N. G. Rudraswami

Evaluating changes in the elemental composition of micrometeorites during entry into the earth`s atmosphere

We evaluate the heating of extraterrestrial particles entering the atmosphere using the comprehensive Chemical ablation model (CABMOD). This model predicts the ablation rates of individual elements in a particle with a defined size, composition, entry velocity and entry angle with respect to the zenith (ZA). In the present study, bulk chemical analyses of 1133 Antarctica micrometeorites (collected from South Pole water well) are interpreted using CABMOD. The marked spread in Fe/Si values in unmelted, partially-melted and melted micrometeorites is explained by the loss of relatively volatile Fe during atmospheric entry. The combined theoretical modelling and elemental composition of the micrometeorites (Mg/Si ratios) suggest that ~85% of particles have a provenance of carbonaceous chondrites, the remaining ~15% are either ordinary or enstatite chondrites. About 65% of the micrometeorites have undergone <20% ablation, while a further 20% have lost between 20 and 60% of their original mass. This has implications for understanding the micrometeorite flux that reaches the Earth’s surface, as well as estimating the pre-atmospheric size of the particles. Our work shows that the unmelted particles which contribute ~50% to the total micrometeorite collection on Earth’s surface have a small entry zone: ZA = 60−90 0 if the entry velocity is ~11 km/s, and ZA = 80

RELICT OLIVINES IN MICROMETEORITES: PRECURSORS AND INTERACTIONS IN THE EARTH’S ATMOSPHERE

Antarctica micrometeorites (~1200) and cosmic spherules (~5000) from deep sea sediments are studied using electron microscopy to identify Mg-rich olivine grains in order to determine the nature of the particle precursors. Mg-rich olivine (FeO < 5wt%) in micrometeorites suffers insignificant chemical modification during its history and is a well-preserved phase. We examine 420 forsterite grains enclosed in 162 micrometeorites of different types—unmelted, scoriaceous, and porphyritic—in this study. Forsterites in micrometeorites of different types are crystallized during their formation in solar nebula; their closest analogues are chondrule components of CV-type chondrites or volatile rich CM chondrites. The forsteritic olivines are suggested to have originated from a cluster of closely related carbonaceous asteroids that have Mg-rich olivines in the narrow range of CaO (0.1–0.3wt%), Al2O3 (0.0–0.3wt%), MnO (0.0–0.3wt%), and Cr2O3 (0.1–0.7wt%). Numerical simulations carried out with the Chemical Ablation Model (CABMOD) enable us to define the physical conditions of atmospheric entry that preserve the original compositions of the Mg-rich olivines in these particles. The chemical compositions of relict olivines affirm the role of heating at peak temperatures and the cooling rates of the micrometeorites. This modeling approach provides a foundation for understanding the ablation of the particles and the circumstances in which the relict grains tend to survive.

Ablation and chemical alteration of cosmic dust particles during entry into the earth`s atmosphere

Most dust-sized cosmic particles undergo ablation and chemical alteration during atmospheric entry, which alters their original properties. A comprehensive understanding of this process is essential in order to decipher their pre-entry characteristics. The purpose of the study is to illustrate the process of vaporization of different elements for various entry parameters. The numerical results for particles of various sizes and various zenith angles are treated in order to understand the changes in chemical composition that the particles undergo as they enter the atmosphere. Particles with large sizes (> few hundred μm) and high entry velocities (>16 km s−1) experience less time at peak temperatures compared to those that have lower velocities. Model calculations suggest that particles can survive with an entry velocity of 11 km s−1 and zenith angles (ZA) of 30°–90°, which accounts for ~66% of the region where particles retain their identities. Our results suggest that the changes in chemical composition of MgO, SiO2, and FeO are not significant for an entry velocity of 11 km s−1 and sizes <300 μm, but the changes in these compositions become significant beyond this size, where FeO is lost to a major extent. However, at 16 km s−1 the changes in MgO, SiO2, and FeO are very intense, which is also reflected in Mg/Si, Fe/Si, Ca/Si, and Al/Si ratios, even for particles with a size of 100 μm. Beyond 400 μm particle sizes at 16 km s−1, most of the major elements are vaporized, leaving the refractory elements, Al and Ca, suspended in the troposphere.