Phase functions of polarization and brightness and the nature of cometary atmosphere particles

Spectropolarimetry of comets Austin and Churyumov-Gerasimenko

After an introduction to the basic history and morphology of comets, the authors consider cometary motion, the physical properties of the neutral gaseous matter, the nucleus, dust particles, cometary ion origin and interactions with the solar wind, the origin of comets, the relation of comets to other astrophysical bodies and the origin of life.

https://iopscience.iop.org/article/10.1088/0034-4885/46/6/001/pdf

Nature and origin of comets

Observations of the sunlight scattered and thermal emission from cometary dust, which may be used to infer the physical properties of the dust grains, are reviewed. Consideration is given to the observed wavelength dependence of the scattered light from cometary coma and tails, the average scattering function of the dust grains, the average grain Bond albedo, the polarization of the scattered light, and grain temperatures deduced from thermal infrared emission. The thermal properties of dust grains are illustrated for models based on magnetite or olivine grain materials, with consideration given to the variation of thermal properties with particle radius and heliocentric distance. Comparison of the models with observations indicates that a disordered or amorphous olivine composition can give a reasonable fit to the data for appropriate grain sizes and temperatures. The observations acquired are noted to indicate an optically important particle size of 1 micron, with silicate particles not larger than a few microns usually present although pure silicate grains can not be responsible for the thermal emission, and the cometary dust grains are most likely not spherical. Further observations needed in the infrared are indicated.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/E61EDC62E87701B93D86983A0CE358E2/S0074180900066766a.pdf/div-class-title-physical-characteristics-of-cometary-dust-from-optical-studies-div.pdf

Physical characteristics of cometary dust from optical studies

Stardust grains are tiny solid samples of stars, newly condensed in an expanding atmosphere of a dying star and injected into interstellar space by stellar wind and radiation pressure.

Light Scattering and Thermal Emission by Primitive Dust Particles in Planetary Systems

Multicolor observations of brightness and polarization in the continuum throughout the tail of Comet 1965 VIII reveal the presence of positive and negative polarization (polarization reversal) characteristic of slightly absorbing particles. The phase angle of the neutral point is found to vary with color and with time, the latter indicating short-term changes in the properties of the particles after perihelion.

Polarization reversal in the tail of comet Ikeya-Seki (1965 VIII).

Claims made by Bandermann and Wolstencroft (1974) that angular structure and day-to-day variations of the polarized component of the zodiacal light were observed near the antisolar point are shown to be unsubstantiated. The data obtained by Bandermann and Wolstencroft are reviewed together with the instruments they used. It is shown that the incorrect results reported were due to observation errors.

J. L. Weinberg

Papers

Polarization reversal in the tail of comet Ikeya-Seki (1965 VIII).

Variations in Gegenschein polarization