Meteorites contain many kinds of microscopic particles of minerals such as metals, silicates, sulfides, carbides, and oxides. Most of these were formed when the Solar system was still a nebula, 4.6 billion years ago. Some, known as presolar grains, are even older than this and were formed in a gas outflow of earlier dying stars. It is important to understand the process of formation of nanometer-sized minerals, because cosmic minerals are the smallest building blocks of planetary systems and they operate as substrates for the formation of molecules in space. Despite extensive studies over a long time, the processes by which solar and presolar minerals were formed remain largely unknown. We have assumed that there are two keys to achieving an understanding of these processes; one is the size effect, and the other is the formation of cosmic minerals under conditions of microgravity.
Many studies on nanometer-sized solid particles were initiated following Kubo’s seminal theoretical study that showed that extremely small metallic particles have physical properties that differ from those of the bulk material as a result of their discrete electronic states [R. Kubo, J. Phys. Soc. Jpn. 17 (1962) 975]; this has come to be known as the Kubo effect. Later studies on nanometer-sized particles have confirmed and progressed that their physical properties differ markedly from those of the bulk materials. For example, the existence of extremely large diffusion coefficients results in the anomalous phenomenon of spontaneous alloying [e.g., H. Yasuda and H. Mori, Phys. Rev. Lett. 69 (1992) 3747]. We have also reported the occurrence of spontaneous mixing of ions in alkali halide nanocrystals [Y. Kimura et al. Phys. Low-Dim. Struct. 1/2 (2000) L1; Physica E 13 (2002) 11].
Because the actual size of cosmic nanomineral particles is of the order of 100 nm or less, I would expect that the singular physical properties and related phenomena that appear on the nanoscale will have to be taken into account if we are to understand the process of formation of nanominerals in the universe. Indeed, we have been able to duplicate in the laboratory several features of cosmic nanominerals, such as the formation of carbonaceous hollow particles [M. Saito & Y. Kimura Astrophys. J. Lett. 703 (2009) 147], the low-temperature crystallization of forsterite [Y. Kimura et al. Astrophys. J. Lett. 680 (2008) 89], the formation of composite particles with a titanium carbide core and a carbon mantle through decomposition of carbon monoxide gas [Y. Kimura et al. Meteorit. & Planet. Sci. 41 (2006) 673], the formation of pyrrhotite by a solid–solid reaction [Y. Kimura et al. ICARUS 177 (2005) 280], and the existence of fullerenes around evolved stars [Y. Kimura et al. Astrophys. J. Lett. 632 (2005) 159], and we have suggested processes for the formation of such nanominerals that are based on crystallization in the mesoscale. I do not claim that all of these minerals were formed by processes identical to those used in the laboratory experiments, but our studies confirm that a knowledge of the significant physical properties of nanoparticles is necessary if we are to understand the life cycles of cosmic minerals.
We recently began a project on homogeneous nucleation that incorporates studies on physical properties of nanoparticles with the aim of achieving a better understanding of the process of formation of nanoparticles and evaluating nucleation theories. Nucleation theory can be used to predict the nucleation temperatures, phase, sizes, size distributions, and number densities of products. Unfortunately, it has become apparent that nucleation rates that are determined experimentally or by means of molecular dynamics simulations always differ by several orders of magnitude from those of classical nucleation theory. Generally, this difference can be considered to be a limitation of nucleation theory. However, on the basis of our experimental results, we have adopted the idea of coalescence growth, which is a process for the growth of nanoparticles, to provide a partial explanation for this discrepancy. We have shown that tiny nuclei can fuse together to form larger particles, thereby reducing their number density by a few orders of magnitude [Y. Kimura et al. J. Crystal Growth 316 (2011) 196; Cryst. Growth & Des. 12 (2012) 3278].
Although cosmic minerals are formed under conditions of microgravity, existing speculations as to mechanisms for their formation have generally been based on experiences on the Earth, where crystallization inevitably involves effects of buoyancy-driven convection and heterogeneous nucleation. When we perform experiments in the laboratory, mineral samples are kept in containers, which form sites for heterogeneous nucleation, and buoyancy-driven convection generates inhomogeneities in the environment for particle formation. We have therefore started a new project that incorporates experiments on homogeneous nucleation under conditions of microgravity and we recently performed such experiments on the sounding rocket S-520-28, launched on December 17th, 2012. I believe that further experiments based on this concept will clarify not only the process of formation of cosmic nanominerals, but also the nucleation process itself.