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New Insights on Micrometeorites

Final Report Summary - MICROMET (New Insights on Micrometeorites)

Project Objectives:
Under MICROMET we sought to systematically characterize and compare micrometeorites from a unique new collection performed on Kwajalein in the mid-Pacific, as well as existing samples from South Pole collections. State-of-the-art facilities were to be used to identify and study rare or unusual micrometeorites, investigate atmospheric entry alteration and terrestrial effects as well as to determine the sources and flux of micrometeorite types over time. It was hoped that these analyses/data would provide new insights into the formation, evolution and current state of the small solar system bodies from which they originate.

Work Performed and Major Results:
The Kwajalein samples were successfully collected and delivered to the Natural History Museum, London (NHM) before the project commenced in April 2013. Additional micrometeorites from the South Pole water well (SPWW) collections were also loaned for study.
The Kwajalein samples were comprised of particles on laser etched polycarbonate membrane filters that had been collected directly from the atmosphere by a high volume air sampler. Upon inspection it was immediately noted that the humid, ocean environment had resulted in filters becoming wet during collections, and salt subsequently precipitating out onto the filter during storage in air-conditioned facilities. As a consequence, particles were now welded to the filters by salt and could not be picked. A further complication was that the filters were approximately 8 by 10 inches in size – too large to efficiently search for micrometeorites (expected to be much less than 250 microns in diameter). We therefore spent the first few months designing, testing and developing a preparation method to remove the salt and concentrate the particles into a smaller, searchable area with minimal damage to, or loss of, samples.
Using this method, five filters were prepared for study by scanning electron microscopy (SEM). Initially, while working on the first filters, all particles were investigated by collecting images and obtaining chemical data via energy dispersive X-ray microanalysis. However, the filters contained far more particles than had been anticipated, and a large proportion were found to have chemistries inconsistent with extraterrestrial materials. Instead, particles were dominated by coral, sand, metal flakes (Cu, Fe, Zn and Sn dominated) and precipitates that result from weathering of the Al casing of the sampler itself. It was deemed unfeasible to study every single particle in the search for bona fide micrometeorites, therefore our investigations to date have focused on particles resembling the cosmic spherule sub-type of micrometeorites due to their low abundance and ease of identification. All particles resembling spherules greater than 5 microns in diameter were identified in image maps collected from roughly equivalent areas of these filters. These particles were then investigated further by SEM to confirm spherule morphology, study surface textures and determine qualitative chemistry. Details of those confirmed as cosmic spherule-like were gathered and we were able to determine how their abundances varied over time (from week to week).
Most recently we have been working on the arduous task of picking (by hand) and preparing (mounting in resin and polishing to reveal their interiors) and analysing cosmic spherule-candidates further by SEM with the aim of confirming or discounting an extraterrestrial origin. This time-consuming task is ongoing but we aim to have studied all candidates greater than 10 microns in diameter within the next few months. To date, several of these spherule particles have shown internal textures and compositions consistent with cosmic spherules (and inconsistent with known terrestrial materials) demonstrating proof-of-concept for this collection approach. During these studies we have discovered several unusual spherules on one filter (representing one week of sampling). Concurrent with these studies we have been analyzing spherules from SPWW collections and although we have not observed anything similar, rare but larger examples of these particles have been previously identified in the SPWW collections by our collaborators. Examples from the SPWW collections and Kwajalein filters are scheduled for further, higher resolution study and comparison by transmission electron microscopy.

Overall Results and Impact:
Despite the unanticipated complications with the Kwajalein collections, we developed a handling and preparation technique that allowed us to survey the contents of five filters to date. We have identified a wealth of cosmic spherule candidates and thus validated this new micrometeorite collection method. From these data we have determined that the flux of these micrometeorite candidates varies over time and may be monitored by this method. We have also identified several unusual spherules that we will study in more detail in the following months. Similar examples have been identified in SPWW micrometeorite collections but are extremely rare and it is our intention to also study and compare these samples in order to determine the origins of these particles. Analysis of atmospheric and terrestrial weathering, and of the sources of micrometeorite types was not possible due to difficulties in identifying all micrometeorite types amongst the Kwajalein collections. However, thanks to the results of this project, we have initiated a contemporary collection that is currently being undertaken in collaboration with the British Antarctic Survey at the Halley VI Research Station in Antarctica. Here we expect the level of terrestrial contamination to be vastly reduced and thus anticipate being able to locate and analyse all collected micrometeorites and hence address our remaining objectives.
Questions relating to this project should be directed to Dr. Penelope Wozniakiewicz at penw@nhm.ac.uk (named Fellow) or Prof. Sara Russell at sarr@nhm.ac.uk (Scientist in Charge).