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Vedder, J. F. 1966, Minor Objects in the Solar System. Space Sci. Rev. 6, 365-414.
Weinberg, J. L. 1967a, Summary Report II on Zodiacal Light, July 1967. Hawaii Inst.
Geophys.
Weinberg, J. L., ed. 1967b, The Zodiacal Light and the Interplanetary Medium. NASA
SP-150.
Weinberg, J. L., Beeson, D. E., and Hutchison, P. B. 1969, Photometry of Lunar Libration
Regions. Bull. Amer. Astron. Soc. 1,368.
Whipple, F. L. 1959, Solid Particles in the Solar System. J. Geophys. Res. 64, 1653-1664.
Whipple, F. L. 1961, The Dust Cloud About the Earth. Nature 189, 127-128.
Whipple, F. L. 1967, On Maintaining the Meteoritic Complex. The Zodiacal Light and the
Interplanetary Medium (ed., J. L. Weinberg), pp. 409-426. NASA SP-150.
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DISCUSSION

DUBIN: The shadow of Earth in the counterglow (and near-Earth dust) is not expected to be observed at all altitudes. You imply that because the shadow could not be observed, the dust of the counterglow is in the vicinity of the asteroid region and that the satellite results of dust measurements near Earth could not be correct. What is the lowest altitude for which the shadow measurements may be applied? ROOSEN: The shadow technique that I used (Roosen, 1970) is useful only above about 6000 km. In the direction in which I was looking, material below that altitude would have been in Earth's umbra and hence could not contribute to the counterglow brightness. DUBIN: The atmosphere extends to several hundred kilometers. It is doubtful that this measurement would work close in where the airglow would interfere. A source of the near-Earth measurements has been identified from the disintegration of the Prairie Network meteoroids, for example. ROOSEN: As I have already mentioned, Peale (1968) has summarized a number of very convincing arguments against a near-Earth geocentric dust cloud. DUBIN: Another point in regard to interstellar particles is the discovery of the penetration of the interstellar wind that has been made by Bertaux and Blamont (1971), and by Thomas and Krassa (1971). The results indicate that a hydrogen wind detected by resonant excitation in Lyman alpha penetrates into the solar system to a distance between 3 and 7 AU. Such an interstellar wind should be accompanied by interstellar grains that, accordingly, should also be able to penetrate into the asteroid region and may contribute to the counterglow without showing an Earth shadow. You indicated that there would be no contribution from interstellar dust based on a recent publication of Bandermann? ROOSEN: I think that Dr. Bandermann should answer that. BANDERMANN: The publication by Bandermann and Wolstencroft (1970) is concerned with the gravitational capture of interstellar dust into the solar system by a single encounter with a planet, rather than with the penetration of dust contained in a gas cloud colliding with the solar system, which involves gas-dust coupling and solar-wind interaction. These authors found a total capture rate of s 10 kg/s for interstellar dust densities of 3 x 10 * g/cm3 and compared this rate with the estimated rate of loss from the zodiacal cloud, ~1 x 10° kg/s. They did not calculate the contribution by captured dust to the zodiacal light or counterglow surface brightness.

DISCUSSION REFERENCES

Bandermann, L. W., and Wolstencroft, R. D. 1970, Three-Body Capture of Interstellar Dust by the Solar System. Mon. Notic. Roy. Astron. Soc. 150, 173-186.

Bertaux, J. L., and Blamont, J. E. 1971, Evidence for a Source of an Extraterrestrial
Hydrogen Lyman-Alpha Emission—The Interstellar Wind. Astron. Astrophys. 11(2),
200–217.
Peale, S. J. 1968, Evidence Against a Geocentric Contribution to the Zodiacal Light. J.
Geophys. Res. 73, 3025-3033.
Roosen, R. G. 1970, The Gegenschein and Interplanetary Dust Outside the Earth's Orbit.
Icarus 13, 184-201.
Thomas, G. E., and Krassa, R. F. 1971, OGO 5 Measurements of the Lyman Alpha Sky
Background. Astron. Astrophys. 11(2), 218-233.

PHYSICAL PROPERTIES OF THE INTERPLANETARY DUST

MARTHA S. HAWWER
Dudley Observatory

The interplanetary dust may be composed of cometary material, interstellar grains, debris from asteroidal collisions, primordial material formed by direct condensation, or contributions from all of these sources. Before we can determine the origin of the dust, we need to know its physical nature, spatial distribution, and the dynamical forces that act on the particles. The spatial distribution and dynamics are separately treated in this symposium by Roosen.' We discuss here the physical characteristics of the dust particles: their size distribution, chemical composition, physical structure, and optical properties.

Data on these properties can be obtained from particle collections, impact craters, thin-film penetrations, acoustic detectors, and the zodiacal light. The results obtained from various methods have been reviewed by Singer and Bandermann (1967) and Bandermann (1968). We shall concentrate on discussing the information that can be derived from observations of the zodiacal light.

ZODIACAL LIGHT MODELS

The zodiacal light provides information on the average properties of the interplanetary dust over a large volume of space. Analysis of the brightness and polarization of the zodiacal light yields, in principle, the large-scale size distribution, spatial distribution, and optical properties of the scattering particles. The optical properties, in turn, contain information on the composition and physical structure of the dust particles.

Figure 1 illustrates the basic scattering geometry. Particles in the volume element at P at a distance r from the Sun, scatter sunlight through an angle 6 into the line of sight along PE. The contribution to the observed brightness from all the particles in the volume element at P will be

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Eo = solar flux at 1 AU
Ro = 1 AU
n(a, r) = particle size and spatial distribution
a1, a2 = the lower and upper limits to the particle size distribution
FA(a, 0) = angular scattering function for particles of radius a

The total surface brightness at an elongation e from the Sun will be the integral of dI(e) over the line of sight

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In practice, there is no simple way of separating the parameters in equation (2). The size distribution can be expected to be a function of r because the dynamical forces acting on the dust depend on particle dimension as well as density. The scattering functions themselves may vary with distance from the Sun because solar radiation can alter the optical properties of the dust particles. The interplanetary material is probably a mixture of several components having different relative concentrations in different parts of the solar system. Cometary and asteroidal debris, for example, would not be injected at the same solar distance.

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