A Three Dimensional Thermal Model of an Irregular Cometary Nucleus
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Thermal evolution of cometary nuclei
The long-term thermal evolution in models of comet nuclei is examined. Models of the nucleus surface and interior are discussed and the equations of comet nucleus evolution are analyzed. The thermal evolution of a nucleus in Comet P/Halley's orbit is outlined. The effects of temperature, composition, and orbital parameters on the evolutionary course are examined. Consideration is given to the implications of the assumption that new comets are pristine objects which have undergone little alteration and constitute a source of original solar nebula material.
Prialnik, Dina↗
Thermal evolution of Comet P/Tempel 1 - Representing the group of targets for the CRAF and CNSR missions
A numerical definition of the thermal evolution of spherically symmetric models of the nucleus in the orbit of Comet P/Tempel-1 is presently used to ascertain the properties of the outer layers of comets under consideration for the future Comet Rendezvous and Asteroid Flyby and the Comet Nucleus Sample Return missions. Evolutionary sequences are computed for different values of density, dust/ice mass ratio, and the dust fraction not lost with ice sublimation. It is found that inner and outer surface temperatures of the dust mantle are comparatively insensitive to parameter changes, and that the total thickness of the crystalline ice layer between the dust mantle and the amorphous ice core will make it difficult for the comet-mission probes to sample pristine ice.
Bar-Nun, Akiva↗
Temperatures and minimum thickness of the inactive surface layer of comet Halley
The effects of a nonvolatile mantle on the thermal state of a comet nucleus are investigated. The original computer model was modified so that temperatures can be computed through a thin dust mantle to the center of a 5 km spherical nucleus in the orbit of P/Halley. No attempt is made to simulate the formation of the mantle. Results are obtained for various specified values of initial mantle thickness and thermal conductivity to determine their effects on temperature profiles through the mantle. The minimum thickness of mantle that can withstand ejection by sublimating gasses is also calculated as a function of mantle thermal conductivity. Calculations were performed for ten or more orbits until temperatures in the mantle reached a near steady state. Results indicate that mantles as thin as 4 cm and 14 cm, for thermal conductivities of 600 and 6000 ergs/cm/s K, respectively, will remain intact. Surface temperatures as high as 511 K at perihelion and 400 K at the position of spacecraft encounter were computed at 0 degree latitude for an upright, rotating nucleus. Ice interface temperatures were raised by different amounts during each orbit, depending on mantle thickness and thermal conductivity, until steady state was reached. These results imply that relatively small nonvolatile masses emplaced randomly in comet nuclei could produce an irregular, permanently mantled surface and could also account for the apparently random location of active areas.
Fanale, F. P.↗
Comet thermal modeling
The past year was one of tremendous activity because of the appearance of Halley's Comet. Observations of the comet were collected from a number of sources and compared with the detailed predictions of the comet thermal modeling program. Spacecraft observations of key physical parameters for cometary nucleus were incorporated into the thermal model and new cases run. These results have led to a much better understanding of physical processes on the nucleus and have pointed the way for further improvements to the modeling program. A model for the large-scale structure of cometary nuclei was proposed in which comets were envisioned as loosely bound agglomerations of smaller icy planetesimals, essentially a rubble pile of primordial dirty snowballs. In addition, a study of the physical history of comets was begun, concentrating on processes during formation and in the Oort cloud which would alter the volatile and nonvolatile materials in cometary nuclei from their pristine state before formation.
Weissman, Paul R.↗
Temperatures within comet nuclei
Knowledge of the temperature distribution within comet nuclei is critical to the understanding of the state and behavior of comets. The present study is concerned with the temperature profile below the surface of the nucleus and below any dust mantle which may be present. With respect to the physics, the considered model is in some respects similar to that discussed by Weissman and Kiefer (1981, 1984). However, the new model is improved in various respects. It represents the first attempt to investigate directly the detailed nature of thermal profiles within a comet nucleus. Included in the investigation are calculations which consider the effects of variations in latitude, nucleus spin axis orientation, surface albedo, and thermal emissivity. It is shown that the application of the model, together with in situ measurements of temperatures within a cometary nucleus, could significantly constrain the range of possible subsurface physical properties.
Squyres, S. W.↗
Multispecies Sublimation of a Comet Including the Influence of the Gas-Phase on Thermal Conductivity
The heat tranfer process that results from gaseous diffusion through the nucleus is shown to play a significant role in modeling comet nulei. In this paper, a comprehensive comet nucleus model is presented which simulates the efflux of gas from surface and interior layers.
gaseous diffusion heat transfer comet nuclei↗
Thermal modeling of comet Kopff: Implications for the CRAF mission
A thermal model of periodic comet Kopff was developed to aid in planning for the Comet Rendezvous Asteroid Flyby mission. The model is based on the comet nucleus model, which estimates the surface temperature distribution and sublimation rates on a rotating icy sphere in heliocentric orbit. The model results are compared with the observed gas production rates from Kopff in 1983. It is found that approximately 15 percent of the sunlit nucleus surface is active, a relatively high fraction for a typical short-period comet. This suggests that the actual nucleus radius may be larger than the model value. The slope of the observed gas production curve with heliocentric distance is steeper than that predicted by the thermal model. This may indicate the existence of a more complex process than direct sublimation of surface ices, such as gas diffusion through a nonvolatile insulating crust.
Weissman, Paul↗
Effects of Nucleus Ice Annealing History on Cometary Outgassing (abstract)
An important consideration for comet modeling is the dependence of the thermal conductivity on the comet structure. The microporosity of the comet ices changes with time as the phase of the ice and its annealing history progresses. Effects of these [modeled] changes on the overall gas production, as a function of time, are presented.
comet outgassing annealing ice dust thermal conduc↗
Numerical models for cometary dust environments
The development of a numerical model for the dust environment near a comet is described. The major components of the model are a simple treatment of the acceleration of the dust by the gas outflow from an isothermal icy nucleus, a detailed treatment of the dust particle trajectories controlled by solar gravity and radiation pressure, including the formation of the dust envelope and tail, and a simple treatment of the radiation from the dust in the visible and thermal infrared. The results of recent comet observations are used to estimate parameter values for the model, including dust-to-gas ratios by light and by mass, the distribution of dust and gas emission on the nuclear surface, the distributions of dust particle size, density, mass, and radiative properties, and other quantities. Gas and dust production rates as functions of time are derived from observed light curves using the simple photometric theory. Sample results are presented for the dust concentration, flux and fluence along the trajectory of a spacecraft which flies by Comet P/Halley in 1986.
Divine, N.↗
Thermal conductivity of comets
The thermal conductivity of two comet models is calculated. Both models assume the comets to be heterogeneous in composition. One model considers the comet to have a nucleus of water-ice mixed with basaltic and meteoritic material in the form of dust and agglomerated particles surrounded by a layer of water-frost. The second model assumes the frost and water-ice layers have sublimated, leaving a porous structure to some depth composed of solid basaltic and meteoritic material with residual gases. K values are calculated as a function of depth, density, temperature and porosity.
Kumar, G. N.↗
The nucleus of comet P/Arend-Rigaux
Simultaneous broadband visual and infrared photometry of comet P/Arend-Rigaux is reported. Its VJHK colors are similar to other periodic comets and the class D asteroids. The visual flux decreased by 0.5 mag within 2 hr, while the thermal flux at 10 microns continued to decrease by 0.7 mag over 3 hr. These observations are consistent with thermal models of a nucleus with a geometric albedo of 0.03 and equivalent radii of 5.1 and 3.8 km.
Veeder, Glenn J.↗
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.↗
The effects of the diffuse radiation fields due to multiple scattering and thermal reradiation by dust on the dynamics and thermodynamics of a dusty cometary atmosphere
A self-consistent model of a dirty, clathrate cometary nucleus is extended to account for diffuse radiation fields caused by multiple scattering and thermal reradiation of the solar continuum by dust. The model is configured to fit conditions expected for the various spacecraft which will encounter Halley's comet at 0.89 AU. The atmosphere is assumed a chemically reactive dust-gas mixture in quasi-steady spherically symmetric expansion. The effect of electron-neutral ion collisions is accounted for, along with rate constants of the various species of clathrate ice particles and radiative transfer of the solar input in the UV, visible and near-IR intervals. The opacity of the circumnuclear dust is projected to be an order of magnitude greater than previous estimates, which severely impacts the potential visibility of the nucleus to the cameras of the flyby satellites.
Marconi, M. L.↗
Numerical simulation of cometary nuclei. III - Internal temperatures of cometary nuclei
The thermal diffusion equation for the internal temperature of cometary nuclei is exactly solved by means of a one-dimensional numerical model in order to shed light on the complex behavior of these temperatures with varying orbital and thermal parameters and in order to consider possible cometary nucleus thermal evolution targets for comet rendezvous and/or sample-return missions. The concept of 'new' and 'old' comets, classified in terms of how many passages around the sun have been made, may take on new meaning in view of the present demonstration that differences in eccentricity between two comets having the same period and number of apparitions may yield significantly different internal temperature profiles.
Herman, Gary↗
Hydrodynamic models of the cometary atmosphere
Self-consistent transonic multifluid solutions for the dynamical and thermal structure of the two-phase dusty gas atmosphere of a comet containing a dirty H2O-clathrate ice nucleus have been obtained. This was done by solving a simultaneous set of differential equations representing conservation of number density, momentum, and energy, as well as the transfer of solar radiation responsible for the major photolytic processes and nuclear heating. The results presented include velocity and Mach number profiles of heavy species and the velocity of dust within the subsonic and supersonic regions, temperature profiles of the dust and gas in the supersonic region, and radial profiles of electron and ion densities in the collision-dominated region.
Mendis, D. A.↗
Model of Dust Thermal Emission of Comet 67p-Churyumov-Gerasimenko for the Rosetta-MIRO Instrument
The ESA's Rosetta spacecraft will arrive at comet 67P/Churyumov-Gerasimenko in 2014. The study of gas and dust emission is primary objective of several instruments on the Rosetta spacecraft, including the Microwave Instrument for the Rosetta Orbiter (MIRO). We developed a model of dust thermal emission to estimate the detectability of dust in the vicinity of the nucleus with MIRO. Our model computes the power received by the MIRO antenna in limb viewing as a function of the geometry of the observations and the physical properties of the grains. We show that detection in the millimeter and submillimeter channels can be achieved near perihelion.
millimeter-submillimeter↗
Thermal modeling of cometary nuclei
A model of the sublimation of volatile ices from a cometary nucleus is presented which includes the effects of (1) diurnal heating and cooling, (2) rotation period and pole orientation, (3) the thermal properties of the ice and subsurface layers, and (4) the contributions from coma opacity, scattering and thermal emission where the properties of the coma are derived from the integrated rate of volatile production by the nucleus. In applying the model to the case of the 1986 apparition of Halley's comet, it is found that the generation of a cometary dust coma increases the total energy reaching the Halley nucleus due to the greater geometrical cross-section of the coma as compared with the bare nucleus. The calculated coma opacity of Halley is about 0.2 at 1 AU from the sun and 1.2 at perihelion. Possible consequences of the results obtained for the generation of nongravitational forces, volatile production rates for comets and cometary lifetimes against sublimation are discussed.
Weissman, P. R.↗