ALMAHATA SITTA

In 2008, October 6 at 5:46 A.M., while on duty at the Catalina Sky Survey at Mount Lemmon Observatory, Tucson, Arizona, Richard Kowalski discovered a small asteroid with a diameter of ~4 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 and light curve observations revealed that the asteroid had acquired both a rapid spin rate of 100 seconds and a tumbling motion (Sánchez and Scheeres, 2014).

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°

US and European satellites tracked the incoming meteor from an altitude of 65 km and obtained photos. At the same time, Sudanese villagers were eyewitness to the fireball as it experienced major ablation at an altitude between 44 and 35 km, and then underwent a catastrophic disruption at 37 km. 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, Peter Jenniskens of the SETI Institute in California flew to the Nubian Desert, Sudan to begin a search for meteorites from the fall. He and Muawia Shaddad, an astronomer from the University of Khartoum, Sudan, along with 45 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 ~40 kg (Shaddad et al., 2011) within a strewn field measuring about 28 × 5 km. Succeeding searches yielded additional specimens which were analyzed and classified at the University of Münster, raising the total recovery to 11.2 kg (Jenniskens et al., 2022).

Portions of the preceding account were gleaned from Nature News Feature, vol. 458, 26 March 2009, by Roberta Kwok, and from 'Asteroid 2008 TC3 and the Fall of Almahata Sitta, a Unique Meteorite Breccia', Cyrena Goodrich et al., Elements, vol. 10 (2014).

Initially, samples of the Almahata Sitta meteorite were sent to NASA's Johnson Space Center in Houston (Zolensky) and Carnegie Institution of Washington (Steele) for analysis and classification. The meteorite was determined to be a polymict ureilite exhibiting anomalous features in some samples, such as low olivine-to-pyroxene ratio (a pigeonite–olivine ureilite), wide ranging silicate compositions, large-sized pores with friability, fine-grained texture, large silicate aggregates, and large carbonaceous aggregates that have experienced higher heating than those in any other ureilite. Some augite-bearing ureilite samples are thought to contain a late shock-melt component. Almahata Sitta is a fragmental breccia composed of various ureilite lithologies (perhaps ten identified), which have been resolved into four general types (Goodrich et al., 2015): 1) coarse-grained, compact, olivine–pyroxene, with petrographic variability; 2) fine-grained, highly porous, highly reduced, with features of high temperature shock metamorphism; 3) metal+sulfide-rich with enclosed ureilitic silicates; 4) feldspar-rich, igneous-textured andesite. Incorporated mineral fragments in Alamahta Sitta include polycrystalline olivine, pigeonite, low-Ca pyroxene, carbon-rich aggregates, kamacite, and troilite.

Coarse-grained samples of Alamahta Sitta are only weakly shocked (S2–3) and exhibit a homogenous carbon composition with more negative δ13C values compared to fine-grained samples. The fine-grained lithology was likely formed by recrystallization of the primary coarse-grained lithology as a result of late-stage impact smelting. This process involved the degassing of lighter 12C as CO producing 13C enrichment (Storz et al., 2021 and references therein). The fine-grained lithology has significant porosity (~20%) composing a 3-D network. The olivine crystals lining the pore walls were formed by vapor deposition processes prior to re-accretion into the daughter asteroid, 2008 TC3. Mosaicism, foliation, and the presence of diamonds in fine-grained samples attest to high temperature shock metamorphism, which likely occurred during re-accretion. The occurrence of significant porosity of this meteorite led some to conclude that it was spalled from relatively unconsolidated material existing in the outer layers (shallow regolith) of the daughter asteroid. This asteroid was known through pre- and post-impact spectral data to have been an F-class asteroid in Tholen taxonomy, measuring ~4.1 m in diameter and having a long axis measuring 6.7 (±0.8) m. This is a class of asteroids located at ~2.45 AU located near the 3:1 mean motion resonance (signifying the ratio between the orbital period of Jupiter and that of an asteroid).

The presence of graphite and microdiamonds in Almahata 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 Almahata Sitta are higher than those in other ureilites, indicating it experienced either greater shock (~6–15 GPa) or less annealing. Diamond and/or lonsdaleite formed from shocked graphite on the parent body is attested by the presence of high-pressure, compressed graphite phases. The predominance of crystalline graphite accompanying diamond leads to the conclusion that shock is likely the major cause of diamond synthesis in ureilites (Ross et al., 2011). Diamond is also considered to have formed by chemical vapor deposition (CVD) in the solar nebula, a process which may be inferred by its N-isotopic composition. In addition, single crystal diamonds measuring at least 40 µm have been identified in coarse-grained ureilite phases of Almahata Sitta, which were also likely formed by the CVD process (Miyahara et al., 2012, 2015).

The occurrence of diamond formed by CVD is also supported by an alternative smelting mechanism involving methane, which has been posited by Langendam and Tomkins (2012) to explain the presence of smelting within fractures and discontinuous smelting at grain boundaries. Further results from an in-depth study of 17 ureilites conducted by Langendam et al. (2021) confirms that ureilites experienced silicate reduction to form Fe metal in a secondary smelting process during parent body decompression. They assert that this smelting process was catalyzed by a highly-reduced gas/fluid system involving H2, CH4, S2, and H2S, accompanied by a low abundance of H2O. This methane-catalyzed smelting was shown to occur at lower temperatures than those required for graphite-catalyzed smelting, and would produce the metal–troilite particles, silicate melt, carbon vein networks, and high porosity that fills fractures and grain boundaries of ureilites.

Nevertheless, large diamond crystallite assemblages have been identified in a low-shock lithology from the coarse-grained ureilite AhS MS-170. These diamonds exhibit almost identical crystallographic orientation and may have originally constituted a large (~100 µm) single-crystal diamond that had been fragmented during a shock-induced graphitization process—back transformation of diamond at low-pressure and high-temperature (Nabiei et al., 2018; see image below). Evidence for this scenario exists as inclusion trails consisting of nm-scale sulfide (plus kamacite and schreibersite) grains transiting multiple diamond segments which are now cut by graphite bands (see photos below). Miyahara et al. (2015) found evidence (e.g., nitrogen sector zoning) that such large diamonds formed relatively slowly in a melt phase located deep within the UPB. Nabiei et al. (2018) argue that the Al- and Mg-free chromite inclusions present within these large diamonds attest to formation in an iron melt phase void of silicates, such as in a core or melt pod. Importantly, the sulfide + kamacite + schreibersite inclusions present within the diamonds are stable at pressures >20 GPa; such pressures would occur at the center of a Mercury-sized planet or at the core–mantle boundary of a Mars-sized planet.

A contrary conclusion was presented by Goodrich et al. (2020) and Nestola et al. (2020) based on XRD, TEM, SEM, EMPA, and micro-Raman spectroscopic analyses of a large ~100 µm single crystal diamond in NWA 7983 (see image below) and nanodiamonds in other highly-shocked (complete mosaicism of olivine) AhS ureilite samples. The diamonds are intermixed with nanographite, lonsdaleite, and nanometric metallic iron. It was concluded that both the nanodiamonds and microdiamonds formed as a result of impact shock transformation from graphite, exhibiting a texture consistent with pseudomorphing of primary graphite laths. No high-pressure polymorphs were observed in the samples, and it was determined that peak impact shock pressures may have reached only 15 GPa for a relatively short duration of 4–5 seconds. The rapid growth of diamonds was due to catalysis by an Fe-Ni-C melt phase which coexisted with the graphite and now present as micron-scale metal–cohenite–sulfide–phosphide spherules. They contend that the similarly-oriented diamond microcrystals in MS-170 never constituted a large single diamond, and that even if convincing evidence for nitrogen sector zoning in ureilite diamonds was found, this would not require a slow growth process under high pressure conditions in a planetary-sized body. Peak temperatures attained during the diamond-forming shock event, which may be associated with the catastrophic breakup of the UPB, were calculated by Barbaro et al. (2020) to be 1200–1300 (±120) °C.

Further analyses of the Almahata Sitta specimens led to the discovery of a wide range of xenolithic clasts representing many different chondritic and achondritic lithologies in a manner similar to the structure of the polymict breccia Kaidun (Bischoff et al., 2010); amazingly, the Almahata Sitta lithologies represent at least 13 different parent asteroids (Goodrich et al., 2019 and references therein). Evidence exists indicating that 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 a rubble pile assemblage derived from a catastrophic collision between ureilte and chondritic objects (Kohout et al., 2010; Sánchez and Scheeres, 2014). Alternatively, it has been considered 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 one or more smaller second-generation asteroids. The daughter asteroid(s) became lightly sintered together through subsequent low-energy impacts resulting in a very weak object having a bulk porosity of ~50%.

Another model which better accounts for the various thermal parameters of the ureilite parent body (UPB), as well as the accumulation of a wide variety of exotic lithologies, was developed (Goodrich et al., 2015; Wilson and Goodrich, 2016). These investigators posit that accretion of the UPB from ordinary chondrite-like precursor material occurred in the outer asteroid belt (beyond the ice line) 0.61–0.64 m.y. after CAIs. Radiogenic heating from 26Al decay led to silicate melting and melt migration to the surface, possibly aided by smelting-generated CO2. After peak temperatures were attained ~4 m.y. after CAIs, a period of slow cooling ensued, during which time gravitational perturbations from large planetary embryos (possibly aided by nebular gas drag) began to influence the UPB orbital mechanics. These forces induced a slow migration sunward over a timescale measured in millions of years, a journey which took the UPB (or possibly a second generation object) inward across the orbital paths of the OC and EC parent bodies. After the severe impact disruption of the primary UPB and re-accretion into one or more daughter bodies—an event dated at 5.4 m.y. after CAIs—the ureilite daughter body (UDB) parental to 2008 TC3 experienced substantial mass accumulation along with large-scale impact gardening. Over time, a deep loosely consolidated regolith was developed on at least a portion of the UDP surface which incorporated a wide variety of foreign impactor material, before it too experienced a disruptive event 20 m.y. ago (based on the 21Ne age of multiple clasts). Ultimately, 2008 TC3 was intercepted by Earth where the complex history of Almahata Sitta could be revealed. Furthermore, it was argued that all of the other ureilite meteorites delivered to Earth also derive from the same UDB, but the exact ejection history for each is still being determined.

Because of the lack of solar-wind-implanted gases in a sample of Almahata Sitta clasts, Bischoff et al. (2010) inferred that the meteorite could not have been derived from a regolith, and it was considered instead to represent a more deeply buried lithology (Hartmann et al., 2011). In contrast, Goodrich et al. (2015) argued that solar gases would only be implanted within the top millimeter of the regolith, and that subsequent gardening may not have accomplished homogeneous mixing into deeper regolith layers. Utilizing measurements of gamma-ray emitting radionuclides along with the calculated density range of the Almahata Sitta samples, Taricco et al. (2010) determined the shielding depth of ureilite fragment AhS 15 on asteroid 2008 TC3; activity of 60Co and 26Al indicate a shielding depth of 41 (±14) cm.

It was suggested by Goodrich et al. (2019 #2107) that the asteroid TC3 may have actually been a CC object that had incorporated ureilitic and other types of xenolithic impactors. After considering the calculated density and porosity of asteroid 2008 TC3, together with its reflectance spectra (F-class) and recovered C1 material (e.g., AhS 91/91A, AhS 202, AhS 671), Bischoff et al. (2022) also agreed that the asteroid likely had a predominant carbonaceous chondrite (C1) composition rather than ureilite composition. Furthermore, based in part on the CRE ages obtained for the individual components recovered from the Almahata Sitta fall, they were able to ascertain that accretion (or re-accretion if a daughter body) of asteroid 2008 TC3 would necessarily have occurred relatively late—possibly 50,000 to 100,000 years after CAIs. However, the lack of solar wind-implanted gases in the recovered Almahata Sitta components, including the few C1 carbonaceous samples, does not favor a scenario in which the various xenolithic impactors were incorporated into the regolith of a C1 asteroid.

Results of a trace element study of a suite of unbrecciated ureilites led Barrat et al. (2016) to propose a revised melting history for the UPB. They concluded that following the segregation of a S-rich metallic core, ureilite precursor material experienced two successive steps of melting and melt extraction. The initial extracted melts were low-density feldspathic lavas containing high abundances of Al, alkalis, and silica, comparable to some clasts identified in polymict ureilites and to two trachyandesitic lithologies identified from the Almahata Sitta fall (24.2 g MS-MU-011 [aka ALM-A] and MS-MU-035). A presumed scenario was presented in which this relatively buoyant silicic liquid was extracted, leaving behind an olivine and pigeonite residue, and ultimately reached the surface to form a crust of some extent. A representative of the next sequential melt to be extracted has not yet been identified among known meteorites, but it is inferred to have been an Al-poor, alkali-depleted liquid possibly more dense than the resulting ureilitic residues. Consequently, this melt material may have been located deeper in the mantle of the UPB where sampling would be a less likely event. Regardless of the actual density of this second-stage melt, the model scenario infers a partial melting degree of ~17–28% that was sustained throughout the entire melting event, and therefore the mass of this second-stage melt would be proportionately less than that represented by the rare trachyandesitic lithologies. The Al–Mg age determined for the ALM-A trachyandesite sample indicates that the disruption of the ureilite parent body occurred no earlier than ~6.5. m.y. after CAIs (Bischoff et al., 2014).

The ultimate parental source melt of the ureilites was enriched in incompatible trace elements relative to CI chondrites and likely contained phosphate in the form of merrillite, the latter being exhausted along with feldspar prior to the crystallization of the ureilitic residues. Based on trace element abundances and REE patterns among the ureilite samples studied, Barrat et al. (2016) resolved two distinct groups which they termed 'A' and 'B' (see more about the two groups here). The REE abundances for the ureilites constituting the smaller group B are consistent with a fractional melting model in which the critical mass porosity of the residue was zero, thereby leaving no residual melt component following extraction. However, the REE abundances for the ureilites constituting the larger group A are more consistent with a dynamic melting process in which the critical mass porosity of the residue was greater than zero (<2%), resulting in the retention of a critical fraction of melt within the residue.

The broad diversity of lithologic types present in 2008 TC3 constituted <30% of all material recovered. However, given that the vast bulk of 2008 TC3 is thought to have been lost as fine dust (≥99.9% of the estimated 42–83 ton pre-atmospheric mass), the asteroid was likely composed predominantly of very fine-grained, highly-porous, weakly-consolidated matrix material, possibly represented by the recovered specimen MS-168 and/or the C1+URE+OC+EH regolith breccia clasts AhS 91/91A and 671; this would be consistent with the reflectance spectra and other data obtained for the asteroid (Goodrich et al., 2015, 2019; Bischoff et al., 2022). Examples of some of the diverse samples that have been recovered are listed below (Bischoff et al., 2010, 2015, 2016, 2018, 2019; Horstmann and Bischoff, 2010, 2014; Hoffmann et al., 2016; Fioretti et al., 2017; Goodrich et al., 2018, 2019):

• fine- to ultrafine-grained ureilites (representing numerous lithologies with varying olivine compositions): MS-185 (ultrafine), -212, -216; MS-MU-001, -018 (high shock, metal–sulfide-rich), -025 (high shock), -027 (high shock), -030 (high shock, metal–sulfide-rich), -032 (high shock, metal–sulfide-rich), -033 (high shock, metal–sulfide-rich), -040 (high shock), -045 (high shock)
• coarse-grained ureilites (representing numerous lithologies with varying olivine compositions: MS-208 (ol-rich), -209 (ol-rich), -214 (ol-rich), -218 (ol-rich), -227 (ol-rich), -210 (px-rich), -219 (px-rich), -220 (px-rich), -222 (px-rich), -226 (px-rich); MS-MU-005, -006, -008, -010, -014 (very coarse), -016, -017, -020, -022, -034, -037, -038
• variable grain-sized ureilite breccias: MS-25, -190, -205, -207, -213, -215, -296; MS-MU-004, -021, -028, -042
• highly porous ureilitic (matrix?) material: MS-168
• enstatite chondrites (41 samples, representing numerous different enstatite chondrites): EH3 (MS-14), EHa3 (MS-224), EH4/5 (MS-192, MS-MU-009), EH5 (MS-MU-041, -044), EL3 (MS-1, -17, -177, MS-MU-002, -023, -031), EL3/4 + melt (MS-17, MS-MU-039 [+ melt]), EL3–5 (MS-179), EL4 (MS-MU-029), EL4/5 (MS-192, MS-MU-009), ELb4 (MS-221), EL5 (MS-196), EL5/6 (MS-7), EL6 (MS-150, MS-MU-007, -015, -024, -026), ELb6 (MS-211, -217, -225), EL breccias (MS-MU-003), and both EL and EH (MS-155) shock-darkened, impact-melt rocks or impact-melt breccias
• ordinary chondrites: H4 (MS-MU-043), H5 (AhS 25, MS-151 [shock-darkened]), H5-an (MS-MU-013), H5/6 (MS-11, with compositional discordance), L3 (MS-MU-060), L4 (AhS A100), LL4 (MS-197)
• 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, 20.057 g, w/ tremolite amphibole (Fioretti et al., 2017 #1846 [photo]; Goodrich et al., 2019 #1551, 2020 #1223 [O plot]; Hamilton et al., 2020 #1122; Hamilton et al., 2021; Miller et al., 2021 #2360 [O–Cr plot]; Dodds et al., 2022 #2158, Hamilton and Goodrich, 2023 #6137 [similar to MIL 090292])
• C1+URE+OC+EH regolith breccia: AhS 91/91A and 671 (Goodrich et al., 2018 #1321 [photo], 2019 #1551, #1312, article, 2021 #1331, article); MS-266 (polymict breccia with C1 clast; Bischoff et al., 2022)
• niningerite-bearing, fine-grained ureilitic fragment (linking E chondrites): MS-20
• sulfide-metal assemblage in a fine-grained ureilitic fragment: MS-158, -166
• ungrouped enstatite- and metal-rich achondrite fragments: MS-MU-019 (complete mass; cut section photo credit: Bischoff et al. 2022; characteristics similar to NWA 8173/10271); MS-MU-036 (similar to MS-MU-019, Itqiy, and NWA 2526 [Bischoff et al., 2016; Zhu et al., 2021]); AhS 38 (similar to MS-MU-019 and Itqiy but contains olivine [Goodrich et al., 2018]); AhS 60 (possible E IMR analogous to Portales Valley [Goodrich et al., 2018])
• the first known plagioclase-bearing olivine–orthopyroxene–augite ureilite lithology: MS-MU-012 (Goodrich et al., 2022)
• trachyandesitic clasts: (1) MS-MU-011 (view 1), MS-MU-011 (view 2), MS-MU-011 (aka ALM-A); plagioclase-enriched (~70 vol%) with pockets of gemmy olivine (see photos below) likely sampling the UPB crust, or possibly an alkali- and water-rich localized melt pocket; calculated Ar–Ar age of ~4.556 b.y. and Pb–Pb 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); (3) MS-277, 11.03 g; (4) MS-MU-065, 54.7 g
• andesitic clast: AhS 3005, 16.84 g, composed of two different sectors: 1) "labradorite–opx" (plag cores = An50–53); 2) "oligoclase–augite" (plag cores = An30–35) (Goodrich et al., 2022 #1065)

A consortium was established to study Almahata Sitta and much has since been learned. This meteorite is a characteristic ureilite that has experienced high degrees of silicate partial melting. It underwent rapid cooling (0.05–2°C/hour) and experienced 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, which consist mostly of graphite and contain extraterrestrial two- to six-carbon aliphatic amino acids interspersed with a distinct range of polycyclic aromatic hydrocarbons (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.

Fitting the measured radionuclide concentrations to the known size of the pre-atmospheric asteroid for a range of estimated densities, and then 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 Almahata Sitta, the pre-atmospheric size of the asteroid was calculated to have been ~4.1 m in diameter with a volume of 28 m³, and it had an oblong shape with a total mass of 51,000 kg.

In a study of oxygen isotopes for Almahata Sitta, Rumble III et al. (2010) established an oxygen three-isotope plot for a small lot of fragments. As with typical ureilites, 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 III et al., 2010). These data are 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. Based on its O-isotopic signature and major element chemistry, Rai et al. (2016) calculated that the precursor material for the UPB had a mixed composition consisting of CI, CM, EH, and Mg-rich chondrules.

By contrast, studies of ε54Cr anomalies in Almahata Sitta samples by Qin et al. (2010) revealed a negative anomaly that is consistent only with ordinary chondrites and achondrites such as HED members, but not with carbonaceous chondrites which have a positive anomaly. The ε54Cr value infers that Almahata Sitta, and possibly all ureilites, were not derived from a carbonaceous chondrite parent body as commonly presumed. Valdes et al. (2016) studied the Ca-isotopic signature for ureilites, which serves as an excellent tool for resolving genetic relationships, and found a wide range in δ44Ca values unique from values for carbonaceous chondrites or any other chondritic material. Goodrich et al. (2015) recognized that the Cr, Ti, and Ni isotopic compositions of ureilites are more consistent with an origin on an OC or EC parent body than a CC parent body. They also showed that the inferred lithophile element composition of ureilite precursor material was most similar to OC and R-chondrites. In a comparison of Si/Mg vs. Mg/Mn (see below), the ureilites plot near OC and R-chondrites rather than CC or E-chondrites.

Warren (2011) determined that the isotope signatures of Δ17O, ε54Cr, ε50Ti, and ε62Ni can be utilized to resolve carbonaceous from non-carbonaceous meteorites; the carbonaceous meteorites have positive values for all of these elements, while the non-carbonaceous meteorites have negative values. An example coupled Δ17O vs. diagram is shown below to demonstrate the separation between carbonaceous and non-carbonaceous meteorites. Ureilites plot in the non-carbonaceous field furthest removed from the carbonaceous field.

In a study conducted by Srinivasan et al. (2018), it was determined that the O-isotopic composition and Al–Mg age data are reasonably consistent among several different achondrites: the ungrouped NWA 11119 and NWA 7325, and the trachyandesite clasts in the Almahata Sitta anomalous polymict ureilite (MS-MU-011/ALM-A and MS-MU-035). In addition, some notable geochemical similarities have been identified. Moreover, it was recognized that these are the only achondrites known that plot on the carbonaceous chondrite anhydrous mineral (CCAM) line at the high-δ18O end. Although these meteorites do have distinct Fe/Mn ratios, the range in values is much less than that within the acapulcoite–lodranite clan, the latter of which is known to represent a single parent body. The many similarities that have been documented among these diverse meteorites suggest that a genetic relationship (i.e., a common differentiated parent body) may exist (see diagram below).

Two stones recovered from the Almahata Sitta strewn field, designated AhS 91/91A and AhS 671, may represent a highly friable regolith breccia component. These two stones are composed of a dominant C1 lithology in sharp contact with multiple different ureilite (olivine + pigeonite + albite [= polymict ureilite "albitic lithology" and AhS trachyandesite MS-MU-011/035]) and xenolithic chondrite lithologies (OC and CC chondrules, H- and EH-group metal-sulfide), silicate clasts (olivine, pyroxene, and plagioclase with ureilitic provenance), and graphite clasts (probable ureilitic provenance). Together these lithologies could represent the bulk matrix of asteroid 2008 TC3, originally constituting a significant portion (est. 40–70%) of the pre-atmospheric volume (Goodrich et al., 2017, #6214; 2018, #1321; 2019, #1312, article; Bischoff et al., 2022). By comparison to the negative Cr-isotopic values obtained from all ureilites analyzed thus far (ε54Cr ~ –0.90), the C1 lithology has the highest ε54Cr composition measured for any meteorite (ε54Cr +1.83 [±0.08]; Sanborn et al., 2017, #6277) (see top diagram below). The bulk mineralogy of AhS 91A was determined to be most similar to CI1 carbonaceous chondrite material, but is unlike any known carbonaceous chondrite group represented in our collections (Yin et al., 2018, #1810). An O-isotopic analysis shows that the C1 material from AhS 91 and AhS 671 plot along the CCAM line in a unique location, while the highly magnetite-rich (>10%) and tremolite-bearing C2 clast AhS 202 plots within the CR/CM field, although its mineralogy and magnetite compositions are unique (see bottom diagram below and Fig. 3 in Goodrich et al., 2019, #1551); however, AhS 202 shows many similarities to the tremolite-bearing, C1-ung MIL 090292, and both meteorites may derive from the same Ceres-sized parent body (Hamilton and Goodrich, 2023 #6137). Notably, a dark clast from the Nilpena polymict ureilite previously studied by Clayton and Mayeda (1988), as well as three clasts in other polymict ureilites identified by Patzek et al. (2018, #6254; see also here) are identical to AhS 91A and AhS 671 in mineralogy and O-isotopic composition. In addition, clast 2 in polymict ureilite NWA 10657 identified by Goodrich et al. (2019, #1312) shares many similarities with AhS 91A and AhS 671, which can add to our understanding of the nature of the ureilite parent body.

It was previously considered by some investigators that the observed correlations between oxygen isotopes and redox trends in ureilites was inherited from accreted material which previously had undergone high-temperature solid–gas exchange in the nebula. Extensive smelting has also been invoked to explain these correlations. However, Sanders et al. (2017) argue that the positive correlation observed between Fa content in olivine and Δ17O, as well as that observed for other O-isotopic and redox trends (e.g., the plot of oxygen isotope ratios along a mass-independent [slope ~1] line; the precise correlation between FeO/MgO and FeO/MnO molar ratios), also exists in equilibrated ordinary and R chondrites. However, studies have shown that these same trends that exist among ordinary chondrite clan meteorites were not derived through accretion of primary nebula material, but instead were the result of varying degrees of secondary low-temperature aqueous alteration on their respective parent bodies (R > LL > L > H > low-FeO chondrites). This alteration process involved oxidation of metal by isotopically heavy ice/water (Δ17O possibly as high as +20‰) to form FeO that was subsequently incorporated into existing silicates to establish the correlated redox trends. Therefore, Sanders et al. (2017) infer that a similar aqueous alteration process, induced by primary accreted water ice, occurred during the early history (pre-disruption) of the chondritic parent body from which the ureilites would ultimately derive. They propose that regions of the ureilite parent body with abundant accreted metal reached a maximum oxidation state with an olivine composition of ~Fa25, while other regions with less accreted metal experienced much less oxidation which is reflected in olivine of ~Fa5.

An alternate model was presented by Rai et al. (2019 [#3265], 2020) to explain the observed correlation between Mg# of olivines and Δ17O as well as other properties of ureilites. The original ureilite parent body, which they termed the Proto-Ureilite Planetesimal (PUP) was initially compositionally zoned due to the difference in accretion temperatures between Mg-rich and Fe-rich nebular components; Mg-rich material condensed at higher temperatures and formed the inner portions of the PUP, while later condensing Fe-rich material formed the outer layers (see schematic illustration below). After undergoing a low-degree of partial melting and differentiation, the PUP was catastrophically disrupted and re-accreted into one or more daughter bodies comprised of a larger proportion of the Fe-rich material from the outer layers of the PUP. One such daughter body is now in a propitious location, such as near the 3:1 mean-motion resonance with Jupiter at 2.50 AU, to enable efficient delivery of fragments to Earth.

1. Intial accretion of the UPB consisted of more MgO-rich material with low Δ17O and δ13C values (see also the scenario of Barrat et al. [2016] above).
2. Accretion transitioned to more FeO-rich material with higher Δ17O and δ13C values, establishing a compositional gradient.
3. Radiogenic heating (²⁶Al) produced an FeS melt which percolated to form a core.
4. Continued heating produced a silicate partial melt including buoyant basalts and trachyandesites which formed a crust.
5. While the UPB was still hot, a severe impact ejected a predominantly Fe-rich outer section which re-accreted to form a rubble-pile daughter body.

Chromium isotopic compositions were studied for Almahata 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 4.5636 (±0.0022) b.y., anchored to the angrite D'Orbigny. It was shown that the ε54Cr systematics for Almahata 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 Almahata Sitta are very similar to those of the HED parent body, and it can be inferred that both the ureilite and HED parent bodies accreted and experienced igneous melting very early in solar system history—within 5 m.y. of CAI formation—in a similar region of the solar nebula. The short-lived Hf–W chronometer was applied to a large number of ureilites to constrain the timing of metal–silicate separation on the UPB (Budde et al., 2015). Their calculations indicate that parent body differentiation occurred as early as 2.6 (±0.9) m.y. after CAI formation. As a consequence of this timing and in accord with thermal history modeling, accretion of the UPB would have occurred ~1.4 m.y. after CAIs. In further studies based on Al–Mg systematics, Hublet et al. (2016) obtained a model age for differentiation on the UPB of 1.09 (±0.75) m.y. after CAIs, indicating that accretion began even earlier in solar system history. Likewise, Zhu et al. (2019) obtained precise ε54Cr values for eleven monomict urelites and the trachyandesite crustal lithology ALM-A (MS-MU-011). They calculated a Mn–Cr isochron age anchored to D'Orbigny of 4.5671 (±0.0010) b.y., only ~1 m.y. after CAIs, which they consider likely represents the formation of planetesimals precursor to the ureilite parent body.

A classification of the Almahata Sitta polymict breccia ureilite was given 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 the scheme of Goodrich et al. (2004), in which ureilites are divided into three distinct subgroups—olivine–pigeonite, olivine–orthopyroxene, and augite-bearing—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, Almahata Sitta 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); a rapid cooling phase followed.

The average CRE age based on 21Ne for four Almahata Sitta ureilite fragments was determined to be 14.5 (±0.9) m.y. (Welten et al., 2010, 2011). This age falls well within the range of typical ureilites and represents the time since asteroid 2008 TC3 broke away from its larger parent asteroid. A more reliable method employed by Welten et al. (2010) gave an average CRE age of 19.5 (±2.5) m.y., which is consistent with other ureilites. Further noble gas studies of ureilite fragments conducted by Nagao et al. (2014) gave similar 3He- and 21Ne-based CRE ages of ~20 m.y. The He-, Ne-, and Ar-based CRE age values of H and L chondrite material from Almahata Sitta were determined by Meier et al. (2010) to be ~13.6–27 m.y., overlapping within error those of Almahata Sitta ureilites. Additional noble gas studies conducted by Riebe et al. (2014) of several chondritic samples from Almahata Sitta (E, LL, CB, R-like chondrites) also support this age. With inclusion of all CRE age results obtained to date, they 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. The gas retention age of the L chondrite samples from Almahata Sitta is distinct from that of the typical L group members, the latter forming an ~500 m.y. age cluster. Notably, one E chondrite sample studied by Riebe et al. (2017) designated MS-179, 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 might 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. Further noble gas data were obtained for the C1 breccia clast AhS 91A by Riebe et al. (2018 #2295), and for the trachyandesite sample MS-MU-011 Riebe et al. (2022 #6258). They found that both of these samples have similar CRE ages of ~7–8 m.y., which is significantly lower than other Almahata Sitta components measured to date. Like all other AhS samples, and consistent with its low CRE age indicating residence at some depth (at least a few meters) within a regolith, no solar-wind-implanted noble gases have yet been definitively observed in AhS 91A (Goodrich et al., 2019). Based on these CRE age data, Riebe et al. (2022) inferred that the actual length of time these fragments spent in space before arrival on Earth was no longer than ~8 m.y., with most fragments experiencing pre-irradiation before parent body breakup.

In an attempt to identify possible common ejection events among the ureilites, Beard and Swindle (2017) conducted a comparative study of 39 different samples utilizing three parameters: CRE age, Fo content in olivine (Mg#), and Δ17O value. They resolved ten potential clusters, several of which show concordance in their CRE age and Mg# but not in Δ17O value (heterogeneous), and three that are concordant in all three parameters (homogeneous). The oldest cluster they resolved, comprising DaG 084, DaG 319, Goalpara, and Haverö (homogeneous, although the Δ17O values for DaG 084 and DaG 319 have not yet been determined), reflects an ejection event that occurred 20.1 (±1.2) m.y. ago. The CRE age of this cluster is consistent with the average of all CRE age results obtained to date (see Riebe et al. [2017] information above). However, Beard and Swindle (2017) utilized alternate data for Almahata Sitta which provided a CRE age of 15.80 m.y., and they have included it along with GRA 98032 and Lahrauli within a heterogeneous cluster dated at 15.5 (±0.3) m.y. Leya and Stephenson (2019) conducted a more advanced 21Ne-based CRE age study employing 40 individual ureilites and correcting for pre-atmospheric radius and shielding depth. They determined that ureilites with short CRE ages are more highly represented than those with long CRE ages (see diagram below). This supports the inference that the ureilite parent body supplying fragments to Earth is currently moving closer to an efficient mean motion resonance.

Trapped noble gas contents and Xe-isotopic compositions in Almahata Sitta are consistent with those of other ureilites (Ott et al., 2010), with sub-µm- to several µm-sized diamonds 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, which seems to correspond with the Late Heavy Bombardment period (Welten et al., 2011).

The magnetic signature of Almahata Sitta was found to be identical to that of previously studied ureilite falls, containing kamacite (two distinct phases), suessite (an iron silicide formed in situ from pre-existing ureilitic metal under low pressure [regolith], high temperature [impact heating], and very reducing conditions [abundant carbon]; Downes et al., 2010; Ross et al., 2019), schreibersite, and troilite, as well as a daubreelite-heideite phase new to ureilites which is possibly a contaminate from 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. As a result of shock re-melting processes in Almahata Sitta, grain boundary FeNi-metal particles are complex assemblages not observed in main group ureilites, comprising variable combinations of taenite, kamacite, Fe-carbide (cohenite), Fe-phosphide (schreibersite), and Fe-sulfide (Aoyagi et al., 2013). Mikouchi et al. (2014) identified an Fe-martensite phase in Almahata Sitta that is a product of very rapid quenching (>1°C/sec) at relatively low temperatures (~300–500°C). They hypothesized that the martensite was formed through an impact-related heating and fragmentation event on a small ureilitic asteroid; the asteroid probably represents a re-accreted daughter object following the collisional disruption of the original ureilite parent body.

Spectral studies of the many asteroid families have revealed some potential matches to 2008 TC3 and Almahata Sitta, especially the F-class Nysa-Polana family located in the inner Main Belt near the 3:1 resonance. The Nysa-Polana family has a low orbital inclination similar to that observed for 2008 TC3 along its trajectory to Earth (Gayon-Markt, 2011). This asteroid 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). There is also a possibility that the foreign clasts in Almahata Sitta were derived from a collision with the S-class Mildred family located in the inner Main Belt (Jenniskens et al, 2010). Asteroid 2008 TC3 is one of 69 members of the '1999TV16' association (orbital cluster) of near-Earth asteroids identified by T. J. Jopek (2020), which are predicted to generate bolides that intercept Earth during the period October 7th to February 8th (see diagram below).

As a quickly recovered fall, Almahata Sitta has a weathering grade of W0. It 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). The specimen of Almahata Sitta shown above is a 0.33 g crusted partslice sectioned from an undesignated coarse-grained lithology, which represents the most abundant lithology recovered (Horstmann and Bischoff, 2014). The bottom photo below is an excellent petrographic thin section micrograph of a coarse-grained sample of Almahata Sitta shared by Peter Marmet.

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.

See the PSRD article by Linda M. V. Martel, "Asteroid, Meteor, Meteorite", April 30, 2010.