Thermal alteration of hydrated minerals during hypervelocity capture to silica aerogel at the flyby speed of Stardust

Outside the Earth's atmosphere, silica aerogel is one of the best materials to capture fine-grained extraterrestrial particles in impacts at hypervelocities. Because silica aerogel is a superior insulator, captured grains are inevitably influenced by frictional heat. Therefore, we performed laboratory simulations of hypervelocity capture by using light-gas guns to impact into aerogels fine-grained powders of serpentine, cronstedtite, and Murchison CM2 meteorite. The samples were shot at > 6 km s(-1) similar to the flyby speed at comet P/Wild-2 in the Stardust mission. We investigated mineralogical changes of each captured particle by using synchrotron radiation X-ray diffraction (SR-XRD), transmission electron microscope (TEM), and field emission scanning electron microscope (FE-SEM). SR-XRD of each grain showed that the majority of the bulk grains keep their original mineralogy. In particular, SR-XRD, and TEM investigations clearly exemplified the presence of tochilinite whose decomposition temperature is about 300 degrees C in the interior of the captured Murchison powder. However, TEM study of these grains also revealed that all the samples experienced melting and vesiculation on the surface. The cronstedtite and the Murchison meteorite powder show remarkable fracturing, disaggregation, melting, and vesiculation. Steep thermal gradients, about 2500 degrees C/mu m were estimated near the surface of the grains (< 2 mu m thick) by TEM observation. Our data suggests that the interior of > 4 mu m across residual grains containing abundant materials that inhibit temperature rise would have not experienced > 300 degrees C at the center.