ALMAHATA SITTA IS NO MORE EXOTIC THAN ANY OTHER POLYMICT UREILITE

Introduction: Almahata Sitta (AhS) fell in 2008 when asteroid 2008 TC3 (~4 m) disintegrated over Sudan [1,2]. The recovered ~cm-sized stones (<1% of the mass of the asteroid) include ureilites (achondrites) and diverse chondrites (OC, EC, RC, CC) [3,4]. Based on the predominance of ureilites among stones studied so far, AhS is classified as an anomalous polymict ureilite [1]. However, how the recovered stones were related to one another in the asteroid, and the composition of the missing 99% of its mass, are unknown [4,5]. The “extraordinary” mix of meteorite types in AhS has led to “exotic” models for formation of 2008 TC3 [3,6]. Main group ureilites are carbon-rich ultramafic achondrites that represent the mantle of a partially differentiated asteroid [7]. They show a characteristic range and distribution of compositions (Fig. 1), and low to high shock levels. Typical polymict ureilites (TPUs) (~7% of all ureilites) are fragmental and regolith breccias consisting of clasts of mixed main group ureilite materials with the same range and distribution of compositions, as well as shock states, as main group ureilites [8-10]. They also contain a few % feldspathic clasts representing ureilitic crustal rocks [10,11], and ~10% xenoliths [12]. They are interpreted as having formed in ureilitic regolith, with the xenoliths being impactor remnants [4,8,9,13,14]. While AhS/2008 TC3 is famous, TPUs are too often ignored. We argue here that AhS and TPUs are similar and most likely share a common origin. Almahata Sitta vs. TPUs – Compositions: The range and distribution of compositions of AhS ureilitic stones studied so far is nearly identical to the range and distribution shown by ureilitic clasts in TPUs and main group ureilites (Fig. 1). AhS also contains feldspathic ureilites with distinctive compositions like those of the feldspathic clasts in TPUs [15]. The range of types of non-ureilites in AhS is no more extraordinary than that in TPUs, which are noted for containing xenoliths from every major chondrite class (OC, EC, RC, CC), including multiple groups (i.e., H, L, LL, EH, EL) and petrologic types (1-6) of each [13]. There are differences, however, in relative abundances. CC clasts are the most common in TPUs, while EC are only found as mineral fragments [16,17]. Conversely, CC are rare in AhS while EC are the most abundant nonureilites. In addition, the achondrite types found as rare xenoliths in TPUs (angrites and NWA 7325-like) have not been found in AhS. The abundance of non-ureilites in AhS/2008 TC3 been regarded as anomalously high [3] compared with the abundance of xenoliths in TPUs (19-35% of AhS stones studied so far are non-ureilites). However, if 2008 TC3 was mainly ureilitic, as has been argued from the asteroid reflectance spectrum [18], the abundance of non-ureilitic clasts must have been much lower than among the surviving stones, possibly similar to or even lower than that in TPUs. Almahata Sitta vs. TPUs – Structure: The AhS stones, which were clasts in 2008 TC3, are ~0.5 to 6 cm in size, larger than clasts in TPUs (<1-2 cm, most being m to a few mm). The significance of this difference is unknown. Most of 2008 TC3 could have had smaller clast sizes than the recovered stones (larger clasts are more likely to have survived), and given that TPUs are usually studied in small sections (1-inch diameter), their largest clasts may not have been recognized (e.g., some sections consist of single ureilitic clasts). Nevertheless, an obvious difference between AhS and TPUs is that 2008 TC3 was weak and disintegrated in the atmosphere [19,20]. This may be because the matrix of the asteroid was similar to the fine-grained, porous, highly-shocked ureilitic stones [3], which are also a plausible spectral match to the asteroid [18]. This suggests the possibility that 2008 TC3 was derived from shallow, loosely-consolidated regolith, while typical polymict ureilites were derived from deeper, more mature regolith, on the same ureilitic body [4]. Alternatively, the matrix of the asteroid may have been similar to AhS stone 91A, which is a very friable, 72 76 80 84 88 92 96 100 0 10 20 30 40 50 60