Sublimation-driven evolution of the local radius and the moment of inertia of a long-period comet (original) (raw)
Related papers
New Astronomy, 2003
We introduce a quasi 3-D thermal evolution model for a spherical comet nucleus, which takes into account the diurnal and latitudinal variation of the solar flux, but neglects lateral heat conduction, which is shown to be insignificant. Comparing this model with the 'fast-rotator', evenly heated nucleus model, we show that, for long-term calculations of the evolution of the interior, the latter provides a good approximation. The 3-D model is applied to comet 46P/ Wirtanen: yielding the distribution of temperature and water production rate over the nucleus surface. A very good agreement with observations is obtained for the integrated production rate. The model is also used in order to derive the change of shape due to uneven erosion. Two cases are considered for different inclination angles of the spin axis; it is concluded that the evolved shape depends strongly on the obliquity of the comet's spin axis.
The effect of local topography and self-heating on the sublimation rate of cometary nuclei
Advances in Space Research, 2006
This work presents the results of calculation of the temperature regime and total flux of sublimation for a new physical model of local active areas on a cometary nucleus. As it was shown before a conical hole in the surface dust layer of a cometary nucleus can work as a concentrator of solar energy and, therefore, intensifies sublimation. The temperature of ice on the bottom and that of dust on the side of the crater is calculated for different geometrical parameters of the crater and different conditions of rotation of the nucleus. The dependence of the effect on the location of the active region as well as the effect of changing the solar zenith angle is studied.
Context. Current theories, models of cometary nuclei and of ice formation in the protoplanetary disk, and laboratory studies suggest that cometary materials could be formed of pure crystalline water ice, amorphous water ice, clathrate hydrate, or a mixture of these structures of water ice. However, current models of cometary nuclei consider only two forms of ice during the thermodynamic evolution of comets: amorphous and crystalline water ices. Aims. In this work, we have developed a model of cometary nucleus that takes into account all water ice structures and phase changes in order to predict the outgassing profile of volatile molecules that could be measured by the Rosetta mission and can be used to constrain the structural type of ice existing in the interior of the Comet 67P/Churyumov-Gerasimenko, the target comet of the Rosetta mission, and, hopefully, its initial composition. Methods. We added the physic of formation/dissociation of clathrate hydrates in addition to others physical processes that are taken into account in models without clathrate hydrates. All thermal changes, as well as the release and trapping of gas with water phase changes are taken into account. Results. This model describes heat transmission, latent heat exchanges, all water ices structures and transitions (amorphous-to-pure crystalline, amorphous-to-clathrate hydrates and pure crystalline-to-clathrate hydrates and vise versa), sublimation/recondensation of volatile molecules in the nucleus, gas diffusion, gas released and trapped by crystallization and clathrate formation/dissociation processes, as well as gas and dust release and mantle formation at the surface. Applying this model to the comet 67P/Churyumov-Gerasimenko, results show different outgassing profiles of volatiles molecules from the nucleus depending on the water ice structure, the distribution of volatile molecules between the "trapped" and "condensed" states in the nucleus and the thermal inertia of its porous matrix. Conclusions. Given these results, we pretend that this model is able to constrain the water ice structure and chemical composition in comets from outgassing profiles of volatile molecules, and especially those of the target comet of the Rosetta mission.
Evolution of Comet Nucleus Rotation
Icarus, 2002
The secular evolution of comet nucleus rotation states subject to outgassing torques is studied. The dynamical model assumes that the nucleus inertia ellipsoid is axially symmetric. The outgassing torques acting on the surface are modeled using standard cometary activity formulae. The general rotational equations of motion are derived and separately averaged over the fast rotational dynamics terms and the comet orbit. Special cases where the averaging assumptions cannot be applied are evaluated separately. The modification of the comet orbit due to comet outgassing is neglected. Resulting from this analysis is a system of secular differential equations that describes the dynamics of the comet nucleus angular momentum and rotation state. We find that the qualitative secular evolution of the rotation state is controlled by a single parameter that combines parameters related to the comet orbit and parameters related to the nucleus surface geometry and activity. From this solution, we find qualitatively different evolutionary paths for comet nuclei whose entire surface is active, as compared to nuclei with only a single active region. For surface activity models between these extremes, we show that certain evolutionary paths are more likely than others. Additionally, our solution indicates that a comet nucleus' rotational angular momentum will tend to increase over time, potentially contributing to the observed phenomenon of comet nucleus splitting. c 2002 Elsevier Science (USA)
Hydrodynamic study of condensation and sublimation of ice particles in cometary atmospheres
Icarus, 1986
Condensation of ice particles in the vicinity of a cometary nucleus as pointed out by Yamamoto and Ashihara (1985, Astron. Astrophys. 152, L17-L20) is fully studied by solving the hydrodynamic equations for ice particles and HzO gas. Formulation is presented for the hydrodynamics including condensation and sublimation of ice particles, and energy exchange between ice particles and the gas in a dustless comet. It is shown that sublimation of ice particles condensed leads to heating of the ambient gas, resulting in the higher gas temperature than those predicted by the models proposed so far. Compared with the previous calculation carried out under the conditions at the encounter of the spacecraft to Halley's Comet, the present results have revealed that the survival distance of ice particles against sublimation is longer, but that their size, which attains its maximum of 6.4/~ at 51 km from the center of the nucleus, is smaller, resulting in a larger fraction of uncondensed H20 gas. Discussion is given on the physical conditions under which condensation of ice particles can take place in cometary comae.
A fully 3-dimensional thermal model of a comet nucleus
New Astronomy, 2007
A 3-D numerical model of comet nuclei is presented. An implicit numerical scheme was developed for the thermal evolution of a spherical nucleus composed of a mixture of ice and dust. The model was tested against analytical solutions, simplified numerical solutions, and 1-D thermal evolution codes. The 3-D code was applied to comet 67P/Churyumov-Gerasimenko; surface temperature maps and the internal thermal structure was obtained as function of depth, longitude and hour angle. The effect of the spin axis tilt on the surface temperature distribution was studied in detail. It was found that for small tilt angles, relatively low temperatures may prevail on near-pole areas, despite lateral heat conduction. A high-resolution run for a comet model of 67P/Churyumov-Gerasimenko with low tilt angle, allowing for crystallization of amorphous ice, showed that the amorphous/crystalline ice boundary varies significantly with depth as a function of cometary latitude.
Advances in Space Research, 1999
Five one-dimensional algorithms for heat and gas transport in porous icy bodies have been applied, each to four simplified compositions of comet nucleus models: (1) pure Hz0 ice, (2) a mixture of Hz0 and CO ices, (3) a mixture of Hz0 ice and dust, and (4) a mixture of Hz0 and CO ices and dust. In addition, two different pore radii were assumed in models 3 and 4. The algorithms were chosen on the basis of recent applications and independent code development and evolution. A sixth algorithm was used only in a few models. Results for a spinning comet nucleus in'the orbit of Comet 46P/Wirtanen were compared to establish credibility for the five algorithms. Discrepancies in results were discussed and isolated for possible causes. The algorithms were then investigated and modified by the presenting team members. After resolution of major differences from the simple models, each team member will be able to address more complex models of their own choosing with greater confidence. These models will include investigations of the effects of amorphous versus crystalline water ice, trapped gases versus frozen gases, multi-dimensional calculations, and dust mantle development and removal. Results and progress on the algorithms, and plans for further development are discussed. 0 1999 COSPAR. Published by Elsevier Science Ltd.
Icarus, 1999
A multidimensional comet nucleus model is used to estimate the temperature and gas production distributions on the surface of a comet nucleus in the orbit of 46 P/Wirtanen. The spherical model nucleus is assumed to be made up of a porous dust-ice (H20,CO)matrix. Heat and gas diffusion inside the rotating nucleus are taken into account in radial and meridional directions. A quasi-3D solution is obtained through the dependency of the boundary conditions on the local solar illumination as the nucleus rotates. As a study case, we consider a homogeneous chemical composition of the surface layer which is assumed to contain water ice, The model results include the distributions of temperature and gas production on the surface. For the chosen test case of a nucleus spin axis perpendicular to the orbital plane we found that the CO gas production on the surface is quasi uniformly distributed in contrast to the non-uniform water outgassing. The mixing ratio at a specific point on the comet nucleus surface is not representative of the overall mixing ratio which is observed in the coma.
Further Investigation of Changes in Cometary Rotation
The Astronomical Journal, 2018
Samarasinha & Mueller (2013) related changes of cometary rotation to other physical parameters for four Jupiter family comets defining a parameter X, which is approximately constant within a factor of two irrespective of the active fraction of a comet. Two additional comets are added to this sample in this paper and the claim of a nearly constant parameter X for these six comets is confirmed, albeit with a larger scatter. Taking the geometric mean of X for all the comets above excluding 2P/Encke (as X for each comet was determined with respect to that of 2P/Encke), the expected changes in the rotation periods for a sample of 24 periodic comets are derived. We identify comets from this sample that are most likely to show observationally detectable changes in their rotation periods. Using this sample and including the six comets used to determine X, we find a correlation between the parameter ζ (i.e. the total water production per unit surface area per orbit approximated by that inside of 4 au) and the perihelion distance q; specifically we derive ζ ∝ q −0.8 and provide a theoretical basis for this in Appendix A. This relationship between ζ and q enables ready comparisons of activity due to insolation between comets. Additionally, a relationship between the nuclear radius R and the rotation period P is found. Specifically, we find that on average smaller nuclei have smaller rotation periods compared to the rotation periods of larger nuclei. This is consistent with expectations for rotational evolution and spin-up of comet nuclei, providing strong observational evidence for sublimation-driven rotational changes in comets.