Silica aerogels are amorphous SiO2 gelled solid. With their unique properties such as transparency and low bulk density (approximately 0.03 g/cm3 in this study) as well as stability in vacuum and in a wide temperature range, many studies proved that they are excellent capture media for hypervelocity particles in space. Indeed, they have been used in several intact capture experiments in space more than 10 years. In planetary science field, collection of micrometeoroids has significance because they retain information of their parent bodies such as asteroids or comets which can be derived to decode the evolution of solid materials in the solar system. However, aerogels are also excellent thermal insulators (thermal conductivity of 0.007 W/mK at 0.01-1 torr). Hence there is a possibility that heat converted from impact energy of a captured particle might alter itself. No one has evaluated the captured samples directly from this point of view so far. In the former studies, both experimental and theoretical works were based on the experimental data set of projectiles that were not good indicators for thermal alteration (e.g. glass, Al, aluminum oxide). The scope of this study includes two objectives. One is to evaluate the thermal alteration of captured particles mineralogically. The second is to give a constraint on thermal fraction of energy partitioning of particle impact energy. The author assesses physical alterations of microparticles on capturing by aerogel at hypervelocities up to 6km/s, which is the flyby speed of STARDUST spacecraft with its target comet. Simulating the hypervelocity capture, the author fired analog microparticles (125 -167 μm) into aerogel (0.026 - 0.030 g/cm3) with a shotgun method. With relatively low decomposition temperatures, serpentine (600 - 660 ℃) and cronstedtite (470 ℃) are suitable to evaluate thermal alteration during capture. After the shots, excavated particles are analyzed by several methods such as optical microscopy, SEM/EDS, SR-XRD, TEM and FE-SEM. In order to assess the mass loss of a particle during capture, survivability of a particle volume S is defined as ( a\u27/ f c2)×((b\u27+c\u27)/2 f c3)×(d\u27/f c4)S(%) = ------------------------------------- ×100 (d aerogel / f cl)3Here, a\u27, (b\u27･c), d\u27 are three axes of the captured particle and daerogel is hole diameter>of the penetration track of the particle measured on its digital images, while fc represents correction factor for each parameter determined in this study. Thus the method of estimation of S is much improved compared to the previous work (Okudaira et al., 2004). As a result, it is revealed that excavated grain measurably reduced their volumes during penetration into aerogel. Albeit wide variation in data, it is found that serpentine has survivability about 43 % (1σ19) at 2 km/s, 38 % (1σ 20) at 3 km/s, 17 % (1σ 17) at4 km/s and that it drops to about 2% (1σ5)at 6 km/s, while cronstedtite has 55 %(1σ29) at 2 km/s,32% (1σ17)at 3 km/s,14%(1σ49) at 4km/s, and 10%(1σ19) at 6 km/s. With these values and latent heat of vaporization of forsterite and fayalite, thermal fraction of energy partition is calculated. In the case of serpentine, it is estimated that nearly 100% of the kinetic energy of a particle is consumed to ablate the particle at 2 km/s, assuming the mass loss is caused only by ablation (vaporization) of the particle. Similarly, 53% is used at 3 km/s, 40% at 4 km/s and 21 % at 6 km/s. As for cronstedtite, 55% at 2 km/s, 32% at 3 km/s, 27% at 4 km/s and 12% is used for ablation at 6 km/s. These values are maximum estimation because it is assumed that the mass loss is due to vaporization alone. The analytical results of excavated particles show that both kinds of mineral grains have slightly melted textures at their surface at about 3 - 4 km/s and that they have greatly vesiculated textures as the impact velocity increases(6km/s). Degree of effervescence is higher in cronstedtite that has lower decomposition temperature than serpentine at the similar velocity. However, the inner parts of remained grains are mineralogically intact even shot at 6 km/s.Applying the results to the STARDUST samples, they may well lose their original surface morphologies and nearly 90% of their volumes, but the original mineralogy can be retained in their interior
