On the Absorption Properties of Metallic Needles
  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.
