H2O + ions in comets: models and observations

An improved magnetohydrodynamic (MHD) model with chemistry is presented. The analysis of the source and sink terms for H₂O ⁺ shows that for small comets up to 11% of water molecules are finally ionized. For large comets (such as Halley) this fraction decreases to less than 3%. From the MHD scaling laws a similarity law for the individual ion densities is deduced which takes into account that the mother molecules are depleted by dissociation. This is applied to H₂O ⁺ ions. Radial density profiles from model calculations, observations by Giotto near comet Halley, and ground based observations of three comets confirm this scaling law for H₂O ⁺ ions. From the similarity law for the density a scaling law for the column density is derived which is more convenient to apply for ground based observations. From these scaling laws methods are derived which allow the determination of the water production rate from the ground based images of the H₂O ⁺ ions. Finally, the two dimensional images of model column densities are compared with observations.     

Negative ions in the Enceladus plume

During Cassini’s Enceladus encounter on 12th March 2008, the Cassini Electron Spectrometer, part of the CAPS instrument, detected fluxes of negative ions in the plumes from Enceladus. It is thought that these ions include negatively charged water group cluster ions associated with the plume and forming part of the ‘plume ionosphere’. In this paper we present our observations, argue that these are negative ions, and present preliminary mass identifications. We also suggest mechanisms for production and loss of the ions as constrained by the observations. Due to their short lifetime, we suggest that the ions are produced in or near the water vapour plume, or from the extended source of ice grains in the plume. We suggest that Enceladus now joins the Earth, Comet Halley and Titan as locations in the Solar System where negative ions have been directly observed although the ions observed in each case have distinctly different characteristics.     

Formaldehyde polymers in comets

The possibility that crystalline formaldehyde polymers are present in cometary dust is discussed. In common with most other parent molecules proposed for comets, (H₂CO) _n is difficult to detect, even if it is present in relatively high concentrations. The optical properties of these polymers in the visual and infrared regions are similar to those of silicate grains, and crystalline formaldehyde polymers provide no emission at 6 cm wavelength. The lifetime of gaseous H₂CO in the solar radiation field is too short, and the expected transitions in the microwave region would be too weak to be detected. However, the available data concerning the physical properties of comets indicate that polymerized formaldehyde cannot be ruled out as a major constituent of cometary material.     

Invisible comets on evolutionary track of short-period comets

We systematically surveyed the orbits of short-period (SP) comets that show a large change of perihelion distance (q) between 1–2 AU (visible comets) and 4–5 AU (invisible comets) during 4400 years. The data are taken from Cosmo-DICE (Nakamura and Yoshikawa 1991a), which is a long-term orbital evolution project for SP comets. Recognizing that q is the most critical element for observability of comets, an invisibility factor (f), defined as the ratio of unobservable time span to observable span during 4400 years, is calculated for each of the large-q-change comets. A detection limit for each comet is obtained from the heliocentric distance at discovery and/or the absolute magnitude at recent apparitions. A mean f value for 35 SP comets with 2.9 J (J is the Tisserand's invariant) is found to be 19.8. This implies that for each visible SP comet of this J-range, at every epoch of time, there exist about 20 invisible comets near the capture orbits by Jupiter, under the assumptions of steady-state flux and ergodicity for the SP-comet population.     

Negative ions in the ionospheres of planetary bodies without atmospheres

Negative ions may be formed in the ionospheres of Mercury, the Moon and Jupiter's satellites with densities about a few % of the ionospheric electron density. The negative ions are produced by three mechanisms at the planetary surface: charge inversion during energetic proton scattering, with simultaneous secondary negative ion emission, and micrometeorite impacts. The density and distribution of negative ions around planetary bodies depends primarily upon the negative ion life-times determined by photodetachment by solar radiation.     

Long-term evolution of Oort Cloud comets: capture of comets

We test different possibilities for the origin of short-period comets captured from the Oort Cloud. We use an efficient Monte Carlo simulation method that takes into account non-gravitational forces, Galactic perturbations, observational selection effects, physical evolution and tidal splittings of comets. We confirm previous results and conclude that the Jupiter family comets cannot originate in the spherically distributed Oort Cloud, since there is no physically possible model of how these comets can be captured from the Oort Cloud flux and produce the observed inclination and Tisserand constant distributions. The extended model of the Oort Cloud predicted by the planetesimal theory consisting of a non-randomly distributed inner core and a classical Oort Cloud also cannot explain the observed distributions of Jupiter family comets. The number of comets captured from the outer region of the Solar system are too high compared with the observations if the inclination distribution of Jupiter family comets is matched with the observed distribution. It is very likely that the Halley-type comets are captured mainly from the classical Oort Cloud, since the distributions in inclination and Tisserand value can be fitted to the observed distributions with very high confidence. Also the expected number of comets is in agreement with the observations when physical evolution of the comets is included. However, the solution is not unique, and other more complicated models can also explain the observed properties of Halley-type comets. The existence of Jupiter family comets can be explained only if they are captured from the extended disc of comets with semimajor axes of the comets   a <5000 au  . The original flattened distribution of comets is conserved as the cometary orbits evolve from the outer Solar system era to the observed region.     

Methane in Oort cloud comets

We detected CH₄ in eight Oort cloud comets using high-dispersion (λ/Δλ∼2×10⁴) infrared spectra acquired with CSHELL at NASA's IRTF and NIRSPEC at the W.M. Keck Observatory. The observed comets were C/1995 O1 (Hale-Bopp), C/1996 B2 (Hyakutake), C/1999 H1 (Lee), C/1999 T1 (McNaught-Hartley), C/1999 S4 (LINEAR), C/2000 WM₁ (LINEAR), C/2001 A2 (LINEAR), and 153/P Ikeya-Zhang (C/2002 C1). We detected the R0 and R1 lines of the ν₃ vibrational band of CH₄ near 3.3 μm in each comet, with the exception of McNaught-Hartley where only the R0 line was measured. In order to obtain production rates, a fluorescence model has been developed for this band of CH₄. We report g-factors for the R0 and R1 transitions at several rotational temperatures typically found in comet comae and relevant to our observations. Using g-factors appropriate to T_rot as determined from HCN, CO and/or H₂O and C₂H₆, CH₄ production rates and mixing ratios are presented. Abundances of CH₄/H₂O are compared among our existing sample of comets, in the context of establishing their place of origin. In addition, CH₄ is compared to native CO, another hypervolatile species, and no correlation is found among the comets observed.     

Detecting active comets in the SDSS

Using a sample of serendipitously discovered active comets in the Sloan Digital Sky Survey (SDSS), we develop well-controlled selection criteria for greatly increasing the efficiency of comet identification in the SDSS catalogs. After follow-up visual inspection of images to reject remaining false positives, the total sample of SDSS comets presented here contains 19 objects, roughly one comet per 10 million other SDSS objects. The good understanding of selection effects allows a study of the population statistics, and we estimate the apparent magnitude distribution to r∼18, the ecliptic latitude distribution, and the comet distribution in SDSS color space. The most surprising results are the extremely narrow range of colors for comets in our sample (e.g. root-mean-square scatter of only ∼0.06 mag for the g-r color), and the similarity of comet colors to those of jovian Trojans. We discuss the relevance of our results for upcoming deep multi-epoch optical surveys such as the Dark Energy Survey, Pan-STARRS, and the Large Synoptic Survey Telescope (LSST), and estimate that LSST may produce a sample of about 10,000 comets over its 10-year lifetime.     

Infrared emission from comets

A brief discussion of the infrared observations from 4 to 20 micrometers of seven comets is presented. The observed infrared emission from comets depends primarily on their heliocentric distance. A model based on grain populations composed of a mixture of silicate and amorphous carbon particles in the mass ratio of about 40 to 1, with a power-law size distribution similar to that inferred for comet Halley, is applied to the observations. The model provides a good match to the observed heliocentric variation of both the 10 micrometers feature and the overall thermal emission from comets West and Halley. Matches to the observations of comet IRAS-Araki-Alcock and the antitail of comet Kohoutek require slightly larger grains. While the model does not match the exact profile and position of the 3.4 micrometers feature discovered in comet Halley, it does produce a qualitative fit to the observed variation of the feature's strength as a function of heliocentric distance. The calculations predict that the continuum under the 3.4 micrometers feature is due primarily to thermal emission from the comet dust when the comet is close to the Sun and to scattered solar radiation at large heliocentric distances, as is observed. A brief discussion of the determination of cometary grain temperatures from the observed infrared emission is presented. It is found that the observed shape of the emission curve from about 4 to 8 micrometers provides the best spectral region for estimating the cometary grain temperature distribution.     

Prediction of deuterium abundance in comets

Using a chemical scheme based on ion-molecule reactions in cool interstellar clouds, the possibility of a deuterium enrichment of volatile material in comets is discussed. It is assumed that the hydrogen-containing molecules are deuterated before accretion and condensation on the coremantle dust particles from which the cometesimals are formed. The D/H ratio in comets may be enhanced in respect to the average value by a factor of 10². Therefore comets are not promising objects for testing the primordial deuterium abundance.     

Viscous relaxation on comets

Three Jupiter family comets have now been observed by spacecraft with the surprising result that these comets lack unambiguous impact craters. Large-scale topography generally appears to be softened on these comets, although sharp topography is preserved at small scales. We find that viscous relaxation of water ice may explain these observations, given reasonable assumptions about ice grain size and temperatures attained in the interiors. We suggest that both the shapes and the cratering records of Jupiter family comets may be substantially modified and no longer reflect cometary formation processes or collisional evolution in the Kuiper Belt.     

Mass spectrometric measurements of fractional ion abundances in the stratosphere—Negative ions

Fractional abundances of stratospheric negative ions are for the first time explicitly reported. The measurements made by balloon-borne ion mass spectrometers also rely on recent studies of electric field induced collisional dissociation of negative cluster ions conducted at our laboratory. These indicate that the negative ion composition measurements around 36 km conducted by our group do not suffer from any significant dissociation. The new composition data support ion identifications NO₃^?(HNO₃)_b and HSO₄^?(H₂SO₄)_c(HNO₃)_d and the underlying ion reactions propo previously. Moreover, it is found that HSO₄^?(H₂SO₄)_g-ions appear to be particularly stable and that H₂SO₄-association is very fast. Implications of the ion composition data for ion processes are discussed.     

Do comets have satellites?

The recent evidence that many minor planets may have satellites, together with recently iscovered physical, chemical, and lightcurve similarities between minor planets and comets, lead naturally to the question, “Might comets have satellites also?” This paper explores several puzzling features of comets which do not fit easily into conventional cometary models, but which can be satisfactorily explained if it is assumed that comets have a full range of gravitationally bound masses, from dust size to the size of the nucleus, in orbit around the principal nucleus. This discussion also implies a higher probability of destruction of a spacecraft near a comet than is usually assumed.     

Coma morphology of Jupiter-family comets

We present a general review of cometary coma morphology, with specific reference to how it used in studies of Jupiter-family comets. We introduce the most common features that are seen in gas and dust observations, and summarize of how they are used to infer the properties of the nucleus and coma. We also expand the discussion to cover other topics relating to morphology, including the general shape of the coma (characterized by radial gradient profiles) and spatial maps of the color, albedo and polarization of the dust. We address the pros and cons of the different approaches used in the interpretation and analysis of the features. Finally, we review the results obtained for specific comets and compare the Jupiter-family comets to those from other classes.     

Orbital properties of Jupiter-family comets

The history of “comet families”, in particular the Jupiter comet family, is reviewed, together with ways in which the Jupiter family has been defined. New criteria are proposed, particularly with regard to distinguishing Jupiter-family comets (JFCs) from centaurs. The effect of nongravitational forces on JFCs is also discussed.     

A simple mapping for comets in resonance

The dynamical evolution of a low inclination, intermediate period comet in mean-motion resonance with a giant planet can be described by a simple mapping giving the comet's heliocentric energy and ecliptic longitude at successive aphelia. Provided that the energy oscillations produced by the resonance are not too large, this mapping is linear. When the amplitude of the oscillations is close to the maximum possible within the resonance, the mapping can be improved by adding non-linear terms.     

Thermal instability and transport effects in comets

The localized ion density depletion region observed at comet Halley can be understood in terms of a thermal instability of the cometary plasma due to the excitation of rotational and vibrational levels of water molecules. The electron energy losses due to these processes peak near 4000 K and at higher temperatures a localized cooling leads to the thermal instability due to the increased radiation loss. The resulting increase in recombination leads to the ion density depletion and the estimates for this depletion at comet Halley agree with the observations. The transport effects at the density depletion region due to the diffusion and thermal conductivity are found have time scales larger than the instability time scale and does not affect the formation of the depletion region.     

Combined mass spectrometric composition measurements of positive and negative ions in the lower ionosphere—II. Negative ions

Negative ion composition measurements of the lower ionosphere made simultaneously with positive ion measurements show some interesting features not previously observed. The higher sensitivity of this instrument allowed for detection of numerous previously undetected species. A layer of heavy ions (> 100 a.m.u.) was observed to exist between 80 and 90 km. The properties of this layer seem to indicate a meteoric source. The heavy ions are seen to exist down to the lowermost heights measured and their abundance increases below 58 km indicating a stratospheric source. Besides these ions, numerous new light ions were also seen. Their abundance level and sources are discussed.     

On the origin of comets

The paper presents a brief history of cometary cosmogony. It discusses critically the eruptive hypothesis, the hypothesis on the relict origin of comets, and the hypothesis on a genetic connection between comets and trans-Plutonian planets. Laplace’s theoretical prediction as to the capture of long-period comets by Jupiter into short-period orbits is confirmed. We conclude that the interstellar hypothesis promising is for the provenance of comets.     

Collision of comets with meteoroids

Statistical analysis of the quantity of dust in the cometary atmosphere in relation to the direction of motion of the comet about the Sun suggests an excess of dust for the retrograde comets. This excess is analyzed in the light of Harwit's theory of the cloud of “boulders” and of Öpik's impact theory. A comparison is also made between these excesses and other cometary phenomena such as splittings and outbursts.