Noble gases and nitrogen in the Almahata Sitta ureilite

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Abstract-On October 7, 2008, a small F-class asteroid, 2008 TC 3 , exploded in Earth's atmosphere, and produced a strewn field of meteorites in the Nubian Desert of Sudan. Subsequent searches yielded several hundred meteorite fragments, known as Almahata Sitta. This fall was classified as a polymict ureilite. We measured cosmogenic radionuclides in six fragments and noble gases in four fragments of the Almahata Sitta ureilite. The concentrations of 10 Be, 26 Al, and 36 Cl in the meteorite fragments of asteroid 2008 TC 3 indicate a preatmospheric radius of 300 ± 30 g cm )2 . Combined with an absolute radius of 1.8 ± 0.2 m, which was derived from its brightness in space, the cosmogenic radionuclides constrain the bulk density of asteroid 2008 TC 3 to 1.66 ± 0.25 g cm )3 and the bulk porosity to 50 ± 7%. The bulk density of asteroid TC 3 is on the low end of the range of 1.6-3.3 g cm )3 determined for 40 of the recovered ureilite fragments. Since the denser materials have a higher chance of surviving atmospheric fragmentation, the lowdensity ureilites are probably more representative of the bulk asteroid. We thus conclude that the high porosity of asteroid 2008 TC 3 is mainly due to microporosity, which implies that not all low-density asteroids are necessarily rubble pile structures. Finally, from the cosmogenic 21 Ne concentrations, we determined a cosmic-ray exposure (CRE) age of 19.5 ± 2.5 Myr. This age represents the time since asteroid 2008 TC 3 was ejected from a large F-class parent body in the asteroid belt, until its collision with Earth.

Section: Resultscontrasting

confidence: 70%

Cosmogenic nuclides in Almahata Sitta ureilites: Cosmic‐ray exposure age, preatmospheric mass, and bulk density of asteroid 2008 TC₃

Welten

Meier

Caffee

et al. 2010

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“…The bulk δ 15 N isotopic composition obtained by Murty et al. () of −36.8‰ is lighter than our bulk value of −24‰ but their peak release, i.e., the single temperature step where the largest amount of N was released, occurred at 1000 °C with a δ 15 N of −94.3‰, which is within error of the composition (−95.6‰) seen in the 650 °C main release in our analysis of AS36.…”

Section: Discussionsupporting

confidence: 59%

“…Murty et al. () also carried out step‐wise combustion analyses of nitrogen isotopes in AS36. Their bulk sample of AS36 contained almost twice the amount of nitrogen (21.1 ppm compared with our value of 9.35 ppm).…”

Section: Discussionmentioning

confidence: 99%

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Isotopic composition of carbon and nitrogen in ureilitic fragments of the Almahata Sitta meteorite

Downes

Abernethy

Smith

et al. 2015

This study characterizes carbon and nitrogen abundances and isotopic compositions in ureilitic fragments of Almahata Sitta. Ureilites are carbon‐rich (containing up to 7 wt% C) and were formed early in solar system history, thus the origin of carbon in ureilites has significance for the origin of solar system carbon. These samples were collected soon after they fell, so they are among the freshest ureilite samples available and were analyzed using stepped combustion mass spectrometry. They contained 1.2–2.3 wt% carbon; most showed the major carbon release at temperatures of 600–700 °C with peak values of δ13C from −7.3 to +0.4‰, similar to literature values for unbrecciated (“monomict”) ureilites. They also contained a minor low temperature (≤500 °C) component (δ13C = ca −25‰). Bulk nitrogen contents (9.4–27 ppm) resemble those of unbrecciated ureilites, with major releases mostly occurring at 600–750 °C. A significant lower temperature release of nitrogen occurred in all samples. Main release δ15N values of −53 to −94‰ fall within the range reported for diamond separates and acid residues from ureilites, and identify an isotopically primordial nitrogen component. However, they differ from common polymict ureilites which are more nitrogen‐rich and isotopically heavier. Thus, although the parent asteroid 2008TC3 was undoubtedly a polymict ureilite breccia, this cannot be deduced from an isotopic study of individual ureilite fragments. The combined main release δ13C and δ15N values do not overlap the fields for carbonaceous or enstatite chondrites, suggesting that carbon in ureilites was not derived from these sources.

“…1993). The extent to which this noble gas composition is nebular has been an enduring discussion (Berkley and Jones 1982; Murty et al. 2010).…”

Section: Discussionmentioning

confidence: 99%

Thermal and fragmentation history of ureilitic asteroids: Insights from the Almahata Sitta fall

Herrin

Zolensky

Ito

et al. 2010

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150

Abstract-The Almahata Sitta fall event provides a unique opportunity to gain insight into the nature of ureilitic objects in space and the delivery of ureilite meteorites to Earth. From thermal events recorded in the mineralogy, petrology, and chemistry of ureilites recovered from the fall area, we reconstruct a timeline of events that led to their genesis. This history is similar to that of other known ureilites and supportive of a disrupted ureilite parent body hypothesis. Temperatures of final mantle equilibrium were 1200-1300°C, but this hightemperature history was abruptly terminated by rapid cooling and reduction associated with pressure loss. The onset of late reduction reactions and onset of rapid cooling must have been essentially simultaneous, most likely engendered by the same event. Cooling rates of 0.05-2°C h )1 determined from reversely zoned olivines and pyroxenes in Almahata Sitta imply rapid disassembly into fragments tens meters in size or smaller. This phenomenon seems to have affected all known portions of the ureilite parent body mantle, implying an event of global significance rather than localized unroofing. Reaccretion of one or more daughter asteroids occurred only after significant heat loss at minimum time scales of weeks to months, during which time the debris cloud surrounding the disrupted parent was inefficient at retaining heat. Fragments initially dislodged from the ureilite parent body mantle underwent subsequent size reduction and mixed with various chondritic bodies, giving rise to polylithologic aggregate objects such as asteroid 2008 TC 3 .