There is a persistent misconception that micrometeorites are small metal spheres. And everywhere we sample for cosmic dust with a magnet, even in desolate areas far from industry, we do find small iron oxide spherules. These, however, are human-made, or anthropogenic particles that are redistributed by wind and atmospheric jet streams.
Most cosmic spherules are rocky, small aerodynamic rocks, composed mainly of magnesium silicates (forsterite) crystals with magnetite in between. Often these “Christmas tree” crystals are what makes the micrometeorite magnetic, enabling star hunters to search for them with a magnet.
From the Antarctic reference collection of micrometeorites (Susan Taylor et al., 2011), we know that approximately one percent of micrometeorites are so called iron (I-type) spherules. This is iron oxide, composed of the minerals magnetite and/or wüstite, like the omnipresent industrial spherules. For the time being the only distinguishing criterion between the two is the presence of small platinum group nuggets. These can sometimes be detected on extraterrestrial spherules by using electron microscope analysis (EDS). One such micrometeorite can be seen in this new collage, at the top of this post in the middle row on the right.
On some micrometeoroids the constituting nickel-iron has undergone an elemental differentiation which then accumulated into a metal bead. During atmospheric deceleration, the bead often migrates to the surface of the micrometeoroid. These are the beautiful “golden pearls” we sometimes can see in the front of cosmic spherules or, if the micrometeoroid had spin during atmospheric entry, a bit off the symmetrical axis. However, the location of the bead is always consistent with the laws of aerodynamics. Which is why the placement of a metal bead may hold the key to distinguishing a human-made industrial spherule from an extraterrestrial one. This is explained in detail in the books available in the Project Stardust bookstore, which are recommended.
At times, the metal bead is larger than usual, and the enclosed collage is a collection of such rarities. On five of these micrometeorites (NMM 976, NMM 3177, NMM 3655, NMM 3999, NMM 2842) the metal bead is surrounded by a thin coating of shiny sulfide, which forms a so-called rim. Since viscous iron sulfide is rapidly weathered, the presence of an intact sulfide rim is an indication of short terrestrial age. These micrometeorites are fresh from space!
In the middle of the top row is micrometeorite NMM 3324, which has a large nickel-iron bead with a thin partial coating of olivine crystals. The difference between the I-type in the middle row, right, is striking.
The yellow-green glass (V-type) micrometeorite NMM 2000 in the centre of the collage is also interesting, because here we can see the entire metal core inside the stone. It constitutes approximately one third of the stone and has a particular surface texture. Still sealed inside the extraterrestrial glass, this is unaltered by terrestrial weathering. The same texture can also be observed on the large metal bead of NMM 338, in the middle of the bottom row.
NMM 2264 is an olive green glass (V-type) micrometeorite measuring approximately 0.3 mm with a large nickel-iron bead in the front. The metal bead has a discrete sulfide rim on one side, to the right in the photo. The stone show no sign of crystallization around the metal bead, which is rare, so it was confirmed ET via chemical analysis.
NMM 3177 is a barred olivine (BO-type) micrometeorite with a large metal bead in the front, which is in the middle of the photo. It is also the subject of a featured post on the Project Stardust blog. The metal is surrounded by a partially weathered sulfide rim. The irony is that this stone was found on the roof of the Norwegian Oil Department, where a large number of skilled geologists are working who joined me for the field search on the roof. They were shocked when they got the results of the search right above their heads; no less than nine beautiful stardust particles!
NMM 976 is a rare green barred olivine micrometeorite with a rare partial green glass area. It is described in detail in a featured post on the Project Stardust blog.
It is interesting how the ratio between nickel-iron and rocky silicate is an echo of Earth itself, with its metal core and stoney mantle. How come this chemical spectrum is found in every single grain of dust in space? The explanation is as intriguing as it is simple; the dust was here first. Earth, the oceans, the atmosphere, and everything on our precious blue marble, including ourselves, are made of stardust.
P.S. The catalogue IDs for the micrometeorites in this post’s collage are, from top to bottom and left to right: NMM 976, NMM 3324, NMM 3177, NMM 2842, NMM 2000, NMM 2807, NMM 3655, NMM 338, NMM 3999.