Abstract
We report and analyze updated molecular abundances in 20 comets obtained by employing modern data reduction procedures and molecular models. Using box and scatter plots, we examine how the ...different molecular species are distributed among the comet population, while by means of pie charts, we investigate the relative proportions of these molecular species in each comet. We compare these results with the orbital parameters of the selected comets to identify trends related to their dynamical history. With these analyses, we tentatively identify at least three chemical classes based mainly on relative abundances of CO, CH
3
OH, CH
4
, C
2
H
6
, HCN, and NH
3
. The combination of relative abundances and orbital parameters is then compared with recent chemical models of planetary system formation. This approach may help in investigating the origins and evolution of the material in cometary nuclei. Among other aspects, we underline the need to increase our sample size, especially for hypervolatiles (i.e., CH
4
and CO) in Jupiter family comets.
Results are presented from the first cometary observations using the Atacama Large Millimeter/Submillimeter Array (ALMA), including measurements of the spatially resolved distributions of HCN, HNC, H ...sub(2)CO, and dust within the comae of two comets: C/2012 F6 (Lemmon) and C/2012 S1 (ISON), observed at heliocentric distances of 1.5 AU and 0.54 AU, respectively. These observations (with angular resolution approximate0".5), reveal an unprecedented level of detail in the distributions of these fundamental cometary molecules, and demonstrate the power of ALMA for quantitative measurements of the distributions of molecules and dust in the inner comae of typical bright comets. In both comets, HCN is found to originate from (or within a few hundred kilometers of) the nucleus, with a spatial distribution largely consistent with spherically symmetric, uniform outflow. By contrast, the HNC distributions are clumpy and asymmetrical, with peaks at cometocentric radii ~500-1000 km, consistent with release of HNC in collimated outflow(s). Compared to HCN, the H sub(2)CO distribution in comet Lemmon is very extended. The interferometric visibility amplitudes are consistent with coma production of H sub(2)CO and HNC from unidentified precursor material(s) in both comets. Adopting a Haser model, the H sub(2)CO parent scale length is found to be a few thousand kilometers in Lemmon and only a few hundred kilometers in ISON, consistent with the destruction of the precursor by photolysis or thermal degradation at a rate that scales in proportion to the solar radiation flux.
Vinyl cyanide (C2H3CN) is theorized to form in Titan's atmosphere via high-altitude photochemistry and is of interest regarding the astrobiology of cold planetary surfaces due to its predicted ...ability to form cell membrane-like structures (azotosomes) in liquid methane. In this work, we follow up on the initial spectroscopic detection of C2H3CN on Titan by Palmer et al. with the detection of three new C2H3CN rotational emission lines at submillimeter frequencies. These new, high-resolution detections have allowed for the first spatial distribution mapping of C2H3CN on Titan. We present simultaneous observations of C2H5CN, HC3N, and CH3CN emission, and obtain the first (tentative) detection of C3H8 (propane) at radio wavelengths. We present disk-averaged vertical abundance profiles, two-dimensional spatial maps, and latitudinal flux profiles for the observed nitriles. Similarly to HC3N and C2H5CN, which are theorized to be short-lived in Titan's atmosphere, C2H3CN is most abundant over the southern (winter) pole, whereas the longer-lived CH3CN is more concentrated in the north. This abundance pattern is consistent with the combined effects of high-altitude photochemical production, poleward advection, and the subsequent reversal of Titan's atmospheric circulation system following the recent transition from northern to southern winter. We confirm that C2H3CN and C2H5CN are most abundant at altitudes above 200 km. Using a 300 km step model, the average abundance of C2H3CN is found to be 3.03 0.29 ppb, with a C2H5CN/C2H3CN abundance ratio of 2.43 0.26. Our HC3N and CH3CN spectra can be accurately modeled using abundance gradients above the tropopause, with fractional scale-heights of 2.05 0.16 and 1.63 0.02, respectively.
Slightly less than a Martian Year of nominal science (March 2018–January 2020) with the ExoMars Trace Gas Orbiter has furthered the ongoing investigation of dayside water vapor column abundance. ...These dayside observations span latitudes between 75°S and 75°N, and all longitudes, which can provide global snapshots of the total water column abundances. In addition to tracking the seasonal transport of water vapor between poles, geographic enhancements are noted, particularly in the southern hemisphere, both in Hellas Basin, and in other regions not obviously correlated to topography. We report consistent water vapor climatology with previous spacecraft observations, however, note a difference in total water vapor content is noted. Finally, we are unable to find evidence for substantial diurnal variation in the total dayside water vapor column.
Plain Language Summary
This work provides the first look at the ExoMars Trace Gas Orbiter's ability to track atmospheric water vapor on the day side of Mars, through downward looking observations. Water vapor is reported in a series of maps, with respect to geography and season, and find consistent water vapor climatology with precious spacecraft observations. These maps inform our understanding of where Martian water vapor moves throughout the year and where it is concentrated.
Key Points
Water vapor can be retrieved robustly from the Nadir and Occultation for MArs Discovery limb and Nadir observatory nadir observations for most dayside conditions
Retrieved water vapor columns demonstrate volatile transport, geographic variations, and lack of evidence for daytime local variations
Trace Gas Orbiter observations continue the legacy of monitoring the Martian water cycle and will contribute to future modeling efforts
(ProQuest: Formulae and/or non-USASCII text omitted)We report production rates for H sub(2) O and eight trace molecules (CO, C sub(2) H sub(6), CH sub(4), CH sub(3) OH, NH sub(3), H sub(2) CO, HCN, C ...sub(2) H sub(2)) in the dynamically new, Sun-grazing Comet C/2012 S1 (ISON), using high-resolution spectroscopy at Keck II and the NASA IRTF on 10 pre-perihelion dates encompassing heliocentric distances R sub(h)= 1.21-0.34 AU. Measured water production rates spanned two orders of magnitude, consistent with a long-term heliocentric power law Q... .Abundance ratios for CO, C sub(2) H sub(6), and CH sub(4) with respect to H sub(2) O remained constant with R sub(h) and below their corresponding mean values measured among a dominant sample of Oort Cloud comets. CH sub(3) OH was also depleted for R sub(h)> 0.5 AU, but was closer to its mean value for R sub(h)< or = 0.5 AU. The remaining four molecules exhibited higher abundance ratios within 0.5 AU: for R sub(h)> 0.8 AU, NH sub(3) and C sub(2) H sub(2) were consistent with their mean values while H sub(2) CO and HCN were depleted. For R sub(h)< 0.5 AU, all four were enriched, with NH sub(3), H sub(2) CO, and HCN increasing most. Spatial profiles of gas emission in ISON consistently peaked sunward of the dust continuum, which was asymmetric antisunward and remained singly peaked for all observations. NH sub(3) within 0.5 AU showed a broad spatial distribution, possibly indicating its release in the coma provided that optical depth effects were unimportant. The column abundance ratio NH sub(2)/H sub(2) O at 0.83 AU was close to the "typical" NH/OH from optical wavelengths, but was higher within 0.5 AU. Establishing its production rate and testing its parentage (e.g., NH sub(3)) require modeling of coma outflow.
► 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.
In 2017 April, we acquired comprehensive high-resolution spectra of newly discovered comet C/2017 E4 (Lovejoy) as it approached perihelion, and before its disintegration. We detected many cometary ...emission lines in the range (2.8-5.3) m, in four customized instrument settings (L1-c, L3, Lp1-c, and M1) of iSHELL-the new near-IR high-resolution immersion echelle spectrograph at NASA/IRTF (Maunakea, Hawaii). We identified 12 molecular species: nine primary volatiles (H2O, HCN, NH3, CO, C2H2, C2H6, CH4, CH3OH, H2CO) and three product species (CN, NH2, OH). We detected 85 H2O emission lines from 12 water vibrational bands across L1-c and M1 settings. The many detected water emission lines enabled retrieval of accurate measures for ortho- and para-H2O independently, thereby reducing systematic uncertainty in the derived ortho-para ratio and nuclear spin temperature. Excitation analyses and emission profile analyses were performed for all species, and molecular abundance ratios relative to water are compared with values found for other Oort Cloud comets in our infrared database. Abundance ratios are consistent for most species, with the exception of underabundant methanol and overabundant ammonia in E4.
ABSTRACT We report the first spectroscopic detection of ethyl cyanide (C2H5CN) in Titan's atmosphere, obtained using spectrally and spatially resolved observations of multiple emission lines with the ...Atacama Large Millimeter/submillimeter Array (ALMA). The presence of C2H5CN in Titan's ionosphere was previously inferred from Cassini ion mass spectrometry measurements of C2H5CNH+. Here we report the detection of 27 rotational lines from C2H5CN (in 19 separate emission features detected at confidence) in the frequency range 222-241 GHz. Simultaneous detections of multiple emission lines from HC3N, CH3CN, and CH3CCH were also obtained. In contrast to HC3N, CH3CN, and CH3CCH, which peak in Titan's northern (spring) hemisphere, the emission from C2H5CN is found to be concentrated in the southern (autumn) hemisphere, suggesting a distinctly different chemistry for this species, consistent with a relatively short chemical lifetime for C2H5CN. Radiative transfer models show that C2H5CN is most concentrated at altitudes km, suggesting production predominantly in the stratosphere and above. Vertical column densities are found to be in the range (1-5) × 1014 cm−2.
We observed short-period comet 252P/LINEAR post-perihelion during its 2016 passage, which presented a favorable opportunity to survey its chemical composition at a close Earth approach (∼0.14 au). We ...characterized the comet's chemical composition on four dates (UT 2016 April 12, 19, 26, and 29) using spectroscopic measurements with the Near-infrared Spectrograph (NIRSPEC) at the Keck Observatory on Maunakea, HI. Our high-resolution infrared spectra yielded production rates for four species (H2O, CH3OH, C2H6, and HCN) and upper limits for five species (NH3, H2CO, C2H2, CO, and CH4). We measured water at an average production rate of 4.9 0.1 × 1027 molec s−1. The chemical properties of 252P suggest a rather typical composition, yet somewhat enriched in methanol and ethane but low in formaldehyde (upper limit) compared to other short-period comets surveyed at infrared wavelengths. Analysis of the ortho/para ratio in water indicates a nuclear spin temperature larger than ∼38 K, consistent with statistical equilibrium (2 ). Spatial distributions of gases, which are representative of possible heterogeneity in the nucleus and/or gas dynamics upon sublimation, showed rather symmetric profiles, with subtle enhancements of the more volatile species C2H6 and HCN toward the sunward hemisphere, while water showed spatial distributions that were extended toward the anti-sunward hemisphere. The continuum was characterized by a narrow distribution. We place our infrared results in the context of observations with the Discovery Channel Telescope, the James Clerk Maxwell submillimeter Telescope, and the Hubble Space Telescope.
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.
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