26Al and 10Be in urban and Antarctic micrometeorites
Dr Jenny Feige1, Alessandro Airo1,2, Dirk Berger1, Dennis Brückner3, Matthew Genge4, Ingo Leya5, Fatemeh Habibi Marekani1, Niko Klingner6, Johannes Lachner6,7, Jörg Nissen1, Beate Patzer1, Nicolas Schley1, Andreas Schropp3, Scott Peterson8, Christof Sager1, Martin Suttle9, Reto Trappitsch10, Joachim Weinhold1
1Technische Universität Berlin, Berlin, Germany, 2Museum für Naturkunde, Berlin, Germany, 3Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany, 4Imperial College, London, United Kingdom, 5Universität Bern, Bern, Switzerland, 6Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany, 7Universität Wien, Vienna, Austria, 8University of Minnesota, Minneapolis, United States of America, 9Natural History Museum, London, United Kingdom, 10Brandeis University, Waltham, United States of America
Roughly 100 tons of extraterrestrial material released from asteroid collision events or cometary sublimation enter the Earth’s atmosphere each day. Part of this material reaches the Earth’s surface as micrometeorites (MMs) – mostly submillimetre-sized spherical melted droplets. For more than a century MMs were collected only in remote environments such as deep-sea sediments or Antarctic firn and ice. However, since 2017 significant numbers of MMs have been recovered from urban areas, particularly the rooftops of buildings. In contrast to MMs originating from slow-accumulating environments that can have terrestrial ages of millions of years, cosmic spherules recovered from rooftops are not older than the buildings themselves and are therefore the youngest MMs ever collected.
The study of the irradiation histories of MMs provides an important step towards identifying the nature and origin of their parent bodies. During their million-year-long space journey on spiral trajectories to Earth, these small interplanetary particles are exposed to cosmic radiation producing long-lived radionuclides such as ²⁶Al and ¹⁰Be. Since the number of cosmogenic nuclides increases with the time the MMs reside in interplanetary space, it is possible to estimate from which heliocentric distance in the Solar System they originated. However, the very small amounts of a few million atoms of the radionuclides within a MM decrease after deposition on Earth, i.e., with increasing terrestrial age. Hence, urban MMs, with insignificant terrestrial ages, provide for the first time the opportunity to measure the highest possible concentrations of long-lived radionuclides within MMs.
We analyzed the ²⁶Al and ¹⁰Be content of six urban MMs and, for comparison, six Antarctic MMs (which have terrestrial ages up to 780 kyr). These MMs with sizes of 90-500 µm were dissolved and, after stable carrier addition, ²⁶Al and ¹⁰Be were chemically extracted and measured by accelerator mass spectrometry (AMS) at the Vienna Environmental Research Accelerator (VERA), Austria. The data was compared to results from numerical simulations calculating ²⁶Al and ¹⁰Be concentrations in micrometeoroids using various orbital parameters, compositions, and irradiation profiles.
The ²⁶Al/²⁷Al and ¹⁰Be/⁹Be measurement results were significantly above the chemistry blank values, except for the smallest (90 µm) Antarctic MM. Conversion to ²⁶Al and ¹⁰Be concentrations yield values between 10⁴ and 10⁷ atoms per sample. Based on the comparison of the ²⁶Al and ¹⁰Be concentrations with the theoretical data we generally favour carbonaceous chondrite objects as the parent bodies of the MMs orbiting with several eccentricities within our Solar System.
Our results are influenced by the following assumptions: no pre-irradiation within the parent body, no mass loss during atmospheric entry, average carbonaceous or ordinary chondrite composition, no significant terrestrial ages, and no uncertainty in the production rates for ²⁶Al and ¹⁰Be. Besides the use of additional methods such as mineralogical and isotope geochemical analysis, better statistics of long-lived radionuclides within MMs may help to constrain some of these assumptions.
2014: PhD of Physics, Vienna Environmental Research Accelerator (VERA), University of Vienna, Austria, Thesis title: “Supernova-produced radionuclides in Deep-Sea sediments Measured with AMS”.
Since 2015: Research scientist at the Department of Astronomy and Astrophysics at the TU Berlin, Germany.
Fields of research: Cosmic traces on Earth;
AMS measurements and modelling of long-lived radionuclides
– ejected from nearby supernovae during the past millions of years that are transported and incorporated into geological records,
– produced within interplanetary dust (originating e.g. from asteroid-asteroid collisions or cometary sublimation) that reaches the Earth’s surface as micrometeorites.