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

TL;DR

This study investigates the high-temperature spectral component of meteor fireballs, focusing on neutral hydrogen H$_\alpha$ and ionised silicon Si II-2, to derive relative elemental abundances under LTE assumptions with plasma parameters around $T\approx10500\ \mathrm{K}$ and $n_e\approx4.8\times10^{13}\ \mathrm{cm}^{-3}$. Using European Fireball Network spectra, the authors find that the H/Si abundance ratio shows no significant dependence on meteor velocity but increases with photometric mass for cometary meteoroids, suggesting enhanced volatile preservation in larger bodies. The results place cometary H/Si values above CI chondrites and, for large meteoroids, at or above Halley dust values under nominal plasma conditions, implying that comets could be important contributors to Earth's volatile inventory. Afterglow contamination is acknowledged and partially corrected, but the mass-dependent hydrogen signal persists, reinforcing a physical, size-dependent retention of volatiles with implications for Solar System water delivery models.

Abstract

We present a study of the high-temperature spectral component in meteor fireballs, with a particular focus on neutral hydrogen at 656.28 nm and ionised silicon 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 emissions, we investigated the relationship between H and Si 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 volatile hydrogen with less volatile silicon in bodies with different velocities. 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. The prevalence of hydrogen in larger cometary meteoroids supports models where comets could be significant contributors to Earth's volatile inventory.

Figures (17)

• Figure 1: Two spectra with different meteor velocities. The bottom panel shows a relatively slow meteor with the low-temperature component in the spectrum. The top panel shows a fast meteor with the high-temperature components marked.
• Figure 2: Normalised spectral sensitivity of SDAFO cameras. The digital camera response was combined with atmospheric absorption (telluric absorption lines).
• Figure 3: Reduced Lyrid meteor spectrum EN230421_013038. Zoomed-in images of the analysed spectral regions are shown in the insets, and the positions of the studied high-temperature component lines are marked.
• Figure 4: Ratio of the integrated intensities in the H$_\alpha$ region at $656$ nm to the silicon doublet Si II - 2 at $634$--$637$ nm as a function of meteor velocity. The different showers are marked in different colours. The circles mark the cases with clearly detectable hydrogen and silicon lines. The triangles mark the upper limits of the ratio when the hydrogen line was either weak, at the noise level, or invisible and only the level of noise was measured. The line ratio corrected for afterglow contamination in one Lyrid meteor is shown as a blue cross (see Sect. \ref{['afterglow']})
• Figure 5: Ratio of the integrated intensities in the H$_\alpha$ region at $656$ nm to the silicon doublet Si II - 2 at $634$--$637$ nm. (a) As a function of absolute magnitude of the meteor. (b) As a function of photometric mass of the meteoroid.