Breakup, Sorting, Mixing, and Melting of Cometary Dust during Capture by Stardust

Introduction: Dust from comet Wild 2 was collected at a velocity of 6.1 km/s by the Stardust spacecraft using low-density silica aerogel [1]. During deceleration, dust particles fragmented, mixed with aerogel, and were thermally modified, often melted. In order to disentangle the properties of impacting cometary particles from those caused by the capture process, Wild 2 material from various locations along the aerogel tracks was analyzed. were studied using TOF-SIMS. Six fragments represent terminal particles from the very end of slender tracks or slender parts of tracks. Three fragments are from bulbous parts of tracks, two are from the slender parts leading to track termini. Five fragments can be described as quasi-terminal particles, since they were found at the termini of tiny sidetracks. Elemental distributions in particle sections were investigated by TOF-SIMS at a lateral resolution of ~0.3 µm. Element ratios were determined for bulk samples as well as for selected regions within the sections using established techniques [2, 3]. Results: The TOF-SIMS results clearly demonstrate that cometary matter extracted from along the tracks is heavily mixed with aerogel, as can be inferred from Si/Mg-ratios up to 64. While two terminal particles also showed high Si/Mg-ratios from aerogel admixture, four other terminal particles are mostly aero-gel-free in their interior. From their elemental composition, they can be identified as being dominated by Mg-rich pyroxene (En 95 and En 96) and olivine (Fo 87) as well as Ca-Al-Mg silicates. These particles are mostly crystalline and compact, whereas the fragments with high Si/Mg-ratios are typically amorphous, probably melted and mixed with melted aerogel during impact [4]. Quasi-terminal particles have chemical and textural properties intermediate between terminal particles and material from track bulbs. They often show some slight enrichment of SiO 2 from aerogel, but are also mostly compact. The fragments extracted from along the tracks have close to CI carbonaceous chondritic chemical composition, whereas individual terminal particles that often consist of single or only a few minerals show large deviations from CI. An almost exact match with CI was found for fine-grained material within tracks [5]. Conclusions: The results corroborate earlier suggestions that Wild 2 particles consist of a mixture of mono-and multi-mineralic grains, embedded in a fine-grained, porous matrix [6]. During their encounter with Stardust aerogel, the matrix, which probably represents the greater part of Wild 2 material, was stripped from the larger mineral grains, melted and mixed with aerogel, and was …