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

Abstract

Abstract-On October 7, 2008, a small F-class asteroid, 2008 TC 3 , 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 10 Be, 26 Al, and 36 Cl in the meteorite fragments of asteroid 2008 TC 3 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 TC 3 to 1.66 ± 0.25 g cm )3 and the bulk porosity to 50 ± 7%. The bulk density of asteroid TC 3 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 lowdensity ureilites are probably more representative of the bulk asteroid. We thus conclude that the high porosity of asteroid 2008 TC 3 is mainly due to microporosity, which implies that not all low-density asteroids are necessarily rubble pile structures. Finally, from the cosmogenic 21 Ne concentrations, we determined a cosmic-ray exposure (CRE) age of 19.5 ± 2.5 Myr. This age represents the time since asteroid 2008 TC 3 was ejected from a large F-class parent body in the asteroid belt, until its collision with Earth.

Noble gases and nitrogen in the Almahata Sitta ureilite

“…The agreement, within experimental uncertainties, between the exposure ages based on 3 He and 21 Ne suggests no 3 He loss in #36. Also, our value of T 21 = 16.0 ± 2.4 Ma for AS#36 is in agreement with the value of 14.1 Ma, as well as the average exposure age of 15 ± 2 Ma, from the four fragments #4, #36, #44, and #47 (Welten et al. 2010).…”

“…(2010) have reported 36 Ar amount (in ccSTP g −1 units) in both AS#36 (13.1 × 10 −8 ) and AS#44 (1390 × 10 −8 ), and they differ by about a factor of 100. Our value of 36 Ar in AS#36 is high by a factor of 3.3, compared to Welten et al. (2010) value.…”

“…2010). The Bern plot of Welten et al. (2010) also does not indicate any 3 He loss, although partial loss of 3 He is indicated for #1 (Ott et al.…”

“…We did not measure the bulk sample AS#44. Welten et al. (2010) have reported 36 Ar amount (in ccSTP g −1 units) in both AS#36 (13.1 × 10 −8 ) and AS#44 (1390 × 10 −8 ), and they differ by about a factor of 100.…”

Internal structure and physical properties of the Asteroid 2008 TC3 inferred from a study of the Almahata Sitta meteorites

“…1). However as Asteroid 2008 TC 3 is compositionally heterogeneous and probably of low mechanical strength we consider it very likely that the asteroid contains significant macroporosity, around 50% or higher as also suggested by Borovička and Charvát (2009) or Welten et al (2010). Thus we conclude that the bulk density and mass of Asteroid 2008 TC 3 should be lower than this estimate.…”