Perihelion history and atmospheric disruption: The Primary Culprits of the Missing Carbonaceous Chondrites

Hydrated, carbonaceous asteroids constitute the majority of the main‑belt [1], whereas carbonaceous chondrites form just ≈4% of the >83,000 meteorites in collections [2]. Additionally, recent meteoroid transfer models from young asteroid families predict >50% of the meteoroid impacts at the top of the atmosphere should be carbonaceous in composition, a mismatch often ascribed—to their inherent material fragility [2,3].

Methods

To test whether Earth’s atmosphere drives the mismatch between the predicted carbonaceous meteoroid flux and the recovered carbonaceous meteorite flux, we compared the orbital distributions of a debiased sporadic asteroidal impact population against those of meteorite‑dropping fireballs. We assembled 7,982 top‑of‑atmosphere sporadic impacts (≥ 10 g) from five global networks (EDMOND, CAMS, GMN, EFN, FRIPON), spanning 19 camera systems across 39 countries. Meteor showers were removed using the Valsecchi DN < 0.1 criterion (≤ 5 % false positives) and cometary contributions (TJ < 2 for LPCs; Tancredi criterion for JFCs with TJ>2), isolating the asteroidal sporadic component. To mitigate velocity‑dependent detection bias for faint meteors, each network was debiased by imposing minimum mass cutoffs. We then identified 540 candidate meteorite‑dropping events (≥ 1 g) from FRIPON, EFN, and GFO using α–β dynamic criteria and photometric mass estimates. Finally, we constructed 10×10 binned, two‑dimensional histograms in orbital element spaces—(a,e), (a,i), (i,q), (i,Q)—and performed χ² tests of independence on each bin (α = 0.0455 for 2 σ, α = 0.0027 for 3 σ) to locate statistically significant discrepancies between the total cm-m impactor flux and the meteorite-dropping subpopulation. Relative density differences, Δ = (f_sub – f_ref)/f_ref × 100 %, were mapped to highlight over‑ and under‑abundant orbital regions between the impactor and fall populations

Results

• Perihelion filter dominates – Objects that have present or past perihelia q << 1 au are up to 10x over-represented in the fall sample relative to all impacts (Fig. 1)
• More high-velocity survivors than expected – there are more meteorite falls at entry speeds > 20 km s−1 despite higher ablation losses, some survival bias in space already removing the weaker meteoroids.
• Atmospheric removal – For impactors ≥ 1 kg, only roughly 30–50% deliver ≥ 50 g fragments to the ground.
• Tidal debris appears fragile – tidally disrupted NEO-cluster fragments contribute 0.2 % of the falls vs 0.4 % to all impactors, i.e., are 2x weaker according to these results. However, further work is necessary to confirm this, as this assumes that NEA clustering can be directly tied to meteor and fireball data despite the lack of statistically significant evidence within these observations alone.

Fig. 1 Comparison of orbital distributions for all impactors versus those that produce meteorites. Each panel shows a heatmap of the percentage difference between the normalized density of impacts ≥ 10 g at the top of the atmosphere and the subset of those that yield meteorite falls ≥ 1 g, binned by orbital parameters. Reds mark regions where meteorite‑dropping events are over‑represented relative to the overall impactor flux, while blues show under‑representation. Bins outlined in bold indicate differences significant at the 3σ level (chi‑square test); unoutlined bins meet the 2σ threshold. The overlaid curves trace the expected Kozai–Lidov resonant exchange between inclination and eccentricity for a semi‑major axis of 2.5 au, illustrating why an excess of meteorite drops appears at high inclinations when perihelion distances approach ~ 1 au.

Discussion

Our findings point to a two‑step selection process—first in interplanetary space and then in Earth’s atmosphere—that reshapes which meteoroids become meteorites and explains the low fraction of carbonaceous falls [4]. First, objects whose orbits currently dip well inside 1 au, or have done so in the recent past, yield up to an order of magnitude more recovered meteorites than their abundance in the overall sporadic flux would predict [4]. Repeated perihelion passages subject fragile, volatile‑rich material to intense thermal cycles, driving micro‑cracking and mass loss that effectively removes the weakest fragments and leaves behind a cohort of heat‑hardened, mechanically robust stones. Second, these survivors—which characteristically enter at higher speeds (> 20 km/s), a regime normally more hostile to meteorite survival—nonetheless endure atmospheric passage more often than slower, less cohesive bodies, demonstrating that intrinsic strength gained from thermal processing outweighs the disadvantage of increased ablation [4]. Even so, once past the thermal fragmentation filter, only about 30-50% of kilogram‑scale meteoroids survive to deposit substantial fragments on the ground (see Supplementary Figure 9 in [4]), confirming that atmospheric breakup remains a significant sieve.

Together, the combined action of solar heating and atmospheric entry preferentially delivers low‑porosity, thermally resilient carbonaceous fragments that have avoided—or survived—severe thermal fatigue. This dual filtering mechanism naturally accounts for the anomalously short cosmic‑ray exposure ages of CI and CM chondrites [3,4] and their pronounced scarcity among collected meteorites [2,3].

 

References: [1] DeMeo, F. E. and B. Carry. (2014) Nature 505:629–634. [2] Brož, M., et al. (2024) Astronomy & Astrophysics 689:A183. [3] Scherer, P. and L. Schultz. (2000) Meteoritics & Planetary Science 35.1: 145-153. [4] Shober P.M. et al. (2025) Nature Astronomy 14:1-4