Micrometeorites from Rooftops of Physics Buildings - Origin and Cosmic Ray Exposure

Each year, about 30,000 tons of extraterrestrial solid material falls onto the Earth's surface. A non-negligible fraction of these materials are micrometeorites (MMs); small (1 µm up to 1 mm), mostly spherical, particles that predominantly originate from the asteroid belt between Mars and Jupiter.

As soon as MMs are released in space, they are affected by the radiation pressure of the Sun causing them to spiral inwards. While travelling towards the Sun, the particles are irradiated by solar and galactic cosmic rays, producing cosmogenic nuclides such as ¹⁰Be (t_1/2=1.4 Myr) and ²⁶Al (t_1/2=0.7 Myr).

In urban areas MMs are accumulating on the rooftops of buildings [1]. They can be easily collected from roofs of high buildings, where anthropogenic influences like pollution (e.g. from traffic) are reduced.

We have taken samples using a simple magnet from the rooftops of three physics buildings of the northern and southern hemisphere: the physics building of the Berlin Institute of Technology (~45 m above street level, Berlin, Germany), the accelerator buildings of the ANU in Canberra (44 m, Australia), and the accelerator building at HZDR, Dresden, Germany (24 m).

The magnetic fraction is investigated non-destructively by SEM-EDX to allow for enrichment of potential magnetic MM particles (~50–200 μm). Proof of extraterrestrial origin will be achieved by the determination of minor elements such as Ni from the interior of the spherules after cutting. We will then perform a radiochemical separation from individual samples and measure ¹⁰Be and ²⁶Al by AMS at the DREAMS facility (HZDR [2]).

The experimental concentrations of those radionuclides will be compared with theoretical production rates [3] to deduce exposure ages and the potential origin of the samples.

[1] Genge et al., Geol., doi:10.1130/G38352.1 (2016)
[2] Rugel et al., NIMB, 370, 94 (2016)
[3] Trappitsch & Leya, Meteorit. Planet. Sci., 48, 195 (2013)