An experimental study of the formation of an ice crust and migration of water vapor in a comet's upper layers

From recent close encounters with Comets Wild-2 and Tempel 1 we learned that their surfaces are very rugged and no simple uniform layers model can be applied to them. Rather, a glaciological approach should be applied for describing their surface features and behavior. Such intrinsically rugged surface is formed in our large scale experiments, where an agglomerate of ∼200 μm gas-laden amorphous ice particles is accumulated to form a 20 cm diameter and few cm high ice sample. The density, tensile strength and thermal inertia of our ice sample were found to be very close to those found by Deep Impact for Comet Tempel 1: density 250-300 kg m⁻³ vs DI 350-400 kg m⁻³; tensile strength 2-4 kPa vs DI 1-10 kPa; thermal inertia 80 W K⁻¹ m⁻² s^1/2 vs <100 W K⁻¹ m⁻² s^1/2 and <50 W K⁻¹ m⁻² s^1/2. From the close agreement between the thermal inertias measured in our ice sample, which had no dust coverage and that of Comet Tempel 1, we deduce that the low thermal inertia is an intrinsic property of the fluffy structure of the ice as a result of its low density, with an addition by the broken terrain and not due to the formation of a dust layer. Upon warming up of the ice, water vapor migrates both outward into the coma and inward. Reaching cooler layers, the water vapor condenses, forming a denser ice crust, as we show experimentally. We also demonstrate the inward and outward flow of water vapor in the outer ice layers through the exchange between layers of D₂O ice and H₂O ice, to form HDO.     

Investigations of the pre-Deep Impact morphology of the dust coma of Comet Tempel-1

The pre-Deep Impact images of Comet Tempel-1 obtained at the Indian Astronomical Observatory are used to investigate the morphology of the dust coma of the comet. We show that the trajectory of a cometary grain under the influence of solar radiation pressure is a reliable diagnostic to estimate its initial velocity. Four main active regions at mean latitudes +45° ± 5°(D), 0° ± 5° (E),−30° ± 5°(A) and−60° ± 5°(F) are found to explain the morphology of the dust coma in the ground-based and published images obtained by the High Resolution Instrument(HRI) cameras aboard the Deep Impact flyby spacecraft. From a χ² fit of the intensity distribution in the observed and the simulated images, we derive the fraction of the productivity of the active vents to the total dust emission of the comet to be 27%. Of this the southern source alone accounts for 19.8%. The grains are found to be ejected with a velocity distribution with an upper limit of 70 ± 7 m s⁻¹. However, the broad region ‘A’ appears to eject slower grains with an upper limit of 24 ± 2.5 m s⁻¹. This source, that is active throughout the cycle is likely to be driven by CO₂ sublimation. We compute the dependence of the percentage contribution of the southern source on the heliocentric distance and show that this ratio varies over the apparition and reaches a maximum at around 260 days before perihelion. The published images of the nucleus of Comet Tempel-1 show significant departure from sphericity. Therefore, the torque exerted by the enhanced activity of the southern region may be significant enough to produce changes in the rotational state of the nucleus before each perihelion passage.     

Formation of smooth terrains on Comet Tempel 1

We suggest that the regions of smooth terrain which were observed on Comet 9P/Tempel 1 by the Deep Impact spacecraft were formed by blowing ice grains in an outburst of gas from the comet interior. When gas is released from 10 to 20 m deep layers which were heated to 135 K, it is released quiescently onto the surface by individual conduits. If large amounts of gas are released, the drainage system cannot release them fast enough and wider interconnected channels are formed, leading to sudden outburst of gas. Instability triggering a sudden shift of flow is well known in subglacial drainage of water. The ballistic trajectory of the ice particles reach a distance of 3 km in the atmosphereless comet, whose gravity is 0.034 cm s⁻¹, if ejected at an angle of 45° at a speed of 95 cm s⁻¹. This speed is close to the speeds measured in laboratory experiments: 167, 140×sini and 167 cm s⁻¹, for particles of 0.3, 1000 and 14-650 μm, respectively. Blowing of ice grains can overcome the 1650 m long horizontal section of smooth terrain i1 (Fig. 1), whereas simple flow of material downhill would stop close to the foot of the hill. The ice particles at the end of their trajectory have a horizontal velocity component and this low velocity ballistic sedimentation would lead to formation of lineaments on the smooth terrain, like in solid-particulate volcanic eruptions.     

In situ observation of penetration process in silica aerogel: Deceleration mechanism of hard spherical projectiles

A large number of cometary dust particles were captured with low-density silica aerogels by NASA’s Stardust Mission. Knowledge of the details of the capture mechanism of hypervelocity particles in silica aerogel is needed in order to correctly derive the original particle features from impact tracks. However, the mechanism has not been fully understood yet. We shot hard spherical projectiles of several different materials into silica aerogel of density 60 mg cm⁻³ and observed their penetration processes using an image converter or a high-speed video camera. In order to observe the deceleration of projectiles clearly, we carried out impact experiments at two velocity ranges; ∼4 km s⁻¹ and ∼200 m s⁻¹. From the movies we took, it was indicated that the projectiles were decelerated by hydrodynamic force which was proportional to v² (v: projectile velocity) during the faster penetration process (∼4 km s⁻¹) and they were merely overcoming the aerogel crushing strength during the slower penetration process (∼200 m s⁻¹). We applied these deceleration mechanisms for whole capture process to calculate the track length. Our model well explains the track length in the experimental data set by Burchell et al. (Burchell, M.J., Creighton, J.A., Cole, M.J., Mann, J., Kearsley, A.T. [2001]. Meteorit. Planet. Sci. 36, 209-221).     

Cometary dust trail associated with Rosetta mission target: 67P/Churyumov-Gerasimenko

A thin, bright dust cloud, which is associated with the Rosetta mission target object (67P/Churyumov-Gerasimenko), was observed after the 2002 perihelion passage. The neckline structure or dust trail nature of this cloud is controversial. In this paper, we definitively identify the dust trail and the neckline structure using a wide-field CCD camera attached to the Kiso 1.05-m Schmidt telescope. The dust trail of 67P/Churyumov-Gerasimenko was evident as scattered sunlight in all images taken between September 9, 2002 and February 1, 2003, whereas the neckline structure became obvious only after late 2002. We compared our images with a semi-analytical dynamic model of dust grains emitted from the nucleus. A fading of the surface brightness of the dust trail near the nucleus enabled us to determine the typical maximum size of the grains. Assuming spherical compact particles with a mass density of 10³ kg m⁻³ and an albedo of 0.04, we deduced that the maximum diameter of the dust particles was approximately 1 cm. We found that the mass-loss rate of the comet at the perihelion was on or before the 1996 apparition, while the mass-loss rate averaged over the orbit reached . The result is consistent with the studies of the dust cloud emitted in the 2002/2003 return. Therefore, we can infer that the activity of 67P/Churyumov-Gerasimenko has showed no major change over the past dozen years or so, and the largest grains are cyclically injected into the dust tube lying along the cometary orbit.     

The dust trail of Comet 67P/Churyumov-Gerasimenko

We report the detection of Comet 67P/Churyumov-Gerasimenko's dust trail and nucleus in 24 μm Spitzer Space Telescope images taken February 2004. The dust trail is not found in optical Palomar images taken June 2003. Both the optical and infrared images show a distinct neck-line tail structure, offset from the projected orbit of the comet. We compare our observations to simulated images using a Monte Carlo approach and a dynamical model for comet dust. We estimate the trail to be at least one orbit old (6.6 years) and consist of particles of size ?100 μm. The neck-line is composed of similar sized particles, but younger in age. Together, our observations and simulations suggest grains 100 μm and larger in size dominate the total mass ejected from the comet. The radiometric effective radius of the nucleus is 1.87±0.08 km, derived from the Spitzer observation. The Rosetta spacecraft is expected to arrive at and orbit this comet in 2014. Assuming the trail is comprised solely of 1 mm radius grains, we compute a low probability (∼¹⁰⁻³) of a trail grain impacting with Rosetta during approach and orbit insertion.     

Cometary dust properties retrieved from polarization observations: Application to C/1995 O1 Hale-Bopp and 1P/Halley

The analysis of the polarized light scattered by cometary dust particles provides information on the physical properties of the solid component of cometary comae for C/1995 O1 Hale-Bopp and 1P/Halley. A model of light scattering by a size distribution of aggregates of up to 256 submicron-sized grains (spherical or spheroidal) mixed with single spheroidal particles has been developed, with its parameters adjusted to fit the phase angle and wavelength dependence of the polarization observations. The particles are built of two materials: a non-absorbing silicates-type material and a more absorbing organic-type material. The model reproduces accurately the inversion angle and the positive branch of the polarization phase curves from the visible to the near-infrared spectral domains. A negative branch of the polarization phase curves appears in our model, although the negative branch is not deep enough to reproduce accurately the observations. Significant differences are shown between the two comets, with dominance of small grains in the coma of Comet C/1995 O1 Hale-Bopp, well fitted by a distribution of the volume-equivalent diameter, a, following a^−3.0 with a lower cutoff around 0.20 μm and an upper cutoff of at least 40 μm. For 1P/Halley, the size distribution follows a^−2.8 with a lower cutoff around 0.26 μm and an upper cutoff of about 38 μm. The relative amount of organic-type particles is larger for 1P/Halley while the amount of aggregates, significant for both comets, is larger for C/1995 O1 Hale-Bopp.     

The dust trail of Comet 67P/Churyumov-Gerasimenko between 2004 and 2006

We report on observations of the dust trail of Comet 67P/Churyumov-Gerasimenko (CG) in visible light with the Wide Field Imager at the ESO/MPG 2.2 m telescope at 4.7 AU before aphelion, and at with the MIPS instrument on board the Spitzer Space Telescope at 5.7 AU both before and after aphelion. The comet did not appear to be active during our observations. Our images probe large dust grains emitted from the comet that have a radiation pressure parameter β<0.01. We compare our observations with simulated images generated with a dynamical model of the cometary dust environment and constrain the emission speeds, size distribution, production rate and geometric albedo of the dust. We achieve the best fit to our data with a differential size distribution exponent of −4.1, and emission speeds for a β=0.01 particle of 25 m/s at perihelion and 2 m/s at 3 AU. The dust production rate in our model is on the order of 1000 kg/s at perihelion and 1 kg/s at 3 AU, and we require a dust geometric albedo between 0.022 and 0.044. The production rates of large (>) particles required to reproduce the brightness of the trail are sufficient to also account for the coma brightness observed while the comet was inside 3 AU, and we infer that the cross-section in the coma of CG may be dominated by grains of the order of .     

The E ring in the vicinity of Enceladus: I. Spatial distribution and properties of the ring particles

Saturn's diffuse E ring is the largest ring of the Solar System and extends from about (Saturn radius RS=60,330 km) to at least encompassing the icy moons Mimas, Enceladus, Tethys, Dione, and Rhea. After Cassini's insertion into her saturnian orbit in July 2004, the spacecraft performed a number of equatorial as well as steep traversals through the E ring inside the orbit of the icy moon Dione. Here, we report about dust impact data we obtained during 2 shallow and 6 steep crossings of the orbit of the dominant ring source—the ice moon Enceladus. Based on impact data of grains exceeding 0.9 μm we conclude that Enceladus feeds a torus populated by grains of at least this size along its orbit. The vertical ring structure at agrees well with a Gaussian with a full-width-half-maximum (FWHM) of ∼4200 km. We show that the FWHM at is due to three-body interactions of dust grains ejected by Enceladus' recently discovered ice volcanoes with the moon during their first orbit. We find that particles with initial speeds between 225 and 235 m s⁻¹ relative to the moon's surface dominate the vertical distribution of dust. Particles with initial velocities exceeding the moon's escape speed of 207 m s⁻¹ but slower than 225 m s⁻¹ re-collide with Enceladus and do not contribute to the ring particle population. We find the peak number density to range between 16×¹⁰⁻² m⁻³ and 21×¹⁰⁻² m⁻³ for grains larger 0.9 μm, and 2.1×¹⁰⁻² m⁻³ and 7.6×¹⁰⁻² m⁻³ for grains larger than 1.6 μm. Our data imply that the densest point is displaced outwards by at least with respect of the Enceladus orbit. This finding provides direct evidence for plume particles dragged outwards by the ambient plasma. The differential size distribution for grains >0.9 μm is described best by a power law with slopes between 4 and 5. We also obtained dust data during ring plane crossings in the vicinity of the orbits of Mimas and Tethys. The vertical distribution of grains >0.8 μm at Mimas orbit is also well described by Gaussian with a FWHM of ∼5400 km and displaced southwards by ∼1200 km with respect to the geometrical equator. The vertical distribution of ring particles in the vicinity of Tethys, however, does not match a Gaussian. We use the FWHM values obtained from the vertical crossings to establish a 2-dimensional model for the ring particle distribution which matches our observations during vertical and equatorial traversals through the E ring.     

Surface temperature of the nucleus of Comet 9P/Tempel 1

The Deep Impact (DI) spacecraft encountered Comet 9P/Tempel 1 on July 4th, 2005 and observed it with several instruments. In particular, we obtained infrared spectra of the nucleus with the HRI-IR spectrometer in the wavelength range of 1.0-4.9 μm. The data were taken before impact, with a maximum resolution of ∼120 m per pixel at the time of observation. From these spectra, we derived the first directly observed temperature map of a comet nucleus. The surface temperature varied from 272±7 to 336±7 K on the sunlit hemisphere, matching the surface topography and incidence angle. The derived thermal inertia is low, most probably <50 W K⁻¹ m⁻² s^1/2. Combined with other arguments, it is consistent with the idea that most of rapidly varying thermal physical processes, in particular the sublimation of volatiles around perihelion, should occur close to the surface. Thermal inertia is sufficient to explain the temperature map of the nucleus of Comet Tempel 1 to first order, but other physical processes like roughness and self-radiation are required to explain the details of the temperature map. Finally, we evaluated that the Standard Thermal Model is a good approximation to derive the effective radius of a cometary nucleus with an uncertainty lower than ∼10% if combined with a thermal infrared light curve.     

Physical properties of morphological units on Comet 9P/Tempel 1 derived from near-IR Deep Impact spectra

In this paper we analyze near-infrared thermal emission spectra of the spatially resolved nucleus of Comet 9P/Tempel 1 obtained by the NASA spacecraft Deep Impact. Maps of spectral reddening, the product X^′ between the beaming function and directional emissivity, as well as surface temperature are constructed. Thermophysical modeling is used to estimate the degree of small scale surface roughness and thermal inertia by detailed reproduction of the empirical temperature map. Mie and Hapke theories are used in combination with numerically calculated beaming functions to analyze the X^′ map and place constraints on composition and grain size of the surface material. We show that it is absolutely mandatory to include small scale surface roughness in thermophysical modeling of this object, since the resulting self heating is vital for reproducing the measured temperatures. A small scale self heating parameter in the range 0.6?ξ?0.75 is common, but smoother areas where 0.2?ξ?0.3 are also found. Contrary to models neglecting small scale surface roughness, we find that the thermal inertia of Comet 9P/Tempel 1 generally is high (1000-3000 J m⁻² K⁻¹ s^−1/2), although it may be substantially lower (40-380 J m⁻² K⁻¹ s^−1/2) in specific areas. We obtain a disk-averaged reddening of 3.5% kÅ⁻¹, with statistically significant local variations around that value on a ±1.0% kÅ−1 level. Vast regions appear covered by small (∼0.1 μm) highly absorbing grains such as carbon or iron-rich silicates. Other regions appear dominated by somewhat larger (∼0.5 μm) and/or less absorbing grains such as troilite or magnesium-rich silicates. Surface variations in reddening, roughness, thermal inertia, composition and/or grain size are moderately to strongly correlated to the locations of morphological units on the surface. The existence of morphological units with differing physical properties may be primordial, hence reflecting a diversity in the building block cometesimals, or resulting from evolutionary processes.     

Detection of large grains in the coma of Comet C/2001 A2 (LINEAR) from Arecibo radar observations

Arecibo S-band () radar observations of Comet C/2001 A2 (LINEAR) on 2001 July 7-9 showed a strong echo from large coma grains. This echo was significantly depolarized. This is the first firm detection of depolarization in a grain-coma radar echo and indicates that the largest grains are at least λ/2π or 2 cm in radius. The grains are moving at tens of m s⁻¹ with respect to the nucleus. The nondetection of the nucleus places an upper limit of 3 km on its diameter. The broad, asymmetric echo power spectrum suggests a fan of grains that have a steep (differential number ∼a⁻⁴) size distribution at cm-scales, though the observed fragmentation of this comet complicates that picture.     

Dust coma morphology in the Deep Impact images of Comet 9P/Tempel 1

We present an overview of the dust coma observations of Comet Tempel 1 that were obtained during the approach and encounter phases of the Deep Impact mission. We use these observations to set constraints on the pre-impact activity of the comet and discuss some preliminary results. The temporal and spatial changes that were observed during approach reveal three distinct jets rotating with a 1.7-day periodicity. The brightest jet produces an arcuate feature that expands outward with a projected velocity of about 12 m s⁻¹, suggesting that the ambient dust coma is dominated by millimeter-sized dust grains. As the spatial resolution improves, more jets and fans are revealed. We use stereo pairs of high-resolution images to put some crude constraints on the source locations of some of the brightest features. We also present a number of interesting coma features that were observed, including surface jets detected at the limb of the nucleus when the exposed ice patches are passing over the horizon, and features that appear to be jets emanating from unilluminated sources near the negative pole. We also provide a list of 10 outbursts of various sizes that were observed in the near-continuous monitoring during the approach phase.     

Comet 9P/Tempel 1: Sublimation beneath the dust cover

In the current work we analyze properties of the dust mantle, its thickness and thermal conductivity, necessary to reproduce observed rate of water production of Comet 9P/Tempel 1. For this purpose we considered simplified shape of the comet nucleus approximated by the symmetric prolate ellipsoid with smooth surface. We have performed simulations, using models with dust mantle of the thickness either constant, but nonuniform (Model A), or evolving (Model B). The simulated profiles of water production versus time were compared with observations. In addition, we compared the calculated surface temperature with the real temperatures derived from IR observations (the Deep Impact mission). This new double-stage verification procedure, shows that our model A is a good representation for the nucleus of Comet Tempel 1. This indicates, that the dust mantle thickness should be nonuniform, but does not change significantly with time. We show, that reproducing observed high temperatures of the nucleus requires dust mantle, that is almost everywhere thick and has extremely low thermal inertia. The latter should be close to zero as already predicted by others. The agreement between the simulated and measured water production can be obtained when the dust is regionally thin and has the thermal inertia higher than average, according to our simulations about 100 W s^1/2 K⁻¹ m⁻². Such regions should be located in the south hemisphere of the nucleus.     

Observations of Comet 9P/Tempel 1 with the Keck 1 HIRES instrument during Deep Impact

We report high-spectral resolution observations of Comet 9P/Tempel 1 before, during and after the impact on 4 July 2005 UT of the Deep Impact spacecraft with the comet. These observations were obtained with the HIRES instrument on Keck 1. We observed brightening of both the dust and gas, but at different rates. We report the behavior of OH, NH, CN, C₃, CH, NH₂ and C₂ gas. From our observations, we determined a CN outflow velocity of at least 0.51 km s⁻¹. The dust color did not change substantially. To date, we see no new species in our spectra, nor do we see any evidence of prompt emission. From our observations, the interior material released by the impact looks the same as the material released from the surface by ambient cometary activity. However, further processing of the data may uncover subtle differences in the material that is released as well as the time evolution of this material.     

CN, C2 radicals and dust in Comets Shoemaker-Levy 1991 T2 and P/deVico 1995 S1

The results of the multiaperture photometry of Comet Shoemaker-Levy 1991 T2 in the pre-perihelion and P/deVico in the post-perihelion period with the narrowband CN, C₂ and Blue Continuum (BC) IHW filters are presented. A Haser model of the molecular coma was used for the determination of the parent and daughter scale-lengths and production rates of the radicals. The comets showed some substantial differences between their parent scale-lengths. The CN parent scale-length (at 1.0 AU) was 16×¹⁰³ km for Comet Shoemaker-Levy and 39×¹⁰³ for P/deVico, the C₂ parent scale-lengths were respectively 29×¹⁰³ and 54×¹⁰³ km. Such divergences could be interpreted in the frame of different scenarios of emission of cometary parents, either from a nucleus or from a volume source. The daughter scale-lengths for these comets were quite similar, namely: 306×¹⁰³ and 318×¹⁰³ km for CN and 69×¹⁰³ and 66×¹⁰³ km for C₂. We determined the Afρ parameter for apertures of different radii. A Monte Carlo model of the dust coma was used to obtain the dust ejection velocity. It was of the order of 0.1 km s⁻¹ for both comets. The power index of the distribution of the β-parameter of dust particles (ratio of light pressure to the solar gravitation) was of the order of 3 for C/Shoemaker-Levy and close to 2 for P/deVico. The dependence on heliocentric distance (rh) of the radical and dust production rates for P/deVico in the range of 0.7-1.0 AU was described by the power law function with a power index equal to: 5.55±0.14 for CN, 5.70±0.24 for C₂ and 5.22±0.19 for dust. Relative abundances of the dynamically new Comet Shoemaker-Levy and short-period P/deVico were quite similar with an enhancement of C₂ comparing with standard values taken from A'Hearn et al. (1995).     

Interplanetary dust detected by the Cassini CDA Chemical Analyser

During its cruise phase, prior to encountering Jupiter, the Cosmic Dust Analyser (CDA) onboard the Cassini spacecraft returned time of flight mass spectra (TOF MS) of two interplanetary dust particles. Both particles were found to be iron-rich, with possible traces of hydrogen, carbon, nickel, chromium, manganese, titanium, vanadium and minor silicates. Carbon, hydrogen, oxygen and potassium are also present as possible contaminants of the impact target of CDA. Silicates and magnesium do not feature predominantly in the spectra; this is surprising considering the expected dominance of silicate-rich minerals in interplanetary dust particles. The particle masses are and . The corresponding radii ranges for the particles, assuming densities from 7874-2500 kg m⁻³ are 0.7-4 μm and 2.6-6.8 μm, respectively. With the same density assumptions the β values (ratio of radiation pressure to gravitational force) are estimated as 0.027-0.21 and 0.016-0.06 respectively, allowing possible orbits to be calculated. The resulting orbits are bound and prograde with semi-major axes, eccentricities and inclinations in the region of 0.3-1.26 AU, 0.4-1.0 and 0-60° for the first particle and 0.8-2.5 AU, 0.2-0.9 and 0-30° for the second. The more probable orbits within these ranges indicate that the first particle is in an Aten-like orbit, whilst the second particle is in an Apollo-like orbit, despite both grains having very similar, predominantly metallic compositions. Other possible orbital solutions for both particles encompass orbits which more closely resemble those of Jupiter-family comets.     

Explosion of Comet 17P/Holmes as revealed by the Spitzer Space Telescope

An explosion on Comet 17P/Holmes occurred on 2007 October 23, projecting particulate debris of a wide range of sizes into the interplanetary medium. We observed the comet using the mid-Infrared Spectrograph (5-40 μm), on 2007 November 10 and 2008 February 27, and the imaging photometer (24 and 70 μm), on 2008 March 13, on board the Spitzer Space Telescope. The 2007 November 10 spectral mapping revealed spatially diffuse emission with detailed mineralogical features, primarily from small crystalline olivine grains. The 2008 February 27 spectra, and the central core of the 2007 November 10 spectral map, reveal nearly featureless spectra, due to much larger grains that were ejected from the nucleus more slowly. Optical images were obtained on multiple dates spanning 2007 October 27-2008 March 10 at the Holloway Comet Observatory and 1.5-m telescope at Palomar Observatory. The images and spectra can be segmented into three components: (1) a hemispherical shell fully 28′ on the sky in 2008 March, due to the fastest (262 m s⁻¹), smallest (2 μm) debris, with a mass ; (2) a ‘blob’ or ‘pseudonucleus’ offset from the true nucleus and subtending some 10′ on the sky, due to intermediate speed (93 m s⁻¹) and size (8 μm) particles, with a total mass ; and (3) a ‘core’ centered on the nucleus due to slower (9 m s⁻¹), larger (200 μm) ejecta, with a total mass . This decomposition of the mid-infrared observations can also explain the temporal evolution of the millimeter-wave flux. The orientation of the leading edge of the ejecta shell and the ejecta ‘blob,’ relative to the nucleus, do not change as the orientation of the Sun changes; instead, the configuration was imprinted by the orientation of the initial explosion. The distribution and speed of ejecta implies an explosion in a conical pattern directed approximately in the solar direction on the date of explosion. The kinetic energy of the ejecta >10²¹ erg is greater than the gravitational binding energy of the nucleus. We model the explosion as being due to crystallization and release of volatiles from interior amorphous ice within a subsurface cavity; once the pressure in the cavity exceeded the surface strength, the material above the cavity was propelled from the comet. The size of the cavity and the tensile strength of the upper layer of the nucleus are constrained by the observed properties of the ejecta; tensile strengths on >10 m scale must be greater than 10 kPa (or else the ejecta energy exceeds the binding energy of the nucleus) and they are plausibly 200 kPa. The appearance of the 2007 outburst is similar to that witnessed in 1892, but the 1892 explosion was less energetic by a factor of about 20.     

Dust tail of the active distant Comet C/2003 WT42 (LINEAR) studied with photometric and spectroscopic observations

We present the study of dust environment of dynamically new Comet C/2003 WT42 (LINEAR) based on spectroscopic and photometric observations. The comet was observed before and after the perihelion passage at heliocentric distances from 5.2 to 9.5 AU. Although the comet moved beyond the zone where water ice sublimation could be significant, its bright coma and extended dust tail evidenced the high level of physical activity. Afρ values exceeded 3000 cm likely reaching its maximum before the perihelion passage. At the same time, the spectrum of the comet did not reveal molecular emission features above the reflected continuum. Reddening of the continuum derived from the cometary spectrum is nonlinear along the dispersion with the steeper slop in the blue region. The pair of the blue and red continuum images was analyzed to estimate a color of the comet. The mean normalized reflectivity gradient derived from the innermost part of the cometary coma equals to 8% per 1000 Å that is typical for Oort cloud objects. However, the color map shows that the reddening of the cometary dust varies over the coma increasing to 15% per 1000 Å along the tail axis. The photometric images were fitted with a Monte Carlo model to construct the theoretical brightness distribution of the cometary coma and tail and to investigate the development of the cometary activity along the orbit. As the dust particles of distant comets are expected to be icy, we propose here the model, which describes the tail formation taking into account sublimation of grains along their orbits. The chemical composition and structure of these particles are assumed to correspond with Greenberg’s interstellar dust model of comet dust. All images were fitted with the close values of the model parameters. According to the results of the modeling, the physical activity of the comet is mainly determined by two active areas with outflows into the wide cones. The obliquity of the rotation axis of the nucleus equals to 20° relative to the comet’s orbital plane. The grains occupying the coma and tail are rather large amounting to 1 mm in size, with the exponential size distribution of a^−4.5. The outflow velocities of the dust particles vary from a few centimeters to tens of meters per second depending on their sizes. Our observations and the model findings evidence that the activity of the nucleus decreased sharply to a low-level phase at the end of April–beginning of May 2007. About 190 days later, in the first half of November 2007 the nucleus stopped any activity, however, the remnant tail did not disappear for more than 1.5 years at least.     

The Umov effect for single irregularly shaped particles with sizes comparable with wavelength

The Umov effect manifests itself as an inverse correlation between the linear polarization maximum of an object’s scattered light P_max and its geometric albedo A. This effect is observed for the Moon, Mercury and Mars, and there are data suggesting this effect is valid for asteroids. The Umov effect is due to the contribution of interparticle multiple scattering that increases albedo and decreases polarization. We here study if the Umov effect can be extended to the case of single irregularly shaped particles with sizes comparable with the wavelength. This, in particular, is important for cometary dust polarimetry. We show the Umov effect being valid for weakly absorbing irregular particles (Im(m) ? 0.02) almost through the entire range of size parameters x considered. Highly absorbing particles (Im(m) > 0.02) follow the Umov effect only if x exceeds 14. In the case of weakly absorbing particles, the inverse correlation is essentially non-linear, which is caused by the contribution of particles with small x. However, averaging over many different types of irregularly shaped particles could make it significantly more linear. The size averaging does not change qualitatively the diagram log(P_max)-log(A) for weakly absorbing particles. For single irregular particles whose sizes are comparable with wavelength, there is no reliable correlation between the slope of the polarization curve h near the inversion phase angle and geometric albedo A. Using the extended Umov Law, we estimate the geometric albedo of dust particles forming cometary circumnuclear haloes A = 0.1 − 0.2, which is a few times larger than the average geometric albedo over the entire comae. Note that, using the obtained values for A of cometary particles, one can derive their number density in circumnuclear haloes from photometric observations.