Vesicle dynamics during the atmospheric entry heating of cosmic spherules

Abstract

Abstract-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-center vesicles occur in both porphyritic and glassy spheres. Numerical simulation of the dynamic behavior of vesicles during atmospheric flight is presented that indicates vesicles rapidly migrate due to deceleration and separate from nonporphyritic particles. Modest rotation rates of tens of radians s À1 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 À1 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 preatmospheric rotations of several thousand radians s À1 . These particles are suggested to represent immature dust, recently released from parent bodies, in which rotations have not been slowed by magnetic damping.

The Extraterrestrial Dust Flux: Size Distribution and Mass Contribution Estimates Inferred From the Transantarctic Mountains (TAM) Micrometeorite Collection

“…Tailed micrometeorites were first reported by Taylor et al () (Figure ) and later found among the TAM collection (Suavet et al, , Figure i; Suttle et al, ); their formation mechanism is unresolved. Metal beads, formed by the segregation of immiscible Fe–Ni metal (or Fe–Ni sulfides) from silicate melt (Genge & Grady, ), were found in 20.4% of S‐type cosmic spherules. Hollow spherules occur at a frequency of 1.6% and contain a single large off‐center vesicle (>40 vol %). They were first reported in Folco and Cordier () and later suggested by Genge () to form by rapid spin rates (of several thousand radians per second), potentially representing immature dust, recently released from their parent body, and whose fast rotation have not yet been slowed by magnetic dampening.…”

“…• Hollow spherules occur at a frequency of 1.6% and contain a single large off-center vesicle (>40 vol %). They were first reported in Folco and Cordier (2015) and later suggested by Genge (2017) to form by rapid spin rates (of several thousand radians per second), potentially representing immature dust, recently released from their parent body, and whose fast rotation have not yet been slowed by magnetic dampening.…”

To be or not to be oxidized: A case study of olivine behavior in the fusion crust of ureilite A 09368 and H chondrites A 09004 and A 09502

“…], and in cosmic spherules [Genge ]). According to numerical modeling for cosmic spherules, for low peak temperature, vesicle migration within the melt is mostly controlled by the melt viscosity, whereas at high peak temperature (superheating, like those experienced in the fusion crust), the migration and coalescence of vesicles is controlled by the dynamic processes during transit through the Earth's atmosphere (Genge ). This implies that due to fast cooling, the vesicles in the fusion crust are quenched and stop migrating relatively soon after formation.…”

“…Nevertheless, information is provided by the occurrence .5 (7) 6.7 (9) 14 avg = average of the indicated number (#) of spot analyses, with standard deviation; Avg ureilite* = average ureilite composition after Jarosewich (1990), only absolute content of selected elements (tot < 100); bdl = below detection limit. of large vesicles, suggesting degassing (e.g., fusion crust of a carbonaceous chondrite [Genge and Grady 1999], of lunar meteorites [e.g., Joy et al 2006], and in cosmic spherules [Genge 2017]). According to numerical modeling for cosmic spherules, for low peak temperature, vesicle migration within the melt is mostly controlled by the melt viscosity, whereas at high peak temperature (superheating, like those experienced in the fusion crust), the migration and coalescence of vesicles is controlled by the dynamic processes during transit through the Earth's atmosphere (Genge 2017).…”

An increased abundance of micrometeorites on Earth owing to vesicular parachutes

“…Initial trapping of gas is, therefore, likely to be efficient with initial vesicle abundances controlled by collapse of foams. Subsequent loss of vesicles by migration under the influence of deceleration, as discussed by Genge [], then decreases vesicle content with further heating.…”