Deciphering mixtures of complex organic compounds in cosmic dust particles using JAXA's Destiny+ Dust Analyzer

Organic compounds are a ubiquitous component of cosmic dust and provide insight into the origin of planetary systems, the availability of carbon for life in the solar system and beyond, and the distribution of potential biosignatures in the universe. Compositional and dynamical analysis of such dust grains can shed insight into their origin. The Destiny Dust Analyzer (DDA) onboard JAXA’s interplanetary space mission DESTINY+ will detect and analyse the composition of (sub-)micron sized dust ejecta during flybys of asteroids Apophis and Phaethon [1,2]. DDA will characterise both interplanetary and interstellar dust grains during the mission’s lifetime [3]. DDA is an impact ionisation time-of-flight mass spectrometer, whereby dust particles incident onto the instrument’s target at hypervelocity (≥ 2 km s^-1) vaporise and partially fragment into various constituent ions and neutrals. Here, we investigate the capability of DDA to detect a mixture of complex organic compounds in single cosmic dust particles. An organic cosmic dust analogue is prepared by coating polycyclic aromatic hydrocarbon, perylene (C₂₀H₁₂), microparticles with an ultrathin overlayer of a conductive polymer, polypyrrole H(C₄H₂NH)_nH, to enable acceleration up to hypervelocities with a high-voltage van de Graaff instrument. Time-of-flight mass spectra obtained at impact speeds ~3-20 km/s are recorded in this calibration campaign. The characteristic parent molecular ion for perylene, [C₂₀H₁₂ (+H)]⁺, is observed at m/z 251 ± 1 in mass spectra arising from impacts between 3 and 8 km s^-1. However, between 8 and 18 km s^-1, no such parent ion is observed. Instead, impact ionisation mass spectra exhibit a characteristic series of homologous [C_nH_m]⁺ fragments originating from both polypyrrole and perylene, alongside some non-sequential ions which may be diagnostic for distinguishing between different organic components in cosmic dust. The contributions of each species to fragmentation patterns in the mass spectra is coupled with the impact velocity. Our results are in agreement with Mikula et al. (2024), who investigated impact ionisation of polypyyrole-coated anthracene particles for the Interstellar Dust EXperiment (IDEX) onboard NASA's Interstellar Mapping and Acceleration Probe (IMAP), and observed a similar relationship between fragmentation pattern and velocity [4].

Additional experiments with a range of PAHs, heterocycles, and lower mass organics at various velocities, will yield further insight into the detection and characterisation of heterogeneous dust likely to be encountered by DDA. Similarly, theoretical chemical calculations could assist in deciphering the contribution of different species to mass spectral features via the analysis of dissociation thermodynamics and kinetics.

[1] Ozaki et al. (2022) https://doi.org/10.1016/j.actaastro.2022.03.029

[2] Simolka et al. (2024) https://doi.org/10.1098/rsta.2023.0199

[3] Krüger et al. (2024) https://doi.org/10.1016/j.pss.2024.106010

[4] Mikula et al. (2024) https://doi.org/10.1021/acsearthspacechem.3c00353