ALMAHATA SITTA


Ureilite
Polymict
Shallow regolith fragmental breccia

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click on image for a magnified view

Fell October 7, 2008
20° 43.04' N., 32° 30.58' E.

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).

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Asteroid 2008 TC3 prior to atmospheric entry.
Credit: M. Kozubal & Ron Dantowitz—Clay Center Observatory

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A sequence of images of asteroid 2008 TC3 taken at Mount Lemmon Observatory.
Credit: Richard Kowalski and Ed Beshore, Catalina Sky Survey

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).

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Artist's conception of the pre-impact shape of asteroid 2008 TC3
based on actual observations as shown by the horizontal line at the top.
Photo Credit: P. Scheirich, P. Jenniskens

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Real-time IR and visual composite image of the 2008 TC3 asteroid impact.
Image from the European Space Agency's Meteosat 8 weather satellite.

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Contrail of 2008 TC3 at dawn distorted by upper atmosheric winds.
Photo Credit: Mohamed Elhassan Abdelatif Mahir (Noub NGO),
Dr. Muawia H. Shaddad (Univ. Khartoum),
Dr. Peter Jenniskens (SETI Institute/NASA Ames)

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Ground path of the meteoroid over Sudan.
Red line is the object's path, terminating where it would have hit the ground.
Green line is the infrasound detection of the explosion.
Orange crosshairs mark the fireball location according to Meteosat 8 IR data.
Map of Sudan: CIA Factbook. Graphic overlay: George W. Herbert, 10-07-2008.

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Ground path indicating the location of recovered meteorites (red dots).

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 strewnfield measuring about 28 × 5 km. Succeeding searches yielded additional specimens which were analyzed and classified at the University of Münster.

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 Alamahta 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. Alamahta 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.

The fine-grained lithology of Alamahta Sitta has significant porosity (~20%) comprising 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. Coarse-grained samples are only weakly shocked (S2–3). The occurrence of significant porosity led some to conclude that the meteorite 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.

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 formed from shocked graphite on the parent body as evidenced by the presence of high-pressure, compressed graphite phases. 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 Alamahta 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. Nevertheless, large diamond assemblages measuring ~100 µm have been identified in low-shock ureilite lithologies in Alamahta Sitta, considered most likely to have formed relatively slowly in a melt phase located deep within the UPB (Miyahara et al., 2015). Still, 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).

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Pore wall lined with crystalline olivine produced by vapor deposition.
Photo Credit: M.E.Zolensky et al., 41st LPSC, #2306 (2010)

Further analyses of 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). 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 is 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 bulk porosity of ~50% and manifesting a very weak object.

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, has been developed (Goodrich et al., 2015; Wilson and Goodrich, 2016). The 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, incorporating 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 the Earth where the complex history of Almahata Sitta could be revealed. Furthermore, it is 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 homogenous 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 a fragment on asteroid 2008 TC3; activity of 60Co and 26Al indicate a shielding depth of 27–55 cm.

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 first 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 (MS-MU-011 and -035). It is presumed that after extraction of this relatively buoyant liquid, leaving behind an olivine and pigeonite residue, it reached the surface and built up a crust to some extent. A representative of the next sequential melt to be extracted has not 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 material might have been located deeper in the mantle of the UPB. Regardless of the actual density of this second-stage melt, the model scenario presented infers a partial melting degree of ~17–28% which 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 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. 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.

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, with the vast bulk of 2008 TC3 thought to have been lost as fine dust (≥99.9% of the estimated 42–83 ton pre-atmospheric mass), the bulk asteroid was likely composed of fine-grained, highly porous, and 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; Fioretti et al., 2017):

MS-MU-011 Trachyandesite clast with gemmy olivines
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click on photos for a magnified view
Photos courtesy of Stephan Decker–Meteorite Shop and Museum

A consortium was established to study Alamahta Sitta, and much has since been learned. This is a characteristic ureilite that has experienced high degrees of silicate partial melting. It 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 calculated to have been ~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. 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.

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Image credit: Rai et al., 47th LPSC, #1702 (2015)

By contrast, studies of ε54Cr anomalies in Alamahta 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. The ε54Cr value infers that Alamahta 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.

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Image credit: Goodrich et al., MAPS, vol. 50, #4, p. 792 (2015)
'Origin and history of ureilitic material in the solar system: The view from asteroid 2008 TC3 and the Almahata Sitta meteorite'
(http://dx.doi.org/10.1111/maps.12401)

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. It was shown that the 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, and 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.

A classification of the Alamahta Sitta polymict breccia ureilite was given by Zolensky et al. (2010). They concluded that its Mg# characterizes place 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, Alamahta 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 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. Further noble gas data from ureilite fragments also indicated a 3He- and 21Ne-based CRE age of ~20 m.y. (Nagao et al., 2014). 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-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 may correspond to the period of 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 kamacite (two distinct phases), suessite (an iron silicide formed in situ from pre-existing ureilitic metal under low pressure [regolith] and very low redox conditions; Downes et al., 2010), schreibersite, and troilite, as well as a daubreelite-heideite phase new to ureilites that is possibly contamination 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 comprising variable combinations of taenite, kamacite, Fe-carbide (cohenite), Fe-phosphide (scheibersite), and Fe-sulfide, which is not observed in main group ureilites (Aoyagi et al., 2013). Mikouchi et al. (2014) identified an Fe-martensite phase in Almahata Sitta, which 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). 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. 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 Alamahta Sitta shown above is a 0.33 g crusted partslice sectioned from an undesignated coarse-grained lithology.

Ureilites Are Finally Figured Out

Image credit: Goodrich et al., MAPS, vol. 50, #4, p. 784 (2015)
'Origin and history of ureilitic material in the solar system: The view from asteroid 2008 TC3 and the Almahata Sitta meteorite'
(http://dx.doi.org/10.1111/maps.12401)

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.