Micrometeorites are small particles of cosmic dust that land on the Earth’s surface. Until 2015, when yours truly, Jon Larsen, discovered the world’s first urban micrometeorite, it was generally accepted that these tiny specks of stardust could only be found in extremely clean and remote areas. For decades, scientists studied micrometeorites from Antarctica and made many remarkable discoveries. In this article I am going to discuss the size and mass distributions of micrometeorites and explain why they are so limited.
The information I impart in this article is based on my personal findings and data from academic sources. In addition to the papers referenced below, I also draw from a lecture given by Dr. Matthew Genge in February 2015 and Dr. Michel Maurette’s legendary book, “Micrometeorites and the Mysteries of Our Origins”. I am deeply grateful for their support over the years and encourage you to explore their publications.
Now, let’s get to it!
How big are micrometeorites?
Most micrometeorites have a size of approximately 300 microns (µm) or 0.3 mm. However, there are both smaller and larger ones.
On one end of the spectrum are particles measuring under 50 µm (0.05 mm) — approximately the thickness of a human hair! As you might imagine, these micrometeorites are extremely difficult to identify under a light microscope. In fact, their delicate features and surface characteristics are difficult to analyze even with powerful technology. Consequently, in my own collection, I concentrate on spherules larger than 150 µm (0.15 mm).
At the other end of the spectrum, a micrometeorite larger than 500 µm (0.5 mm) is considered to be a giant, and when it approaches twice that size, a supergiant. Though I have collected thousands of micrometeorites, I have only collected a handful of giants and supergiants, as both are exceptionally rare.
Note: All of the supergiants I have found are included in the full version of the collage featured in this post!
Perhaps you may be wondering, “Why are supergiants so rare?”
The Mysterious Size Gap
The illustration below is from my book, “In Search of Stardust”, and depicts the total cosmic influx (amount of cosmic dust entering into the Earth’s atmosphere) vs the size of meteoritic material.
There is a distinct peak between 0.2-0.4 mm, then the mass distribution quickly decreases to zero between 2-10 mm. Since particles measuring less than 10 mm (1 cm) are considered micrometeorites, it is clear that these tiny particles make up the vast majority of meteoritic mass that comes to Earth. But, why?
With larger mass and kinetic energy, most particles measuring between 2-10 mm (0.2-1 cm) burn up in the atmosphere, leaving behind only nano-sized meteoritic smoke particles. Objects measuring around 1 cm and upwards to a few meters are considered meteorites, while larger objects, which are thankfully exceptionally rare, are asteroids.
How much stardust is there really?
So, now that we’ve discussed the size gap, one critical piece remains: How much stardust falls to Earth each day?
The exact mass of the cosmic influx is not known, but according to a recent publication by Rojas et al. (2021), the total amount of micrometeorites per year is at least one thousand times larger than meteorites. This adds up to somewhere between 5000—36000 metric tons of cosmic dust per year, or somewhere between 14—100 tons per day.
According to Genge (2008) the rule of thumb for micrometeorites is approximately one object with a diameter of 0.1 mm per square meter per year. However, the average cosmic spherule has a diameter of approximately 0.3 mm and may contain up to 27 times more mass than one at 0.1 mm. Consequently, in my hunt for micrometeorites in populated areas, on a roof of 50 square meters, I expect to find two, and not fifty cosmic spherules per year.
If you have questions about this or how to find your own stardust please leave me a comment on Facebook, Instagram, or Twitter. If you’d like to hang stardust photos on your wall, please join the Project Stardust VIP List!
P.S. Here’s the full version of the collage! Enjoy!
Genge, M. J. (2007). Micrometeorites and Their Implications for Meteors. Earth, Moon, and Planets, 102(1-4), 525–535. https://doi.org/10.1007/s11038-007-9185-z
Rojas, J., Duprat, J., Engrand, C., Dartois, E., Delauche, L., Godard, M., Gounelle, M., Carrillo-Sánchez, J. D., Pokorný, P., & Plane, J. M. C. (2021). The micrometeorite flux at Dome C (Antarctica), monitoring the accretion of extraterrestrial dust on Earth. Earth and Planetary Science Letters, 560(116794), 116794. https://doi.org/10.1016/j.epsl.2021.116794