Cometary Species Research Articles

Light scattering by cometary dust: Large-particle contribution

Using the discrete dipole approximation (DDA) and a model of agglomerated debris particles, we study the contribution of large particles on the light scattering by cometary dust. The numerical simulations show that moderately and highly absorbing particles larger than 3 µm do not significantly affect the backscattering phenomena observed at visible wavelengths in comets at phase angles α ≤ 30°, such as the brightness surge and the negative polarization branch. Weakly and non-absorbing particles may play some role even if they are larger than 3 µm; but they are not abundant species in comets. However, the contribution of large particles on polarizations at phase angles α > 30° can be significant, even though they are highly absorbing; simultaneously, the phase dependence of brightness remains insensitive to the contribution of large particles.

Hydrogen sulfide clathrate hydrate FTIR spectroscopy: A help gas for clathrate formation in the Solar System?

Clathrate hydrates may affect the stability and evolution of volatile species in comets and planets. Among abundant volatiles species, hydrogen sulfide forms one of the most stable clathrate hydrate and may play a major role in the initiation of the clathration process, as well as significantly affect the fraction of each volatiles species encaged in the clathrate structure with respect to the initial gaseous reservoir. In this study we investigate the hydrogen sulfide clathrate hydrate infrared signatures. Several pure and co-mixed hydrogen sulfide clathrate hydrates are produced and analyzed by infrared spectroscopy. The H2S clathrate hydrate specific vibrational transitions are recorded for an potential future search in planetary science and astrophysics, and its role as a clathrate hydrate promoter addressed.

The formation heritage of Jupiter Family Comet 10P/Tempel 2 as revealed by infrared spectroscopy

We present spectral and spatial information for major volatile species in Comet 10P/Tempel 2, based on high-dispersion infrared spectra acquired on UT 2010 July 26 (heliocentric distance Rh=1.44AU) and September 18 (Rh=1.62AU), following the comet’s perihelion passage on UT 2010 July 04. The total production rate for water on July 26 was (1.90±0.12)×1028moleculess−1, and abundances of six trace gases (relative to water) were: CH3OH (1.58%±0.23%), C2H6 (0.39%±0.04%), NH3 (0.83%±0.20%), and HCN (0.13%±0.02%). A detailed analysis of intensities for water emission lines provided a rotational temperature of 35±3K. The mean OPR is consistent with nuclear spin populations in statistical equilibrium (OPR=3.01±0.18), and the (1σ) lower bound corresponds to a spin temperature >38K. Our measurements were contemporaneous with a jet-like feature observed at optical wavelengths. The spatial profiles of four primary volatiles display strong enhancements in the jet direction, which favors release from a localized vent on the nucleus. The measured IR continuum is much more sharply peaked and is consistent with a dominant contribution from the nucleus itself. The peak intensities for H2O, CH3OH, and C2H6 are offset by ∼200km in the jet direction, suggesting the possible existence of a distributed source, such as the release of icy grains that subsequently sublimed in the coma. On UT September 18, no obvious emission lines were present in our spectra, nevertheless we obtained a 3σ upper limit Q(H2O)<2.86×1027moleculess−1.

Complex aromatic hydrocarbons in Stardust samples collected from comet 81P/Wild 2

Abstract– The successful return of the Stardust spacecraft provides a unique opportunity to investigate the nature and distribution of organic matter in cometary dust particles collected from comet 81P/Wild 2. Analysis of individual cometary impact tracks in silica aerogel using the technique of two‐step laser mass spectrometry demonstrates the presence of complex aromatic organic matter. While concerns remain as to the organic purity of the aerogel collection medium and the thermal effects associated with hypervelocity capture, the majority of the observed organic species appear indigenous to the impacting particles and are hence of cometary origin. While the aromatic fraction of the total organic matter present is believed to be small, it is notable in that it appears to be N rich. Spectral analysis in combination with instrumental detection sensitivies suggest that N is incorporated predominantly in the form of aromatic nitriles (R–C≡N). While organic species in the Stardust samples do share some similarities with those present in the matrices of carbonaceous chondrites, the closest match is found with stratospherically collected interplanetary dust particles. These findings are consistent with the notion that a fraction of interplanetary dust is of cometary origin. The presence of complex organic N containing species in comets has astrobiological implications as comets are likely to have contributed to the prebiotic chemical inventory of both the Earth and Mars.

The Deep Space 1 encounter with comet 19P/Borrelly

The Deep Space 1 Mission successfully encountered comet 19P/Borrelly on 22 September 2001, returning a wealth of images and in situ plasma measurements. To enhance the scientific return of the mission, it is desired to gain knowledge of basic properties of the cometary environment through the interpretation and analysis of the spacecraft data. For this purpose, a coma model for 19P/Borrelly during encounter is presented that consistently represents the cometary environment from the nucleus surface to the solar wind interaction region with detailed chemistry. Neutral gas and plasma dynamics and spatial distributions of various cometary species from the model are presented for interpreting in situ plasma measurements obtained during the encounter. Whereas several features of the plasma environment were well modeled, such as, the standoff distance of the bow shock and the ion velocity profile; the abundances of water-group ions around closest approach are not easily understood in terms of the coma chemistry model. Further data analysis and interpretation is needed.

Basic scientific results from the investigation of the halley comet coma glow by data of the three-channel spectrometer on board the venus-halley interplanetary station

More than 3000 unique spectra of the Halley comet in the near UV and visible region on 9, 10, and 11 March 1986 are registered and processed. Two-dimensional monochromatic images are constructed for different cometary species and the continuum. An original method for the dust continuum glow and gas emissions separation is worked out. Images of the emission spatial distribution of the cometary atmosphere are obtained. The strongest are the emissions of OH (considered here in details), C 2, CN, C 3 and CH. The neutral gas production rate and quantities describing their spatial distribution are determined by the measured intensities (Q OH ∼ 8×1029 mol/s). Results show strong anisotropic distribution of the dust and gas emission with the decreasing of the VEGA-comet distance. The hypothesis of jets structures origin near the nucleus is affirmed for the radicals and the dust continuum. A tomographic reconstruction of the dust continuum distribution around the nucleus is made. A model of isotropic and anisotropic dust ejection can approximately describe it. Two strong jets and dust shells are obtained.

Chemistry of O( 1D) atoms in the coma: implications for cometary missions

The forbidden red oxygen lines at 6300 and 6364 Å, which results due to 1D→ 3P transition, provide an important diagnostic tool in the study of comets. These lines cannot be produced by resonance fluorescence excitation of the ground-state oxygen atom, and therefore are mainly produced due to dissociation of H 2O and other O-containing species in comets (e.g., OH, CO, CO 2, H 2CO etc.) by photon and electron impact and in other collision reactions. Since the lifetime of 1D state is quite long (∼110 sec) collisional de-excitation processes are important. We have used a coupled chemistry-transport model in conjunction with an efficient emission production code to study the chemistry of O( 1D) atoms and the production of OI 6300 Å emission in comets. The model calculations are made for comet 46P/Wirtanen: the target of the ROSETTA mission. It is found that in the inner coma the density profile of O( 1D) is controlled dominantly by the H 2O. The model predicts ∼300 R of OI 6300 A brightness on comet Wirtanen.

Observational Constraints on the Abundance and Evolution of “XCN” in Interstellar Grain Mantles

The 4.62 μm "XCN" absorption feature, attributed to CN-bearing molecules in solids, is potentially an important diagnostic of the evolution of organic matter in the interstellar medium and the envelopes of newly formed stars. We report quantitative limits on the strength of this feature in the diffuse interstellar medium toward the reddened B-type star Cyg OB2 No. 12 and in the dark cloud toward the young stellar object R CrA IRS 2. On the basis of an assumed band strength for the carrier species, we estimate that less than 0.3% and less than 0.1% of the elemental nitrogen is in CN bonds along these lines of sight, respectively; if they are typical of diffuse and dense environments, it follows that the carrier of XCN is no more than a trace constituent of either organic-refractory or icy interstellar grain mantles. Appreciable XCN abundances seem to occur only in the envelopes of certain young stellar objects (YSOs), most notably the high-mass objects W33A and AFGL 7009S. We confirm the presence of XCN in the spectrum of the low-mass YSO R CrA IRS 7. The strengths of the XCN absorptions in R CrA IRS 7 and other low-mass YSOs indicate mean XCN concentrations relative to H2O in the ices of ~1%, comparable with the abundance of CN-bearing species in comets.

Measurement of Charge Exchange and X-Ray Emission Cross Sections for Solar Wind-Comet Interactions.

X-ray emission from a comet was observed for the first time in 1996. One of the mechanisms believed to be contributing to this surprisingly strong emission is the interaction of highly charged solar wind ions with cometary gases. Reported herein are total absolute charge-exchange and normalized line-emission (X-ray) cross sections for collisions of high-charge state (+3 to +10) C, N, O, and Ne ions with the cometary species H2O and CO2. It is found that in several cases the double charge-exchange cross sections can be large, and in the case of C3+ they are equal to those for single charge exchange. Present results are compared to cross section values used in recent comet models. The importance of applying accurate cross sections, including double charge exchange, to obtain absolute line-emission intensities is emphasized.

Excitation of X rays in comet Hyakutake (C/1996 B2)

There is great disagreement between current evaluations of the role of bremsstrahlung and line excitation by electron impact and recombination in cometary X rays. To study the problem in detail, we use the exact values of the Roentgen satellite (ROSAT) high‐resolution imager and wide‐field camera instrument response functions and apply them to all nine exposure intervals in the ROSAT observation of comet Hyakutake to determine X ray two‐parameter power law and thermal bremsstrahlung spectra. Thermal bremsstrahlung is a convenient form for two‐parameter spectrum and not a physical reality in comets. Then spectral index is α = 1.97 ± 0.18 ≈ 2, spectral temperature is kT = 360 eV, and the exposure‐mean photon emission is 3 × 1025 ph s−1 and 1.3 × 1016 ergs s−1 in a range of 90–2000 eV and in an aperture radius of 1.2 × 105 km. The exposure‐mean ROSAT spectrum is in excellent agreement with the Extreme Ultarviolet Explorer (EUVE) observation of 7.5 × 1024 ph s−1 in a range of 100–165 eV from comet Hyakutake. Then we follow the same methods as Northrop et al. [1997] to calculate a volume emission rate at the comet brightness maximum and to calculate that from bremsstrahlung with the Vega 2 electron flux. Yet we obtain a much lower emission than Northrop et al., at a level of 0.5% of the observed emission in accordance with results of Krasnopolsky [1996, 1997a] and Bingham et al. [1997]. Electron impacts of molecular, atomic, and ion species in comets produce mostly excitation of the X ray lines C 277 eV and O 525 eV at a level of 2% of the total X ray emission from comet Hyakutake. This value is calculated for the Vega 2 electron flux. The value for the Giotto electron flux is 3 × 10−6 of the measured emission. Processes which are responsible for impurity radiation in fusion plasma either are inefficient or produce ultraviolet photons in cometary plasma, and the method of Bingham et al. is inapplicable to excitation of X ray lines in comets. The calculations show that the expected effects of bremsstrahlung and electron impact are very low in cometary X rays. The obtained spectral index α = 2 differs significantly from α ∼ 5 for scattering of solar X rays by attogram dust. These facts favor charge transfer of heavy solar wind ions as the dominant process of X ray excitation in comet Hyakutake. Charge transfer is not efficient in production of X rays on the Moon and other solid bodies.

Spectra of Comet Hale-Bopp (C/1995 O1) Obtained with 6-M BTA Reflector at Mount Pastukhov

Between July 1996 and April 1997, 92 spectra of comet Hale-Bopp were obtained with the 6-meter BTA telescope of the SAO of the RAS at Mount Pastukhov. The spectra are two-dimensional, which allows one to determine the energy distribution for each emission along the slit and the energy distribution in wavelength for each individual position in the slit. From the 92 two-dimensional spectra covering the inner coma, detailed spectral maps of the total near-nuclear region of the comet are available for July 10, 11 and 12, 1996 and April 15, 1997. We propose an hypothesis about an unknown cometary species near λ = 620 nm in the spectrum obtained July 10, 1996. We also find an effect which may be caused by fluorescence of cometary dust.

Narrow-band IPD images of cometary CN and C2: the effect of solar activity on coma scales

Comet Halley and other cometary targets have been observed over a five-year period extending from 1985 August to 1990 May. 1-m and 24-inch telescopes have been used for near-nucleus studies and a selection of wide-angle Nikon camera lenses for large-scale structures. The imaging systems employed an Imaging Photon Detector in conjunction with narrow-band interference filters to isolate the emissions of various cometary species. Integration over these images give profiles extending over a large range in radius, facilitating accurate determinations of the CN and C 2 parent scalelengths, as well as the CN destruction scale. Dependence on solar activity is found and the results analysed separately for solar maximum and minimum conditions

Cometary constraints on the planet forming environment

Molecular, elemental and isotopic abundances of comets provide sensitive diagnostics for models of the primitive solar nebula. New measurements of the N 2 NH and NH 2 abundances in comets together with the in situ Giotto mass spectrometer and dust analyzer data provide new constraints for models of the comet forming environment in the solar nebula. An inventory of nitrogen-containing species in comet Halley indicates that NH 3 and CN are the dominant N carriers observed in the coma gas. The elemental nitrogen abundance in the gas component of the coma is found to be depleted by a factor ∼75 relative to the solar photosphere. Combined with the Giotto dust analyzer results for the coma dust component, we find for comet Halley N gas + dust ∼ 1 6 the solar value. The measurement of the CN carbon isotope ratio from the bulk coma gas and dust in comet Halley indicates a significantly lower value, 12 C 13 C = 65 ± 9 than the solar system value of 89 ± 2. Because the dominant CN carrier species in comets remains unidentified, it is not yet possible to attribute the low isotope ratio predominantly to the bulk gas or dust components. The large chemical and isotopic inhomogeneities discovered in the Halley dust particles on 1μ scales are indicative of preserved circumstellar grains which survived processing in the interstellar clouds, and may be related to the presolar silicon carbide, diamond and graphite grains recently discovered in carbonaceous chondrites. Less than 0.1% of the bulk mass in the primitive meteorites studied consists of these cosmically important grains. A larger mass fraction (∼5%) of chemically heterogeneous organic grains is found in the nucleus of comet Halley. The isotopic anomalies discovered in the PUMA 1 Giotto data in comet Halley are probably also attributable to preserved circumstellar grains. Thus the extent of grain processing in the interstellar environment is much less than predicted by interstellar grain models, and a significant fraction of comet nuclei (∼5%) may be in the form of preserved circumstellar matter. Comet nuclei probably formed in much more benign environments than primitive meteorites.

Cometary Molecules

Which molecular species are firmly identified as cometary species and which are also interstellar? How may excitation effects bias the data interpretation?

Dinosaurs victims of cyanide poisoning?

The Eos article, Comets and Life (March 28, 1989), reports on the work of Paul Thomas, Christophere Chyba, Carl Sagan and Leigh Brookshaw on cometary impact production of cyanides and other organics that may have been precursors of life. The article was based on material presented at the 20th Lunar and Planetary Science Conference [Thomas et al, 1989].As pointed out in the article, comets may contain 20% organic matter in the form of a complex interbonded mass. This kerogen‐like material contains CN bonds as well as CC and CO bonds. Evidence for cyanide protection from light‐element “CHON” solids was observed in the recent Halley's comet encounter in the form of CN plumes [Eberhardt et al., 1986; Schlosser et al., 1986]. An additional, and possibly more important source of cyanide, is HCN, which was observed [Schloerb et al., 1986] to be emitted from Halley's comet as part of the normal neutral gas emission with an abundance equal to 10−3 that of H2O. H2O is the dominant volatile species in comets and appears to constitute 80% or more of the total molecular release [Mendis, 1986].

Radio detection of formaldehyde emission from Comet Halley

The J(K-1 K1) = -1(11) -10(10) transition of H2CO was detected in emission at 4829.659 MHz from Comet Halley. The H2CO emission line had a peak intensity of 2.66 + or - 0.78 mJy/beam with a small blueshift of -0.76 + or - 0.40 km/s, which is consistent with the anisotropic outgassing of the nucleus in the solar direction found for other cometary species. Data analysis suggests that cometary H2CO was produced from an extended source in the coma as well as directly from the nucleus and that it was not refrigerated as in interstellar dark nebulae. The derived H2CO production rate of 1.5 x 10 to the 28th molecules/s is obtained which is consistent with observational and theoretical findings.

Interstellar molecules

Some 70 different molecular species have so far been detected variously in diffuse interstellar clouds, dense interstellar clouds and circumstellar shells. Only simple (diatomic and triatomic) species exist in diffuse clouds because of the penetration of destructive ultraviolet radiations, whereas more complex (polyatomic) molecules survive in dense clouds as a result of the shielding against this ultraviolet radiation provided by dust grains. A current list of interstellar molecules is given together with a few other molecular species that have so far been detected only in circumstellar shells. Also listed are those interstellar species that contain rare isotopes of several elements. The gas phase ion chemistry is outlined via which the observed molecules are synthesized, and the process by which enrichment of the rare isotopes occurs in some interstellar molecules is described. Reference is also made briefly to some very recent work in interstellar ion chemistry. A list of the atomic and molecular species that have been detected in cometary atmospheres is given and attention is drawn to the similarities and differences between interstellar and cometary molecules. The physical and chemical processes by which these observed cometary species may be generated from material that sublimes from the cometary nucleus are discussed.

Convolution of cometary brightness profiles by circular diaphragms

A method for determining the production and decay scale lengths of neutral cometary species from filtered photometric scans of cometary comae has been developed. The column densities determined from Haser's (1957) two-parameter model have been integrated over a representative sample of relative diaphragm sizes to produce brightness profiles and photometric gradients for the usual set of scale-length ratios. The convolved model has been applied to brightness profiles of C2, CN, and NH, and their scale lengths are determined. The scale lengths at a heliocentric distance of 0.664 AU for radical and parent, respectively, measured in 10,000 kilometers were found to be 6.7 and 0.41 for C2, 8.5 and 1.3 for CN, and 18 and 0.25 for NH. The results for CN and C2 are consistent with those found for other comets. The scale lengths for C2 and parent at a heliocentric distance of 0.713 AU are 12 and 0.90, respectively. This anomalously large scale length for C2 is directly attributed to the non-steady-state activity of the nucleus.