Thermal history, partial preservation and sampling bias recorded by Stardust cometary grains during their capture

The nature and origin of cometary grains are poorly known. For this reason the first in situ sampling of cometary materials, the Stardust mission, raised considerable expectations. The Stardust spacecraft captured cometary dusts at a relative velocity of 6.1 km/s in a low density silica aerogel medium. Calculations and laboratory experiments on the shock behaviour of aerogel and hypothetical dust-analogue material indicate that an intense but short heating stage (on the order of the microsecond) occurred during the collection; however, results from the preliminary team emphasized that a large fraction of the collected material was deeply affected by the thermal event, indirectly suggesting that the heating was more protracted. Here we constrain its parameters based on diffusion of MgO and SiO2 between a molten cometary dust and the embedding molten aerogel. After the shock, this assemblage stayed at ∼ 2100 K for duration of up to 0.1 ms. This timescale is characteristic for friable and fine-grained aggregates whereas consolidated grains were quenched faster. Therefore, a sampling bias occurred during the collection. It may explain the recent finding that preserved cometary grains look more similar to inner solar nebula material than to chondritic porous interplanetary dust particles (CP-IDP). Indeed, CP-IDPs and other primitive materials are friable materials and consequently, they must have been selectively damaged. The strong thermal modification precludes detailed studies of mineralogy, but molten cometary dusts are only partially mixed with aerogel, giving an opportunity to analyze bulk and local compositions of the impacting aggregate fragments.