Ablation Rates of Organic Compounds in Cosmic Dust: Implications for Fragmentation during Atmospheric Entry

Cosmic dust consists of mineral grains that are held together by a refractory organic "glue", and it has been proposed that loss of the organics during atmospheric entry can lead to the fragmentation of dust particles into sub-micron sized fragments (Campbell-Brown 2019). If this happens, there are several important implications in the Earth’s atmosphere: 1) slow-moving particles may be undetectable by radar, so that the total dust input could be considerably larger than current estimate of around 30 tonnes per day that is required to explain the measured vertical fluxes of Na and Fe atoms in the mesosphere, and the accumulation rate of cosmic spherules and unmelted micrometeorites at the surface (Carrillo-Sánchez et al. 2020, Rojas et al. 2021); 2) meteoritic fragments may freeze stratospheric droplets in the polar lower stratosphere, producing polar stratospheric clouds that cause ozone depletion (James et al. 2018); and 3) the anomalously large measured accumulation rates of meteoritic material in polar ice cores may be better explained (Brooke et al. 2017). Meteoritic fragmentation may also supply nuclei for the formation of ice clouds in other planetary atmospheres, such as Mars (Plane et al. 2018).  

At Leeds we have developed a new experimental system for studying the pyrolysis of the refractory organic constituents in cosmic dust during atmospheric entry (Bones et al. 2022). The pyrolysis kinetics of meteoritic fragments was measured by mass spectrometric detection of CO₂ at temperatures between 625 and 1300 K. The complex time-resolved kinetic behaviour is consistent with two organic components – one significantly more refractory than the other, probably corresponding to the insoluble and soluble organic fractions, respectively (Alexander et al. 2017). The measured temperature-dependent pyrolysis rates were then incorporated into the Leeds Chemical Ablation Model (CABMOD) (Vondrak et al. 2008), which demonstrates that organic pyrolysis should be detectable using high performance large aperture radars (Bones et al. 2022). Atomic force microscopy was used to show that although the residual meteoritic particles became more brittle after organic pyrolysis, they will nevertheless withstand stresses that are at least 3 orders of magnitude higher than would be encountered during atmospheric entry. This suggests that most small cosmic dust particles (radius < 100 μm) will not fragment during entry into the atmosphere as a result of organic pyrolysis (Bones et al. 2022).

However, a subset of slow-moving, low density particles with a large organic component, as observed in fresh cometary particles such as those in the coma of comet 67/P (Mannel et al. 2019), could fragment into sub-micron meteoritic particles that would survive entry. In fact, meteoritic fragments with a size distribution peaking around radius = 250 nm have been observed in the Arctic polar vortex (Schneider et al. 2021). Experiments in our laboratory show that meteoritic fragments, as well the nanometre-sized meteoric smoke particles which form from the condensation of metallic vapours produced by meteoric ablation in the upper mesosphere, are very effective ice nuclei. On Earth, these particles can facilitate the freezing of polar stratospheric cloud droplets, and may also play a role in the freezing of clouds in the middle atmospheres of Mars and Venus.

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