In 2008, October 6 at 5:46 A.M., asteroid 2008 TC3 fell to Earth in northern Sudan. See the Almahata Sitta webpage for the complete story of the discovery of this meteorite, results of the consortium analyses, and new models for the petrogenetic history of the ureilite parent body.
The 2008 TC3 meteorite was sent to NASA's Johnson Space Center in Houston (Zolensky) and Carnegie Institution of Washington (Steele) for analysis and classification, and Alamahta Sitta was determined to be a polymict ureilite fragmental breccia composed of three main ureilite lithologies, along with a wide range of xenolithic clasts representing many different chondritic and achondritic lithologies in an assemblage similar to the polymict breccia Kaidun (Bischoff et al., 2010). Results of the analyses indicate that all of the clasts came from the Almahata Sitta fall; e.g., detection of short-lived cosmogenic nuclides, very low weathering grade (W0W0/1), multiple lithologies among fragments delimiting a strewn field, a high number of rare E-chondrite rock types found together, diffusion of PAHs among clasts [Sabbah et al., 2010], and the finding of new and unique meteorite fragments within a small area.
With inclusion of most all CRE age results obtained to date for both ureilite and chondrite samples, Riebe et al. (2017) derived an average age of 19.7 (±2.8) m.y. This relatively young CRE age for the various components of 2008 TC3 indicates that it was located in close proximity to a mean-motion resonance. Notably, one E chondrite sample designated MS-179 that was studied by Riebe et al. (2017), which is linked to the Almahata Sitta fall through multiple lines of evidence, presents a younger CRE age of 11 (±1.4) m.y. They suggest this lower CRE age may represent the actual age of the 2008 TC3 meteoroid as an independent object in space subjected to galactic cosmic rays (4π exposure), while the older ages of the other components could be attributed to pre-irradiation (2π exposure) within a regolith setting on the parent asteroid.
The heterogeneous composition of Almahata Sitta could reflect an assemblage derived from a catastrophic collision(s) between ureilte and chondrite objects (Kohout et al., 2010). Alternatively, it is considered likely that these diverse clasts could have become gravitationally bound within a common debris disk composed of a disrupted ureilite asteroid, and that this disk then re-accreted into one or more smaller second-generation asteroids. This second-generation asteroid later became lightly sintered together through subsequent low-energy impacts, resulting in a bulk porosity of ~50%. The highly porous ureilite material recovered from the Almahata Sitta fall, such as MS-168, is consistent with the hypothesized lightly-sintered matrix of the second-generation asteroid 2008 TC3.
Inclusion MS-181 is the first carbonaceous chondrite found associated with the Almahata Sitta fall to date. MS-181 originally weighed 58.63 g, but only 42 g remained after cut losses. An analysis of cosmogenic radionuclides present in MS-181, conducted at the Max-Planck Institute in Heidelberg, Germany, has definitively confirmed that the timing of its fall event is consistent with that of Almahata Sitta. Results from analyses of MS-181 were presented at the 75th Annual Meeting of the Meteoritical Society in August 2012, and further information about Almahata Sitta and its wide variety of inclusions is scheduled to be published in a special issue of Meteoritics & Planetary Science.
Exclusive of the primary ureilite components, there was a broad diversity of lithologic types present in 2008 TC3, constituting <30% of all material recovered. However, since the vast bulk of 2008 TC3 is thought to have been lost as fine dust (≥99.9% of the estimated 4283 ton pre-atmospheric mass), the asteroid was likely composed primarily of fine-grained, highly-porous, weakly-consolidated ureilitic matrix material consistent with the reflectance spectra obtained for the asteroid (Goodrich et al., 2015). Examples of some of the diverse samples that have been recovered are listed below (Bischoff et al., 2010, 2015, 2016; Horstmann and Bischoff, 2010; Hoffmann et al., 2016):
unique chondrite: MS-CH, type 3.8 [± 0.1], has petrographic and isotopic affinities to R-chondrites, but is mineralogically anomalous
Bencubbin-like carbonaceous chondrite: MS-181, a 58.6 g chondrule-like clast containing metal globules and silicates in a 60:40 ratio, having an O-isotopic composition consistent with bencubbinites
C2 carbonaceous chondrite: AhS 202 (photo; Fioretti et al., 2017)
niningerite-bearing, fine-grained ureilitic fragment (linking E chondrites)
sulfide-metal assemblage in a fine-grained ureilitic fragment: MS-158, -166
ungrouped enstatite- and metal-rich achondrite fragments: MS-MU-019 (characteristics similar to NWA 8173/10271; MS-MU-036 (similar to MS-MU-019 and Itqiy (Bischoff et al., 2016)
the first known plagioclase-bearing olivineaugite ureilite lithology: MS-MU-012
trachyandesitic clasts: 1) MS-MU-011 (view 1), MS-MU-011 (view 2), sample ALM-A; plagioclase-enriched (~70 vol%) with pockets of gemmy olivine (photo courtesy of Stephan Decker), likely sampling the UPB crust (or possibly an alkali- and water-rich localized melt pocket); calculated ArAr age of ~4.556 b.y. and PbPb age of ~4.562 b.y. (Bischoff et al., 2013, 2014; Delaney et al., 2015; Turrin et al., 2015; Amelin et al., 2015); 2) MS-MU-035; anorthoclase and/or plagioclase-enriched (~65 vol%) (Bischoff et al., 2016)
Special thanks to Siegfried Haberer and Stephan Decker for providing specimens of this special meteorite and many of its xenolithic inclusions to the scientific and collector communities. The photo of the MS-181 bencubbinite inclusion shown above, and the reverse below, is a 0.107 g cut fragment.