Urban micrometeorites: A myth?
Jon Larsen (Norway)
Is it possible to find micrometeorites in populated areas? The question has been raised for nearly a century and, despite numerous attempts to find them, the answer up to this day has been a very short “no”. Meanwhile, our knowledge about these amazing stones has gradually increased. There is a continuous evolutionary line in the research on micrometeorites, from the early pioneers, John Murray and Adolf Erik Nordenskiöld in the nineteenth century, to Lucien Rudaux and Harvey H Nininger. With Donald E Brownlee and Michel Maurette in the 1960s, micrometeoritics became real science.
During the past two decades, this research has accelerated thanks to, among others, Susan Taylor, who extracted micrometeorites from the water well at the South Pole, Matthew Genge, who figured out the classification, and other splendid researchers, in addition to the space probes that have returned to Earth with dust samples from comets and asteroids. Today, there is a growing literature about micrometeorites, but still the answer to the initial question is “no” and urban micrometeorites have been considered an urban myth.
Micrometeorites have been found in the Antarctic, but also, to some extent, in prehistoric sediments, remote deserts and in glaciers – places that are clear of the confusing anthropogenic influence. The wall of contamination has been considered insurmountable. It is therefore with pride and joy that I can report here about a project involving the systematic examination of all sorts of anthropogenic and naturally occurring spherules in an empirical search for micrometeorites in populated areas. This research has resulted in a new urban collection of pristine cosmic spherules.
The findings have been analysed at several different institutions and, in January 2017, a randomly selected subset of 47 objects from the new collection was prepared for wide beam electron microprobe analysis at the Natural History Museum, London by Dr Matthew Genge (of Imperial College). Nine porphyritic olivine, 23 barred olivine and 15 cryptocrystalline spherules were identified and have textures and mineral compositions identical to Antarctic cosmic spherules. A scientific paper about these new discoveries (Genge et al.) is pending publication, but meanwhile, I can present the results here in Deposits.
The project was initiated in 2009 with a minimum of equipment: a magnet, plastic bags, a sieve and a microscope. To begin with, I sampled accumulated mineral particles from skywards facing hard surfaces like roads, roofs and parking lots in Oslo, and then graduated to look in industrial areas, other cities, countries, mountains, soil, glaciers, beach sand, volcanoes and deserts – that is, everywhere. Now, seven years later, I can look back on nearly with one thousand field searches of about 50 to 5,000µm size particles from nearly 50 countries, all continents represented.
The samples were examined in a Zeiss binocular microscope, and interesting particles picked out and photographed with a USB microscope with higher magnification. Promising candidates were analysed using SEM/EDS. I established a photo database (now containing photos of more than 40,000 individual objects) and kept an illustrated journal while I tried to look for patterns. Due to consistently contradictory data in the literature, I put my complete trust in pure empiricism.
To begin with, the various types of anthropogenic and naturally occurring terrestrial spherules seemed infinite and chaotic, but with time, I started to recognise the most common ones. Gradually, I could start the process of systematisation. There are surprisingly small variations in the types of spherules found in comparable environments around the globe. My recently published book, In Search of Stardust, is an atlas of the various types of spherules, and the approximately 30 most common types that represent most of all spherules found anywhere on Earth. In our search for micrometeorites, the knowledge of these contaminants makes it possible for the first time to separate the extra-terrestrial particles from the terrestrial ones. The most recent field searches with improved methodology for processing the samples before the microscopy have given up to one micrometeorite per gram, which is a near match of the Antarctic results.
Cosmic dust belongs to the oldest matter there is: mineral remnants from before the planets were formed. They may even contain real stardust – interstellar particles older than the Sun, that is, particles which have travelled further than anything else on Earth. There is a widespread misconception that micrometeorites are fragments of ordinary meteorites, ablated during their atmospheric flight, but these ablation spherules are not real micrometeorites in the scientific sense.
Furthermore, there is a common misunderstanding that micrometeorites are “metal spheres”, but that is only about 2% of them. Most of the cosmic spherules are stony, mainly olivine/orthopyroxene in glass with interstitial magnetite. Their next of kin are the primitive C-chondrites and their origin may lie beyond Pluto. We are just beginning to explore these alien stones, yet they are everywhere around us.
The breakthrough in the search for micrometeorites in populated areas came at last in February 2015, with the discovery of an approximately 0.3mm barred olivine beauty with dendritic magnetite crystals sprinkled over the surface. I started immediately to search for similar stones and found them. At the end of the first season, I had a collection of more than 500 pristine micrometeorites in the size range of between 150 to 600µm, with all the most common types from the classification represented.
For many years, meteorite hunters have built micrometeorite traps of various types. Some have succeeded, like water pools to catch interplanetary dust particles (IDPs), but, given the low influx rate, a really efficient trap would have to be much larger. To catch thousands of cosmic spherules, the trap would have to be the size of a football field (or larger) and accumulate particles over decades. The challenges connected with the construction of something like that have discouraged more than one good scientist. However, there are such areas already in place, possibly in your neighbourhood, and ripe for harvesting: roofs.
The micrometeorites in the new collection were mainly found on the roofs of buildings with a maximum of 50 years of age, so it can be assumed that the stones have a terrestrial age of 0 to 50 years, which make them fresh compared with most of the micrometeorites in the other collections. As a result, some of the surface structures of the micrometeorites in the new collection are different from previous observations, with the glass still intact. With the exception of the Concordia collection from melted snow, most of the Antarctic micrometeorites have a terrestrial age of one thousand to one million years and are weathered accordingly.
By monitoring a skyward facing area like a roof at regular time intervals, it should be possible to be even more precise in future sampling, perhaps down to the week (or even day) that the micrometeorite fell to Earth. With careful preparation (that is, cleaning the collecting area) around the annually reoccurring meteor showers, it should be possible to identify material from some of the comets and possibly also to detect variations in the influx rate over time.
Without knowing what micrometeorites really look like, it would not have been possible to find them, and it is a pleasure also to present here for the very first time micrometeorites in high resolution colour photography. This has become possible thanks to new micro photographic techniques developed especially for this project in co-operation with Dr Jan Braly Kihle. The photo rack was created using a modified Olympus camera together with prototypes and newly invented components (both hardware and software). However, study of the morphological details and textures in high resolution is crucial to understand what to search for in the field samples.
Verifying micrometeorites
One of the returning questions about micrometeorites is how they can be verified. The short answer is: as long as they are chondritic and have the right textures. The definitive evidence for the extra-terrestrial origin of micrometeorites came more than 25 years ago on the basis of noble gas measurements and analysis of cosmogenic nuclei. All particles exposed to the high energy cosmic radiation outside Earth’s magnetosphere are altered, and these changes in the atomic structure can be measured in mass spectrometric analysis.
There are also a number of non-isotopic criteria for a positive identification of micrometeorites. First of all, most micrometeorites have a chondritic bulk composition for major and minor elements (at least for particles with a small grain-size relative to particle size), which is easy to check in an EDS analysis. Secondly, the presence of nickel bearing metal in a spherule may suggest an extra-terrestrial origin. However, lack of nickel does not exclude the possibility. It may not be present or the heavier elements may have sunk into a core inside the micrometeorite and is not detectable on the surface. The third main criterion is the presence of a partial or complete rim of magnetite around the micrometeorite. In addition to these three criteria, there are supporting but less definitive features, like high CaO, Cr2O3 olivines and very FeO-poor olivines, which are exceedingly rare in terrestrial rocks.
Until now, little attention has been given to the morphology of micrometeorites. In most reference publications, the stones have been moulded in resin and, therefore, we are presented with black and white SEM section images for study, identification and classification. However, these are poor representations of what the micrometeorites actually look like under the microscope, and the lack of morphological documentation has for a long time caused confusion in the search for micrometeorites in populated areas. Because of the rarity of the micrometeorites up until now, very few meteorite researchers have had access to study real micrometeorites, which is why the precision level about micrometeorites in general is accordingly low outside academic circles.
It is my hope that future studies will include documentation of the micrometeorite morphology in the publications and that easy access to these incredible space rocks will result in a new branch of popular meteoritics. When a micrometeoroid enters the Earth’s atmosphere at a steep angle, it goes through a rapid and unique transformation through melting, differentiation, quenching, recrystallisation and ablation. The result is a stone different from everything else down on Earth. The morphology and the surface textures are characteristic and significant: barred or porphyritic olivine; “turtleback” topography; aerodynamic forms; dendritic magnetite crystals on the surface; a partial magnetite rim; strategically placed metal beads; and so on. In most cases and with experience, a visual identification of a cosmic spherule is unproblematic. When in doubt, a chemical analysis is always recommended.
Origin, formation, influx and classification
There are almost as many explanations as to where the micrometeorites have their origin as there are researchers in the field. Depending on who you ask, the answer may vary from the asteroid belt between Mars and Jupiter, comet related objects in the Kuiper belt or Oort cloud, various planetary ejecta and interstellar matter, to name but a few. It is estimated that up to 0.1% of the matter in primitive meteorites and micrometeorites are presolar grains. On the other hand, there are achondritic (igneous) micrometeorites from differentiated bodies like the Moon and Vesta.
Throughout history, large asteroid impacts on the rocky planets and their moons have ejected substantial quantities of rocks into space, and it is possible to imagine an extensive exchange of matter between all the planetary bodies and their surrounding dust rings, with the zodiacal cloud as a temporary storage pool. Hopefully, access to a new, potentially large and renewable source of micrometeorites in populated areas may contribute to a systematic mapping of the isotopic variations of a substantial number of micrometeorites in the years to come, with more data about the micrometeorites parent bodies as a result. It should not come as a surprise if the origins of the micrometeorites turn out to be a combination of all dust producing bodies in the solar system and beyond.
The micrometeoroids enter the Earth’s atmosphere with a speed of up to 50 times that of a rifle bullet. Depending on the entry angle relative to Earth’s rotation, the peak temperature from the frictional heat will cause a substantial variation in the alteration process. Approximately half of the micrometeoroids smaller than 0.1mm receive a soft deceleration and end on the ground as unmelted micrometeorites. The rest reach peak temperatures of between 1,350°C and greater than 2,000°C, which is enough to create the various types of melted cosmic spherules. There are transitional forms between the types, but the chemistry of the micrometeorites is surprisingly homogenous. From the morphology alone, we cannot yet reveal the parent body (origin) of the micrometeorites. A sphere is nature’s solution to maximum volume with the smallest possible surface and is created by the surface tension in a liquid state.
At the same time, a rapid differentiation takes place, in which the heavier elements (iron, nickel and so on) move inwards to form a core and volatile elements escape. Iron from the stone reacts with oxygen in the atmosphere and creates dendritic magnetite, looking like small Christmas trees on the surface. Still in flight but decelerating, the inertia of the heavy core may push it forwards in the direction of travel, often spinning. The whole formation is over in the blink of an eye before the micrometeoroids fall to Earth at terminal velocity.
Based on radar measurements, the general influx rate of micrometeorites is estimated to approximately one object with a diameter 0.1mm for each square meter every year. This does not sound like much, but adds up to about 100 metric tons a day of mainly micrometeorites like the ones in the photos. The size distribution of the cosmic spherules has a distinct peak of about 0.3mm, and an object this large contains 27 times more mass than an object with a diameter 0.1mm as described in the influx rate. Consequently, on a roof of 50m2, we cannot expect to find 50 new micrometeorites every year, but rather a statistical possibility of two average cosmic spherules is possible. Future research may add varieties to the present classification, and with more hands and eyes in the field, micrometeoritics can evolve into an exciting new branch of the popular study of space rocks.
About the author
Jon Larsen (1959- ) Norwegian geologist who has studied micrometeorites (MMs) and cosmic dust particles, and published books and articles about these. In 2016 he found a method to retrieve MMs from populated areas, whis had been considered impossible by NASA and other scientific researchers, the discovery was published in Geology (http://geology.gsapubs.org/content/early/2016/12/05/G38352.1.abstract). During an expedition to search for the first Americal urban micrometeorite (Feb 2017) he found a collection of MMs on the roofs of NASA’s Stardust loboratories. Larsen is afiliated to the University of Oslo (UiO). This article first appeared in Spacerock Magazine: spacerocksmagazine@gmail.com.
Further reading
The new collection of micrometeorites on Facebook (http://www.facebook.com/micrometeorites) In Search of Star Dust: Amazing Micro-Meteorites and Their Terrestrial Imposters by Jon Larsen. Published by Voyageur Press, Minneapolis (2017). ISBN: 978-0760352649.