• extraterrestrial dust
• mass distribution
• scoriaceous micrometeorites
• size distribution
• Transantarctic Mountains micrometeorites
• unmelted micrometeorites

Micrometeorites (MMs) are extraterrestrial particles (< 2 mm) originating from the interplanetary dust complex that are continuously deposited on Earth's surface, with an annual average of about 40000 metric tonnes. They are classified according to the degree of melting experienced during atmospheric entry heating into three groups: melted micrometeorites or cosmic spherules (CSs), unmelted micrometeorites (UnMMs) and partially melted or scoriaceous micrometeorites (ScMMs). It is estimated that approximately 10 to 20% of micrometeorites are unmelted or scoriaceous upon reaching the Earth’s surface, thus preserving the primary features and characteristics of the parent material. The study of micrometeorites is of significant importance to planetary science as their analysis provides invaluable insights into the diverse range of bodies within the Solar System. Indeed, a knowledge of the physical and compositional properties of micrometeorites provides constraints for modelling the source regions and dynamic evolution of the cosmic dust complex in the near-Earth space. Furthermore, a systematic study of micrometeorite collections allows investigation of the flux in terms of amount and composition and thus of the cycles of extraterrestrial input to the global geochemical budget of planet Earth. The majority of micrometeorite collections are derived from ice, snow and glacial sediments from polar regions, deep-sea sediments, hot deserts and rooftop. During expeditions conducted by the Italian National Antarctic Research Program (PNRA), a considerable quantity of micrometeorites was discovered in the micrometeorite traps within loose, fine-grained sediments accumulated in bedrock weathering pits and fractures at the summit of the Victoria Land Transantarctic Mountains (TAM).
The present study was conducted on extraordinarily large UnMMs and ScMMs micrometeorites from the TAM collection with the aim of filling knowledge in micrometeorite studies. A total of 219 particles were identified based on their morphological and petrographic features using a stereomicroscope. The micrometeorites were weighed using a microbalance and imaged under a FEG-SEM (Field Emission Gun Scanning Electron Microscope) in order to define their size and mass distribution. Micrometeorites with diameters ranging from ~200 to 2800 μm and masses from ~10 to 23000 μg were identified. The (rank-) size distribution of all UnMMs and ScMMs, with diameters < 2000 μm (n = 212), is fitted to a power law with a slope exponent of -0.62 (R² = 0.94) over the size range 200-400 μm and with a slope exponent of -2.49 (R² = 0.99) over the size range 400-1300 μm. The size distribution is bimodal, with two main peaks at approximately 300 and 500 μm. The (rank-) mass distribution, of the aforementioned particles, is fitted to a power law with a slope exponent of -0.25 (R² = 0.96) over the range 10-100 μg and with a slope exponent of -0.83 (R² = 0.99) over the range 100-1000 μg. The mass distribution is characterised by two main peaks at approximately 30 and 120 μg.
The slope exponents obtained from the size distribution were found to be lower than those previously reported in other Antarctic micrometeorite collections and in the TAM reference collection. However, it should be noted that in these studies, the entire extraterrestrial particles collected were analysed, with CSs representing the vast majority. Consequently, any comparative analysis with the other collections is necessarily related to a comparison of unmelted and scoriaceous micrometeorites with melted ones. It was observed that there was a notable lack of particles with a diameter below 200 µm, which is in contrast with the data provided by the TAM collection. This observation is reflected in the size distribution value, whereby the prevalence of large micrometeorites in the collections is indicated by shallow slopes (low coefficients). The discrepancy between the observed number of small UnMMs and ScMMs and that of extraterrestrial particles documented in the TAM collection may be attributed to a combination of factors. It is plausible that fragmentation may occur during melting as a consequence of aerodynamic drag, which could contribute to an increase of the small samples of CSs identified in the TAM collection. An additional potential cause for the loss of these particles is the possibility that they were lost due to the sampling methodology and/or within the TAM traps. This may be attributed to the challenging detection and selection of these particles during manual separation procedures or as a consequence of selective weathering within the TAM traps.
The SE (secondary electron) and BSE (backscattered electron) images obtained represent a valuable source of information for guiding future research studies. It is noteworthy that the considerable number of large scoriaceous and unmelted micrometeorites identified in the TAM collection is unique, according previous reports in the literature. It can thus be stated that the present TAM micrometeorite collection represent valuable research material for filling knowledge gaps in micrometeotite studies. The particles show a diverse range of morphologies, with varying degrees and types of terrestrial weathering, which may be related to variable residential times and parent body composition. Further investigation combining petrographic, geochemical and oxygen isotopic compositional analyses is thus desirable. Because of the large number of particles, we expect to find new micrometeorite types including few from new parent bodies.