ALMAHATA SITTA

On October 6, 2008, while on duty at the Catalina Sky Survey at Mount Lemmon, Tucson, Arizona, Richard Kowalski discovered a small asteroid with a diameter of ~4.1 m, quickly designated 2008 TC3, that was calculated to have a near certainty of impacting Earth in 20 hours. This would be the first time an asteroid was spotted before it impacted Earth. Calculations indicated that the impact would occur in northern Sudan at 5:46 A.M. local time. After notification was made to several government agencies (e.g., NASA/JPL Near-Earth Object Program Office, the Jet Propulsion Laboratory, the Minor Planet Center, Sandia National Laboratories), observations of the asteroid began at many observatories around the world. Telescopic observations revealed that the asteroid was tumbling before it hit.

From the cockpit of Air France, KLM flight 592, pilot Ron de Poorter received a message from Jacob Kuiper of the Royal Netherlands Meteorological Institute regarding the possibility of observing the atmospheric entry of asteroid 2008 TC3. The crew witnessed the entrance as a flash of light below the horizon. It was later calculated that the entry velocity of the asteroid was 12.78 km per second at a height of 100 km (Welten et al., 2010), with an entry angle of 20°.

Sudanese villagers were eyewitness to the fireball as it experienced major ablation at an altitude between 40 and 35 km, and then underwent a catastrophic disruption at 37 km. At the same time, US and European satellites tracked the incoming meteor from an altitude of 65 km and obtained photos. Infrasound monitoring stations in Kenya detected airwaves equivalent to 1.1–2.1 kilotons of TNT, about one-tenth the size of the Hiroshima atomic bomb. The meteor was comparable to a PE type IIIa/b fireball, akin to a fragile cometary mass (Ceplecha et al., 1998).

On December 6, 2008, P. Jenniskens of the SETI Institute in California flew to the Nubian Desert, Sudan to begin a search for meteorites from the fall. He and M. H. Shaddad, an astronomer from the University of Khartoum, along with numerous university students and staff, enlisted the help of eyewitnesses to pinpoint the likely fall location. After an organized search, a student, Mohammed Alameen, found the first fragment—a 4.4 g fragment from the first meteorite recovered following its detection as an asteroid in space. The nearest identifiable landmark was Station 6 along the railroad between Wadi Halfa and Abu Hamad, and so the meteorite was named Almahata Sitta, Arabic for Station 6. Over the next few weeks repeated field expeditions yielded over 600 samples of a brecciated meteorite having a combined weight of 10.7 kg, part of an estimated total fall of 39 (±6) kg (Shaddad et al., 2011) within a strewnfield measuring 28 × 5 km.

Portions of the preceeding account were gleaned from the NATURE NEWS FEATURE, Vol 458, 26 March 2009, by Roberta Kwok.

A sample of 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. Alamahta Sitta was determined to be a polymict ureilite exhibiting anomalous features in some samples such as low olivine-to-pyroxene ratio (a pigeonite–olivine ureilte), wide range of silicate compositions, large pores with friability, a fine-grained texture, large silicate aggregates, and large carbonaceous aggregates which have experienced higher heating than any other ureilite. Some augite-bearing ureilites are thought to contain a late shock melt component. Alamahta Sitta is a fragmental breccia composed of three main ureilite lithologies (of perhaps ten identified): 1) pyroxene-dominated, highly porous, and highly reduced, 2) pyroxene-dominated and compact, and 3) olivine-dominated and compact (Zolensky et al., 2010).

The fine-grained component of Alamahta Sitta has significant porosity (~20%) comprising a 3-D network, with the pore wall olivine crystals being formed by vapor deposition processes prior to the re-accretion of the asteroid. Mosaicism, foliation, and the presence of diamonds in fine-grained samples of Alamahta Sitta attest to high temperature shock metamorphism, likely during re-accretion. Coarse-grained samples are only weakly (S2–3) shocked. The occurrence of significant porosity led some to conclude that the meteorite was spalled from relatively unconsolidated material existing in the outer layers of an ~4.1-m-sized, fragmented daughter asteroid, 2008 TC3. The asteroid, which was established through pre- and post-impact spectral data to have been an F-class asteroid in Tholen taxonomy, had a long axis measuring 6.7 (±0.8) m. This is a class of asteroids located at ~2.45 AU and near the 3:1 mean motion resonance. Mineral fragments include polycrystalline olivine, pigeonite, low-Ca pyroxene, carbon-rich aggregates, kamacite, and troilite.

The presence of graphite and microdiamonds in Alamahta Sitta have also been verified, but the diamond exhibits Raman spectra that are distinct from those of other brecciated and unbrecciated ureilites (Ross et al., 2010, 2011). Nevertheless, the diamond lies within the spectral range of unbrecciated ureilites. The measured diamond strain values in Alamahta Sitta are higher than those in other ureilites, indicating that it experienced either greater shock (~6–15 GPa) or less annealing. Diamond and/or lonsdaleite are thought to have formed from shocked graphite on the parent body, as evidenced by the presence of high-pressure, compressed graphite phases. Alternatively, diamond is also thought to be formed by chemical vapor deposition in the solar nebula, a process which may be inferred by the N-isotopic composition. Still, the invariable presence of crystalline graphite accompanying diamond leads to the conclusion that shock is likely the cause of synthesis of diamonds in ureilites (Ross et al., 2011).

Further analyses has led to the discovery of a wide range of xenolithic clasts representing many different chondritic and achondritic lithologies in a manner that is similar to the structure of the polymict breccia Kaidun (Bischoff et al., 2010). Evidence exists indicating all of these clasts came from the Almahata Sitta fall (e.g., detection of short-lived cosmogenic nuclides, very low weathering grade, multiple lithologies among fragments, a high number of rare E-chondrite rock types found, diffusion of PAHs among clasts (Sabbah et al., 2010), and the finding of new and unique meteorite fragments within a small area). The heterogeneous composition of Almahata Sitta could reflect an assemblage derived from a catastrophic collision between ureilte and chondrite objects (Kohout et al., 2010). Alternatively, it is considered likely that these diverse clasts became gravitationally bound within a common debris disk composed of a disrupted ureilite asteroid, and this disk was then re-accreted into a smaller second-generation asteroid(s). This second-generation asteroid later became lightly sintered together through subsequent low-energy impacts, resulting in a bulk porosity of ~50%. The lack of solar wind gases in the Almahata Sitta clasts excludes the possibility that it was a regolith breccia (Bischoff et al., 2010), but likely a component of a shallow regolith (Hartmann et al., 2011). The diverse lithologic types comprising 2008 TC3 constitute ~20–30% of all material recovered, and includes the following (Bischoff et al., 2010; Horstmann and Bischoff, 2010):

• ultra-fine-grained ureilites (representing at least seven lithologies with varying olivine compositions)
• coarse-grained ureilites (representing at least five lithologies)
• enstatite chondrites (representing at least six to seven different enstatite chondrites: e.g., EH3, EL3, EL6, EL breccias, and both EL and EH shock-darkened, impact melt rocks or impact melt breccias)
• ordinary chondrites: (e.g., H5 and H5/6, with compositional discordance)
• unique chondrite: 5.68 g, type 3.8 [±] 0.1, Fa35–37; has some petrographic and isotopic affinities to R and LL chondrites, but is mineralogically different from all known chondrite groups (Horstmann et al., 2010)
• niningerite-bearing, fine-grained ureilitic fragment (linking E chondrites)
• sulfide-metal assemblage in a fine-grained ureilitic fragment

A consortium was established to study Alamahta Sitta and much has since been learned. This is a characteristic ureilte that has experienced high degrees of silicate partial melting and underwent rapid cooling (0.05–2°C/hour) with reduction following a sudden pressure loss, possibly due to parent body disruption and re-accretion into a second generation of objects 10–100 m in diameter (Herrin et al., 2010). Large carbonaceous grains are present, consisting mostly of graphite, and containing extraterrestrial two- to six-carbon aliphatic amino acids interspersed with a distinct range of PAHs (Callahan et al., 2009; Zare et al., 2009). The presence of various amino acid decomposition products attests to thermal alteration of organic compounds on the parent asteroid.

By fitting the measured radionuclide concentrations to the known size of the pre-atmospheric asteroid for a range of estimated densities, and utilizing the average grain density for ureilites, the total porosity of the 2008 TC3 asteroid was determined to be ~55%, consistent with its early disruption upon atmospheric entry (Welten et al., 2010). Notably, the bulk density (~1.8 g/cm³) of the asteroid was lower than that of the recovered meteorites (1.77–3.26 g/cm³). Given the density of Alamahta Sitta, the pre-atmospheric size of the asteroid was ~4.1 m in diameter in a volume of 28 m³ having an oblong shape and a mass of 51,000 kg.

In a study of O-isotopes for Alamahta Sitta, Rumble et al. (2010) established an oxygen three-isotope plot for a small lot of fragments. As with typical ureiltes, the values are heterogeneous and plot in the upper one-third of the carbonaceous chondrite anhydrous mineral (CCAM) trend line, spanning the entire compositional range of known monomict and polymict ureilites (Rumble et al., 2010). This data is consistent with the hypothesis that ureilites are derived from a minimally molten (20–30%) carbonaceous chondrite body, and that ureilites spanning the entire O-isotopic range constituted a common parent body. In contrast, studies of ε54Cr anomalies in Alamahta Sitta samples by Qin et al. (2010) revealed a negative anomaly, heretofore only consistent with ordinary chondrites and achondrites such as HED members but not with carbonaceous chondrites. The ε54Cr value infers that Alamahta Sitta, and possibly all ureilites, were not derived from a carbonaceous chondrite parent body as commonly presumed.

Chromium isotopic compositions were studied for Alamahta Sitta and two other polymict ureilites by Qin et al. (2010), and a similar isochron was found for all samples corresponding to an absolute age of 4,563.6 (±2.2) m.y., with reference to the angrite D'Orbigny. ε54Cr systematics for Alamahta Sitta and other ureilites are unlike those from any of the carbonaceous chondrite groups, attesting to their origin on distinct parent bodies. It is noteworthy that the ε54Cr systematics for Alamahta Sitta are very similar to those of the HED parent body, and it can be inferred that the ureilite and HED parent bodies accreted and experienced igneous melting very early in solar system history, within 5 m.y. of CAI formation, within a similar region of the solar nebula.

A classification of the Alamahta Sitta polymict ureilite has been attempted by Zolensky et al. (2010). They concluded that its Mg# characterizes it as a member of Berkley's Group II, but that it falls at the extreme Mg-rich range. However, other fragments studied revealed a ferroan composition (Mikouchi et al., 2010). In Goodrich's scheme Alamahta Sitta is a partial melt residue best described as a pigeonite-olivine ureilite due to its greater abundance of pigeonite over olivine. Unusually, due to its low Fe content it plots among the olivine-orthopyroxene subgroup. In contrast, 39 IR spectra taken from 26 stones resulted in a heterogeneous profile of silicates ranging from nearly pure olivine to nearly pure pyroxene (pigeonite), while a combined mass weighted average of all of the spectra resulted in an olivine:pyroxene ratio of 74:26; well within the range of the spectra from other ureilites (Sandford et al., 2010). The fine-grained lithology in Almahata Sitta shows evidence of reduction through impact smelting accompanied by abundant release of CO2 gas and shock granulation of both olivine and pyroxene. This reduction process occurred at the time of the catastrophic disruption of the UPB during peak temperatures which resulted in the production of interstitial silica and tiny Fe-metal particles (Warren and Rubin, 2010). Rapid cooling ensued.

The CRE age based on 21Ne was calculated for specific ureilite fragments of Alamahta Sitta to be 14.5 (±0.9) m.y., which falls well within the range of typical ureilites (Welten et al., 2010, 2011) and represents the time since 2008 TC3 broke from a larger asteroid. A more reliable method gave an average CRE age of 20 (±3) m.y., consistent with that of other ureilites, and indicating its close proximity to a mean-motion resonance. The He-, Ne-, and Ar-based CRE age values of H and L chondritic material present in Alamahta Sitta are ~13.6–27 m.y. (Meier et al., 2010), overlapping within error of Alamahta Sitta ureilites. Notably, the gas retention age of the L chondrite samples from Almahata Sitta are distinct from those of the typical L group 500 m.y. age cluster. Trapped noble gas contents and Xe-isotopic compositions in Alamahta Sitta are consistent with those of other ureilites (Ott et al., 2010), with sub-µm- to several µm-size diamond serving as the main noble gas carrier (Murty et al., 2010). The U,Th–He ages of chondrite clasts of ~3.8 b.y. may represent the time of their incorporation into the ureilite host, and may correspond to the Late Heavy Bombardment (Welten et al., 2011).

The magnetic signature of Alamahta Sitta was found to be identical to that of previously studied ureilite falls, containing two distinct phases of kamacite, along with suessite, schreibersite, troilite, and a daubreelite-heideite phase new to ureilites; possibly contamination by xenolithic clasts (Hoffmann et al., 2010). Ordinary and enstatite chondrite xenolithic clasts tend to have a much higher magnetic susceptibility. Hochleitner et al. (2010) investigated the mineralogy of Almahata Sitta and discovered up to five distinct kamacite phases. The metal phase (Kamacite I) that forms large sheets between silicate grains has a unique composition, and it was likely introduced through the impact of a Ni-poor iron meteorite. Troilite blebs located mainly in carbon-rich veins are thought to be the result of olivine reduction processes.

Measurements of gamma-ray emitting radionuclides allowed Taricco et al. (2010) to determine the shielding depth while part of asteroid 2008 TC3 of certain samples given the density range calculated. Activity of 60Co and 26Al indicate a shielding depth for the fragment studied of 27–55 cm. Almahata Sitta has a shock stage of S0, which is consistent with its low abundance of nanodiamonds but inconsistent with the granoblastic, mosaicized textures observed in olivine and pyroxene (Mikouchi et al., 2010).

Studies of the many asteroid families have revealed some potential matches to 2008 TC3 and Almahata Sitta, especially the F-class Polana (Nysa) family located in the inner Main Belt near the 3:1 resonance. The Polana family has a low orbital inclination similar to that observed for 2008 TC3 along its trajectory to Earth (Gayon-Markt, 2011). The Polana family contains three different types of material, including rare primitive B-type asteroids akin to ureilites, and thermally altered stony S-type and intermediate X-type asteroids. Spectrally, each of these three asteroid types have been identified in the Almahata Sitta fragments, suggesting that this mixture of asteroid types was the result of multiple low-velocity collisions.

Another possible asteroid match is the Hoffmeister family located near the 5:2 resonance (Welten et al, 2010). The possibility exists that the foreign clasts in Almahata Sitta were derived from a collision with the S-class Mildred family in the inner Main Belt, or the terrestrial zone of the nebula (Jenniskens et al, 2010). As a quickly recovered fall, Almahata Sitta has a weathering grade of W0. The specimen of Alamahta Sitta shown above is a 0.33 g crusted partslice.

Special thanks to Siegfried Haberer for providing specimens of this special meteorite to collectors.