The exploration of fresh micrometeorites from urban environments is still in its infancy. Despite examining these small treasures from space for six years, the status quo is still more questions than answers. Here is one new discovery, presented for the first time here at Project Stardust.
Last week, while we were photographing the glass (V-type) micrometeorite NMM 3369 (Photos 1 and 2), Jan and I noticed a phenomenon we had seen before: Rainbow colored Newton’s rings. These “rings”, which often have an irregular shape, occur frequently on fresh micrometeorites. This feature is not referred to in the scientific literature and, honestly, the first hundred times I observed this fascinating shimmer of colorful oxidation (Photo 3) I wondered if it was remnants of the dishwasher detergent I sometimes use when I rinse field samples.
NMM 3369, Photos 1 & 2
NMM 3999, Photo 3
Consequently, I have experimented with different types of detergent, and none at all. But the small rainbows still occurred, so it was obviously a real phenomenon on the micrometeorites. The enigmatic colorful oxidation has been discussed among stardust hunters, but none of us had any idea about what caused it. Jan Kihle, who is a professional mineralogist, once made a remark when we photographed a micrometeorite with extraordinarily strong oxidation around the metal bead, that “it is related to temperature.” But in which way?
This week, we observed small colorful newton rings on the highly vesicular glass micrometeorite NMM 3369, and this time we noticed that the newton rings are located exactly at the thinnest spot of the glass on the four vesicles that are still sealed. We also observed that a barely punctuated vesicle (top left in Photo 1) still has remnants of the newton rings around the edge!
This is the missing piece in the puzzle, indicating that the newton rings are indeed related to temperature, as Jan predicted. When a gas vesicle occurs inside a molten micrometeoroid due to expansion and degassing of the volatiles during atmospheric entry flash heating, the gas vesicles serve as small “parachutes”. This is splendidly explained by Matthew Genge in his 2017 research letter on the topic.
When the gas vesicles get near the surface, the thin glass experiences a relative lowering of the temperature due to inferior heat conductivity in the gas beneath compared with the exterior glass. This allows oxidation to take place locally on the surface. In the case of the many gas vesicles in micrometeorite NMM 3369 (Photos 1 and 2) this has formed perfect newton rings above each of the still sealed vesicles. As soon as this Eureka moment sunk in, I went through some older photos of similar rainbow-colored rings, like the brown micrometeorite NMM 2840 (Photo 4), and noticed that there is indeed a large gas vesicle right beneath the Newton’s ring. I am proud to present this new discovery here for the first time.
NMM 2840, Photo 4
What then, about other colorful oxidation that occurs without cooling gas vesicles beneath? One such example is the crackelated glass (V-type) micrometeorite NMM 1930 (Photo 5), where the entire surface of the spherule is slightly rainbow colored due to the large void inside. Or the frequently occurring oxidation that I often see on micrometeorites that have metal beads in the front, like the umbrella shaped NMM 3854 (Photo 6). It has oxidation with a rare pink appearance on the metal bead. And what about the small oxidation areas that are sprinkled over most micrometeorites?
NMM 1930, Photo 5
NMM 3854, Photo 6
Yesterday Jan and I photographed a new micrometeorite, NMM 4000, which not only marked my transition into a new millennium in my collection but also features beautiful colorful oxidation on several of the clear translucent olivine crystal faces. Why?
It can be explained with common crystallography. During crystallization, the olivine crystals transmit heat to the surrounding glass matrix and thus suffer a local cooling effect. As a result, oxidation can take place on the coolest parts of the surface area, like on the large rectangular crystal face in Photo 7.
NMM 4000, Photo 7
We will continue to explore these phenomena and publish everything here at Project Stardust. Hope you will enjoy it.