²⁶Al and ¹⁰Be in urban and Antarctic micrometeorites


²⁶Al and ¹⁰Be in urban and Antarctic micrometeorites

Feige, J.; Airo, A.; Berger, D.; Brückner, D.; Genge, M.; Laya, I.; Habibi Marekani, F.; Klingner, N.; Lachner, J.; Nissen, J.; Patzer, A. B. C.; Schley, N.; Schropp, A.; Peterson, S.; Sager, C.; Suttle, M.; Trappitsch, R.; Weinhold, J.

Roughly 100 tons of extraterrestrial material released from asteroid collision events or cometary sublimation enter the Earth’s atmosphere each day. Some of this material reaches the surface as micrometeorites (MMs) – mostly submillimetre spherical melting 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 were found in urban areas, particularly on rooftops of buildings. In contrast to MMs originating from slow-accumulating environments that can have been deposited millions of years ago, the particles from the rooftops are not older than the buildings and currently are the youngest extraterrestrial particles 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 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 travel through space it is possible to estimate from how far out in the Solar System they originated from. 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 six urban MMs and, for comparison with MMs that have terrestrial ages up to 780 kyr, six MMs collected from Antarctic Moraine sediments for their 26Al and 10Be content. The MMs with sizes of 90-500 µm were dissolved and, after stable carrier addition, 10Be an 26Al were chemically extracted and measured with AMS at the Vienna Environmental Research Accelerator (VERA), Vienna, Austria. These experimental data were compared to results from numerical simulations yielding 26Al and 10Be concentrations in micrometeoroids having 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 concentration yields values between 104 and 107 atoms per sample. Comparison with the theoretical data generally favours 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 the theoretical production rates for 26Al and 10Be are correct. 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.

Related publications

  • Lecture (Conference) (Online presentation)
    The 15th International Conference on Accelerator Mass Spectrometry (AMS-15), 15.-19.11.2021, Online, Online

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Publ.-Id: 33054