Springer Nature is making SARS-CoV-2 and COVID-19 research free. View research | View latest news | Sign up for updates

A multi-fluid model of an H2O-dominated dusty cometary atmosphere


A self-consistent multi-fluid solution of the dynamical and thermal structure of an H2O-dominated, two-phase dusty-gas cometary atmosphere has been obtained by solving the simultaneous set of differential equations representing conservation of number density, momentum and energy, together with the transfer of solar radiation in streams responsible for the major photolytic processes and the heating of the nucleus. The validity of this model, as in the earlier single-fluid ones, is restricted to the collision-dominated region where all the heavy species (ions and neutrals) are assumed to achieve a common temperature and velocity. However, recognizing that the photo-produced hydrogen is rather inefficient in exchanging energy with the heavier species we treat the hydrogen separately: it is assumed to be composed of a thermalized component (the second fluid) and a pre-thermal component.

The present model, which is transonic due to the presence of the dust in the inner coma, causes the heavy species to expand subsonically from the nucleus and to smoothly traverse the sonic point within about 45 m of the nucleus, although the dust-gas coupling persists to about 50 km. While the temperature of the heavy species goes through a strong inversion within about 100 km from the nucleus, due to the effects of IR cooling and expansion, it increases to about 300–400 K in the outermost part of the collision-dominated coma due to UV photolytic heating. These temperatures are smaller by a factor of 2–3 from the predictions of the earlier single-fluid models, which assumed instant thermalization of the photo-produced hydrogen.

While the velocities of the heavy species and the thermal hydrogen increase to, respectively, 1.1 km s−1 and 1.6 km s−1 in the outer (collisional) coma, the velocity of the pre-thermal component reaches about 15 km s−1. This latter value is consistent with Ly-α observations of a number of comets, which implies a fast (∼20 km s−1) hydrogen component in the outer coma. The boundary of the exosphere, where the non-thermal hydrogen dominates, is predicted to be around 1.5×104 km from the nucleus. The calculations are for a comet of radius 2.5 km with a dust/gas ratio of 1, at a heliocentric distance of 1 AU.

This is a preview of subscription content, log in to check access.


  1. Allen, C. W.: 1976,Astrophysical Quantities. Atlantic Highlands, N.J.: Athlone Press, p. 172.

  2. Allison, A. C. and Dalgarno, A.: 1967,Proc. Phys. Soc. 90, 609.

  3. Hanner, M. S.: 1980, in I. Halliday and B. A. McIntosh (eds.),Solid Particles in the Solar System, D. Reidel Publ. Co., Dordrecht, Holland. p. 223.

  4. Hellmich, R.: 1979, Ph.D. Thesis, University of Göttingen, West Germany.

  5. Hellmich, R.: 1981,Astron. Astrophys. 93, 341.

  6. Houpis, H. L. F. and Mendis, D. A.: 1980,Astrophys. J. 239, 1107.

  7. Houpis, H. L. F. and Mendis, D. A.: 1981,Astrophys. J. 234, 1088.

  8. Huebner, W. F.: 1981, private communication.

  9. Huffman, D. R.: 1982, private communication.

  10. Ip. W.-H.: 1982, On the Photochemical Heating of Cometary Comas: The Cases of H2O-and CO-Rich Comets,Astrophys. J. (in press).

  11. JANAF,Thermo-Chemical Tables, 2nd edition: 1971. Washington, D.C.: National Bureau of Standards.

  12. Keller, H. U.: 1973,Astron. Astrophys. 23, 269.

  13. Marconi, M. L. and Mendis, D. A.: 1982a,Astrophys. J. 260, 386.

  14. Marconi, M. L. and Mendis, D. A.: 1982b,The Moon and the Planets 26, 345.

  15. Marconi, M. L. and Mendis, D. A.: 1982c,The Moon and the Planets 27, 431 (this issue).

  16. Marconi, M. L. and Mendis, D. A.: 1982d,The Moon and the Planets 27, 27.

  17. Meier, R. R., Opal, C. B., Keller, H. U., Page, T. L., and Carruther, G. R.: 1976,Astron. Astrophys. 52, 283.

  18. Mendis, D. A. and Houpis, H. L. F.: 1982,Rev. Geophys. Space Phys. (in press).

  19. Probstein, R. F.: 1968, in M. Lavrent'ev (ed.),Problems of Hydrodynamics and Continuum Mechanics Philadelphia: SIAM, p. 568.

  20. Sekanina, Z. and Miller, F. D.: 1973,Science 197, 565.

  21. Shimizu, M.: 1975, in B. Donn, M. Mumma, W. Jackson, M. A'Hearn and R. Harrington (eds.),The Study of Comets, NASA-SP 393, p. 763.

  22. Shimizu, M.: 1976,Astrophys. Space Sci. 40, 149.

  23. Shul'man, L. M.: 1981,Astrometriya i Astrofizika 4, 101, Kiev (NASA TT F-599).

  24. van de Hulst, H. C.: 1981,Light Scattering by Small Particles. New York: Dover Pub. Co.

  25. Weissman, P. R. and Kieffer, H. H.: 1981,Icarus 47, 302

  26. Zucrow, M. J. and Hoffman, J. D.: 1976,Gas Dynamics, Vol. 1. New York: John Wiley and Sons, p. 56.

Download references

Author information

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Marconi, M.L., Mendis, D.A. A multi-fluid model of an H2O-dominated dusty cometary atmosphere. The Moon and the Planets 27, 431–452 (1982).

Download citation


  • Heliocentric Distance
  • Thermalized Component
  • Strong Inversion
  • Sonic Point
  • Outermost Part