Physical properties of heterogeneous dust grain

点击翻译按钮获取中文摘要

Exploration of Comet Halley has provided many valuable information of the cometary dust grains. Then, it becomes possible to do more qualitative analysis concerning the origin and evolution of dust in the solar system. Based on the heterogeneous grain model, the following topics were examined.(1) The complex refractive index <m^*> of heterogeneous material measured for mixtures of <H_2> O/ and <NH_3> ices are compared with those estimated based on the Maxwell-Garnett mixing rule. We found that when there are no newly formed chemical bonds between ice and inclusions, the MG rule can give the reasonable values of <m^*> for heterogeneous material.(2) Dirty ice attains higher temperature than pute ice does. This result preferrs to explain higher production rate of OH molecules in a distant comet.(3) Since a lifetime of dirty water-ice grain ejected from the cometary nucleus is short, we can't expect the detection of large halo of icy particles around the nucleus when comet enters to the comet-sun distance less than 1 AU.(4) Dirty ice grains look 'dark' in distant place from the sun. Time evolution of these dark grains causes brighter particles because loss of <H_2> O due to sublimation leads refractory grains with high albedos. Therefore, it can explain the observed evidence that the grains located far from the sun have lower albedos compared with those near the sun.It is found that our heterogeneous grain model is a powerful tool to analyse the observed results in comets, as well as in interplanetary space.