Engineering topics

DiSanti, Michael A.

Publications and source records attributed to DiSanti, Michael A..

At least 19 records

DOE OSTI2021

Chemical Composition of Outbursting Comet C/2015 ER61 (PanSTARRS)

Comet C/2015 ER61 (PanSTARRS) is a long-period Oort cloud comet whose favorable approach to the inner parts of the solar system in 2017 April–May enabled us to characterize its primary volatile composition using the iSHELL spectrograph mounted on the 3 m NASA Infrared Telescope Facility (IRTF) on Maunakea, Hawaii. We used three iSHELL settings (covering ~2.8–5.2 μm) to sample fluorescent emissions from H{sub 2}O, OH, CH{sub 3}OH, HCN, C{sub 2} H{sub 2}, NH{sub 3}, CO, CH{sub 4}, C{sub 2}H{sub 6}, H{sub 2}CO, and OCS on multiple dates ranging from UT 2017 April 15 (shortly after its April 4 outburst) to May 13, nearly 30 days before the detection of its double nucleus. Our observations also offered the opportunity to obtain sensitive 3σ upper limits for cyanoacetylene (HC{sub 3}N). We report rotational temperatures (T{sub rot}), production rates (Qs), abundance (mixing) ratios (relative to H{sub 2}O and C{sub 2}H{sub 6}), and spatial distributions in the coma. ER61 exhibits variability in production rates of many species on short (day-to-day) and long (pre- versus post-perihelion) timescales. The relative abundances of these volatile species remained consistent within uncertainties during our pre-perihelion observations but tended to decrease during our post-perihelion observations (with the exception of CH{sub 3}OH and HCN). The short-timescale variability in the production rates of these volatiles could be due to diurnal effects (over the course of the rotation of the nucleus) and/or the effect of its outburst. The decrease in the production rates and hence the mixing ratios in some volatiles in post-perihelion dates could be due to the presence of seasonal effects in ER61.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Carbonyl Sulfide (OCS): Detections in Comets C/2002 T7 (LINEAR), C/2015 ER61 (PanSTARRS), and 21P/Giacobini–Zinner and Stringent Upper Limits in 46P/Wirtanen

Carbonyl sulfide (OCS) is one of the sulfur-bearing molecules detected in different astronomical environments, including comets. The present-day sulfur chemistry in comets may reveal much about the origin of these ices and their subsequent processing history. Cometary sulfur molecules such as H{sub 2}S, H{sub 2}CS, SO{sub 2}, SO, CS, CS{sub 2}, S{sub 2}, and NS have been detected in many comets. However, OCS, the only sulfur-bearing species with fluorescence emission lines at infrared wavelengths, is under-represented in comet volatile studies, having been reported in only six comets so far. We targeted OCS with the NASA Infrared Telescope Facility in comets 46P/Wirtanen, 21P/Giacobini–Zinner, and C/2015 ER61 (PanSTARRS) in 2017–2018 using the high-resolution iSHELL spectrograph, and in C/2002 T7 (LINEAR) in 2004 using the heritage CSHELL spectrograph. In comet C/2015 ER61, the OCS abundance was similar to those measured in bright comets such as comets C/2012 S1 (ISON) and C/1996 B2 (Hyakutake), whereas in C/2002 T7 it was relatively depleted. Our OCS measurement in 21P/Giacobini–Zinner is the first definitive detection of this molecule in a Jupiter-family comet from a ground-based facility and is close to the average OCS abundance determined in comet 67P/Churyumov–Gerasimenko by the Rosetta mission. Our 3σ upper limit for comet 46P/Wirtanen is the lowest reported OCS abundance in any comet. We present production rates and mixing ratios (with respect to H{sub 2}O) for these comets and place our results in the context of comets measured to date.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2020

Detection of a Water Tracer in Interstellar Comet 2I/Borisov

We present high spectral resolution optical spectra obtained with the ARCES instrument at Apache Point Observatory showing detection of the [O i] 6300 Å line in interstellar comet 2I/Borisov. We employ the observed flux in this line to derive an H2O production rate of (6.3 ± 1.5) × 1026 mol s−1. Comparing to previously reported observations of CN, this implies a CN/H2O ratio of ~0.3%–0.6%. The lower end of this range is consistent with the average value in comets, while the upper end is higher than the average value for solar system comets, but still within the range of observed values. C2/H2O is depleted, with a value likely less than 0.1%. The dust-to-gas ratio is consistent with the normal value for solar system comets. Using a simple sublimation model we estimate an H2O active area of 1.7 km2, which for current estimates for the size of Borisov suggests active fractions between 1% and 150%, consistent with values measured in solar system comets. More detailed characterization of 2I/Borisov, including compositional information and properties of the nucleus, is needed to fully interpret the observed H2O production rate.

McKay, Adam J.↗

NASA NTRS2020

Probing the Evolutionary History of Comets: an Investigation of the Hypervolatiles CO, CH4, and C2H6 in the Jupiter-Family Comet 21P/Giacobini–Zinner

Understanding the cosmogonic record encoded in the parent volatiles stored in cometary nuclei requires investigating whether evolution (thermal or otherwise) has modified the composition of short-period comets during successive perihelion passages. As the most volatile molecules systematically observed in comets, the abundances of CO, CH4, and C2H6 in short-period comets may serve to elucidate the interplay between natal conditions and post-formative evolution in setting present-day composition, yet secure measurements of CO and CH4 in Jupiter-family comets (JFCs) are especially sparse. The highly favorable 2018 apparition of JFC 21P/Giacobini–Zinner enabled a sensitive search for these "hypervolatiles" in a prototypical carbon-chain depleted comet. We observed 21P/Giacobini–Zinner with the iSHELL spectrograph at the NASA Infrared Telescope Facility on four pre-perihelion dates, two dates near-perihelion, and one post-perihelion date. We obtained detections of CO, CH4, and C2H6 simultaneously with H2O on multiple dates. We present rotational temperatures, production rates, and mixing ratios. Combined with previous work, our results may indicate that the hypervolatile coma composition of 21P/Giacobini–Zinner was variable across apparitions as well as within a particular perihelion passage, yet the spread in these measurements is a relatively small fraction of the variation in each molecule from comet to comet. We discuss the implications of our measured hypervolatile content of 21P/Giacobini–Zinner for the evolution of JFCs, and place our results in the context of findings from the Rosetta mission and ground-based studies of comets.

Roth, Nathan X.↗

NASA NTRS2019

The Peculiar Volatile Composition of CO-Dominated Comet C/2016 R2 (PanSTARRS)

Comet C/2016 R2 (PanSTARRS) has a peculiar volatile composition, with CO being the dominant volatile, as opposed to H2O, and one of the largest N2/CO ratios ever observed in a comet. Using observations obtained with the Spitzer Space Telescope, NASA’s Infrared Telescope Facility, the 3.5 m Astrophysical Research Consortium telescope at Apache Point Observatory, the Discovery Channel Telescope at Lowell Observatory, and the Arizona Radio Observatory 10 m Submillimeter Telescope, we quantified the abundances of 12 different species in the coma of R2 PanSTARRS: CO, CO2, H2O, CH4, C2H6, HCN, CH3OH, H2CO, OCS, C2H2, NH3, and N2. We confirm the high abundances of CO and N2 and heavy depletions of H2O, HCN, CH3OH, and H2CO compared to CO reported by previous studies. We provide the first measurements (or most sensitive measurements/constraints) on H2O, CO2, CH4, C2H6, OCS, C2H2, and NH3, all of which are depleted relative to CO by at least 1–2 orders of magnitude compared to values commonly observed in comets. The observed species also show strong enhancements relative to H2O, and, even when compared to other species like CH4 or CH3OH, most species show deviations from typical comets by at least a factor of 2–3. The only mixing ratios found to be close to typical are CH3OH/CO2 and CH3OH/CH4. The CO2/CO ratio is within a factor of 2 of those observed for C/1995 O1 (Hale- Bopp) and C/2006 W3 (Christensen) at a similar heliocentric distance, though it is at least an order of magnitude lower than many other comets observed with AKARI. While R2 PanSTARRS was located at a heliocentric distance of 2.8 au at the time of our observations in 2018 January/February, we argue, using sublimation models and comparison to other comets observed at similar heliocentric distance, that this alone cannot account for the peculiar observed composition of this comet and therefore must reflect its intrinsic composition. We discuss possible implications for this clear outlier in compositional studies of comets obtained to date and encourage future dynamical and chemical modeling in order to better understand what the composition of R2 PanSTARRS tells us about the early solar system.

McKay, Adam J.↗

NASA NTRS2019

Post-Perihelion Volatile Production and Release from Jupiter-Family Comet 45P/Honda-Mrkos-Pajdušáková

High-resolution infrared spectra of Jupiter-family comet 45P/Honda-Mrkos-Pajdušáková were obtained with NIRSPEC at the W. M. Keck Observatory on two post- perihelion dates (UT 2017 February 13 and 19), when the comet was at heliocentric distances of 1.01 and 1.10 AU, respectively. On UT February 13, H2O was measured simultaneously with six trace parent molecules: CH3OH, C2H6, HCN, NH3, C2H2, and H2CO. On UT February 19, CH4 and CO were also targeted in addition to the species measured on UT February 13. Abundances of CO, CH4, and C2H2 relative to H2O are consistent with values obtained from IRTF/iSHELL observations of 45P in early January 2017 just after perihelion when the heliocentric distance was 0.55–0.56 AU. Differences are seen in H2CO/H2O, C2H6/H2O, CH3OH/H2O, and HCN/H2O in February compared to January. Additionally, NH3 abundances appear highly variable during the February measurements, suggesting possible fluctuations of a factor of ten; however, there is significant uncertainty in quantifying NH3 owing to the marginal detections of only one or two lines on each of the two dates. Combining all infrared spectroscopic observations of 45P in January and February 2017, a post-perihelion relationship of Q(H2O) = (2.81 ± 0.25) × 10(exp 27) [R(sub h) (sup −3.83 ± 0.18)] molecules/s is derived. However, all measurements suggest significant variability in H O production on timescales of hours and days. Compared to other comets, volatile abundances relative to H2O in 45P are as follows: CO (depleted relative to all measured comets), CH3OH (enriched/all comets), CH4 and C2H6 (typical/all comets, enriched/Jupiter-family comets), C2H2 (depleted/all comets, typical/Jupiter-family comets), H2CO (January: typical, February depleted/all comets), HCN (January: severely depleted, February: typical/Jupiter-family comets). The small geocentric distances of the comet in February 2017 provide high spatial resolution in the coma of 45P (~12 and 19 km/pixel on February 13 and 19, respectively). Overall, the spatial distributions of volatiles and dust suggest a relatively symmetric and uniform coma during the UT February 13 and 19 observations, with small spatial differences noted between some volatile species. Measured C2H2 and HCN abundances are consistent with HCN as the primary parent of CN and C2H2 as a significant but not primary parent for C2, based on C2 and CN 2017 and previous apparitions. Extracted spectra show an increase in the dust-to-gas ratio in 45P from February 13 to 19 in agreement with contemporaneous narrowband optical measurements.

Russo, Neil Dello↗

NASA NTRS2018

Comet C/2013 V5 (Oukaimeden): Evidence for Depleted Organic Volatiles and Compositional Heterogeneity as Revealed through Infrared Spectroscopy

We obtained high-resolution (lambda divided by delta times lambda equals approximately 25,000) pre-perihelion spectra of Comet C/2013 V5 (Oukaimeden) using NIRSPEC at Keck II on UT 2014 September 5-6, and CSHELL (Cryogenic Echelle Spectrograph) at the NASA-Infrared Telescope Facility on September 11-13, altogether spanning a range in heliocentric distance R (sub h) equals 0.789-0.698 astronomical units. We report water production rates, and production rates and abundance ratios relative to co-measured H2O for eight trace molecules: CO, H2CO, CH3OH, CH4, C2H2, C2H6, HCN, and NH3. Our measured water production rates from NIRSPEC ( Near Infra-Red Spectrograph) and CSHELL observations remained relatively constant and were close to those from SOHO (Solar and Heliospheric Observatory) / Solar Wind Anisotropies observations that encompassed our dates, suggesting H2O production dominated by release directly from or within approximately 2000 kilometers of the nucleus. All trace volatiles were depleted relative to their respective median abundances among comets, excepting NH3, which was consistent with its median abundance. Most surprising were pronounced increases in abundance ratios for CH3OH (by 51 percent relative to simultaneously measured H2O) and especially C2H6 (by 87 percent) between September 5 and 6. On September 5, C2H6 was severely depleted, consistent with its lowest abundance yet measured for any comet. It also tracked the spatial profile of H2O, suggesting C2H6 was associated with a polar ice phase dominating gas production. On September 6, C2H6 was only moderately depleted and was spatially distinct from H2O, suggesting both polar- and nonpolar-dominated ice phases contributed to the activity then. Our results are consistent with a nonhomogeneous volatile composition for C/2013V5, implying differential processing of its constituent ices.

DiSanti, Michael A.↗

NASA NTRS2018

A Tale of "Two" Comets: The Primary Volatile Composition of Comet 2P/Encke Across Apparitions and Implications for Cometary Science

The highly favorable 2017 apparition of 2P/Encke allowed the first comprehensive comparison of primary volatile abundances in a given comet across multiple apparitions. This apparition offered opportunities to address pressing questions in cometary science, including investigating evolutionary and/or heliocentric distance (R(sub h)) effects on volatile production, sampling the hypervolatiles CO and CH4 in an ecliptic comet, and measuring volatile release at small R(sub h). The faintness and frequently low geocentric velocity of ecliptic comets during most apparitions make our near-infrared observations of these hypervolatiles rare and of high scientific impact. We characterized the volatile composition of 2P/Encke on three post-perihelion dates using the iSHELL spectrograph at the NASA Infrared Telescope Facility on Maunakea, HI. We detected fluorescent emission from nine primary volatiles (H2O, CO, C2H6, CH3OH, CH4, H2CO, NH3, OCS, and HCN) and three fragment species (OH*, NH2, and CN), and obtained a sensitive upper limit for C2H2. We report rotational temperatures, production rates, and mixing ratios (abundances relative to H2O). Compared to mean abundances in comets observed to date in the near-infrared, mixing ratios of trace gases in 2P/Encke were depleted for all species except H2CO and NH3, which were “normal.” The detection of the hypervolatiles CO and CH4 is particularly notable given the paucity of measurements in ecliptic comets. We observed significant differences in primary volatile composition compared to published pre-perihelion results from 2003 at larger R(sub h). We discuss possible mechanisms for these differences and discuss these results in the context of findings from the Rosetta mission and ground-based studies of comets.

Roth, Nathan X.↗

NASA NTRS2018

Evolution of H2O Production in Comet C/2012 S1 (ISON) as Inferred from Forbidden Oxygen and OH Emission

We present H2O production rates for comet C/2012 S1 (ISON) derived from observations of [O I] and OH emission during its inbound leg, covering a heliocentric distance range of 1.8–0.44 AU. Our production rates are in agreement with previous measurements using a variety of instruments and techniques and with data from the various observatories greatly differing in their projected fields of view. The consistent results across all data suggest the absence of an extended source of H2O production, for example sublimation of icy grains in the coma, or a source with spatial extent confined to the dimensions of the smallest projected field of view (in this case < 1000 km). We find that ISON had an active area of around 10 sq km for heliocentric distances Rh > 1.2 AU, which then decreased to about half this value from Rh = 1.2–0.9 AU. This was followed by a rapid increase in active area at about Rh = 0.6 AU, corresponding to the first of three major outbursts ISON experienced inside of 1 AU. The combination of a detected outburst in the light curve and rapid increase in active area likely indicates a major nucleus fragmentation event. The 5–10 sq km active area observed outside of Rh = 0.6 AU is consistent with a 50–100% active fraction for the nucleus, larger than typically observed for cometary nuclei. Although the absolute value of the active area is somewhat dependent on the thermal model employed, the changes in active area observed are consistent among models. The conclusion of a 50–100+% active fraction is robust for realistic thermal models of the nucleus. However the possibility of a contribution of a spatially unresolved distribution of icy grains cannot be discounted. As our [OI]-derived H2O production rates are consistent with values derived using other methods, we conclude that the contribution of O2 photodissociation to the observed [O I] emission is at most 5–10% that of the contribution of H2O for ISON. This is consistent with the expected contribution of O2 photodissociation if O2/H2O ∼ 4%, meaning [O I] emission can still be utilized as a reliable proxy for H2O production in comets as long as O2/H2O ≲ 4%, similar to the abundance measured by the ROSINA instrument on Rosetta at comet 67P/Churyumov–Gerasimenko.

Coma↗

NASA NTRS2017

Hypervolatiles in a Jupiter-Family Comet: Observations of 45P/Honda-Mrkos-Pajdusakova Using iSHELL at the NASA-IRTF

We used the new high spectral resolution cross-dispersed facility spectrograph, iSHELL, at the NASA Infrared Telescope Facility on Maunakea, HI, to observe Jupiter-family comet (JFC) 45P/Honda-Mrkos-Pajdusakova. We report water production rates, as well as production rates and abundance ratios relative to H2O, for eight trace parent molecules (native ices), CO, CH4, H2CO, CH3OH, HCN, NH3, C2H2, and C2H6, on 2 days spanning UT 2017 January 6/7 and 7/8, shortly following perihelion. Trace species were measured simultaneously with H2O and/or OH prompt emission, a proxy for H2O production, thereby providing a robust and consistent means of establishing the native ice composition of 45P. Its favorable geocentric radial velocity (approximately minus 35 kilometers per second) permitted sensitive measures of the "hypervolatiles" CO and CH4, which are substantially undercharacterized in JFCs. Our results represent the most precise ground-based measures of CO and CH4 to date in a JFC, providing a foundation for building meaningful statistics regarding their abundances. The abundance ratio for CH4 in 45P (0.79 percent plus or minus 0.06 percent relative to H2O) was consistent with its median value as measured among Oort Cloud comets, whereas CO (0.60 percent plus or minus 0.04 percent) was strongly depleted. Compared with all measured comets, HCN (0.049 percent plus or minus 0.012 percent) was strongly depleted, CH3OH (3.6 percent plus or minus 0.3 percent) was enriched, and the remaining species were consistent with their respective median abundances. The volatile composition measured for 45P could indicate processing of ices prior to their incorporation into its nucleus. Spatial analysis of emissions suggests enhanced release of more volatile species into the sunward-facing hemisphere of the coma.

DiSanti, Michael A.↗

NASA NTRS2016

An Infrared Search for HDO in Comet D/2012 S1 (ISON) and Implications for iSHELL

We performed a sensitive search for HDO in comet D/2012 S1 (ISON) on 2013 November 16, 17, and 22 using CSHELL and the NASA Infrared Telescope Facility. We constrained the HDO/H2O ratio to less than 2.0 VSMOW (the terrestrial ocean value) at the 3 sigma uncertainty level from two independent measurements corresponding to different H2O outgassing rates. This represents the best constrained HDO/H2O ratio for a comet using a small (3 m) telescope and illustrates that when CSHELL is replaced with iSHELL, 3 m class telescopes are still strong contenders for detecting minor volatile species in moderately bright comets.

Gibb, Erika L.↗

NASA NTRS2015

The CO2 Abundance in Comets C2012 K1 (PanSTARRS), C2012 K5 (LINEAR), and 290P Jager as Measured with Spitzer

Carbon dioxide is one of the most abundant ices present in comets and is therefore important for understanding cometary composition and activity. We present analysis of observations of CO2 and [O I] emission in three comets to measure the CO2 abundance and evaluate the possibility of employing observations of [O I] emission in comets as a proxy for CO2. We obtained NIR imaging sensitive to CO2 of comets C/2012 K1 (PanSTARRS), C/2012 K5 (LINEAR), and 290P/Jager with the IRAC instrument on Spitzer. We acquired observations of [O I] emission in these comets with the ARCES echelle spectrometer mounted on the 3.5-m telescope at Apache Point Observatory and observations of OH with the Swift observatory (PanSTARRS) and with Keck HIRES (Jager). The CO2/H2O ratios derived from the Spitzer images are 12.6 +/- 1.3% (PanSTARRS), 28.9 +/- 3.6% (LINEAR), and 31.3 +/- 4.2% (Jager). These abundances are derived under the assumption that contamination from CO emission is negligible. The CO2 abundance for PanSTARRS is close to the average abundance measured in comets at similar heliocentric distance to date, while the abundances measured for LINEAR and Jager are significantly larger than the average abundance. From the coma morphology observed in PanSTARRS and the assumed gas expansion velocity, we derive a rotation period for the nucleus of about 9.2 h. Comparison of H2O production rates derived from ARCES and Swift data, as well as other observations, suggest the possibility of sublimation from icy grains in the inner coma. We evaluate the possibility that the [O I] emission can be employed as a proxy for CO2 by comparing CO2/H2O ratios inferred from the [O I] lines to those measured directly by Spitzer. We find that for PanSTARRS we can reproduce the observed CO2 abundance to an accuracy of approximately 20%. For LINEAR and Jager, we were only able to obtain upper limits on the CO2 abundance inferred from the [O I] lines. These upper limits are consistent with the CO2 abundances measured by Spitzer.

McKay, Adam J.↗

NASA NTRS2013

Rotational Variation of Daughter Species Production Rates in Comet 103P/Hartley: Implications for the Progeny of Daughter Species and the Degree of Chemical Heterogeneity

We present analysis of high spectral resolution optical spectra of Comet 103P/Hartley taken during its Fall 2010 apparition. These spectra include transitions belonging to CN, C2, CH, NH2, and OI. We measure production rates and mixing ratios from these spectra. We find evidence for large changes in production rates (factors of a few) over the course of a nucleus rotation, in agreement with other measurements. We also measure variability with rotational phase in the CN/H2O and C2/CN ratios, which has not been previously reported for any comet. There may also be variability in the NH2/H2O ratio with rotational phase, but this trend is not as clear as for CN/H2O. We interpret the changing mixing ratios as due to H2O and C2 being released primarily from the icy grain halo, while the CN parent molecule comes directly from the nucleus. There is evidence that the CH/CN ratio is higher pre-perihelion than post-perihelion. We conclude that the observed CN and NH2 abundances are consistent with HCN and NH3 being the dominant parent molecules for these species. The C2 and CH abundances are higher than those of candidate parent molecules (C2H2 and CH4 respectively), so there must be another source for these molecules in 103P's coma. Carbonaceous dust grains could serve as this source.

Comets↗

NASA NTRS2013

Multiwavelength Observations of Recent Comets

Comets provide important clues to the physical and chemical processes that occurred during the formation and early evolution of the Solar System, and could also have been important for initiating prebiotic chemistry on the early Earth. Comets are comprised of molecular ices, that may be pristine inter-stellar remnants of Solar System formation, along with high-temperature crystalline silicate dust that is indicative of a more thermally varied history in the protosolar nebula. Comparing abundances of cometary parent volatiles, and isotopic fractionation ratios, to those found in the interstellar medium, in disks around young stars, and between cometary families, is vital to understanding planetary system formation and the processing history experienced by organic matter in the so-called interstellar-comet connection. We will present a comparison of molecular abundances in these comets to those observed in others, supporting a long-term effort of building a comet taxonomy based on composition.

ground-based observations↗

NASA NTRS2013

High-Resolution Infrared Spectroscopic Measurements of Comet 2PlEncke: Unusual Organic Composition and Low Rotational Temperatures

We present high-resolution infrared spectroscopic measurements of the ecliptic comet 2P/Encke, observed on 4-6 Nov. 2003 during its close approach to the Earth, using the Near Infrared Echelle Spectrograph on the Keck II telescope. We present flux-calibrated spectra, production rates, and mixing ratios for H2O, CH3OH, HCN, H2CO, C2H2, C2H6, CH4 and CO. Comet 2P/Encke is a dynamical end-member among comets because of its short period of 3.3 years. Relative to "organics-normal" comets, we determined that 2PlEncke is depleted in HCN, H2CO, C2H2, C2H6, CH4 and CO, but it is enriched in CH3OH. We compared mixing ratios of these organic species measured on separate dates, and we see no evidence of macroscopic chemical heterogeneity in the nucleus of 2P/Encke, however, this conclusion is limited by sparse temporal sampling. The depleted abundances of most measured species suggest that 2P/Encke may have formed closer to the young Sun, before its insertion to the Kuiper belt, compared with "organics-normal" comets - as was previously suggested for other depleted comets (e.g. C/1999 S4 (LINEAR)). We measured very low rotational temperatures of 20 - 30 K for H2O, CH3OH and HCN in the near nucleus region of 2P/Encke, which correlate with one of the lowest cometary gas production rates (approx. 2.6 x 10(exp 27) molecules/s) measured thus far in the infrared. This suggests that we are seeing the effects of more efficient radiative cooling, insufficient collisional excitation, and/or inefficient heating by fast H-atoms (and icy grains) in the observed region of the coma. Its extremely short orbital period, very low gas production rate, and classification as an ecliptic comet, make 2PlEncke an important addition to our growing database, and contribute significantly to the establishment of a chemical taxonomy of comets.

Radeva, Yana L.↗

NASA NTRS2012

Characterizing Abundances of Volatiles in Comets Through Multiwavelength Observations

Recently, there have been complimentary observations from multiple facilities to try to unravel the chemical complexity of comets. Incorporating results from various techniques, including: single-dish millimeter wavelength observations, interferometers, and/or IR spectroscopy, one can gain further insight into the abundances, production rates, distributions, and formation mechanisms of molecules in these objects [I]. Such studies have provided great detail towards molecules with a-typical chemistries, such as H2CO [2]. We report spectral observations of C/2007 N3 (Lulin), C/2009 R1 (McNaught), 103P/Hartley 2, and C/2009 P1 (Garradd) with the Arizona Radio Observatory's SMT and 12-m telescopes, as well as the NRAO Greenbank telescope and IRTF-CSHELL. Multiple parent volatiles (HCN, CH3OH, CO, CH4, C2H6, and H2O) as well as a number of daughter products (CS and OH) have been detected in these objects. We will present a comparison of molecular abundances in these comets to those observed in others, supporting a long-term effort of building a comet taxonomy based on composition. Previous work has revealed a range of abundances of parent species (from "organics-poor" to "organics-rich") with respect to water among comets [3,4,5], however the statistics are still poorly constrained and interpretations of the observed compositional diversity are uncertain. We gratefully acknowledge support from the NSF Astronomy and Astrophysics Program, the NASA Planetary Astronomy Program, NASA Planetary Atmospheres Program, and the NASA Astrobiology Program.

Milam, Stefanie N.↗

NASA NTRS2011

Multiwavelength Observations of Volatiles in Comets

Recently, there have been complimentary observations from multiple facilities to try to unravel the chemical complexity of comets. Incorporating results from various techniques, including: single-dish millimeter wavelength observations, interferometers, and/or IR spectroscopy, one can gain further insight into the abundances, production rates, distributions, and formation mechanisms of molecules in these objects [I]. Such studies have provided great detail towards molecules with a-typical chemistries, such as H2CO [2]. We report spectral observations of C/2006 M4 (SWAN), C/2007 N3 (Lulin), and C/2009 RI (McNaught) with the Arizona Radio Observatory's SMT and 12-m telescopes, as well as the NRAO Greenbank telescope and IRTFCSHELL. Multiple parent volatiles (HCN, CH3OH, CO, CH4, C2H6, and H2O) plus two photodissociation products (CS and OH) have been detected in these objects. We will present a comparison of molecular abundances in these comets to those observed in others, supporting a long-term effort of building a comet taxonomy based on composition. Previous work has revealed a range of abundances of parent species (from "organics-poor" to "organics-rich") with respect to water among comets [3,4,5], however the statistics are stiII poorly constrained and interpretations of the observed compositional diversity are uncertain.

Milam, Stefanie N.↗

NASA NTRS2010

Infrared Spectroscopy of Parent Volatiles in Comets: Implications for Astrobiology

Current cometary orbits provide information on their recent dynamical history. However, determining a given comet's formation region from its current dynamical state alone is complicated by radial migration in the proto-planetary disk and by dynamical interactions with the growing giant planets. Because comets reside for long periods of time in the outer Solar System, the ices contained in their nuclei (native ices) retain a relatively well-preserved footprint of when and where they formed, and this in turn can provide clues to conditions in the formation epoch. As a comet approaches the Sun, sublimation of its native ices releases parent volatiles into the coma where they can be measured spectroscopically. The past to - 15 years have seen the advent of infrared spectrometers with high sensitivity between about 2.8 and 5.0 micron, enabling a taxonomy among comets based on abundances of parent volatiles (e.g., H2O, CO, CH4, C2H6, HCN, CH30H, H2CO, NH3). Such molecules are of keen interest to Astrobiology, as they include important pre-biotic species that likely were required for the emergence of life on Earth and perhaps elsewhere. Approximately 20 comets have thus far been characterized, beginning with C/1996 82 (Hyakutake) in 1996. Molecular production rates are established through comparison of observed emission line intensities with those predicted by quantum mechanical fluorescence models. Abundances of parent volatiles (relative to H2O) vary among even the relatively small number of comets sampled, with the most volatile species (CO and CH4) displaying the largest variations. Techniques developed for measuring parent volatile abundances in comets will be discussed, as will possible implications for their formation.

DiSanti, Michael A.↗