Dust Properties of Comets Observed by Spitzer
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David E. Harker↗
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Comet West and the scattering function of cometary dust
Observations of Comet West (1975n) at wavelengths from 0.5 to 18 microns and at a variety of scattering angles are used to infer the scattering phase function for the cometary dust. This function is strongly peaked in the forward direction. The form of the function indicates that the particles are dielectric grains with radii of approximately 1 micron. Abrupt increases in the intrinsic brightness of the coma (both in scattered sunlight and in thermal emission) are consistent with the projected times of comet fragmentation.
Ney, E. P.↗
Comet Shoemaker-Levy 9 Dust Size and Velocity Distributions
Pre-impact observations of Comet Shoemaker-Levy 9 (S-L9) obtained with the Hubble Space Telescope are examined, and a model of an active, dust-producing comet is fitted to images of fragments G, H, K, and L. The model assumes steady isotropic dust emission from each fragment's sunlit hemisphere. Best-fit results indicate that the dominant light-scatterers in these fragments' comae were relatively large dust grains of radii 10 micrometers < R < 3 mm. The fragments' dust size distributions were rather flat in comparison to other comets, dN(R) proportional to R(sup -2.3 +/- 0.1), and the dust ejection speeds were approximately 0.5-1.5 m/s. The S-L9 fragments themselves were not detected directly, and upper limits on their radii are 1.0-1.5 km assuming an albedo a = 0.04. However, these fragments' vigorous production of dust, which ranges from 6 to 22 kg/s, places a lower limit of approximately 100 m on their radii at the moment of tidal breakup. Any fragments smaller than this limit, yet experiencing similar mass loss rates, would have dissipated prior to impact. Such bodies would fail to leave an impact scar at Jupiter's atmosphere, as was realized by fragments F, J, P(sub 1), P(sub 2), T, and U.
Hahn, Joseph M.↗
Coma morphology and dust-emission pattern of periodic Comet Halley. IV - Spin vector refinement and map of discrete dust sources for 1910
A hypothetical continuous ejection of dust from discrete emission sources on the sunlit hemisphere of a rotating comet nucleus is tested against 34 digitally processed plates of Comet Halley over the May 5-June 6, 1910 period, as well as the reported projected lengths of six antisunward jets observed between May 14 and June 6, 1910. A map of the comet's nucleus is presented of whose 13 discrete dust sources only two can be modeled as point-like regions. Attention is given to curious cases of 'chain' activation in which the sources that intersect one another on the surface emit dust sequentially, in successive rotations.
Sekanina, Z.↗
Interpretation of the continuous spectra of comets.
Cometary dust studied by analysis of scattered sunlight in heads and tails of comets
LIGHT SCATTERING↗
Molecular and Elemental Dust Analyzer (MEDA) for the comet rendezvous asteroid flyby mission
Comets are some to the most primitive bodies in the solar system and therefore should contain elemental, chemical, and isotopic records of the early history of the solar system. An opportunity to perform in situ analyses of a comet nucleus exists with the Comet Rendezvous Asteroid Flyby (CRAF) mission. An integrated gas chromatograph/X-ray fluorescence instrument (MEDA), being proposed for inclusion onboard the CRAF spacecraft, will measure the molecular and elemental constituents of collected dust grains and ices. The gas chromatograph, employing helium ionization detectors and three columns designed to separate light gases, polar gases, and hydrocarbons will measure the volatile compounds of the biogenic elements thermally released from collected dust grains. The sensitivity of the GC for compounds of interest is at the picogram level. X-ray fluorescence utilized cryogenically cooled Si(Li) solid state detectors of nominal 150 eV resolution at 5.9 keV. Based on laboratory work with carbonaceous meteorites, both the GC and XRF can perform meaningful analyses with a few micrograms of collected comet dust.
Ohara, B. J.↗
Coma morphology and dust-emission pattern of periodic Comet Halley. II - Nucleus spin vector and modeling of major dust features in 1910
The continuous ejection of dust from discrete emission sources on the rotating nucleus of the Comet Halley is modelled in order to explain the evolution of spiral jets which unwind from the nucleus condensation into envelopes or halos in the comet head. The model is applied to digitally processed images of three features of the comet taken from Mount Wilson plates during the 1910 fly-by. The model permits a determination of the motion and spin vector for each emission source, its cometocentric coordinates, and a function relating particle ejection velocity to the solar radiation pressure exerted on the ejecta. It is found that the obliquity of the comet orbit's plane to its equatorial plane is 45 deg, the axis of rotation period of 17.3 days. The derived function of particle ejection velocity to the solar radiation pressure implied no contribution from grains larger than 10 microns in radius. High dust loading of gas flows from the June 1910 emission sources is indicated. It is estimated that because of the favorable approach geometry of the Gioto spacecraft during its 1986 flyby, the likelihood of encountering dense jets of dust is small.
Sekanina, Z.↗
Silicate Stardust from Comets
Interplanetary dust particles (IDPs) collected in the Earth s stratosphere are 5-50 pm fragments of asteroids and comets. Anhydrous IDPs have escaped significant parent body hydrothermal alteration and are further distinct from meteorites in their greater abundances of volatile elements and C, fine grained (l00 - 500 nm) textures, and unequilibrated mineralogies. Many anhydrous IDPs contain high abundances interstellar organic compounds and grains of silicate stardust. These observations are consistent with properties of comets inferred from remote astronomical observations. Comets have been thought to be pristine aggregates of interstellar materials. However, spectroscopic observations of crystalline silicates in comets has challenged this notion, given their apparent absence in the interstellar medium.
Messenger, S.↗
SOFIA FORCAST Far-IR Photometry of Comet ISON and Constraints on the Coma Grain Size Distribution
Comet C/2012 S1 (ISON) was unique in that it was a dynamically new comet derived from the nearly isotropic Oort cloud reservoir of comets with a sun-grazing orbit. Infrared (IR) observations were executed on NASA's Stratospheric Observatory For Infrared Astronomy (SOFIA) by the FORCAST instrument on 2013 October 25 UT (r(sub h)=1.18 AU, Delta=1.5AU). Photometry was obtained in FORCAST filters centered at 11.1, 19.7, and 31.5 micron. The observations compliment a large world-wide effort to observe and characterize comet ISON.
Comet dust properties↗
Dust in space and comets; Proceedings of the Topical Meeting, Graz, Austria, June 25-July 7, 1984
Consideration is given to: the mass loading of planetary magnetospheres by rocky satellites; the effects of electrostatic forces on the vertical structure of planetary rings; and dust motion in Jupiter's tilted magnetic field. Other topics include: IRAS observations of cometary dust; dust environment models for Comet P/Halley; plasma processes and solar wind interaction; and cometary interplanetary field enhancements in the solar wind. Consideration is also given to: the impact of dust grains on fast fly-by spacecraft; EUV observations of Comet Halley; and the thermal model and thermo-mechanical stresses in cometary nuclei.
Morfill, G. E.↗
Lightcurve of comet Austin(1989c1) and its dust mantle development
Brightness variations of comet Austin(1989c1) were investigated in terms of the variations of water production rate. We translated the visual brightness data into water production rates using Newburn's semi-empirical law. The curve of the water production rates as a function of heliocentric distance was compared with the model calculations that assumed energy balance between the solar incident and vaporization of water. Thermal flow in a dust mantle at a surface of the nucleus is also included in the model. The model calculations including the dust mantle are more favorable for the observed rate than non-dust mantle cases. The extinction after the perihelion passage suggests that the dust mantle developed gradually.
Hasegawa, Hitoshi↗
A comparison of the dust properties in recent periodic comets
Measurements of the thermal emission from the cometary dust coma can be used to derive the rate of dust production from the nucleus as well as the size distribution of absorbing grains. More than ten short-period comets have now been observed in the infrared over a wide range in heliocentric distance. Dust production rates are derived for these comets based on theoretical models of the thermal emission from small absorbing grains and calculations of dust grain velocities. The mean size and albedo of the dust grains is similar in these comets, with the exception of Comet Crommelin, which seems to have had larger, darker grains.
Hanner, M. S.↗
Sulfur in Cometary Dust
The computer-intensive project consisted of the analysis and synthesis of existing data on composition of comet Halley dust particles. The main objective was to obtain a complete inventory of sulfur containing compounds in the comet Halley dust by building upon the existing classification of organic and inorganic compounds and applying a variety of statistical techniques for cluster and cross-correlational analyses. A student hired for this project wrote and tested the software to perform cluster analysis. The following tasks were carried out: (1) selecting the data from existing database for the proposed project; (2) finding access to a standard library of statistical routines for cluster analysis; (3) reformatting the data as necessary for input into the library routines; (4) performing cluster analysis and constructing hierarchical cluster trees using three methods to define the proximity of clusters; (5) presenting the output results in different formats to facilitate the interpretation of the obtained cluster trees; (6) selecting groups of data points common for all three trees as stable clusters. We have also considered the chemistry of sulfur in inorganic compounds.
Fomenkova, M. N.↗
The fragmentation of dust in the innermost comae of comets: Possible evidence from ground-based images
Dust particles when released from the nucleus of a comet are entrained in the expanding gas flow created by the vaporization of ices (mainly water ice). Traditional approaches to dusty-gas dynamics in the inner comae of comets consider there to be an initial distribution of dust particle sizes which do not fragment or evaporate. The standard Finson-Probstein model (and subsequent variations) yields a one-to-one-to-one correspondence between the size of a dust particle, its terminal velocity owing to gas drag, and its radiation pressure acceleration which creates the notable cometary dust tail. The comparison of a newly developed dust coma model shows that the typical elongated shapes of isophotes in the dust comae of comets on the scale of greater than 10(exp 4) km from the nucleus requires that the one-to-one-to-one relationship between particle size, terminal velocity and radiation pressure acceleration cannot in general be correct. There must be a broad range of particles including those having a small velocity but large radiation pressure acceleration in order to explain the elongated shape. A straightforward way to create such a distribution is if particle fragmentation, or some combination of fragmentation with vaporization, routinely occurs within and/or just outside of the dusty-gas dynamic acceleration region (i.e., up to several hundred km). In this way initially large particles, which are accelerated to fairly slow velocities by gas-drag, fragment to form small particles which still move slowly but are subject to a relatively large radiation pressure acceleration. Fragmentation has already been suggested as one possible interpretation for the flattened gradient in the spatial profiles of dust extracted from Giotto images of Comet Halley. Grain vaporization has been suggested as a possible spatially extended source of coma gases. The general elongated isophote shapes seen in ground-based images for many years represents another possible signature of fragmentation.
Combi, Michael R.↗
Dust release rates and dust-to-gas mass ratios of eight comets
Mass release rates of dust and mass ratios of dust-to-gas release rates of Comets Thiele (1985m), Wilson (1986l), P/Borrelly (1987p), Liller (1988a), Bradfield (1987s), Hartley-Good (1985l), P/Giacobini-Zinner (1984e), and P/Halley (1982i) are estimated from the analysis of continuum flux measurements at optical wavelengths. An attempt is made to estimate the size of each comet nucleus on the basis of water-ice sublimation (vaporization), assuming that the nucleus is spherical and only a fraction of its surface area is active. Where possible, the dust mass release rates are compared with those obtained by other investigators in the optical and IR wavelength regions. Good agreement with results based on IR observations is found.
Singh, P. D.↗
Dust density and mass distribution near comet Halley from Giotto observations
The density and the mass spectrum of the dust near comet Halley have been measured by the Giotto space probe's dust impact detection system. The dust spectrum obtained at 291,000 km from the comet nucleus show depletion in small and intermediate masses; at about 600 km from the nucleus, however, the dust activity rises and the spectrum is dominated by larger masses. Most of the mass striking Giotto is noted to reside in the few large particles penetrating the dust shield. Momentum balances and energy considerations applied to an observed deceleration suggest that a large mass of the spacecraft was detached by an impact.
Mcdonnell, J. A. M.↗
Gas and dust in some recent periodic comets
Results are presented for narrow-band photometry of four recent periodic comets. The observed fluxes are used to estimate carbon radical production rates, gas-to-dust ratios, and the colors of the dust grains. It is found that in most respects the periodic comets exhibit the same range of properties typical of parabolic comets. Comet P/Chernykh is one of the dustiest comets on record, and its dust is physically different from that of the other comets considered. A correlation between the gas-to-dust ratio and the color of the grains is tentatively proposed. The need for homogeneous data on a large number of comets is emphasized.
Ahearn, M. F.↗
Stardust Sample Collection at Wild 2 and Its Preliminary Examination
The primary objective of STARDUST is to collect coma samples from 81P/Wild 2. This was made on January 2, 2004. Before the encounter three significant model predictions existed for the number and size of samples to be captured. Three investigations during the Wild 2 encounter (Dust Flux Monitor, Comet and Interstellar Dust Analyzer and Dynamic Science) made in situ measurements of the dust. Spectacular images were captured of the Wild 2 nucleus and dust jets. This abstract compares the model predictions with the in situ measurements and Wild 2 images and assesses the likely samples to be returned for analysis on January 15, 2006. To give some lead time for sample analysts to prepare for the analyses of the returned samples, the organization of the Preliminary Examination is presented.
Tsou, P.↗