, of monomeric H sub(2)CO 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) a correlation analysis that provided a direct global comparison of the observed cometary emissions with the model 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 T sub(rot) clustering near 100 K. Through simultaneous measurement of OH prompt emission, which we use as a proxy for H sub(2)O, we obtained native production rates and mixing ratios for H sub(2)CO. The native production of H sub(2)CO varied from day to day, but its abundance relative to H sub(2)O, X sub(native), remained approximately constant within the errors, which may suggest an overall homogeneous composition of the nucleus. We measured a mean mixing ratio X sub(native) = (0.79 c 0.09) x 10 super(-2) for the three dates.
Earth- and space-based observations provide synergistic information for space mission encounters by providing data over longer timescales, at different wavelengths and using techniques that are ...impossible with an in situ flyby. We report here such observations in support of the EPOXI spacecraft flyby of comet 103P/Hartley 2. The nucleus is small and dark, and exhibited a very rapidly changing rotation period. Prior to the onset of activity, the period was ~16.4 hr. Starting in 2010 August the period changed from 16.6 hr to near 19 hr in December. With respect to dust composition, most volatiles and carbon and nitrogen isotope ratios, the comet is similar to other Jupiter-family comets. What is unusual is the dominance of CO2-driven activity near perihelion, which likely persists out to aphelion. Near perihelion the comet nucleus was surrounded by a large halo of water-ice grains that contributed significantly to the total water production.
Water hot-band lines were detected in comets C/1999 H1 (Lee), C/1999 S4 (LINEAR), and C/2001 A2 (LINEAR) in the 2.9 mu m spectral region using high-dispersion ( lambda / Delta lambda similar to 2 x ...10 super(4)) infrared spectroscopy with NIRSPEC at the W. M. Keck Observatory. The density of H sub(2)O emissions in this spectral region, the spectral coverage and resolution of NIRSPEC, and fluorescence models developed for these hot bands enabled the determination of H sub(2)O production rates, rotational temperatures, and ortho-to-para ratios (OPRs) in these comets. Previous studies revealed clear diversity in the volatile organic chemistries of these comets, suggesting that they may have formed in different regions of the early solar nebula. The nuclear spin temperature of H sub(2)O as derived from its OPR is another possible indicator of cometary formation temperature and region. Nuclear spin temperatures for H sub(2)O were derived on one date in comet S4 and two dates in Lee and A2. Derived spin temperatures for H sub(2)O in these comets are greater than or equal to 30, 30 super(+) sub(-) super(1) sub(6) super(5), and 23 super(+) sub(-) super(4) sub(3) K for S4, Lee, and A2, respectively. Measurements are consistent with a possible link between nuclear spin temperatures and volatile abundances, but studies of more comets and continued improvements in water hot-band fluorescence models are needed to more stringently test this.
In the current paradigm, Oort cloud comets formed in the giant planets' region of the solar nebula, where temperatures and other conditions varied greatly. The measured compositions of four such ...comets (Halley, Hyakutake, Hale-Bopp, and Lee) are consistent with formation from interstellar ices in the cold nebular region beyond Uranus. The composition of comet C/1999 S4 (LINEAR) differs greatly, which suggests that its ices condensed from processed nebular gas, probably in the Jupiter-Saturn region. Its unusual organic composition may require reevaluation of the prebiotic organic material delivered to the young Earth by comets.
Ethane and other hydrocarbon gases have strong rovibrational transitions in the 3.3 μm spectral region owing to C‐H, CH2, and CH3 vibrational modes, making this spectral region prime for searching ...possible biomarker gases in extraterrestrial atmospheres (e.g., Mars, exoplanets) and organic molecules in comets. However, removing ethane spectral signatures from high‐resolution terrestrial transmittance spectra has been imperfect because existing quantum mechanical models have been unable to reproduce the observed spectra with sufficient accuracy. To redress this problem, we constructed a line‐by‐line model for the ν7 band of ethane (C2H6) and applied it to compute telluric transmittances and cometary fluorescence efficiencies. Our model considers accurate spectral parameters, vibration‐rotation interactions, and a functional characterization of the torsional hot band. We integrated the new band model into an advanced radiative transfer code for synthesizing the terrestrial atmosphere (LBLRTM), achieving excellent agreement with transmittance data recorded against Mars using three different instruments located in the Northern and Southern hemispheres. The retrieved ethane abundances demonstrate the strong hemispheric asymmetry noted in prior surveys of volatile hydrocarbons. We also retrieved sensitive limits for the abundance of ethane on Mars. The most critical validation of the model was obtained by comparing simulations of C2H6 fluorescent emission with spectra of three hydrocarbon‐rich comets: C/2004 Q2 (Machholz), 8P/Tuttle, and C/2007 W1 (Boattini). The new model accurately describes the complex emission morphology of the ν7 band at low rotational temperatures and greatly increases the confidence of the retrieved production rates (and rotational temperatures) with respect to previously available fluorescence models.
Key Points
A line‐by‐line model for the ν7 band of ethane (C2H6)
Applications for the remote sensing of ethane
Organic volatiles and water in Oort Cloud comets were investigated at infrared wavelengths. The detected species include H
2O, CO, CH
3OH, CH
4, C
2H
2, C
2H
6, OCS, HCN, NH
3, and H
2CO. Several ...daughter fragments (CN, OH, NH
2, etc.) are also measured, and OH prompt emission provides a proxy for water. Long-slit spectra are taken at high spectral dispersion and high spatial resolution, eliminating several sources of systematic error. The resulting parent volatile production rates are highly robust, permitting a sensitive search for compositional diversity among comets. Here, seven OC comets are compared. Six (including Halley) exhibit similar compositions (excepting CO and CH
4). Their low formation temperatures (∼30 K) suggest this group probably formed beyond 30 AU from the young sun. However, C/1999 S4 is severely depleted in hypervolatiles and also in methanol, and it likely formed near 5–10 AU. C/2001 A2 is discussed briefly to illustrate future prospects.
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