Cosmogenic nuclides in Almahata Sitta ureilites: Cosmic-ray exposure age, preatmospheric mass, and bulk density of asteroid 2008 TC3

On October 7, 2008, a small F-class asteroid, 2008 TC3, exploded in Earth's atmosphere, and produced a strewn field of meteorites in the Nubian Desert of Sudan. Subsequent searches yielded several hundred meteorite fragments, known as Almahata Sitta. This fall was classified as a polymict ureilite. We measured cosmogenic radionuclides in six fragments and noble gases in four fragments of the Almahata Sitta ureilite. The concentrations of 10Be, 26Al, and 36Cl in the meteorite fragments of asteroid 2008 TC3 indicate a preatmospheric radius of 300 +/- 30 g cm-2. Combined with an absolute radius of 1.8 +/- 0.2 m, which was derived from its brightness in space, the cosmogenic radionuclides constrain the bulk density of asteroid 2008 TC3 to 1.66 +/- 0.25 g cm-3 and the bulk porosity to 50 +/- 7%. The bulk density of asteroid TC3 is on the low end of the range of 1.6-3.3 g cm-3 determined for 40 of the recovered ureilite fragments. Since the denser materials have a higher chance of surviving atmospheric fragmentation, the low-density ureilites are probably more representative of the bulk asteroid. We thus conclude that the high porosity of asteroid 2008 TC3 is mainly due to microporosity, which implies that not all low-density asteroids are necessarily rubble pile structures. Finally, from the cosmogenic 21Ne concentrations, we determined a cosmic-ray exposure (CRE) age of 19.5 +/- 2.5 Myr. This age represents the time since asteroid 2008 TC3 was ejected from a large F-class parent body in the asteroid belt, until its collision with Earth.