Tadashi Mukai

Optical constants of olivine particles between wavelengths of 7 and 200 μm

Abstract Laboratory measurements of the optical constants of crystalline olivine particles are presented for wavelengths (λ) from 7 to 200μm. Assuming a spherical particle of olivine, the emission features are computed for the grain radii (s) of 0.03, 0.03, 3, and 30mum, it is found that for s ⪅ 0.3 μm, the emission peaks are seen at at λ = 16.3, 18.6, 23, 28, and 33.6 μm, as well as a twin-peaked structure in the 10-μm region, whereas for s ⪆ 3 μ m , these features become weak. If we apply the Halley dust size distribution to the olivine particles, not remarkable twin-peaked emission feature would be expected in spite of its appearance in Comet P/Halley. This implies the presence of only small olivine particles with radii less than roughly 1 μm or that the enhancement of such smaller olivines might pvvur in the comet whose spectrum shows a significant twin-peaked structure in the region of the 10-μm silicate band.

Heterogeneous grain model in comets

Abstract Based on the Maxwell-Garnet expression for the optical constant of heterogeneous material, the temperature of grain consisting of homogeneous matrix and small sized impurity can be examined. It is found that the heterogeneous grain model can explain the evidence observed in the comets, i.e. (i) higher production rate of water molecules at large solar distance due to sublimation from water-ice with magnetite inclusions, and (ii) elevated color temperature, which frequently coexists with a 10 μm-silicate peak, as the thermal emission of obsidian contaminated by small magnetite inclusions.

Interpretation of the infrared polarization of Venus

Abstract A contradiction in the sulfuric acid cloud hypothesis of Venus, i.e., nondetection of 4.8 μm polarization by Landau (1975) , is examined on the basis of the multiple scattering calculations for the cloud model of Hansen and Hovenier (1974) including an internal heat source. Results show that the polarized thermal component cannot depolarize the scattered sunlight, and therefore a large polarization of about 13% is expected at a phase angle of 110° and wavelength of 4.8 μm, in contrast with Landaus measurements. Our computations are, however, in agreement with the measurements by S. Sato et al. ( in “Proceedings, 10th Lunar and Planetary Symposium,” pp. 179–182. Institute of Space and Aeronautical Science, University of Tokyo, July 11–13, 1977).

Analysis of photopolarimetric data of comets at small phase angles by rough surface scattering

Abstract Photopolarimetric data for comets in the backscattering region, including negative polarization and the opposition effect, are interpreted on the basis of rough scattering by large irregular particles combined with Mie scattering by submicron grains. Both types of grains are assumed to have the same average optical constants, i.e., n = 1.385 and k = 0.035 at wavelength λ = 0.62 μm , where n and k , respectively, denote a refractive index and absorption coefficient of grain material. Scattering from large, rough particles reduces the strong negative polarization in the backward direction contributed by Mie scattering from small grains and is adequate to model observed amplitudes of negative polarization in cometary comae. The introduction of rough scattering also leads to a somewhat steeper increase of intensity with decreasing phase angle, α, in the range 0° ⩽ α ⩽ 20°, and is more consistent with observations than that predicted from Mie theory alone and Halleys grain-size distribution.

Scattering Properties of Cometary Dust Based on Polarimetric Data

Referring to the dust model in Mukai and Mukai(1990), where the scattering by large rough particles and Mie scattering by small particles are taken into account, a phase function of linear polarization of several comets is examined, especially in a region of phase angles Ci near a maximum polarization. A lower maximum polarization observed in comet Austin(1989c1) than those in comets West(1975n) and PfHalley leads a speculation that a mixing ratio of rough scattering to Mie scattering in comet Austin increases from a sun-comet distance r of 0.6 AUto 1.2 AU. This implies that a shortage of large particles in comet Austin occured in r < 1 AU. ABSTRACT. Referring to the dust model in Mukai and Mukai(1990), where the scattering by large rough particles and Mie scattering by small particles are taken into account, a phase function of linear polarization of several comets is examined, especially in a region of phase angles Ci near a maximum polarization. A lower maximum polarization observed in comet Austin(1989c1) than those in comets West(1975n) and PfHalley leads a speculation that a mixing ratio of rough scattering to Mie scattering in comet Austin increases from a sun-comet distance r of 0.6 AUto 1.2 AU. This implies that a shortage of large particles in comet Austin occured in r < 1 AU.

Is water-ice the carrier of the 3μm-absorption in infrared objects?

It is shown that Mie theory predictions of extinction for pure water-ice with the optical constant measured at 100 K do not fit in detail the observed ‘ice’ absorption feature in infrared objects, although we attempt to explain the observations by considering size distribution and shape of the grains.In addition, based on a similarity between the ‘ice’ band and the absorption band found in carbon stars, we feel it is questionable whether or not the ‘ice’ band can really be attributed to the interstellar water-ice.

Infrared polarization of venus: Its periodic fluctuations and evidence for thin haze

Abstract Infrared polarimetry of Venus over the phase angles from 18 to 171° has been made extending previous measurements ( S. Sato, K. Kawara, Y. Kobayashi, H. Okuda, K. Noguchi, T. Mukai, and S. Mukai (1980). Icarus 43 , 288 ) in both wavelength λ and phase angle θ. The results of polarization measurements at 2.25 μ m ⩽ λ ⩽ 5.0 μ m are (i) small positive and negative values at K(2.25 μm), (ii) a remarkable variation with λ in the CVF (2.2−4.2 μ m) filter region, (iii) a nearly smooth curve as a function of θ having a peak value of ∼36% at θ ∼ 90° at both 3.6 μm and L′(3.8 μm), and (iv) a decrease with increasing field of view at M(5.0 μm) due to the contamination of thermal emission from the dark crescent. Furthermore, at 3.6 μm and L′(3.8 μm), (v) higher values at the poles than at the equator and (vi) 4.5- to 5.9-day periodic fluctuations are also found. From a comparison with model calculations, the results confirm the existence of a thin haze layer consisting of submicron-size particles above the main clouds of Venus; e.g., its optical thickness is about 0.1 at λ ∼ 0.94 μ m. In addition, result (vi) could be explained by a variation of the optical thickness of the haze layer or that of the brightness temperature of the main clouds.

Infrared properties of haze particles of Venus

Abstract The computed variation of the infrared flux and polarization of Venus as a function of phase angle, based upon multiple-scattering calculations for the cloud model of Kawabata et al. (1980) with an internal heat source, precludes the possibility of sulfuric acid as the composition of the haze particles located above the main cloud. Furthermore, our calculations reveal that the hazticle should have a large absorption coefficient at these wavelengths, i.e, k( imaginary part of the complex refractive index ) ⪆ 1.3 at a wavelength λ = 3.4 μ m . The optical thickness of the haze layer must be about 0.15 at λ = 3.4 μm.

Infrared polarization of Venus

Abstract Infrared polarization of Venus was measured at several phase angles during 1977, 1978, and 1979. Observations revealed a significant degree of polarization at wavelengths of both 3.6 and 4.8 μm in contrast with little or no polarization in wavelengths shorter than 3 μm. Although the sulfuric acid cloud model seems a likely explanation of our measurements, we found a small discrepancy between these results and the calculations by S. Mukai and T. Mukai (1979, Icarus 38 , 90–99).

Dust from the Comets

We have found a similarity between the size spectra of the observed interplanetary dust and the survived cometary dust, referring to the dynamical behaviour of the dust leaving the comet. As a result, we can suggest that short-period comets with relatively higher eccentricities are major source of the interplanetary dust, especially those with radii less than 10 μm. It is predicted, furthermore, that a supply rate of the dust, which move on bound orbits after leaving the comet, becomes about 8x10 g/s from 85 short-period comets, and nearly 3xl04 g/s from 101 long-period comets.