Every day, the Earth runs into hundreds of pounds of space rock, much of it as tiny as a grain of sand. On any night, in a dark location, you can look up and see bright streaks in the sky, meteors often originating from tiny specks. While a minuscule fraction of the rock that creates these brilliant flashes makes it to Earth’s surface, some can be recovered. We usually picture meteorites as large, exotic looking rocks, but often it takes the form of little bits of rock which is friction melted by Earth’s atmosphere into droplets which cool into tiny spheres during their fall. When these spheres make it to the ground, they are called micrometeorites, specifically, micrometeoritic spherules. There are also unmelted or partially melted micrometeorites, but it is easier to identify the melted ones by their spherical shape, surface texture, and composition.

In numerous lesson plans, websites, and books there are claims that a casual amateur can discover micrometeorite spherules on rooftops or even in their backyard. What a wonderful way to feel connected to the universe, and learn about the flux, composition, and formation of these objects! Unfortunately, these resources tend to be unrealistically optimistic about the success of such a search. I hate to be the harbinger of bad news, but love to be the realist.

I don’t doubt it is possible for amateurs to find micrometeorites in the environment. Members of the public have found objects confirmed as micrometeorites by scientists using the type of advanced analysis we undertook, so it is possible. What wasn’t clear is if this is a viable activity for the public or students and educators to undertake with the expectation of success or the ability to accurately assess success. What me and my students, Adele Antalek, Audrey Lew, Stivaly Paulino, Franky Telles, and Namshik Yoon, discovered was that a casual citizen scientist should not expect to find micrometeorites from urban rooftop searches by following widely publicized procedures, and that it is difficult to impossible to determine if you’ve found a micrometeorite without access to advanced instrumentation for chemical analysis.

The students were participants in the summer research course, the “research practicum,” as a part of the Master of Arts in Teaching program at the American Museum of Natural History (AMNH). After the program they went on to become earth science teachers, and they come into the program with undergraduate and sometimes graduate degrees in geology or related fields. During the 5 week course we undertook a detailed and painstaking search for micrometeorites from debris collected from rooftops.

I knew this might be a difficult project, and that we might not find any micrometeorites (spoiler alert: we didn’t), but I still thought it worthwhile for several reasons. One, if we did find them, micrometeorites provide information about incoming material from space. They could help us understand the extraterrestrial geochemical flux and can reveal information about both the early solar system as well as the conditions experienced upon atmospheric entry. Because they presumably would have fallen since the roof was built or since it was last cleaned or surfaced, they provide a recent sample to compare to antarctic or sea bed micrometeorites, which can be tens of thousands of years old.

Another motivation for the project is the potential for this as a citizen science and or classroom project. If micrometeorites can be easily found, especially in urban environments, then this is an exciting activity teachers can undertake with their students. Recently a step-by-step guide to amateur micrometeorite searching was published, called On the Trail of Stardust, by jazz musician and micrometeorite enthusiast Jon Larsen, which has greatly expanded interest in micrometeorite searches. We followed the general procedure outlined there, with the addition of further analysis.

My background is in astronomy, not geology, and the last time I took a geology focused course was my high school earth science class. This was a fun opportunity for me to learn from my students, whose backgrounds often involved geological field research. The project also made excellent use of the instrumentation available at AMNH. Most of my astronomy research leans heavily toward data analysis and image processing using Python, but with participants coming in with varying levels of coding expertise and a very limited time I thought it would be better to play into my student’s prior experience. As an astronomer, I never got to “touch my science”, and this was an opportunity to literally get dirty, as well as use an scanning electron microscope, which was as satisfying as I had imagined it would be.

We took debris swept from two rooftops, washed and sanitized it, separated the particles into several size ranges between 150 and 425 microns, and separated out the magnetic particles using strong magnets. Using a dissecting or picking microscope, we identified and isolated spherical particles, and then used their morphology to select a subset of the spherules for SEM imaging and chemical characterization.

We identified 3,445 spherules under the optical microscope, and imaged and chemically characterized 290 of those with the scanning electron microscope. The vast majority were dominated by iron, with many containing trace elements which indicated they were human pollution. There is a small possibility that contained in the iron-dominated spherules there were “I-type” (iron dominated, usually in the form of wüstite) micrometeorites, but unfortunately there is very little possibility of being able to confidently confirm their extraterrestrial origin because they are so similar in characteristics to anthropogenic spherules. Instead we focused our search on silicon-containing micrometeorites which are either dominated by silicates (S-type) or of mixed silicate/iron composition (G-type). Our chemical analysis showed very few spherules contained significant amounts of silicon, and none of them had a similar mix of elements to confirmed sea-bed or Antarctic micrometeorites (see the tertiary diagram of our spherules compared to the tertiary diagram of Genge 16, for example). In other words, it’s very unlikely that any of our spherules were of extraterrestrial origin.

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All we can say is that on the roofs we had access to, with great effort on the part of the students, and with access to specialized tools, we weren’t able to find a micrometeorite. Amateur micrometeorite hunters have been successful at finding confirmed micrometeorites, so this is not to say it can’t be done, but I think it’s important to have realistic expectations for success if you undertake such a search.

I also would like to debunk misleading lesson plans on the subject, like this one, that imply that an non-negligible fraction of magnetic spherules students discover are micrometeorites. I think a student could learn a lot from a similar lesson if also presented with information that human pollution is ubiquitous and almost all spherules they find are likely to be anthropogenic in origin. Something that would be quite interesting but out of the scope of this project would be to identify the sources of these spherules. Some possibilities include road dust which is thought to arise from vehicle breaking, welding byproducts, firework debris, and fly ash from coal-fired electric or steam generating plants.

I might be willing to try a micrometeorite search again, but only if I could secure access to an absolutely ideal roof: huge, old, plastic lined, uncleaned for a long period of time, and with raised edges. Unfortunately my experience was that getting access is to such a roof is difficult to impossible in NYC. I spent hours and hours on the phone with corporate offices of building supply companies and warehouses trying to get access to their big, beautiful, disgustingly dirty roofs, but in the end almost everyone said no. Because I wanted the students to be involved in collecting we were further limited to the greater New York City area. It’s possible the search could be more fruitful in less urban environments.