Crystalline Silicates in Comets: Modeling Irregularly-Shaped Forsterite Crystals and Its Implications on Condensation Conditions.

摘要

Crystalline silicates in comets are a product of the condensation in the hot inner regions (T > or approx. equals 1400 K (1)) of our proto-planetary disk or annealing at somewhat lower temperatures (T > or approx. equals 1000-1200 K) (2, 3, 4) in shocks coupled with disk evolutionary processes that include radial transport of crystals from their formation locations out to the cold outer regions where comet nuclei formed. The grain shape of forsterite (crystals) could be indicative of their formation pathways at high temperatures through vapor-solid condensation or at lower temperatures through vapor-liquid-solid formation and growth (5, 6, 7). Experiments demonstrate that crystals that formed from a rapidly cooled highly supersaturated silicate vapor are characterized by bulky, platy, columnar/needle and droplet shapes for values of temperature and supersaturation, T and sigma, of 1000-1450 C and < 97, 700-1000 C and 97-161, 580-820 C and 131-230, and <500 C and > 230, respectively (7). The experimental columnar/needle shapes, which form by vapor-liquid-solid at lower temperatures (<820 C), are extended stacks of plates, where the extension is not correlated with an axial direction: columnar/needles may be extended in the c-axis or a-axis direction, can change directions, and/or are off-kilter or a bit askew extending in a combination of the a- and c-axis direction.