2020MNRAS.498.3560X

Needle-like metallic particles have been suggested to explain a wide variety of astrophysical phenomena, ranging from the mid-infrared interstellar extinction to the thermalization of starlight to generate the cosmic microwave background. These suggestions rely on the amplitude and the wavelength dependence of the absorption cross-sections of metallic needles. On the absence of an exact solution to the absorption properties of metallic needles, their absorption cross-sections are often derived from the antenna approximation. However, it is shown here that the antenna approximation is not an appropriate representation, since it violates the Kramers-Kronig relation. Stimulated by the recent discovery of iron whiskers in asteroid Itokawa and graphite whiskers in carbonaceous chondrites, we call for rigorous calculations of the absorption cross-sections of metallic needle-like particles, presumably with the discrete dipole approximation. We also call for experimental studies of the formation and growth mechanisms of metallic needle-like particles as well as experimental measurements of the absorption cross-sections of metallic needles of various aspect ratios over a wide wavelength range to bound theoretical calculations.