Description
Authors (affiliation): Z. Tymiński (POLATOM RC NCBJ, Poland), M. Hult (EC JRC, Belgium), A.M. Krzesińska (CEED UO, Norway), K. Pelczar (EC JRC, Belgium), P. Saganowski (POLATOM RC NCBJ, Poland), G. Lutter (DEE TUD, Denmark), G. Marissens (EC JRC, Belgium), H. Stroh (EC JRC, Belgium), R. Broda (EC JRC, Belgium), K. Tymińska (POLATOM RC NCBJ, Poland), T. Ziemek (POLATOM RC NCBJ, Poland), T. Kubalczak
Asteroids, 10-20 meters in size, enter the Earth atmosphere on relatively regular basis posing an impact threat to the society. The bodies travel typically with hypersonic velocity in excess of 10 km/s and this induces a pressure which usually exceeds the body's strength, causing its disruption and disintegration. Such disintegration significantly reduces potential damage that the impact could cause. Therefore, understanding the sizes of entering bodies, their strength and fragmentation has major implications for assessing the real impact hazard. For this, radionuclide studies are helpful. Before entering the Earth's atmosphere, asteroids orbit in space as unshielded rocks, exposed to solar wind (SW) particles and galactic cosmic rays (GCR). This leads to production of radionuclides, dependent on the size of meteoroid/asteroid and on the depth (shielding) within the body. To understand the dynamics of atmospheric breakup and pre-atmospheric size of stony asteroids the study of the Ghubara meteorite was performed. Ghubara was a massive meteorite, with multiple fragments (1.8 t of total mass) recovered. To test hypotheses of whether 1. a large asteroid entered the atmosphere and disrupted to non-hazardous fragments or 2. a multiple associated asteroids entered and fragmentation was minimal, the quantitative analyses of 26Al massic activities in two unrelated meteorite samples were performed at the HADES underground laboratory. Twelve drill core samples were measured in a high purity germanium well detector. The massic activities of 26Al were determined together with a sensitivity analysis of the detection efficiency. The distribution and concentrations of cosmogenic 26Al inside two pieces of the Ghubara meteorite appears homogeneous (no outliers detected). However, a small decrease in activity concentration can be noted at depth of ~4-10 cm in both meteorite pieces.
The homogeneous distribution of 26Al and lack of edge effects indicates that both pieces came from the same radial location, plausibly deep in the asteroid. The average concentration of 26Al activity measured in the specimens, 0.893±0.051 mBq/g, was compared to the literature data and suggest that the pre-atmospheric size of the asteroid was at least 70 cm, which corresponds to the mass exceeding 5 tonnes.
