Methods and Difficulties in Laboratory Studies of Cosmic Dust Analogues

Abstract

The effort to understand the nature of cosmic dust, such as circumstellar, interstellar, and interplanetary dust, relies on the comparison of optical measurements such as scattering, extinction and emission with models ultimately derived from some sort of laboratory data. In this long-standing effort, there are two distinctly different approaches which I would like to outline in the beginning by reference to figure 1. First there is the approach in which the basic optical properties of matter, as summarized by wavelength-dependent optical constants, are used in small-particle calculations relating to the observations. This approach is diagrammed on the left side of figure 1. Although the starting point of this process is often the middle of the left column labeled n(λ), k(λ), which sometimes may be found in the published literature, someone had to do a lot of hard work preceding this point to furnish those optical constants. In such determinations, bulk samples of solids (or liquids) have to be prepared in slabs with polished surfaces, for example, and optical measurements such as trasmission and/or reflectance as functions of wavelength performed. From these measurements, an appropriate theory must be used such as Fresnel’s equations for reflectance from a slab. Out of this process comes the wavelength dependent “measured” optical constants. At this point it is possible to transform these bulk optical properties into small-particle optical effects by using the optical constants in an appropriate and doable small-particle theory such as Mie theory. The resulting extinction, absorption, and scattering parameters can the be compared with observed properties of cosmic dust, and parameters such as the size distribution and possibly the shape can be varied.