Dust Particles From the Edge of the Solar System on TU Berlin Roof

Origin of micrometeorites determined for the first time using complex computer simulations and experiments

Citizen scientist identifies micrometeorite

"We asked Scott to take a look at our samples because he really has the sharpest eye for identifying micrometeorites under a microscope," says Dr. Jenny Feige, who conducts research into cosmic dust funded by a European Research Council Starting Grant. She previously worked at the Zentrum für Astronomie und Astrophysik (ZAA) at TU Berlin before moving to the Museum für Naturkunde Berlin, where further projects exploring micrometeorites are carried out in collaboration with citizen scientists. Prior to Scott Peterson's analysis, researchers from TU Berlin had climbed onto the roof of the Physics Building with the telescope dome to sweep up debris from the corners. "Everything we collect is soaked in water to get rid of tiny leaves and similar dirt. The sediment is then heated to 600 degrees to eliminate all microbes and other organic material. The material is then sifted, and the search for micrometeorites begins," says Feige.

Noses and tortoise shells

The sample contained innumerable spheres between 100 and 500 micrometers in size, the vast majority of which were declared "anthropogenic contaminants" – i.e. originating from human activities such as welding, fireworks, or simply metal abrasion from road traffic. In the very last portion of the sample, Scott Peterson did indeed find two micrometeorites, each of which could be categorized into a specific class based on their characteristic structures. These structures are formed when cosmic dust particles hurtle into the Earth's atmosphere and are subsequently decelerated and heated by friction with the air particles until they melt. After shedding an average of 90% of their mass, the rest of the particle crystallizes as it cools, depending on the angle and speed at which it enters the atmosphere, its composition, and the ambient conditions.

Due to the specific crystallization process it underwent, one micrometeorite bears a visual resemblance to a tortoise shell. Meanwhile in the other, the nickel and iron had separated from the rest of the elements as it melted, only to solidify as an extra globule on the micrometeorite as it cooled. "From this micrometeorite's 'nose' we can even determine how it entered the atmosphere, namely globule first," says Feige.

Micrometeorites can tell us about the conditions in the Solar System

"It is still a tremendous challenge for science to determine where micrometeorites found on Earth come from," says Dr. Beate Patzer, theoretical astrophysicist from the ZAA at TU Berlin. "But it is valuable information, because micrometeorites can come from very different parts of our Solar System with very different conditions. The Earth catches around 100 tons of mostly interplanetary dust per day. Micrometeorites are much more abundant than larger meteorites, so we could use them to collect far more data and learn a lot about our Solar System in the process."

Flight time to Earth

One method used to determine the origin of a micrometeorite is to analyze long-lived, radioactive isotopes that have formed on it during its journey through space from exposure to the cosmic radiation that is ubiquitous in outer space. "It is possible to determine the flight time the extraterrestrial dust particles had to reach Earth – and thus where in the Solar System they came from – based on the ratio of different isotopes with various half-lives and a physical model that describes the formation of these isotopes," says Patzer.

First time using computer simulations for analysis

"As a first in the field, we have created a complex computer simulation for this analysis that factors in possible orbits of the interplanetary dust particles, the size of the dust grains, their composition and density, radiation profiles from the Sun and cosmic rays from interstellar space, evaporation rates as they enter the Earth's atmosphere, and a host of other parameters," says Jenny Feige. The researchers focused on the radioactive isotopes aluminum-26 and beryllium-10.

To measure the very small amounts of isotopes in the tiny micrometeorites, the research team worked with the VERA particle accelerator facility in Vienna. The "accelerator mass spectrometry" performed there sorts the chemical elements not only according to their mass, but also according to the number of protons in their nucleus. This makes it possible to clearly identify the isotopes.

Tortoises from the far reaches of the solar system

The concentrations of aluminum-26 and beryllium-10 in the micrometeorites were then compared with the results of the computer simulation, which predicts the accumulation of these radioisotopes in the micrometeorites depending on the time of flight, and thus the place of origin in space. The origin of six micrometeorites collected at other sites remained unclear; however, six other micrometeorites could be assigned to an original site with a high degree of certainty, including the two found on the roof of TU Berlin: The micrometeorite with the tortoise shell pattern comes from the Outer Solar System and could have separated from comets passing by Jupiter or from rocky material in the Kuiper belt – at a distance of about 40 times the distance between the Earth and the Sun. Meanwhile the micrometeorite with the "nose" comes from the Inner Solar System, from near-Earth objects or even objects as far away as the asteroid belt between Mars and Jupiter.

"With this result, we were able to demonstrate the fundamental suitability of our method," says Jenny Feige. Going forward, the method will make it possible for us to learn even more about the cosmos through micrometeorites. "The micrometeorites on the roofs of our homes are particularly valuable, because we can pinpoint their time on earth very precisely: They cannot be older than the roof itself. For meteorites found on the ocean floor or in Antarctica, there's no way to prove that they haven't been there for millions of years, making the results more uncertain."

Publikation: J. Feige et al., "Transport of dust across the Solar System: Constraints on the spatial origin of individual micrometeorites from cosmic-ray exposure," Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, vol. 382, no. 2273, Jun. 2024. https://doi.org/10.1098/rsta.2023.0197