Formation of Cosmic Crystals in Highly Supersaturated Silicate Vapor Produced by Planetesimal Bow Shocks

Abstract

Several lines of evidence suggest that fine silicate crystals observed in primitive meteorite and interplanetary dust particles (IDPs) nucleated in a supersaturated silicate vapor followed by crystalline growth. We investigated evaporation of μm-sized silicate particles heated by a bow shock produced by a planetesimal orbiting in the gas in the early solar nebula and condensation of crystalline silicate from the vapor thus produced. Our numerical simulation of shock-wave heating showed that these μm-sized particles evaporate almost completely when the bow shock is strong enough to cause melting of chondrule precursor dust particles. We found that the silicate vapor cools very rapidly with expansion into the ambient unshocked nebular region; for instance, the cooling rate is estimated to be as high as 2000 K s −1 for a vapor heated by a bow shock associated with a planetesimal of radius 1 km. The rapid cooling of the vapor leads to nonequilibrium gas-phase condensation of dust at temperatures much lower than those expected from the equilibrium condensation. It was found that the condensation temperatures are lower by a few hundred K or more than the equilibrium temperatures. This explains the results of the recent experimental studies of condensation from a silicate vapor that condensation in such large supercooling reproduces morphologies similar to those of silicate crystals found in meteorites. Our results strongly suggest that the planetesimal bow shock is one of the plausible sites for formation of not only chondrules but also other cosmic crystals in the early solar system.

Interferometric in-situ observation during nucleation and growth of WO3 nanocrystals in vapor phase

“…The mean radius and formation rate of the sample are 10.2 nm and 1.7 Â 10 11 cm À 3 s À 1 , respectively. [20,21]. Terminal mean radius a N can be obtained from L value, which is 63.2 in present experiment, as drown by a bold broken line.…”

“…Since the WO 3 particles condense in a cooling stage, to treat an environment dynamically changing, a diagram based on the classical nucleation theory shown in Fig. 5 has been used [20,21]. Using this diagram, the mean terminal radius of grown particle can be determined from a supersaturation with respect to DT/T e and…”

Chondrule Formation in Bow Shocks Around Eccentric Planetary Embryos

“…It is likely that solids will be fully or partially melted in these regions, and experience rapid cooling rates, resulting in objects that do not fit the classical definition of chondrules. For example, Miura et al (2010) proposed that silicate crystals found in meteorites could be produced by supercooling of a silicate vapor in bow shocks. Alexander et al (2007) found that the microstructure of pyroxene in interplanetary dust particles (IDPs) indicates that it formed at temperatures >1258 K and cooled relatively rapidly (∼1000 K hr −1 ), suggesting some type of shock heating.…”

“…suggest that ferroan olivine overgrowths on chondrules and chondrule fragments are produced by cooling rates orders of magnitude greater than those experienced by the chondrules themselves. Formation outside the chondrule-forming impact parameter in a planetary embryo bow shock may explain objects such as the silicates found in meteorites (Scott & Krot 2005;Miura et al 2010), pyroxene in IDPs (Alexander et al 2007), and overgrowths on chondrules . Further detailed modeling of the thermal histories of objects formed in bow shocks around planetary embryos is necessary to address this intriguing possibility.…”

Laboratory annealing experiments of refractory silicate grain analogs using differential scanning calorimetry

“…dust particles evaporated near the protostar and recondensed in the bipolar outflow (Patel et al 2000) and ⁄ or evaporated by shockwave heating and recondensed behind it (Kimura et al 2008;Miura et al 2010). The results of our annealing experiments show that amorphous silicate dust has never been heated to more than 1003 K in an oxidizing atmosphere (after it recondensed from the vapor), because amorphous silicate particles quickly crystallize above this temperature.…”