Photophoresis as a Source of Crystalline Silicates in Comets

Introduction: Crystalline silicates have been detected in a growing number of comets [1] [2] [3]. Since these bodies are presumed to be formed in the cold outer part of the solar nebula, several processes have been invoked in order to explain the origin of their crystalline silicates. It has been suggested that shock waves in the outer solar nebula could anneal the amorphous silicates to crystallinity in situ prior to their incorporation in comets [4]. However, there is no observational evidence for the existence of such processes. In contrast, the formation of crystalline silicates in the inner hot region of the solar nebula and their diffusive transport out to the comet formation zone has been proposed as another possibility [5]. On the other hand, since the diffusive transport of crystalline silicates leads to a homogeneous abundance of grains within the solar nebula, this mechanism is not compatible with the composition gradient observed in comets [3]. In this work, we present calculations relevant to the photophoretic force in the solar nebula [6] [7]. We show that photophoresis can be considered as an alternative transport mechanism that allows crystalline silicates formed in the inner nebula to move outward in the formation regions of cometary reservoirs. Moreover, we find that this mechanism is consistent with the heterogeneity of crystalline silicates abundances observed in comets [3].