Since the discovery of the Antarctic micrometeorites, scanning electron microscope (SEM) images have documented how nickel-iron beads form by elemental differentiation in the core of a cosmic particle during atmospheric entry. These valuable images also show how a dense metal bead may migrate forward in the direction of speed by inertia during the subsequent deceleration. In most cases the nickel-iron bead is held back by surface tension during solidification and ends its journey visible on the surface as a beautiful metal pearl at the front. Or in the case of rapid spin during atmospheric entry, divided and migrated by centrifugal force out to the extremities.
Alternatively, the G-force during atmospheric deceleration is enough for the metal bead to escape the micrometeoroid entirely and leave a crater-like hole on the surface. These holes appear frequently, especially on barred olivine (BO-type) micrometeorites. And most often they are easy to distinguish from holes that formed due to escaping volatiles, known as gas vesicles.
Micrometeorite NMM 4030 is a small cryptocrystalline (CC-type) “turtleback” variety measuring a very slight 0.15 mm, which was found in the rain gutter of the local children’s school in Denmark last month. In Photo 1 we can see the beautiful aerodynamic symmetry of the little space rock. The front is up, and if we look closely at Photo 2, we will see a small “crater” from where a metal bead has fallen off. This probably happened during formation, at the very last moment before solidification. So somewhere in the neighborhood I suppose there is a loose ten micron nickel-iron bead that is stirred up in the road dust, stuck in a gutter, or resting on a rooftop. If the metal pearl had remained attached to the stone, it would have been similar to the micrometeorite in Photo 3.
PHOTO 1, NMM 4030
PHOTO 2, Detailed View of NMM 4030
PHOTO 3, NMM 244
In rare cases, however, we can observe a middle-stage between micrometeorites with a metal bead on the surface, and the ones with a hole where a bead has escaped. Here the metal bead has almost escaped the particle but has reached solidus and was held back just in the nick of time. We can see in the color photo by Jan Kihle and I that the bead appears to be “glued” to the stone by iron sulfide. If solidification had occurred just one millisecond later, the micrometeorite would have lost its nickel-iron bead.
A collection of these rarities is shown in the collage at the top of this post. This is, to my knowledge, the first presentation of this phenomenon, and we can see that the micrometeorites presented have several things in common. For instance, the placement of the almost loose metal bead slightly off the symmetrical length axis.
I would like to extend my special thanks to the four other micrometeorite researchers who let me include their rare findings together with my own in the collage presented as the featured image of this blog entry: Jon Wallace (top left), Thilo Hasse (top right), Jesus Cejas (bottom left), Dusty Segretto (middle center), and Scott Peterson (middle left, middle right, bottom center, and bottom right). The age of stardust is now, as always.