Interstellar grain composition: a model based on elemental depletions

Recent results on interstellar atomic depletions are reviewed and used to place constraints on the density and composition of grains in the diffuse interstellar medium. The total density of depleted material is $$\sim 1.8 \times 10^{-26} \text {g cm}^{-3}$$, consistent with that required in grains to explain the rate of interstellar extinction in the galactic plane. Carbon appears to be less important as a grain constituent than has previously been assumed, and models which attribute the optical extinction to bacteria are excluded on the basis of both the carbon depletion and the phosphorus abundance. The metallic elements constitute approximately a third of the grain mass: this can be reconciled with dielectric optical properties only if the dominant chemical configurations are silicates and oxides, as in collected interplanetary dust. Problems involved in attributing the bulk of the optical extinction to silicates are briefly discussed.