- Email: [email protected]
Thermal emission from cometary dust
Ado. Space Rca. Vol.2, No.12, p.151, 1983 Printed in Great Britain.
All rights reserved.
0273—117]/831120157—O1$03.OO/O Copyright © COSPAR
THERMAL EMISSION FROM COMETARY DUST M. S. Hanner Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, U.S.A.
ABSTRACT
The thermal emission from the dust coma of a comet can be analyzed to yield the flux and size distribution of the dust grains and the relative abundance of silicate and absorbing grains. SUMMARY Modals are being developed to interpret the thermal emission from cometary dust in terms of grain composition, size distribution, and dust production rate [1]. The models are based on measured optical constants for minerals expected to be present in cometary materials, including silicates, graphite, isagnetite, and ices. The calculations for graphite properly take into account the anisotropy in the optical constants. Small absorbing grains, 1 micron radius, are necessary to explain the observed infrared flux at wavelengths 3 to 8.5 microns, while small silicate grains are necessary to explain the excess flux near 10 microns and 18 microns. The computed temperatures versus grain size and heliocentric distance are very similar for magaetite, graphite, and glassy carbon. This resilt is significant in showing that grain size is the major determinant of temperature for absorbing grains ~ 10 micronS radius Consequently, the observed thermal energy distribution can be translated directly into grain size. The flux ratio 1(4.8 pm)/I(3.5 tim) Is shown to be a sensitive Indicator of grain size. One size distribution has been found which fits this observed flux ratio at heliocentric distances less than 0.64 AU in Comets Kohoutek, Bradfield, West, and Encke [2]. All of the well—observed comets can be fit with a change in mean particle size less than a factor of 2 This result has particular signifIcance for predicting the dust environment in Comet Halley. Amorphous olivine grains 13], with a slight admixture of absorbing material (“dirty’S silicate) can explain the observed emission feature near 10 microns and the excess flux at 18 microns. The visibility of the feature at 10 microns depends on the relative temperatures of the absorbing and silicate grains. A two—component model of absorbing grains and amorphous olivine grains has been developed for Comet iCohoutek fitting both the observed infrared flux and the optical scattered light [5]. The numbers of silicate and absorbing grains are roughly comparable in this model. Both components contribute to the observed scattered radiation, while the relatively cool silicate grains are invisible in the thermal emission, 3 5 to 8 5 microns This 2—component model for Comet Kohoutek, suitably scaled, will form the basis of a dust model for Comet Halley. Icy grains can be expected to vaporize within a few hundred kilometers of the nucleus at heliocentric distances less than approximately 2.5 AU [4). Consequently, they will not contribute significantly to the observed scattered light from the visible coma. The research described in this paper was performed at the .1st Propulsion Laboratory, California Institute of Technology, under Contract with the National Aeronautics and Space Administration. References 1. 2. 3. 4. 5.
M.S. Hanner, in Solid Particles in the Solar System, IAU Symposium 90, Reidel, Dordrecht, 1980, p. 223. H. Campins and M.S. Manner, in Comets, University of Arizona Press, Tucson, 1982, p. 341. W. KrHtschtner and O.K. Huffman, Astrophys. Space Sd. 61, 195 (1979). M.S. Manner, Icarus 47, 342 (1981). M.S. Hartner, to be published.
All rights reserved.
0273—117]/831120157—O1$03.OO/O Copyright © COSPAR
THERMAL EMISSION FROM COMETARY DUST M. S. Hanner Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, U.S.A.
ABSTRACT
The thermal emission from the dust coma of a comet can be analyzed to yield the flux and size distribution of the dust grains and the relative abundance of silicate and absorbing grains. SUMMARY Modals are being developed to interpret the thermal emission from cometary dust in terms of grain composition, size distribution, and dust production rate [1]. The models are based on measured optical constants for minerals expected to be present in cometary materials, including silicates, graphite, isagnetite, and ices. The calculations for graphite properly take into account the anisotropy in the optical constants. Small absorbing grains, 1 micron radius, are necessary to explain the observed infrared flux at wavelengths 3 to 8.5 microns, while small silicate grains are necessary to explain the excess flux near 10 microns and 18 microns. The computed temperatures versus grain size and heliocentric distance are very similar for magaetite, graphite, and glassy carbon. This resilt is significant in showing that grain size is the major determinant of temperature for absorbing grains ~ 10 micronS radius Consequently, the observed thermal energy distribution can be translated directly into grain size. The flux ratio 1(4.8 pm)/I(3.5 tim) Is shown to be a sensitive Indicator of grain size. One size distribution has been found which fits this observed flux ratio at heliocentric distances less than 0.64 AU in Comets Kohoutek, Bradfield, West, and Encke [2]. All of the well—observed comets can be fit with a change in mean particle size less than a factor of 2 This result has particular signifIcance for predicting the dust environment in Comet Halley. Amorphous olivine grains 13], with a slight admixture of absorbing material (“dirty’S silicate) can explain the observed emission feature near 10 microns and the excess flux at 18 microns. The visibility of the feature at 10 microns depends on the relative temperatures of the absorbing and silicate grains. A two—component model of absorbing grains and amorphous olivine grains has been developed for Comet iCohoutek fitting both the observed infrared flux and the optical scattered light [5]. The numbers of silicate and absorbing grains are roughly comparable in this model. Both components contribute to the observed scattered radiation, while the relatively cool silicate grains are invisible in the thermal emission, 3 5 to 8 5 microns This 2—component model for Comet Kohoutek, suitably scaled, will form the basis of a dust model for Comet Halley. Icy grains can be expected to vaporize within a few hundred kilometers of the nucleus at heliocentric distances less than approximately 2.5 AU [4). Consequently, they will not contribute significantly to the observed scattered light from the visible coma. The research described in this paper was performed at the .1st Propulsion Laboratory, California Institute of Technology, under Contract with the National Aeronautics and Space Administration. References 1. 2. 3. 4. 5.
M.S. Hanner, in Solid Particles in the Solar System, IAU Symposium 90, Reidel, Dordrecht, 1980, p. 223. H. Campins and M.S. Manner, in Comets, University of Arizona Press, Tucson, 1982, p. 341. W. KrHtschtner and O.K. Huffman, Astrophys. Space Sd. 61, 195 (1979). M.S. Manner, Icarus 47, 342 (1981). M.S. Hartner, to be published.