Impact ionization dust detection with compact, hollow and fluffy dust analogs

Hunziker, S.; Moragas-Klostermeyer, G.; Hillier, J.K.; Fielding, L.A.; Hornung, K.; Lovett, J.R.; Armes, S.P.; Fontanese, J.; James, D.; Hsu, H.W.; Herrmann, I.; Fechler, N.; Poch, O.; Pommerol, A.; Srama, R.; Malaspina, D.; Sterken, V.J. (2022). Impact ionization dust detection with compact, hollow and fluffy dust analogs. Planetary and space science, 220, p. 105536. Elsevier 10.1016/j.pss.2022.105536

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Impact ionization of high-velocity cosmic dust particles has been used as a basic principle for dust detectors in
space for many decades. It has provided optimum means to gain insight into the dust environment in the solar
system. The Ulysses Dust Detector System provided for the first time impact ionization-based detection of
interstellar dust (ISD) in the solar system and discovered surprisingly heavy ISD particles with sizes up to a few
microns. Studies based on astronomical observations of the local interstellar medium, on the other hand, suggested
a much smaller upper limit of around 0.25 μm (silica) or 1 μm (graphite) for the size distribution of ISD
particles. Therefore, it has been suggested that low-density fluffy dust particles may mimic the impact signals of
heavier compact particles. In this work, we discuss a series of impact experiments that have been performed at the
Heidelberg dust accelerator facility with the Cosmic Dust Analyzer flight spare unit, to compare the high-velocity
impact ionization properties of compact and hollow silicate particles, and carbon aerogel particles with each other
and with literature data. The experiments indicate differences in the collected total amount of impact charges and
how quickly the charges are collected, between impacts from compact particles and their non-compact counterparts.
The results of this first study suggest that fluffy particles generate less ions upon impact than their
compact counterparts, opposite to the suggested explanation for the heavy ISD particles. Data from the performed
impact experiments indicate that a secondary process (e.g. secondary impacts from ejecta or more target material
ionization) could be the main cause for the observed differences. These results imply that the previously detected
heavy ISD particles may be real. We identify the key problems with the performed dust experiments and advise
that future impact ionization instruments should additionally be calibrated with improved low-density fluffy dust
particles that better represent the properties of cosmic dust particles.

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