Vesicle dynamics during the atmospheric entry heating of cosmic spherules

• Published in: Meteoritics & Planetary Science
• Main Author: Genge, MJ
• Other Authors: Science and Technology Facilities Council (STFC)
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Wiley 2016
• Subjects: Science & Technology; Physical Sciences; Geochemistry & Geophysics; ANTARCTIC MICROMETEORITES; TRANSANTARCTIC MOUNTAINS; SILICATE MELTS; COLLECTION; ICE; LIQUIDS; RECORD; MODEL; DUST; 0201 Astronomical And Space Sciences; 0402 Geochemistry; 0403 Geology; Antarctic; Transantarctic Mountains; Antarc*
• Online Access: http://hdl.handle.net/10044/1/42446; https://doi.org/10.1111/maps.12805

Cosmic spherules are unique igneous objects that form by melting due to gas drag heating during atmospheric entry heating. Vesicles are an important component of many cosmic spherules since they suggest their precursors had finite volatile contents. Vesicle abundances in spherules decrease through the series porphyritic, glassy, barred, to cryptocrystalline spherules. Anomalous hollow spherules, with large off-centre vesicles occur in both porphyritic and glassy spheres. Numerical simulation of the dynamic behaviour of vesicles during atmospheric flight is presented that indicates vesicles rapidly migrate due to deceleration and separate from non-porphyritic particles. Modest rotation rates of tens of radians s-1 are, however, sufficient to impede loss of vesicles and may explain the presence of small solitary vesicles in barred, cryptocrystalline and glassy spherules. Rapid rotation at spin rates of several thousand radians s-1 are required to concentrate vesicles at the rotational axis and leads to rapid growth by coalescence and either separation or retention depending on the orientation of the rotational axis. Complex rapid rotations that concentrate vesicles in the core of particles are proposed as a mechanism for the formation of hollow spherules. High vesicle contents in porphyritic spherules suggest volatile-rich precursors, however, calculation of volatile retention indicates these have lost >99.9% of volatiles to degassing prior to melting. The formation of hollow spherules, by rapid spin, necessarily implies pre-atmospheric rotations of several thousand radians s-1. These particles are suggested to represent immature dust, recently released from parent bodies, in which rotations have not been slowed by magnetic damping.