Hydrogen Emission in Meteors as a Potential Marker for the Exogenous Delivery of Organics and Water

Abstract

We detected hydrogen Balmer-alpha (H_α) emission in the spectra of bright meteors and investigated its potential use as a tracer for exogenous delivery of organic matter. We found that it is critical to observe the meteors with high enough spatial resolution to distinguish the 656.46 nm H_α emission from the 657.46 nm intercombination line of neutral calcium, which was bright in the meteor afterglow. The H_α line peak stayed in constant ratio to the atmospheric emissions of nitrogen during descent of the meteoroid. If all of the hydrogen originates in the Earth's atmosphere, the hydrogen atoms are expected to have been excited at T = 4,400 K. In that case, we measured an H₂O abundance in excess of 150 ± 20 ppm at 80-90 km altitude(assuming local thermodynamic equilibrium in the air plasma). This compares with an expected <20 ppm from H₂O in the gas phase. Alternatively, meteoric refractory organic matter (and water bound in meteoroid minerals) could have caused the observed H_α emission, but only if the line is excited in a hot T ∼10,000 K plasma component that is unique to meteoric ablation vapor emissions such as Si⁺. Assuming that the Si⁺ lines of the Leonid spectrum would need the same hot excitation conditions, and a typical [H]/[C] = 1 in cometary refractory organics, we calculated an abundance ratio [C]/[Si] = 3.9 ± 1.4 for the dust of comet 55P/Tempel-Tuttle. This range agreed with the value of [C]/[Si] = 4.4 measured for comet 1P/Halley dust. Unless there is 10 times more water vapor in the upper atmosphere than expected, we conclude that a significant fraction of the hydrogen atoms in the observed meteor plasma originated in the meteoroid.