Nonthermal crystallization of amorphous silicates in comets

Abstract

Recent observations show the ubiquity of crystalline silicate in various objects, among which comets provide the best opportunity to study possible processing of pristine matter during their formation and evolution. While thermal processing of bare amorphous silicates in the hot inner solar nebula has been invoked, its drawback is a difficulty in explaining the interstellar composition of cometary ices. Here we apply a model of core-mantle interstellar grains to propose nonthermal crystallization of the amorphous silicate core due to the energy released by chemical reactions in the organic refractory mantle when moderately heated by solar radiation. By formulating the nonthermal crystallization, we find that the degree of crystallinity is determined by a single parameter that is proportional to the amount of the released energy. We show that the present mechanism of crystallization is capable of reproducing the strengths of crystalline silicate features observed in comets. Our model can reconcile crystalline silicate in the comae of comets with the interstellar composition of ices in their nuclei.

Low-temperature crystallization of thin silicate layer on crystalline Fe dust

“…The parallel relation at the interface between Fe and SiO 2 occurred due to a minimum in interface energy. This result supports the theory of lowtemperature crystallization by Yamamoto et al (2010). The oxidation energy of Si crystallites drives the crystallization of the SiO 2 layer.…”

“…To explain how cometary silicates crystallize under pressure in volatile interstellar ice in their parent comets, we experimentally demonstrated the possibility of a chemical reaction-driven crystallization , which is called nonthermal crystallization (Yamamoto and Chigai, 2005;Yamamoto et al, 2010), using laboratory-synthesized amorphous Mg-bearing silicate grains (Kamitsuji et al, 2005). If the crystalline grain surface is covered with an amorphous silicate layer, the crystallization of the amorphous silicate layer may be different due to the different rates of thermal diffusion between amorphous and crystal layers, as observed during the crystallization of a carbon layer covering Pt clusters (Shintaku et al, 2006;.…”

Revisiting Jovian-Resonance Induced Chondrule Formation

“…In contrast, the composition of the gas in cometary comae indicates the preservation of interstellar ice in the cold outer nebula (Biermann, Giguere, & Huebner 1982). It is unclear why these two materials, which have contradicting heating records, co-exist in comets (see Yamamoto & Chigai 2005;Yamamoto et al 2010). It is possible that the amorphous silicates crystallize through shock heating.…”

On the anomalous silicate emission features of active galactic nuclei: a possible interpretation based on porous dust

“…But we are not aware of any amorphous silicate species that could give rise to a Si–O feature peaking at λ > 10 μm. Crystalline olivine has its Si–O feature peaks at ∼11.2 μm (see Yamamoto et al 2008). But this feature is too sharp compared to the broad Si–O emission features of AGN.…”