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PubMed1985

Temperatures within comet nuclei.

We have performed a theoretical study of temperatures beneath the surface of a comet's nucleus. We solve the one-dimensional heat conduction equation for the outer portion of the comet. The upper boundary condition of the model is given by energy balance at the surface of the nucleus, including conduction of heat inward, radiation, insolation as modified by the coma, and sublimation. Our coma model assumes single scattering and includes attenuation of direct sunlight by dust grains, scattering of light onto the nucleus, and infrared radiation by dust grains. The lower boundary condition is zero net heat flux around an orbit. The thermal conductivity expression for the nucleus includes direct conduction at grain boundaries, radiative conduction, and Knudsen flow vapor diffusion. The thermal diffusivity of the nucleus and the resultant temperature profiles are shown to be strongly dependent on the physical properties of the material, including porosity, pore size, and compaction. The temperature profiles and the equilibrium temperature deep within the comet also depend on the functional relationship between thermal conductivity and temperature; the highest deep equilibrium temperatures are found for models where the thermal conductivity increases strongly with increasing temperature. The dependence of temperatures on the albedo and thermal emissivity of the nucleus is also calculated, as well as the variation of temperature with latitude for a variety of pole orientations. The effect of a dust mantle on subsurface temperatures is also investigated. All calculations are presented for short-period comets with orbits that make them accessible for exploration by spacecraft rendezvous. In situ measurements of the thermal profile in the upper meter of a comet nucleus can substantially constrain the thermal diffusivity of the material, which in turn can provide significant information about the physical properties of the nucleus.

Cosmic Dust↗
PubMed2003

Importance of surface morphology in interstellar H2 formation.

Detailed laboratory experiments on the formation of HD from atom recombination on amorphous solid water films show that this process is extremely efficient in a temperature range of 8 to 20 kelvin, temperatures relevant for H2 formation on dust grain surfaces in the interstellar medium (ISM). The fate of the 4.5 electron volt recombination energy is highly dependent on film morphology. These results suggest that grain morphology, rather than the detailed chemical nature of the grain surface, is most important in determining the energy content of the H2 as it is released from the grain into the ISM.

Astronomical Phenomena↗
PubMed2002

Detection of carbonates in dust shells around evolved stars.

Carbonates on large Solar System bodies like Earth and Mars (the latter represented by the meteorite ALH84001) form through the weathering of silicates in a watery (CO3)2- solution. The presence of carbonates in interplanetary dust particles and asteroids (again, represented by meteorites) is not completely understood, but has been attributed to aqueous alteration on a large parent body, which was subsequently shattered into smaller pieces. Despite efforts, the presence of carbonates outside the Solar System has hitherto not been established. Here we report the discovery of the carbonates calcite and dolomite in the dust shells of evolved stars, where the conditions are too primitive for the formation of large parent bodies with liquid water. These carbonates, therefore, are not formed by aqueous alteration, but perhaps through processes on the surfaces of dust or ice grains or gas phase condensation. The presence of carbonates which did not form by aqueous alteration suggests that some of the carbonates found in Solar System bodies no longer provide direct evidence that liquid water was present on large parent bodies early in the history of the Solar System.

Astronomical Phenomena↗
PubMed2002

Observations of comet 19P/Borrelly by the miniature integrated camera and spectrometer aboard Deep Space 1.

The nucleus of the Jupiter-family comet 19P/Borrelly was closely observed by the Miniature Integrated Camera and Spectrometer aboard the Deep Space 1 spacecraft on 22 September 2001. The 8-kilometer-long body is highly variegated on a scale of 200 meters, exhibiting large albedo variations (0.01 to 0.03) and complex geologic relationships. Short-wavelength infrared spectra (1.3 to 2.6 micrometers) show a slope toward the red and a hot, dry surface (</=345 kelvin, with no trace of water ice or hydrated minerals), consistent with approximately 10% or less of the surface actively sublimating. Borrelly's coma exhibits two types of dust features: fans and highly collimated jets. At encounter, the near-nucleus coma was dominated by a prominent dust jet that resolved into at least three smaller jets emanating from a broad basin in the middle of the nucleus. Because the major dust jet remained fixed in orientation, it is evidently aligned near the rotation axis of the nucleus.

Carbon↗
PubMed2006

Early inner solar system impactors: physical properties of comet nuclei and dust particles revisited.

During the epoch of early bombardment, terrestrial planets have been heavily impacted by cometary nuclei and cometary dust particles progressively injected in the interplanetary medium. Stardust and Deep Impact missions confirm that the nuclei are porous, loosely consolidated objects, with densities below 1,000 kg m(-3), and that they often release small fragments of ices and dust. Recent numerical simulations of the light scattering properties of cometary dust particles indicate that they are highly porous, most likely fractal, and rich in absorbing organics compounds (with a mixture ratio of e.g. 33 to 60% in mass for comet Hale-Bopp). Taking into account the fact that porous structures survive more easily than compact ones during atmospheric entry, such results reinforce the scenario of the early terrestrial planets enrichment--in organics needed for life to originate--by comets.

Cosmic Dust↗
PubMed1989

On the temperature and gas composition in the region of comet formation.

The findings of the Giotto and Vega spacecrafts on the gas composition of comet Halley, together with an experimental study on the trapping of gas mixtures in amorphous water ice, enable estimation of the gas composition and temperature in the region of comet Halley's formation: If Halley was formed in the solar nebula by condensation of water vapor in the presence of gas, in the region of its formation the CO/CH4 ratio had to be at least 100 and the temperature about 48 K. The ice particles that formed the comet could not have condensed at a higher temperature and subsequently cool down because then the 7% CO found as a parent molecule could not have been trapped in the ice. A approximately 48 K formation implies that the ice was in amorphous form. This temperature is surprisingly close to the temperatures observed by IRAS for the circumstellar dust shells around alpha PsA (55 K) and epsilon Eri (45 K) and supports the suggestion that short-period comets were formed outside the region of planet formation. The CO content of comet Halley and sensitivity to explosion of irradiated, ice-coated, interstellar grains seem to exclude the possibility of their direct incorporation into comets. Yet, they might have provided the condensed organics--the "CHON" materials.

Ammonium Chloride↗
PubMed1988

The formation of a permanent dust mantle and its effect on cometary activity.

The growth of a permanent, permeable, dust mantle on the surface of a comet nucleus, composed initially of dusty amorphous water ice, is investigated. Numerical simulations of the evolution of one-dimensional comet nucleus models, in Comet Halley's orbit, are carried out for various parameters, allowing for the crystallization of the amorphous ice. It is assumed that the mantle forms gradually, by the accumulation of a constant fraction (0.001-0.01) of the dust, which is not carried away with the sublimating ice. It is found that an approximately 1-cm-thick dust mantle diminishes the average sublimation rate by a factor of approximately 5, and a further growth of the dust mantle may decrease the surface activity of the nucleus by another factor of 10. Therefore, the activity of a dust-covered nucleus is expected to result mainly from exposed patches of ice and from craters, such as were observed on Comet Halley by Giotto. These are formed by explosions of gas-filled pockets in the crystalline outer layer of the nucleus. The insulating effect of the dust mantle causes the crystallization of the amorphous ice to proceed at a slower rate than in the case of a bare icy nucleus. Thus, the thickness of the outer crystalline shell, overlying the amorphous ice core, is always greater than 15 m, but does not exceed a few tens of meters. This size range is compatible with the amount of gas released in the numerous small explosions which were observed on Comet Halley.

Cosmic Dust↗
PubMed1997

Evidence for interacting gas flows and an extended volatile source distribution in the coma of comet C/1996 B2 (Hyakutake).

Images of comet C/1996 B2 (Hyakutake) taken during its close approach to Earth show differences in the distribution of gas and dust in the inner coma and reveal two arc-shaped molecular resonant emission features. The morphology of these features, as well as the apparent decoupling gas from dust in the inner coma, suggest that an extended region of icy grains surrounds the nucleus of Hyakutake and contributes substantially to the production of volatiles. Model simulations suggest the same conclusion and indicate that the brighter arc is explainable by the presence of a trailing condensation of ice-bearing granules with a rate of volatile production approximately 23 percent of that of the nucleus.

Cosmic Dust↗
PubMed2006

Infrared spectroscopy of comet 81P/Wild 2 samples returned by Stardust.

Infrared spectra of material captured from comet 81P/Wild 2 by the Stardust spacecraft reveal indigenous aliphatic hydrocarbons similar to those in interplanetary dust particles thought to be derived from comets, but with longer chain lengths than those observed in the diffuse interstellar medium. Similarly, the Stardust samples contain abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene. The presence of crystalline silicates in Wild 2 is consistent with mixing of solar system and interstellar matter. No hydrous silicates or carbonate minerals were detected, which suggests a lack of aqueous processing of Wild 2 dust.

Cosmic Dust↗
PubMed1997

Comets as porous aggregates of interstellar dust.

A comet model is derived based on the interstellar dust chemical composition in dense molecular and diffuse clouds resulting from their subsequent chemical interactions and UV photoprocessing. The collapsing presolar nebula leads to a porous aggregate model for comet nuclei, from which is derived certain physical properties that include thermal conductivity and tensile strength. The porous morphological structure is also shown to imply anomalous (expansion rather than contraction) behavior when subjected to strong shock waves, which is supported by recent shock experiments on (porous) carbonaceous chondrite material.

Cosmic Dust↗
PubMed1995

Coagulation of grains and gas-grain interactions.

Three-dimensional off-lattice Monte Carlo simulations have been used to simulate the production of interstellar dust analogues on the computer. At first, the two rather well known extreme cases of ballistic growth, i.e. particle-cluster (BPCA) and cluster-cluster aggregation (BCCA) were investigated. In a second step, we included rotation of the clusters during the growth process. Following this, gas particles were deposited on these aggregates taking variable sticking probabilities due to the growth of monolayers into account. This is of interest to the problem of molecular abundances in dark clouds.

Computer Simulation↗
PubMed1995

Cometary origin of the biosphere: a progress report.

Empirical evidence of the accretion temperature for undifferentiated meteorites coming from the asteroid belt, combined with any reasonable temperature gradient extending from the asteroid belt to the Earth's zone, suggests that the Earth accreted from very hot dust grains that were degassed from all volatile elements and depleted in labile compounds. Isotopic evidence from the atmospheric noble gases also shows that no primary atmosphere has survived on the Earth. The only possible source for the atmosphere and the oceans is therefore the cometary bombardment that is predicted as the inescapable consequence of the formation of the giant planets. This implies that comets are the only source of organic carbon, nitrogen and water, hence of the total biosphere of the Earth.

Atmosphere↗
PubMed1999

Processing of amorphous carbon grains produced in hydrogen-rich atmosphere.

Recent laboratory experiments, compared with observations, have strongly suggested that the amount of hydrogen, present in the grain formation regions, around C-rich stars, can play a major role in the definition of the spectral characteristics of the freshly formed carbon dust particles. Such spectral characteristics could also be modified, during the thermal processing, due to the shock waves and/or the strong radiation field, that the grains undergo after their formation. In this work we report about a series of laboratory tests, during which submicronic dust particles of amorphous carbon, condensed in an hydrogen-rich atmosphere, have been processed, by means of two different methods. Some samples have been annealed into an oven at different temperatures, while others have been exposed to strong ultraviolet radiation. Their spectra have been obtained before and after the processing in the full range from ultraviolet (200 nm) to far infrared (0.1 mm). The results of the experiment are presented and discussed, together with their astrophysical implications.

Argon↗
PubMed1985

Photochemical reactions in interstellar grains photolysis of CO, NH3, and H2O.

A simulation of the organic layer accreted onto interstellar dust particles was prepared by slow deposition of a CO:NH2:H2O gas mixture on an Al block at 10 K, with concomitant irradiation with vacuum UV. The residues were analyzed by GC-MS, HPLC, and near IR; a reaction pathway leading from NH3 to complex alcohol, fatty acid, and amide products in 27 stages is postulated. The astronomical relevance and significance of the observations are discussed.

Ammonia↗
PubMed1987

Observations of OCS and a search for OC3S in the interstellar medium.

Observations of OCS and a search for OC3S are reported, with particular reference to cold dust clouds. OCS has been detected for the first time in dark clouds with a mean fractional abundance relative to hydrogen of approximately 3 x 10(-9); this is approximately 4 times greater than that observed for giant molecular clouds. This results is discussed in the context of molecule formation mechanisms. Observations of the J = 1 --> 0 transition of OCS indicate that this transition is amplifying the background continuum radiation in the direction of Sgr B2.

Astronomical Phenomena↗
PubMed2005

The onset of planet formation in brown dwarf disks.

The onset of planet formation in protoplanetary disks is marked by the growth and crystallization of sub-micrometer-sized dust grains accompanied by dust settling toward the disk mid-plane. Here, we present infrared spectra of disks around brown dwarfs and brown dwarf candidates. We show that all three processes occur in such cool disks in a way similar or identical to that in disks around low- and intermediate-mass stars. These results indicate that the onset of planet formation extends to disks around brown dwarfs, suggesting that planet formation is a robust process occurring in most young circumstellar disks.

Astronomical Phenomena↗
PubMed1997

Photometric behavior of comet Hale-Bopp (C/1995 O1) before perihelion.

Narrowband photometric observations of comet Hale-Bopp (C/1995 O1) between 25 July 1995 and 15 February 1997 indicated gas and dust production rates of 20 and 100 times greater, respectively, than observed at the same heliocentric distances for comet P/Halley in 1985. Hale-Bopp produced dust at a rate greater than has been observed for any other comet at any distance since at least 1977. On the basis of the observed production rate of the hydroxyl molecule, the calculated minimum effective diameter of Hale-Bopp's nucleus is 17 kilometers, but the actual diameter of the nucleus is likely to be at least two to three times larger. The chemical composition of Hale-Bopp is consistent with that of other long-period comets originating from the Oort Cloud.

Carbon↗
PubMed1992

Cometary origin of carbon and water on the terrestrial planets.

An early high-temperature phase of the protosolar accretion disk is implied by at least three different telltales in chondrites and confirmed by peculiarities in the dust grains of comet Halley. The existence this high-temperature phase implies a large accretion rate hence a massive early disk. This clarifies the origin of the Kuiper Belt and of the Oort cloud, those two cometary populations of different symmetry that subsist today. Later, when the dust sedimented and was removed from the thermal equilibrium with the gas phase, a somewhat lower temperature of the disk explains the future planets' densities as well as the location beyond 2.6 AU of the carbonaceous chondrite chemistry. This lower temperature remains however large enough to require an exogenous origin for all carbon and all water now present in the Earth. The later orbital diffusion of planetesimals, which is required by protoplanelary growth, is needed to explain the origin of the terrestrial biosphere (atmosphere, oceans, carbonates and organic compounds) by a veneer mostly made of comets.

Carbon↗