Daniel C. Boice

Cometary gas and plasma flow with detailed chemistry

Abstract This paper describes recent hydrodynamic and magnetohydrodynamic (MHD) simulations for the gas and plasma flow around a comet with detailed photo and chemical reaction network of 59 neutral and 76 ionized chemical species. The method allows for a separate energy balance of the electrons, separate flow of neutral gas, fast neutral atomic and molecular hydrogen, and plasma with momentum exchange by elastic collisions, mass-loading of the plasma flow by ion pick-up, and Lorentz-forces of the advected magnetic field. The contact surface or magnetopause is resolved. The simulation is applied to comet Halley at the time of the Giotto encounter. The results can be compared with the data from the Giotto mission as they become available. The chemical composition of the nucleus can be inferred from these data by iteration.

Cometary MHD and chemistry

Our magneto-hydrodynamical (MHD) and chemical comet-coma model has been applied to describe and analyze the plasma flow, the magnetic field and the ion abundances in comet P/Halley in a consistent manner. We assume the volatile composition to consist of 80% water and 20% carbon-, nitrogen-, oxygen-, and sulfur-compounds. The radius of the nucleus is 3.36 km. With hemispherical illumination, this is equivalent to the active area on the nucleus during the Giotto encounter. The physics and chemistry of the coma are modeled in great detail, including photoprocesses, gas-phase chemical kinetics, energy balance with a separate electron temperature, multifluid hydrodynamics with a transition to free molecular flow, fast-streaming atomic and molecular hydrogen, counter and cross-streaming of the ionized species relative to the neutral species in the coma-solar wind interaction region with momentum exchange by elastic collisions, mass-loading through ion pick-up, and Lorentz-forces of the advected magnetic field. A comparison of the results is made with the data from the Giotto mission, especially from the HIS ion mass spectrometer. We resolve the contact surface (magnetopause); its position is in agreement with observations. We also find the enhancement of the ion density just outside of the contact surface and agreement for the three groups in the ion mass spectra, particularly for the first group up to 21 amu.

ORTHO-TO-PARA RATIOS OF WATER AND AMMONIA IN COMET C/2001 Q4 (NEAT): COMPARISON OF NUCLEAR SPIN TEMPERATURES OF WATER, AMMONIA, AND METHANE

Cold nuclear spin temperatures found in cometary molecules have been simply interpreted as the physical temperature when the population distribution among different nuclear spin isomers was determined through thermal equilibrium processes. However, the real meaning of cold nuclear spin temperatures is unclear due to a lack of experimental studies about the ortho-to-para ratios (OPRs) of molecules in cometary ice analogs. Here we report the nuclear spin temperatures (Tspin) of water and ammonia in comet C/2001 Q4 (NEAT). Measurements of the nuclear spin temperatures of these species and methane from previous work are all consistent with ~30 K. Consistency of the nuclear spin temperatures among different molecular species may suggest that OPRs (or abundance ratios of different nuclear spin isomers) of these molecules were last determined in thermal equilibrium. The obtained nuclear spin temperature of cometary ices is not consistent with molecular formation by hydrogen-atom addition reactions on cold grains, where the H atoms accreted from the gas phase onto grains. The condensation process on the grains might control the ortho-to-para ratios of the precometary ices, or conversion of OPRs within the ices might occur. The small diversity of the nuclear spin temperatures and lack of clear correlation between Tspin and chemical composition in several comets are consistent with the hypothesis that Tspin reflects the temperatures in the presolar nebula.

The chemistry of C2 and C3 in the coma of Comet C/1995 O1 (Hale-Bopp) at heliocentric distances rh ≥ 2.9 AU

The extraordinary activity of comet C/1995 O1 (Hale-Bopp) made it possible to observe the emission bands of the radicals C2 and C3 in the optical wavelengths range at heliocentric distances larger than 3 AU. Based on these observations, we perform an analysis of the formation of C2 and C3 in a comet coma at large heliocentric distances. We present the most complete chemical reaction network used until today, computing the formation of C2 and C3 from C2H2 ,C 2H6 ,a nd C 3H4 as their parent molecules. The required photodissociation rates of C3H2 and C3 had to be derived based on the observations. The spatial distributions of C2 and C3 calculated with the chemical model show good agreement with the observations over the whole range of heliocentric distances covered in this work. Based on the production rates for C2H2 ,C 2H6 ,a nd C 3H4, abundance ratios are obtained for heliocentric distances rh ≥ 3 AU. In comet Hale-Bopp, C2H2 and C2H6 were measured directly by infrared observations only at heliocentric distance rh ≤ 3 AU (Dello Russo et al. 2001). The model presented here greatly extends the heliocentric distance range over which hydrocarbons can be studied in the coma of comet Hale-Bopp. We discuss possible indications of these abundance ratios for the formation region of comet Hale-Bopp.

Nuclear Spin Temperature and Deuterium-to-Hydrogen Ratio of Methane in Comet C/2001 Q4 (NEAT)*

We carried out high-dispersion, spectroscopic observations of comet C/2001 Q4 (NEAT) in the near-infrared with the 8 m Subaru telescope and detected the R-branch emission series of the ν3 vibrational band of methane. The signal-to-noise ratio of the observed spectrum was sufficient to make the first determination of the nuclear spin temperature of methane, derived to be 33 K, which reflects the temperature of formation or condensation of molecules on cold grains. The upper limit of the CH3D/CH4 ratio was determined to be 0.04 (95% confidence limit), indicating the formation of methane in a dense molecular cloud at temperatures higher than about 30 K. On the basis of these observational results, we conclude that the Sun was born in a warm molecular cloud near 30 K, not in a cold dark cloud near 10 K, as is usually assumed.

The deep space 1 encounter with comet 19P/Borrelly

NASAs Deep Space 1 (DS1) spacecraft successfully encountered comet 19P/Borrelly near perihelion and the Miniature Integrated Camera and Spectrometer (MICAS) imaging system onboard DS1 returned the first high-resolution images of a Jupiter-family comet nucleus and surrounding environment. The images span solar phase angles from 88° to 52°, providing stereoscopic coverage of the dust coma and nucleus. Numerous surface features are revealed on the 8-km long nucleus in the highest resolution images(47–58 m pixel). A smooth, broad basin containing brighter regions and mesa-likestructures is present in the central part of the nucleus that seems to be the source ofjet-like dust features seen in the coma. High ridges seen along the jagged terminator lead to rugged terrain on both ends of the nucleus containing dark patches and smaller series of parallel grooves. No evidence of impact craters with diameters larger thanabout 200-m are present, indicating a young and active surface. The nucleus is very dark with albedo variations from 0.007 to 0.035. Short-wavelength, infrared spectra from 1.3 to 2.6 μm revealed a hot, dry surface consistent with less than about10% actively sublimating. Two types of dust features are seen: broad fans and highlycollimated “jets” in the sunward hemisphere that can be traced to the surface. The source region of the main jet feature, which resolved into at least three smaller “jets” near the surface, is consistent with an area around the rotation pole that is constantly illuminated by the sun during the encounter. Within a few nuclear radii, entrained dustis rapidly accelerated and fragmented and geometrical effects caused from extended source regions are present, as evidenced in radial intensity profiles centered on the jet features that show an increase in source strength with increasing cometocentric distance. Asymmetries in the dust from dayside to nightside are pronounced and may show evidence of lateral flow transporting dust to structures observed in the nightside coma. A summary of the initial results of the Deep Space 1 Mission is provided, highlighting the new knowledge that has been gained thus far.

14NH2/15NH2 RATIO IN COMET C/2012 S1 (ISON) OBSERVED DURING ITS OUTBURST IN 2013 NOVEMBER*

We performed high-dispersion optical spectroscopic observations of comet C/2012 S1 (ISON) using the High Dispersion Spectrograph (R = 72,000) at the Subaru Telescope on UT 2013 November 15.6, during an outburst that started on UT 2013 November 14. Due to the high gas-production rate of NH2 during the outburst, we successfully detected weak emission lines of 15NH2 and many strong emission lines of 14NH2 in the optical wavelength region from 5500 to 8200 ?. The ratio of 14NH2/15NH2 is derived to be 139 ? 38 in comet C/2012 S1 (ISON). This ratio is close to that recently revealed based on the averaged spectrum of 12 comets, ~130. This is also comparable to the typical cometary isotopic ratio of CN (12C14N/12C15N, observed in optical) and HCN (H12C14N/H12C15N, observed in radio), ~150. However, these ratios are much smaller than the protosolar value, 14N/15N = 441 ? 5. Because NH2 is considered to be a photodissociation product of NH3 in cometary coma, our result implies the occurrence of 15N fractionation of NH3 in the solar nebula or in the presolar molecular cloud.

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.

Neutral sodium tails in comets

Abstract In the last few years, after a new type of tail composed only of neutral sodium atoms had been discovered in Comet Hale-Bopp, much attention has been given to the release and distribution of sodium in comets. It was not the first time that a sodium tail had been observed in a comet, but the amount of data collected from Comet Hale-Bopp in general and on the sodium emissions in particular is much larger than for any other comet. Several authors tried to identify the sources of the sodium and explain its distribution. However, no clear solution how neutral sodium atoms can be correlated with gas and dust particles has resulted. The presence of a sodium tail superimposed on the dust tail suggests a strong relation of sodium with the dust emitted by the nucleus. Furthermore, analysis of the dust data in the coma and tail provides important insights in the chemical and physical processes relevant to dust particles. In order to investigate whether release of sodium atoms from a Na-bearing molecule may form an distributed source, we have calculated photodissociation rate coefficients (inverse lifetimes) and excess energies for several Na-bearing molecules. The calculations are based on measured cross sections near the dissociation threshold combined with cross sections at short wavelengths from the separated atom approximation for several Na-bearing molecules in the solar radiation field. Sodium can be used as a tracer of mechanisms working on other elements that are more difficult to observe. Wide interest in the sodium emissions stimulated reanalysis of wide-field images of Comet Hyakutake. It was found that this comet also had a neutral sodium atom tail, showing that Na tails might be more common than previously thought. Our preliminary conclusion is that the sodium in the fast, narrow tail may also originate from fragmenting dust in the inner coma.

ORTHO-TO-PARA ABUNDANCE RATIO OF WATER ION IN COMET C/2001 Q4 (NEAT): IMPLICATION FOR ORTHO-TO-PARA ABUNDANCE RATIO OF WATER*

The ortho-to-para abundance ratio (OPR) of cometary molecules is considered to be one of the primordial characteristics of cometary ices, and contains information concerning their formation. Water is the most abundant species in cometary ices, and OPRs of water in comets have been determined from infrared spectroscopic observations of H2O rovibrational transitions so far. In this paper, we present a new method to derive OPR of water in comets from the high-dispersion spectrum of the rovibronic emission of H2O+ in the optical wavelength region. The rovibronic emission lines of H2O+ are sometimes contaminated by other molecular emission lines but they are not affected seriously by telluric absorption compared with near-infrared observations. Since H2O+ ions are mainly produced from H2O by photoionization in the coma, the OPR of H2O+ is considered to be equal to that of water based on the nuclear spin conservation through the reaction. We have developed a fluorescence excitation model of H2O+ and applied it to the spectrum of comet C/2001 Q4 (NEAT). The derived OPR of water is 2.54+0.32 – 0.25, which corresponds to a nuclear spin temperature (T spin) of 30+10 – 4 K. This is consistent with the previous value determined in the near-infrared for the same comet (OPR = 2.6 ± 0.3, T spin = 31+11 – 5 K).