Geminids are initially cracked by atmospheric thermal stress

TL;DR

This study investigates the mechanical properties of Geminid meteoroids across a broad mass range by combining fragmentation modeling of 39 Geminid fireballs and faint meteors with numerical thermal-stress analysis. Using a semiautomatic, GA-based approach (and manual validation for some faint events), the authors distinguish mass-loss regimes (erosion, ablation, and gross fragmentation) and derive fragmentation pressures and bulk densities, finding that thermal stress in the atmosphere initiates fragmentation before ablation dominates. The results indicate two distinct strength regimes: smaller and larger meteoroids are largely destroyed by an initial thermal- stress-driven process, with moderate-mass objects undergoing more abrupt fragmentation, while large meteoroids house compact, non-fragmenting parts released by erosion. Bulk densities of smaller Geminids span roughly $1400$–$2800$ kg m$^{-3}$, climbing toward the grain density (≈$3000$ kg m$^{-3}$) for larger bodies, and the most compact meteoroids lie in the $20$–$200$ g mass range, providing insight into the internal structure and resilience of Geminids.

Abstract

Geminids have the highest bulk density of all major meteor showers and their mechanical strength appears to depend on their mass. They are also the most active annual shower, enabling detailed studies of the dependence of their physical and mechanical properties on mass. We calculated the fragmentation cascades of 39 bright Geminid fireballs, as well as faint video meteors, to derive fragmentation pressures and other physical properties characterizing the meteoroids, such as their bulk densities. Our goal is to describe the mechanical properties across a broad range of initial masses and explain the cause of the observed behavior. We used a physical fragmentation model with a semiautomatic method based on parallel genetic algorithms to fit the radiometric and regular light curve and dynamics data. We also calculated the thermal stress of model bodies with the type of physical properties and trajectories as the observed Geminids. Then, we compared the outcomes of these simulations to our observations. We find that the Geminids are probably cracked by thermal stress in the atmosphere first and then eroded by mechanical forces. The most compact Geminids are in the 20-200 g mass range. The largest observed meteoroids have a wide range of grain sizes, from about 20 um to large, non-fragmenting parts of 1-20 mm in size. The derived bulk densities range from about 1400 to 2800 kg/m3 for smaller meteoroids and approach the assumed grain density of 3000 kg/m3 for larger Geminids.

Figures (12)

• Figure 1: Illustration image of Geminids we used for fragmentation modeling. Panel a: Crop of the all-sky image of the most massive modeled fireball (Geminid 5, Šindelová station). Panel b: Stacked image of the narrow-field-of-view video observation of a faint Geminid 21 (Kunžak station).
• Figure 2: Geminid 10 fragmentation model is compared to the observed radiometric curve (dark-blue pluses) and a composite photometric light curve from various instruments (sky-blue disks). The total model brightness is shown as a solid red line, blue curves show the brightness of regular fragments, green curves signify eroding fragments, and violet lines indicate dust particles released from these fragments. Vertical dashed lines show fragmentation times, and numbers indicate the height above the ground in kilometers of the fragmentations.
• Figure 3: Geminid 10 length residuals. Different colors indicate the stations of the EN that were used to calculate the model.
• Figure 4: Statistical values of aerodynamic pressure for Geminids. Blue triangles show the pressure for the first observed fragmentation, green diamonds, magenta disks, and orange squares show the average pressure in the volume of meteoroids, calculated in different ways, and red triangles show the maximum attained pressure. The gray arrow indicates that the maximum pressure is only a lower limit. The numbers in the plot are meteor designations from Table \ref{['gem_tab1']}. The dashed lines are power-law fits to minimum or maximum dynamic pressures. Both axes are logarithmic.
• Figure 5: Regimes of mass loss vs. initial meteoroid mass. Both axes are logarithmic.