T. Mukai

Complex refractive index of grain material deduced from the visible polarimetry of comet P/Halley

Visible polarimetry of comet Halley over the phase angle α (sun-comet-earth angle) from 1°.6 to 64°.4 has shown negative values of the polarization p in α ≤20° and linearly increase of p with α > 20°. In addition, a slight increase of p with wavelength λ is likely to exist. By applying the Mie calculations to explain both α -and λ-dependences of p, we have found that it’s likely for the cometary dust to consist of the material with the average complex refractive index (m = n − i• k), i.e. (n, k) = (1.392, 0.024) at λ = 0.365 μm, (1.387, 0.031) at λ= 0.484 μm, (1.385, 0.035) at λ= 0.62 μm and (1.383, 0.038) at λ = 0.73 μm, for grains assumed to have the size spectrum reported by the Vega missions.

Polarimetry of comet P/Halley

The linear polarization P of comet P/Halley was observed simultaneously at eight wavelengths ranging from 0.36 to 0.90µm, for phase angles α (sun-comet-earth angle) between 2° and 65°. In addition, observations in the J and H bands were made for α = 30°. In the continuum bands, we have obtained (i) negative polarization for α ≲ 20°, (ii) a quasi-linear increase of the polarization level with phase angle for 20° < α < 50°, and (iii) a slight increase of P with wavelength from the visible through to the near infrared.

Icy particles from comets

The survival probability of the particles leaving a comet is examined based on their orbital changes at the moment of ejection. Considering the repulsive radiation pressure on them plus their emission velocities relative to the cometary nucleus, the critical radii s1 and sc are defined in a grain radius s ≧ 10−5 cm, where the ejected grains with s1 ≦ s ≦ sc cannot stay in the Solar System. We have found empirical formulae of s1 and sc; e.g., for long-period comets with semimajor axes a ≧ 103 AU and perihelion distances q ≧ 3 AU, sc (cm) = 2.3 × 10−6 a (AU) and s1 (cm) < 10−5 in the case of zero emission velocity. Water-ice particles with radii s ≧ 100 μm contaminated by “dirty” inclusions, which are injected into bound orbits with q ≧ 7 AU, are controlled by catastrophic collisions with interplanetary dust, and those with s ≦ 100 μ are controlled by the Poynting-Robertson drag forces. Contrary to that, almost all dirty water-ice grains on orbits with q ≦ 7 AU lose their ice parts due mainly to sublimation.

Optical constants of the mixture of ices

The Maxwell-Garnett mixing rule (MG) was applied to estimate the optical constant m* of the mixture of ices. A fairly good agreement between the resulting values of m* and those of our laboratory measurements suggests that the MG rule becomes a powerful tool to derive the unknown optical constants of natural dirty-ice, when there are no chemical bonds between the guest and host materials. Our results would provide the knowledge of the optical constants of dirty-ice for studies of icy substances on the surface of satellites, cometary nucleus and interplanetary dust grains based on their reflectance spectra.

Heterogeneous grain destruction near the Sun

Catastrophic fracture of heterogeneous grains consisting of refractory matrix and trapped icy inclusions is examined. As such a grain approaches the Sun after release from the comet, the saturation pressure of icy inclusions increases, and finally exceeds the crack-extension-force necessary for onset of unstable fast fracturing. Subsequently a growth of cracks results in a facture of the grain.The calculation revealed that obsidian and magnetite grain, respectively, of radius 100 Μm with 10% water-ice inclusions in volume suddenly disrupt near the solar distance of 0.25 AU and of 0.6 AU. Since the small debris of fragmentation comes from the direction of the sun on a relatively higher eccentric orbit, this fragmentation mechanism seems favourable to explain the β-meteoroids observed at 1 AU.

Dirty ice grains in comets

Abstract Based on the computed equilibrium temperature of evaporating dirty water-ice grains, dirty water-ice halo is examined, taking into account of a size dependence of terminal velocity of dust at P/Halley. It is found that due to an enhanced grains temperature caused by dirtiness, icy halo cannot extend over 100 km from the nucleus when comet approaches inside a solar distance r of 1 AU. Therefore, it is unlikely that the ice bands in the near infrared wavelengths could be detected in the cometary coma at r

Intensity profiles of the multiple scattering light near the cometary nucleus

Abstract The multiple scattering of solar radiation in the cometary atmosphere is treated with the method of successive scattering. Referring to in situ measurements of comet Halley about the size and spatial distributions of dust, the optical thickness τ 1 of dust has been estimated, i.e. τ 1 =0.03 at wavelength λ =0.62 μm in a quiet time, but τ 1 =0.3 when the outbursts/jets occur. In the derivation of τ 1 , optical properties of dust including a mixing ratio of absorbing to silicate grains, are determined based on the polarimetry of P/Halley at λ =0.62 μm observed during the phase angles over Nov. 1985 to May 1986 at the Dodaira Station of Tokyo Astronomical Observatory. It is found that a temporary enhancement of τ 1 leads an increase of the upward reflected intensity when the surface albedo A of the nucleus is less than 0.04, but the reverse is true when A >0.04. On the other hand, the intensity of the downward radiation at the surface of the nucleus always decreases as an increase of τ 1 .

Mass loss rate of the zodiacal dust cloud

The mass loss rate of the zodiacal dust cloud near the Sun has been estimated on the basis of the orbital behaviour of circumsolar dust grains suffering sublimation. It is found that the solar dust ring located at 4 solar radii from the Sun, which consists of grains whose inward spiraling due to the Poynting-Robertson effect is stopped by the influence of sublimation, loses its mass at a rate of 3.5∼0.35 tons per second.