Towards a model of cometary nuclei for engineering studies for future space missions to comets

doi:10.1016/0032-0633(96)00067-0

Planetary and Space Science

Volume 44, Issue 7, July 1996, Pages 637-653

https://doi.org/10.1016/0032-0633(96)00067-0 Get rights and content

Abstract

A brief review is given of the present state of knowledge on comets. Existing comet models are critically analysed in view of these observational facts. This analysis leads to the conclusion that the most promising model approach is one where the comet nucleus is considered as a porous medium containing an intimate mixture of dust particles and different ices. Compact nuclei can be treated as limiting cases. An outline of a pseudo-tridimensional model based on this approach is given. The hypothesis is made that comets initially contain a mixture of amorphous water ice, solid carbon monoxide and silicate dust. As examples, local production rates of CO and integrated production rates of H₂O and CO for comets P/Schwassmann-Wachmann 1 and P/Schwassmann-Wachmann 3 are calculated, making the extreme assumption that all the devolatilized dust remains on the surface. The depletion of CO in the near surface layers is determined after ten revolutions. Under the assumptions that all the dust remains on the surface or that all the dust is lifted off, thermal profiles at the equator of a nucleus on the orbit of comet P/Wirtanen with the rotation axis perpendicular to the orbital plane are determined when the surface temperature is maximum or minimum. The evolution of the maximum and minimum surface temperature is computed over ten orbital periods for two values of the bulk thermal conductivity of the dust coverage. For the same comet, the relative H₂O and CO content as a function of depth are calculated for a dust covered and a non-dust covered nucleus. Improvements of the present day model are suggested and a strategy is proposed for adapting this model for complex and thus more realistic situations.

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The production rates of species coming out from the nucleus have been investigated via a numerical model. The model is based upon work carried out by Klinger in the 1990s (e.g. Klinger et al., 1996; Klinger, 1999) but the specific implementation is that of Marboeuf (2008), Marboeuf et al. (2012), Marboeuf and Schmitt (2014). We note that several other models of the evolution of cometary nuclei have been published and used in the past.
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There is no direct evidence about the internal structure of cometary nuclei, which are mostly hidden by their gas and dust comae, and have not yet been orbited by any spacecraft. Their densities are low, typically of about 400 kg m⁻³ for 9P/Tempel 1 (that was impacted by the Deep Impact probe) and 67P/Churyumov–Gerasimenko (that is the target of the Rosetta mission). Such low densities are in favour of a high macro-porosity, or a high micro-porosity, or both. Observations of disruption or splitting of nuclei indeed suggest that some huge sub-nuclei or some meter-sized fragments could be the building blocks of comets. Analysis, from in-situ measurements and from remote light scattering observations, of the structure of the dust particles, which significantly consist of fluffy aggregates of submicron-sized grains, could be in favour of a fractal structure. However, the presence of huge icy grains in the innermost coma, and of flat layers on the surface of 9P/Tempel 1, are clues to the complexity of these objects, which have suffered drastic erosion phenomena on their elongated orbits. It is expected that the Rosetta mission will provide a fair understanding of the structure of the deep interior of the nucleus of 67P/Churyumov–Gerasimenko, thanks to the on-board CONSERT experiment.
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Planetesimals up to sizes of at least 100 m (Weidenschilling, 1995) or larger formed from these aggregates and should therefore be marked by a porous particulate structure with micrometer-sized constituents. Comets and Kuiper belt objects are believed to have developed from planetesimals with fewer changes than other present Solar System objects and to have preserved porosity to a certain extend (Klinger et al., 1996). Meteoritic studies support that micrometer size is typical for preplanetary dust before particle accretion (Kerridge, 1993).
I describe a new method to make particle layers which consist of SiO₂ spheres with 0.78 μm radius. The layers were produced by sedimentation of aggregates which had grown in ballistic particle collisions, and the layers had a porosity of 0.95. They were used for experiments on sintering, i.e., the samples were heated in an oven at varying temperatures and heating durations, and the samples were analyzed by scanning electron microscopy. Based on the change of particle diameter, surface diffusion sintering and viscous flow are identified as important transformation mechanisms. The first effect dominated at the start of restructuring and the latter at higher temperatures. The neck growth of adjacent particles was fitted to a surface diffusion sintering model and predicts neck radii as a heating temperature and duration function. Between the temperature range of neck formation and of melting, further restructuring occurred which lead to dissolution of particulate structure and to densification and which resulted in a porous object consisting of straight elongated substructures which connected kinks of higher material density. The thermal transformation is important for the change of strength, collisional behavior, light-scattering properties, and thermal conductivity with relevance to dust aggregates, planetesimals, comets, interplanetary dust particles, and regolith-covered celestial bodies.
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Towards a model of cometary nuclei for engineering studies for future space missions to comets

Abstract

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Planet. Space Sci.

Icarus

Icarus

Icarus

Icarus

Icarus

Astrophys. J.

Astrophys. J.

Astrophys. J.

Ann. Geophysicae

Density of comet Shoemaker-Levy 9 by modelling breakup of the parent “rubble pile”

Nature

Global properties of the nucleus of Halley's comet from studies of its spin state

Results of a coupled heat and mass transfer model applied to KOSI sublimation experiments

Clathrate hydrate formation in amorphous cometary ice analogs in vacuo

Science

Elaboration d'un modèle thermo-chimique de la matière cométaire: contribution à l'étude physico-chimique des milieux poreux hétérogènes

Dust release and mantle development in comets

Astrophys. J.

Two approaches to discuss the unexpected activity of comets at large heliocentric distances. Accepted for publication

Complementary studies on the unexpected activity of comet Schwassmann-Wachmann 1

Planet. Space Sci.

Detection of faint Kuiper belt objects

Infrared sounding of comet Halley from Vega 1

Nature

Molecular abundances in comets

Hydrates de gaz dans les noyaux cométaires es les grains interstellaires

Ann. Astrophys.

The icy rings of the outer planets

The accumulation and structure of comets

On the mechanisms of comet outbursts and the chemical composition of comets

Astrophys. J.

Low-temperature forms of ice as studied by X-ray diffraction

Nature

Numerical modelling of gas production curves of a dust covered comet nucleus. Accepted for publication

Modelisation du comportement thermique et de la différenciation chimique des noyaux de cometes

A method of estimating temperature profiles and chemical differentiation in the near surface layers of porous comet nuclei—first results for comet P/Churyurnov-Gerasimenko

ESA SP-302

Comets

European Southern Observatory Scientific Preprint No. 960

Thermal models of the nucleus of comet P/Schwassmann-Wachmann 1

Interpretation of ion mass spectra in the mass per charge range 25–35 amu/e obtained in the inner coma of Halley's comet by the HIS-sensor of the Giotto IMS experiment

Astron. Astrophys.

Enthalpy changes and heat capacity in the transformations form high surface area amorphous ice to stable hexagonal ice

J. Chem. Phys.

A radar study of comet Iras-Araki-Alcock 1983d

Astron. J.

An icy-glue model of cometary nuclei

Nature

What are comets made of? A model based on interstellar dust

Fluffy comets

The friable sponge model of cometary nucleus

The friable sponge model of cometary nucleus

CO2 depth profiles in cometary model substances of KOSI

ESA SP-302

Structure of the coma: chemistry and solar wind interaction

CO₂ depth profiles in cometary model substances of KOSI