Cometary dust collected by MIDAS on board Rosetta

Abstract

Context. The Micro-Imaging Dust Analysis System (MIDAS) atomic force microscope (AFM) on board the Rosetta comet orbiter has been dedicated to the collection and 3D topographical investigation of cometary dust in the size range of a few hundreds of nanometers to tens of micrometers with a resolution down to a few nanometers. Aims. We aim to catalog all dust particles collected and analyzed by MIDAS, together with their main statistical properties such as size, height, basic shape descriptors, and collection time. Furthermore, we aim to present the scientific results that can be extracted from the catalog, such as the size distribution and statistical characteristics of cometary dust particles. Methods. The starting point for this work was the existing MIDAS particle catalog, which has been greatly improved by a careful re-analysis of the AFM images, leading to the addition of more dust particles and a detailed description of the particle properties. The final product is a comprehensive list of all possible cometary dust particles detected by MIDAS. The catalog documents all images of identified dust particles and includes a variety of derived information tabulated one record per particle. Furthermore, the best image of each particle was chosen for subsequent studies. Finally, we created dust coverage maps and clustering maps of the MIDAS collection targets and traced any possible fragmentation of collected particles with a detailed algorithm. Results. The revised MIDAS catalog includes 3523 MIDAS particles in total, where 1857 particles are expected to be usable for further analysis (418 scans of particles before perihelion + 1439 scans of particles after perihelion, both after the removal of duplicates), ranging from about 40 nm to about 8 µm in size. The mean value of the equivalent radius derived from the 2D projection of the particles is 0.91 ± 0.79 µm. A slightly improved equivalent radius based on the particle's volume coincides in the range of uncertainties with a value of 0.56 ± 0.45 µm. We note that those sizes and all following MIDAS particle size distributions are expected to be influenced by the fragmentation of MIDAS particles upon impact on the collection targets. Furthermore, fitting the slope of the MIDAS particle size distribution with a power law of a • r b yields an index b of ∼ -1.67 to -1.88. Lastly, based on the created dust coverage maps and clustering maps of the MIDAS collection targets, we determined the particle fragmentation ratio of 4.09 for nominal activity and 11.8 for the outburst, which underlines that parent particles with faster impact velocity are more likely to be fragmented during dust collection.

Cometary dust collected by MIDAS on board Rosetta

“…The detailed list of target exposure history and duration can be found in Appendix Table B.1. in Kim et al (2023).…”

“…In the retrieved scans identifying dust grains and marking them with a mask is performed by hand with the open-source package Gwyddion (Nečas & Klapetek 2012). More detailed information about MIDAS and its dataset can be found in previous MIDAS studies (e.g., Riedler et al 2007;Bentley et al 2016a;Mannel et al 2016Mannel et al , 2019Kim et al 2023).…”

“…The Micro-Imaging Dust Analysis System (MIDAS; Riedler et al 2007;Bentley et al 2016a), one of three in-situ dust instruments on board, is the first generation Atomic Force Microscope (AFM) that was launched into space to collect the smallest cometary dust particles with sizes from hundreds of nanometers to tens of micrometers. It recorded the 3D topographic/textural information with nanometer resolution and collected statistical parameters (Kim et al 2023). Most importantly, dust particles were collected close to the comet (less than ∼ 30 km afar) with low relative velocities between spacecraft and nucleus, roughly between some cm s −1 and 10 m s −1 (e.g., Della Corte et al 2015), which enables to collect dust particles with only a small degree of alteration.…”

“…Thus, the investigation of unaltered and thus more realistic particles could open up new possibilities for unraveling unresolved questions in space science, particularly in gas flow experiments in cometary environments (Laddha et al 2023). The importance of shape descriptors of the non-spherical MIDAS particles (Bentley et al 2016b;Kim et al 2023) is further enhanced as traditional models using purely spherical particles might not be sufficient to significantly improve gas flow models further.…”

“…Thus, it has to be judged case by case in which sense a particle's properties reflect properties of cometary dust in the coma, the comet, or even in the early solar system. Using the final MIDAS particle catalog that contains all cometary dust particles collected by MIDAS together with their basic properties, called particle catalog V6 1 (Kim et al 2023), we focus on the development of MIDAS particle shape descriptors to understand the degree of dust alteration and we determine the structural properties of the MIDAS dust particles that remained pristine. This paper 2 is organized as follows: We provide a description of the MIDAS instrument and its collection procedure in Sect.…”