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one wavelength will not fit at another wavelength. In addition, the sizes of the spheres will be much smaller than the characteristic lengths of the actual particles.

Holland and Gagne (1970) measured the scattering matrix for a polydisperse system of silica particles smaller than 1 p.m., m = 1.55. Their data for the matrix elements S11 and S22 at 546 mm are reproduced in figure 3 (Holland, 1969). The solid curve was computed from Mie theory for the observed size distribution. Mie theory fits the data fairly well at small scattering angles, but it predicts a steep rise toward the backscatter direction that is absent in the laboratory data. Data for S12 = S21 indicate that the polarization is positive at 03. 160°, whereas Mie theory predicts negative polarization. There is, however, an indication that the polarization changes sign near 160° at 546 nm and near 150° at 486 mm. The position of the neutral point varies with wavelength in the opposite direction from the shift observed by Weinberg and Mann in the zodiacal light.

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Figure 3.-Variation of matrix elements S11(0) and S22(0) with scattering angle 9. Solid line is theoretical curve computed from Mie theory (Holland, 1969, fig. 6).

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m = refractive index; Xo = size distribution parameter.

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The scattering properties of nonspherical particles also can be studied by microwave scattering from scaled particle models (Greenberg, Pedersen, and Pedersen, 1961). Greenberg, Wang, and Bangs (1971) have found that the measured extinction for particle models with roughened surfaces differs widely from the extinction by smooth spheres. Microwave scattering data over the whole range of scattering angles for many values of a/\ are needed before we can conclude whether the peculiar scattering patterns observed for individual particles average out to resemble scattering by spheres over an extended size distribution.

Scattering functions can be computed analytically for the case of long cylinders (Kerker, 1969; Lind, 1966). Detailed comparison of the scattering functions for spheres and cylinders, and possibly ellipsoids, can provide useful information on the effects of particle shape. For example, there is a significant change in the polarization, even for an extended size distribution, when the angle between the cylinder axis and the incident radiation is varied. However, the polarization by randomly oriented cylinders with n(a) or exp [-5(a|0.5)*] was quite similar to that for spheres with the same size distribution (Hanner, 1969).

The limitations of Mie theory and the importance of computing zodiacal light models for nonspherical particles have been emphasized by Greenberg (1970). Richter (1966) has discussed experimental phase functions and polarization curves for irregular particles over the size range 107° to 10 cm.

CONCLUSION

The zodiacal light data sample the average properties of the interplanetary dust particles over a large volume of space. Intensity and polarization measurements in the ultraviolet and infrared, together with the wavelength dependence of polarization throughout the visible spectral region, will provide information on the physical nature and size distribution of the dust particles. However, we cannot expect to obtain a complete model of the interplanetary dust from zodiacal light observations alone. The data will be most valuable when combined with the results of particle collections and other methods used to study in detail the physical properties of individual particles.

Giese and Dziembowski (1969) and Giese (1970) have discussed the value of zodiacal light observations from space probes in determining the spatial distribution of the interplanetary dust. Zodiacal light experiments will be included on both the Helios inner solar system probes and the Pioneer F and G asteroid-Jupiter probes.

ACKNOWLEDGMENTS

It is a pleasure to thank Dr. J. L. Weinberg for his interest and his helpful discussions. This research has received support from NSF grant GA-12400 and NASA grant NGR 33-017011.

REFERENCES

Allen, C. W. 1946, The Spectrum of the Corona at the Eclipse of 1940 October 1. Mon.
Notic. Roy. Astron. Soc. 106, 137.
Aller, L. H., Duffner, G., Dworetsky, M., Gudehus, D., Kilston, S., Leckrone, D.,
Montgomery, J., Oliver, J., and Zimmerman, E. 1967, Some Models of the Zodiacal
Cloud. The Zodiacal Light and the Interplanetary Medium (ed., J. L. Weinberg), pp.
243-256. NASA SP-150.
Bandermann, L. W. 1968, Physical Properties and Dynamics of Interplanetary Dust. Ph.D.
Thesis. Univ. of Maryland.
Blackwell, D. E., Dewhirst, D. W., and Ingham, M. F. 1967, The Zodiacal Light. Advances
in Astronomy and Astrophysics (ed., Z. Kopal), vol. 5, p. 1. Academic Press, Inc. New
York.
Blackwell, D. E., and Ingham, M. F. 1967, Toward a Unification of Eclipse and
Zodiacal-Light Data. The Zodiacal Light and the Interplanetary Medium (ed., J. L.
Weinberg), pp. 17-21. NASA SP-150.
Blackwell, D. E., Ingham, M. F., and Petford, A. D. 1967, The Distribution of Dust in
Interplanetary Space. Mon. Notic. Roy. Astron. Soc. 136, 313.
Donn, B., and Powell, R. S. 1962, Angular Scattering From Irregularly Shaped Particles
With Application to Astronomy. Electromagnetic Scattering (ed., M. Kerker), p. 151.
Pergamon Press, Inc. New York.
Elsásser, H. 1963, The Zodiacal Light. Planet. Space Sci. 11, 1015.
Elsásser, H. 1970, The Zodiacal Light: Space Observations. Space Research X. (eds., T. M.
Donahue, P. A. Smith, and L. Thomas), p. 244. North-Holland Pub.Co. Amsterdam.
Elsásser, H., and Schmidt, T. 1966, Rayleigh-Teilchen (a 3 10-5 cm) im interplanetaren
Raum? Z. Naturforsh. A 21, 1116.
Field, G. B., Partridge, R. B., and Sobel, H. 1967, Effects of Absorption Spectra of Ices on
the Ultraviolet Extinction by Interstellar Grains. Interstellar Grains (ed., J. M.
Greenberg and T. P. Roark), p. 207. NASA SP-140.
Giese, R. H. 1961, Streuung elektromagnetischer Wellen an absorbierenden und
dielektrischen Kugelförmigen Einzelteilchen und an Gemischen solcher Teilchen. Z.
Astrophys. 51, 119.
Giese, R. H. 1963, Light Scattering by Small Particles and Models of Interplanetary Matter
Derived From the Zodiacal Light. Space Sci. Rev. 1, 589.
Giese, R. H. 1970, Model Computations Concerning Zodiacal Light Measurements by
Space Missions. Paper presented at COSPAR, 13th meeting (Leningrad).
Giese, R. H., and Dziembowski, C. v. 1967, On Optical Models Approximating
Observations of the Zodiacal Light Outside the Ecliptic. The Zodiacal Light and the
Interplanetary Medium (ed., J. L. Weinberg), pp. 271-276. NASA SP-150.
Giese, R. H., and Dziembowski, C. v. 1969, Suggested Zodiacal Light Measurements From
Space Probes. Planet. Space Sci. 17,949.
Giese, R. H., and Siedentopf, H. 1962, Optische Eigenschaften von Modellen der
interplanetaren Materie. Z. Astrophys. 54, 200.
Greenberg, J. M. 1970, Models of the Zodiacal Light. Space Research X (eds., T. M.
Donahue, P. A. Smith, and L. Thomas), p. 225. North-Holland Pub.Co. Amsterdam.
Greenberg, J. M., Pedersen, N. E., and Pedersen, J. C. 1961, Microwave Analog to the
Scattering by Nonspherical Particles. J. Appl. Phys. 32, 233.
Greenberg, J. M., Wang, R. T., and Bangs, L. 1971, Extinction by Rough Particles and the
Use of Mie Theory. Nature (London) Phys. Sci. 230, 110-112.
Hanner, M. S. 1969, Light Scattering in Reflection Nebulae. Ph. D. Thesis. Rensselaer
Polytechnic Institute.
Hanner, M. S. 1970, Zodiacal Light Models Based on Scattering by Silicate Particles. Paper
presented at Amer. Astron. Soc. Meeting (Boulder, Colo.).

Harwit, M. 1963, Infrared Appearance of Different Zodiacal Light Cloud Models. Paper
presented at the 12th Int. Colloq. Astrophys. Inst. (Liege, Belgium).
Hemenway, C. L., and Hallgren, D. S. 1970, Time Variation of the Altitude Distribution
of the Cosmic Dust Layer in the Upper Atmosphere. Space Research X (ed., T. M.
Donahue, P. A. Smith, and L. Thomas), p. 272. North-Holland Pub. Co. Amsterdam.
Hemenway, C. L., Hallgren, D. S., and Coon, R. E. 1967, High Altitude Balloon-Top
Collections of Cosmic Dust. Space Research VII (ed., R. L. Smith-Rose), p. 1423.
North-Holland Pub. Co. Amsterdam.
Hemenway, C. L., Hallgren, D. S., Laudate, A. T., Patashnick, H., Renzema, T. S., and
Griffith, O. K. 1970, A New High Altitude Balloon-Top Cosmic Dust Collection
Technique. Paper presented at COSPAR, 13th meeting (Leningrad).
Holland, A. C. 1969, The Scattering of Polarized Light by Polydisperse Systems of
Irregular Particles. NASA TN D-5458.
Holland, A. C., and Gagne, G. 1970, The Scattering of Polarized Light by Polydisperse
Systems of Irregular Particles. Appl. Opt. 9, 1113.
Hulst, H. C., van de. 1947, Zodiacal Light in the Solar Corona. Astrophys.J. 105,471.
Hulst, H. C., van de. 1957, Light Scattering by Small Particles. John Wiley & Sons, Inc.
New York.
Ingham, M. F. 1961, Observations of the Zodiacal Light From a Very High Altitude
Station IV. The Nature and Distribution of the Interplanetary Dust. Mon. Notic. Roy.
Astron. Soc. 122, 157.
Ingham, M. F., and Jameson, R. F. 1968, Observations of the Polarization of the Night
Sky and a Model of the Zodiacal Cloud Normal to the Ecliptic Plane. Mon. Notic. Roy.
Astron. Soc. 140,473.
Jameson, R. F. 1970, Observations and a Model of the Zodiacal Light. Mon. Notic. Roy.
Astron. Soc. 150, 207.
Kerker, M. 1969, The Scattering of Light and Other Electromagnetic Radiation. Academic
Press, Inc. New York.
Lind, A. C. 1966, Resonance Electromagnetic Scattering by Finite Circular Cylinders.
Ph. D. Thesis. Rensselaer Polytechnic Institute.
Little, S.J., O'Mara, B.J., and Aller, L. H. 1965, Light Scattering by Small Particles in the
Zodiacal Cloud. Astron. J. 70,346.
Peterson, A. W. 1963, Thermal Radiation From Interplanetary Dust. Astrophys. J. 138,
1218.
Peterson, A. W. 1964, Thermal Radiation From Interplanetary Dust II. Distribution of the
Dust. Ann. N.Y. Acad. Sci. 119, 72.
Peterson, A. W., and MacQueen, R. M. 1967, Infrared Observations of Thermal Radiation
From Interplanetary Dust at the Eclipse of November 12, 1966 (abstract). The
Zodiacal Light and the Interplanetary Medium (ed., J. L. Weinberg), p. 89. NASA
SP-150.
Powell, R. S., Woodson, P. E., III, Alexander, M. A., Circle, R. R., Konheim, A. G., Vogel,
D. C., and McElfresh, T. W. 1967, Analysis of all Available Zodiacal Light
Observations. The Zodiacal Light and the Interplanetary Medium (ed., J. L. Weinberg),
pp. 225-241. NASA SP-150.
Richter, N. B. 1966, The Photometric Properties of Interplanetary Matter. Quart. J. Roy.
Astron. Soc. 3, 179.
Singer, S. F., and Bandermann, L. W. 1967, Nature and Origin of Zodiacal Light. The
Zodiacal Light and the Interplanetary Medium (ed., J. L. Weinberg), p. 379. NASA
SP-150.
Smith, L. L., Roach, F. E., and Owen, R. W. 1965, The Absolute Photometry of the
Zodiacal Light. Planet. Space Sci. 13, 207.
Stecher, T. P. 1965, Interstellar Extinction in the Ultraviolet. Astrophys. J. 142, 1683.
Taft, E. A., and Philipp, H. R. 1965, Optical Properties of Graphite. Phys. Rev. 138A,
197.

Weinberg, J. L. 1964, The Zodiacal Light at 5300A. Ann. Astrophys. 27, 718.

Weinberg, J. L., 1970, Current Problems in the Zodiacal Light. Space Research X (eds., T. M. Donahue, P. A. Smith, and L. Thomas), p. 233. North-Holland Pub. Co. Amsterdam.

Weinberg, J. L., and Mann, H. M. 1968, Negative Polarization in the Zodiacal Light. Astrophys. J. 152, 665.

DISCUSSION

BANDERMANN: Purely photometric, spectroscopic, polarimetric observations of the zodiacal light will not lead to definite answers about the physical properties of the particles. One must try to consider all the different types of evidences (zodiacal light as well as impact counts, deep sea sediments, etc.) and then look for collaboration toward an answer.

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