Six years ago, before the discovery of the fresh (urban) micrometeorites, the reference collections consisted of old micrometeorites from Antarctica. These particles from space are weathered on the outside, so in most cases their visual appearance is not enough for identification and verification. Instead it is necessary to analyze the inner structures in an electron microscope.
Now that we have learned what fresh micrometeorites look like, the study of these enigmatic particles has radically changed. With two exceptions:
First, the rare iron (I-type) micrometeorites. These particles cannot be distinguished from anthropogenic iron oxide spherules generated in industrial processes, unless platinum group nuggets are detected in a chemical analysis.
Secondly, completely melted glass or vitreous (V-type) micrometeorites without metal beads and recrystallization. These cosmic dust grains have been heated so much during atmospheric entry that all metal has evaporated and the result is a non-magnetic glass spherule. From the Antarctic collections, we know that up to 20% of micrometeorites may be of this type.
In urban collections, however, these glass spherules are rare. Both because they are not magnetic and because similar looking spherules from mineral wool, such as that found in insulation, is rather common in urban dust, at least in temperate zones. An energy-dispersive X-ray spectroscopy (EDS) analysis in the electron microscope can confirm a chondritic spectrum (content), which is a clear indicator of extraterrestrial origin.
Cosequently, in the case of the ~0.2 mm olive green glass spherule in the photo at the top of this post, I simply cannot know wether it is extraterrestrial or not. It is what we call a “micrometeorite candidate”, an object of unknown origin optimistically designated as NMM 4018, which has been put aside for future analysis. The spherule was found in the rain gutter of a school at Tårup, Funen, Denmark.
Guessing or believing one way or the other will not be of any help. I call this uncertainty the joy of not knowing; a moment of Zen. Note by the way, the small gas vesicles inside the object.
Shortly I’ll analyze the candidate in the electron microscope lab and then we’ll find out whether or not this fascinating spherule is of extraterrestrial origin. Meanwhile, if you want to find out more about micrometeorites, I recommend that you study the comprehensive “Atlas of Micrometeorites”. If you have questions or perhaps a comment, which are both appreciated, please leave it under this post on Facebook, Instagram, or Twitter and I’ll answer asap. Enjoy!