Searching for stardust is searching for the proverbial needle in a haystack. But with experience, it becomes possible to confirm the criteria of extraterrestrial provenance by visually scanning through a batch of urban dust. In most cases micrometeorites look like black aerodynamic stones of the correct size, with one of the known textures: barred olivine, cryptocrystalline, and so on. Once an interesting stone is identified, the process then is followed by a check for excluding criterions, like fused spherules, splash marks, etc.
In most cases this is enough for a quick positive identification of a micrometeorite, based on the morphology of the stone. But since we are still in the infancy of the exploration of micrometeorites, once in a while an unusual object attracts our attention. Naturally, as experience increases, the more candidates may be picked out for further exploration. A quick analysis under the scanning electron microscope (SEM) is usually enough to erase the question mark, thereby confirming or rejecting a candidate as extraterrestrial. This is why I always recommend that stardust hunters put aside strange but promising candidates for a future analysis. Access to a scanning electron microscope may be hard to come by, but it is always worthwhile if one is properly prepared. My favorite analyses are those of dubious objects where we may learn something new; a dark horse throwing light upon a new aspect of these majestic cosmic dust particles.
One such anomaly is that of pale micrometeorites. While most micrometeorites are pitch-black or some variation of gray, an occasional rarity is micrometeorite that is pale or even white. In the case of glass (V-type) micrometeorites, we have even discovered colorless micrometeorites. How is that possible?
In this context we are not talking about the rare CAT-spherules, which none of us urban micrometeorite hunters have found yet. I will write a blog about them later. But there are pale varieties of all the common micrometeorite types. Like NMM 3871, found recently on the roof of a shopping mall. It is an oriented cryptocrystalline (CC type) stone with a large metal bead in the front and an elongated tail. The size is an average 0.3 mm. Micrometeorites of this type are relatively common, but they are usually opaque black in color or have a pale halo around the metal bead, which possibly drained the surrounding material of iron.
I have not seen any explanation of colorless or pale micrometeorites in scientific literature, but in The Atlas of Micrometeorites I present one hypothesis: oxidation. Perhaps a longer exposure to Earth’s oxygen-rich atmosphere during formation may have caused this visually striking phenomenon. I suggest that this could have occurred due to a low entry angle and/or a grazing orbit with multiple attempts to enter the atmosphere. Furthermore, a low entry angle may be an indication of an unusual origin. While this hypothesis is plausible, to be honest, I am not entirely convinced about it myself. Science is not always about finding answers, but rather asking new questions.
There is a possibility that the rare pale color is an echo of an unusual parent body in space. In my work I have not seen any chemical differences between the ordinary black and the rare pale varieties. The iron content of the matrix is low in general and, as we know, even small traces of iron may cause color changes. I do not have access to mass spectroscopy or microprobe analysis, which may be able to detect variations in the isotopes of these strange micrometeorites. Indeed, gaining access to these instruments is a future goal.
Part of a new project with Professor Martin Bizzarro at the University of Copenhagen, will include spectrometric analysis of fresh urban micrometeorites. So, within a couple of months, it is possible that we may find out more about this phenomenon. Stay tuned to learn more about the progression of this co-operative study here at Project Stardust!
Perhaps one day we will know the answer to why these beautiful micrometeorites are colorless. Meanwhile, we can enjoy this new collage of twelve pale cosmic stones and wonder about what they have in common, apart from the lack of the ordinary black color?
There is also one beautiful pale micrometeorite available in the Winter 2022 Fine Art Collection, NMM 1359. This exquisite cryptocrystalline (CC-type) micrometeorite features an aerodynamic triangular shape with a spectacular nickel-iron bead.
Now you too may have a cosmic print on your wall! What will it remind you of? Whenever I look at a micrometeorite, I am reminded that no matter where I am or how dreary things appear, I am always surrounded by cosmic beauty.