Spectroscopic analysis of hydrogen and silicon in bright fireballs: New insights into meteoroid composition

Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 251 65 Ondřejov, Czech Republic

^★ Corresponding author: vojacek@asu.cas.cz

Received: 11 August 2025
Accepted: 20 October 2025

Abstract

Aims. We present a study of the high-temperature spectral component in meteor fireballs, with a particular focus on neutral hydrogen (H_α at 656.28 nm) and ionised silicon (Si II–2 doublet at 634.71 nm and 637.14 nm). By analysing spectra from the European Fireball Network (EN) that exhibit H_α and Si II–2 emissions, we investigated the relationship between hydrogen and silicon abundances across different meteoroid types. The plasma temperature of the high-temperature component remains independent of meteor velocity. This allows us to directly compare relative intensities of hydrogen, bound in more volatile materials, with silicon, bound in less volatile materials, in bodies with different velocities.

Methods. We analysed 31 meteor spectra from the EN, focusing on H_α (656.28 nm) and Si II–2 (634.71 nm, 637.14 nm) emissions to determine the elemental abundances and their relationships with the meteor parameters. The spectroscopic data were reduced following established procedures to derive the line intensities. We employed direct line integration and applied ionisation corrections through Saha equations to estimate the relative atomic abundances.

Results. Our results confirmed that the H/Si value remains largely independent of meteor velocity. We show a positive correlation with photometric mass for cometary meteoroids, suggesting that larger bodies better preserve their volatile content, namely hydrogen. This correlation persists across the meteor showers, showing a physical process related to volatile preservation rather than specific parent body composition. Our data suggest that the abundance of hydrogen in large cometary meteoroids is not only higher than in CI chondrites, but is also comparable to or higher than the measured abundances in small particles of dust from Halley’s comet, depending on the assumed plasma conditions. This work brought new constraints on the distribution and preservation of volatile elements in Solar System bodies and new insights into the potential delivery mechanisms of water to Earth.

Conclusions. The H/Si values show no correlation with meteor velocity, but increase with photometric mass for cometary meteoroids. The prevalence of hydrogen in larger cometary meteoroids supports models where comets could be significant contributors to Earth’s volatile inventory.