Fell October 7, 2008
20° 43.04′ N., 32° 30.58′ E. In 2008, October 6 at 5:46 A.M., asteroid 2008 TC3 fell to Earth in northern Sudan. See the Almahata Sitta webpage for the complete story of the discovery of this meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and Click on Term to Read More, results of the consortium analyses, and new models for the petrogenetic history of the ureilite parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. Click on Term to Read More.
The 2008 TC3
meteorite was sent to NASA’s Johnson Space Center in Houston (Zolensky) and Carnegie Institution of Washington (Steele) for analysis and classification, and Alamahta Sitta was determined to be a polymict ureilite fragmental brecciaWork in Progress ... A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts). A breccia may also be distinguished by the origin of its clasts: (monomict breccia: monogenetic or monolithologic, and polymict breccia: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material Click on Term to Read More
composed of three main ureilite lithologies, along with a wide range of xenolithic clasts representing many different chondritic and achondritic lithologies in an assemblage similar to the polymict brecciaGeneral term for all breccias that are neither monomict nor dimict. Modified from image source: http://www.saharamet.com/meteorite/gallery/HED/index.html.
Kaidun (Bischoff et al.
, 2010). Results of the analyses indicate that all of the clasts came from the Almahata Sitta fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More
, detection of short-lived cosmogenic nuclides, very low weathering grade (W0–W0/1), multiple lithologies among fragments delimiting a strewn fieldArea on the surface containing meteorites and fragments from a single fall. Also applied to the area covered by tektites, which are produced by large meteorite impacts. Strewnfields are often oval-shaped with the largest specimens found at one end. Given that the largest specimens go the greatest distance, a meteoroid's
, a high number of rare E-chondriteType of meteorite high in the mineral enstatite and also referred to as E-chondrites. Although they contain substantial amounts of Fe, it is in the form of Ni-Fe metal or sulfide rather than as oxides in silicates. Their highly reduced nature indicates that they formed in an area of the Click on Term to Read More
rock types found together, diffusionMovement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be represented by a change in concentration). Diffusion, the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomena. Because diffusion is thermally activated, coefficients for diffusion Click on Term to Read More
of PAHs among clasts [Sabbah et al.
, 2010], and the finding of new and unique meteorite fragments within a small area.
With inclusionFragment of foreign (xeno-) material enclosed within the primary matrix of a rock or meteorite. Click on Term to Read More
of most all CRE age results obtained to date for both ureilite and chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More
samples, Riebe et al.
(2017) derived an average age of 19.7 (±2.8) m.y. This relatively young CRE age for the various components of 2008 TC3
indicates that it was located in close proximity to a mean-motion resonance. Notably, one E chondrite sample designated MS-179 that was studied by Riebe et al.
(2017), which is linked to the Almahata Sitta fall through multiple lines of evidence, presents a younger CRE age of 11 (±1.4) m.y. They suggest this lower CRE age may represent the actual age of the 2008 TC3 meteoroidSmall rocky or metallic object in orbit around the Sun (or another star).
as an independent object in space subjected to galactic cosmic raysHigh-energy subatomic particles mainly originating outside the Solar System that continuously bombard the Earth from all directions. They represent one of the few direct samples of matter from outside our solar system and travel through space at nearly the speed of light. These charged particles – positively charged protons or Click on Term to Read More
(4π exposure), while the older ages of the other components could be attributed to pre-irradiation (2π exposure) within a regolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles). Click on Term to Read More
setting on the parent asteroid.
The heterogeneous composition of Almahata Sitta could reflect an assemblage derived from a catastrophic collision(s) between ureilte and chondrite objects (Kohout et al.
, 2010). Alternatively, it is considered likely that these diverse clasts could have become gravitationally bound within a common debris disk composed of a disrupted ureilite asteroid, and that this disk then re-accreted into one or more smaller second-generation asteroids. This second-generation asteroid later became lightly sintered together through subsequent low-energy impacts, resulting in a bulk porosityThe volume percentage of a rock that consists of void space. Vesicular porosity is a type of porosity resulting from the presence of vesicles, or gas bubbles, in igneous rock such as the pumice presented here. Vesicular porosity is very rare in meteorites and is often associated with slag, one Click on Term to Read More
of ~50%. The highly porous ureilite material recovered from the Almahata Sitta fall, such as MS-168
, is consistent with the hypothesized lightly-sintered matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Click on Term to Read More
of the second-generation asteroid 2008 TC3
Inclusion MS-181 is the first carbonaceous chondriteCarbonaceous chondrites represent the most primitive rock samples of our solar system. This rare (less than 5% of all meteorite falls) class of meteorites are a time capsule from the earliest days in the formation of our solar system. They are divided into the following compositional groups that, other than Click on Term to Read More
found associated with the Almahata Sitta fall to date. MS-181 originally weighed 58.63 g, but only 42 g remained after cut losses. An analysis of cosmogenic radionuclides present in MS-181, conducted at the Max-Planck Institute in Heidelberg, Germany, has definitively confirmed that the timing of its fall event is consistent with that of Almahata Sitta. Results from analyses of MS-181 were presented at the 75th Annual Meeting of the Meteoritical Society in August 2012, and further information about Almahata Sitta and its wide variety of inclusions is scheduled to be published in a special issue of MeteoriticsScience involved in the study of meteorites and related materials. Meteoritics are closely connected to cosmochemistry, mineralogy and geochemistry. A scientist that specializes in meteoritics is called a meteoriticist. Click on Term to Read More
& Planetary Science.
Exclusive of the primary ureilite components, there was a broad diversity of lithologic types present in 2008 TC3
, constituting <30% of all material recovered. However, since the vast bulk of 2008 TC3
is thought to have been lost as fine dust (≥99.9% of the estimated 42–83 ton pre-atmospheric mass), the asteroid was likely composed primarily of fine-grained, highly-porous, weakly-consolidated ureilitic matrix material consistent with the reflectance spectra obtained for the asteroid (Goodrich et al.
, 2015). Examples of some of the diverse samples that have been recovered are listed below (Bischoff et al.
, 2010, 2015, 2016, 2018; Horstmann and Bischoff, 2010, 2014; Hoffmann et al.
- ultrafine- to fine-grained ureilites (representing numerous lithologies with varying olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion Click on Term to Read More compositions): MS-185 (ultrafine-grained), 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-MU-005, -006, -008, -010, -014 (very coarse), -016, -017, -020, -022, -034, -037, -038
- variable grain-sized ureilite breccias: MS-25, -205, -190; MS-MU-004, -021, -028, -042
- highly porous ureilitic (matrix) material: MS-168
- enstatiteA mineral that is composed of Mg-rich pyroxene, MgSiO3. It is the magnesium endmember of the pyroxene silicate mineral series - enstatite (MgSiO3) to ferrosilite (FeSiO3). Click on Term to Read More chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More (36 representing numerous different enstatite chondrites): EH3 (MS-14), 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), EL5 (MS-196), EL5/6 (MS-7), EL6 (MS-150, MS-MU-007, -015, -024, -026), 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]), H5/6 (MS-11, with compositional discordance), L4 (AhS A100), LL4/5 (MS-197), H5-an (MS-MU-013)
- 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 clastA mineral or rock fragment embedded in another rock. Click on Term to Read More containing metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More globules and silicates in a 60:40 ratio, having an O-isotopic composition consistent with bencubbinites
- C2 carbonaceous chondrite: AhS 202 (photo; Fioretti et al., 2017, #1846)
- C1 carbonaceous chondrite: AhS 91/91A and 671 (photo; Goodrich et al., 2018, #1321)
- niningerite-bearing, fine-grained ureilitic fragment (linking E chondrites): MS-20
- sulfide-metal assemblage in a fine-grained ureilitic fragment: MS-158, -166
- ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). Click on Term to Read More enstatite- and metal-rich achondriteAn achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent body reached a sufficient size that through heating due to radioactive decay of 26Al (aluminum isotope) and gravitational Click on Term to Read More fragments: MS-MU-019 (characteristics similar to NWA 8173/10271); MS-MU-036 (similar to MS-MU-019 and Itqiy [Bischoff et al., 2016]); 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–augiteHigh-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, that occurs in many igneous rocks, particularly those of basaltic composition. In order to be considered augite, the clinopyroxene must contain 20 to 45 mol % of calcium (Wo20 - 45). An important and unique Martian meteorite is NWA 8159, that has been classified as an augite Click on Term to Read More ureilite lithology: MS-MU-012
- trachyandesitic clasts: 1) MS-MU-011 (view 1), MS-MU-011 (view 2), sample ALM-A; plagioclase-enriched (~70 vol%) with pockets of gemmy olivine (photo courtesy of Stephan Decker), likely sampling the UPB crustOutermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of ices, frozen gases, and accumulated meteoritic material. On Earth, the Click on Term to Read More (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)
Special thanks to Siegfried Haberer and Stephan Decker for providing specimens of this special meteorite and many of its xenolithic inclusions to the scientific and collector communities. The photo of the MS-181 bencubbinite inclusion shown above, and the reverse below, is a 0.107 g cut fragment.
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