The moon and the planets

, Volume 27, Issue 4, pp 431–452 | Cite as

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

  • M. L. Marconi
  • D. A. Mendis


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.


Heliocentric Distance Thermalized Component Strong Inversion Sonic Point Outermost Part 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. Allen, C. W.: 1976,Astrophysical Quantities. Atlantic Highlands, N.J.: Athlone Press, p. 172.Google Scholar
  2. Allison, A. C. and Dalgarno, A.: 1967,Proc. Phys. Soc. 90, 609.Google Scholar
  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.Google Scholar
  4. Hellmich, R.: 1979, Ph.D. Thesis, University of Göttingen, West Germany.Google Scholar
  5. Hellmich, R.: 1981,Astron. Astrophys. 93, 341.Google Scholar
  6. Houpis, H. L. F. and Mendis, D. A.: 1980,Astrophys. J. 239, 1107.Google Scholar
  7. Houpis, H. L. F. and Mendis, D. A.: 1981,Astrophys. J. 234, 1088.Google Scholar
  8. Huebner, W. F.: 1981, private communication.Google Scholar
  9. Huffman, D. R.: 1982, private communication.Google Scholar
  10. Ip. W.-H.: 1982, On the Photochemical Heating of Cometary Comas: The Cases of H2O-and CO-Rich Comets,Astrophys. J. (in press).Google Scholar
  11. JANAF,Thermo-Chemical Tables, 2nd edition: 1971. Washington, D.C.: National Bureau of Standards.Google Scholar
  12. Keller, H. U.: 1973,Astron. Astrophys. 23, 269.Google Scholar
  13. Marconi, M. L. and Mendis, D. A.: 1982a,Astrophys. J. 260, 386.Google Scholar
  14. Marconi, M. L. and Mendis, D. A.: 1982b,The Moon and the Planets 26, 345.Google Scholar
  15. Marconi, M. L. and Mendis, D. A.: 1982c,The Moon and the Planets 27, 431 (this issue).Google Scholar
  16. Marconi, M. L. and Mendis, D. A.: 1982d,The Moon and the Planets 27, 27.Google Scholar
  17. Meier, R. R., Opal, C. B., Keller, H. U., Page, T. L., and Carruther, G. R.: 1976,Astron. Astrophys. 52, 283.Google Scholar
  18. Mendis, D. A. and Houpis, H. L. F.: 1982,Rev. Geophys. Space Phys. (in press).Google Scholar
  19. Probstein, R. F.: 1968, in M. Lavrent'ev (ed.),Problems of Hydrodynamics and Continuum Mechanics Philadelphia: SIAM, p. 568.Google Scholar
  20. Sekanina, Z. and Miller, F. D.: 1973,Science 197, 565.Google Scholar
  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.Google Scholar
  22. Shimizu, M.: 1976,Astrophys. Space Sci. 40, 149.Google Scholar
  23. Shul'man, L. M.: 1981,Astrometriya i Astrofizika 4, 101, Kiev (NASA TT F-599).Google Scholar
  24. van de Hulst, H. C.: 1981,Light Scattering by Small Particles. New York: Dover Pub. Co.Google Scholar
  25. Weissman, P. R. and Kieffer, H. H.: 1981,Icarus 47, 302Google Scholar
  26. Zucrow, M. J. and Hoffman, J. D.: 1976,Gas Dynamics, Vol. 1. New York: John Wiley and Sons, p. 56.Google Scholar

Copyright information

© D. Reidel Publishing Co. 1982

Authors and Affiliations

  • M. L. Marconi
    • 1
  • D. A. Mendis
    • 2
  1. 1.Department of Physics, Center for Astrophysics and Space ScienceUniversity of CaliforniaSan Diego, La JollaU.S.A.
  2. 2.Department of Electrical Engineering and Computer Sciences, Center for Astrophysics and Space ScienceUniversity of CaliforniaSan Diego, La JollaU.S.A.

Personalised recommendations