Compositional and Textural Diversity in Cosmic Dust: Investigating Micrometeorites from Cap Prud'homme, Antarctica

Cosmic dust is a significant source of extra-terrestrial material delivered to the Earth: estimates suggest between 3 and 300 tonnes of dust enter the atmosphere each day (Plane, 2012). Furthermore, approximately 10¹⁶ kg of dust particles with diameters of 100-200 µm reside within 5 AU (Nesvorný et al., 2010). It is therefore a substantial reservoir of material that can be examined on Earth. Cosmic dust can be classified into two groups: micrometeorites (MMs), typically 100-200 µm, retrieved from terrestrial settings, and the smaller (5-50 µm) interplanetary dust particles (IDPs), gathered from the stratosphere using specialised collectors on aeroplanes. Bradley (1994) notes the presence of cometary material within IDP collections, whilst Genge et al. (1997) report compositional similarities of MMs to some asteroidal material. Even within these collections, there is variability in both morphology and composition. Furthermore, these particles are thought to undergo different processes as they enter the atmosphere, due to the trajectories associated with their source material.

Here, we present the analysis of a collection of MMs from blue ice at Cap Prud’homme, Antarctica, recovered in 1994 (Maurette et al., 1992). We obtain compositional and textural information using backscattered (BSE) and secondary electron imaging (SEI) with a field emission-scanning electron microscope (FE-SEM). One MM, approximately 70 µm in diameter, shows a rounded shape in 2D, with an elongate structure adjoined to one side, 100 µm in length. BSE imaging reveals a difference in composition between the core of the circular region and the adjoined material. Another particle, 70 x 100 µm in size, shows little evidence of fractures or vesicles and a primarily homogeneous texture. However, two edges and a corner region show compositionally “speckled” areas with heavier elements. SEI reveals these surfaces to be rough, compared to the smooth interior. Some MMs show clearly defined regions in BSE images up to 20 µm in diameter that are compositionally dissimilar to the surrounding material, whilst others display distinct core and rim areas. We identify regions of interest for further analyses on nanometre-scales, including investigating compositional and textural boundaries. These may reveal 3D compositional trends, such as elemental diffusion, unseen at lower resolutions.

1. Bradley, J. (1994) Science, 265, 925-929

2. Genge M. et al. (1997) Geochim. Cosmochim. Acta., 61, 5149-5162

3. Maurette M. et al. (1992) Meteoritics, 27, 473-475

4. Nesvorný D. et al. (2010) Astrophys. J., 713, 816-836

5. Plane J. (2012) Chem. Soc. Rev., 41, 6507-6518