Introduction: Asteroid 2008 TC3 was the first near-Earth object to be detected and studied in space before it hit the Earth [1-6]. More than 700 cm-sized stones (~0.2-400 g in mass) were recovered from the predicted fall area and named the Almahata Sitta (AhS) meteorite [7]. AhS is the first meteorite observed to originate from a spectrally classified asteroid. It pro-vides an unprecedented opportunity to correlate spec-tral, compositional, and physical properties of a mete-orite with those of the asteroid from which it was de-rived. The reflectance spectrum of 2008 TC3 was meas-ured in the 0.5-1 ?m range [4] and most closely matches F-type asteroids [8]. The average F-type al-bedo of 0.046 [9] is consistent with independent esti-mates of the asteroid's size [4]. F-type asteroids belong to the C complex of dark asteroids commonly identi-fied with carbonaceous chondrites [8-11]. If Almahata Sitta had not been recovered, 2008 TC3 would have been assumed to be a carbonaceous chondrite-type asteroid. However, AhS turned out to be a unique and com-plex meteorite. The AhS stones are diverse, with ~70-80% of those studied so far being various types of ureilites (achondrites from the mantle of a differenti-ated asteroid), and 20-30% being various types of chondrites (mainly enstatite-, ordinary- and Rumuruti-types) [12-14]. Based on the apparent dominance of ureilites, AhS was classified as an anomalous polymict ureilite [4]. It has been inferred that 2008 TC3 was a loosely-consolidated, heterogeneous breccia that disin-tegrated in the atmosphere, with its clasts landing on Earth separately and most of its mass lost [4,13]. De-termining its structure and composition has been hin-dered so far because none of the studied AhS stones showed contacts between ureilitic and chondritic lithologies. We have now discovered AhS 91A and AhS 671, the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC3. Combined petrologic, geochemical, physical, and spectroscopic studies of these stones [14] provide clues and cautions to distinguishing differentated dark aster-oids from primitive dark asteroids in remote spectros-copy.
DISTINGUISHING DIFFERENTIATED DARK ASTEROIDS FROM PRIMITIVE DARK ASTEROIDS: CLUES AND CAUTIONS FROM ASTEROID 2008 TC3 AND THE ALMAHATA SITTA METEORITE
AM Fioretti;
2019
Abstract
Introduction: Asteroid 2008 TC3 was the first near-Earth object to be detected and studied in space before it hit the Earth [1-6]. More than 700 cm-sized stones (~0.2-400 g in mass) were recovered from the predicted fall area and named the Almahata Sitta (AhS) meteorite [7]. AhS is the first meteorite observed to originate from a spectrally classified asteroid. It pro-vides an unprecedented opportunity to correlate spec-tral, compositional, and physical properties of a mete-orite with those of the asteroid from which it was de-rived. The reflectance spectrum of 2008 TC3 was meas-ured in the 0.5-1 ?m range [4] and most closely matches F-type asteroids [8]. The average F-type al-bedo of 0.046 [9] is consistent with independent esti-mates of the asteroid's size [4]. F-type asteroids belong to the C complex of dark asteroids commonly identi-fied with carbonaceous chondrites [8-11]. If Almahata Sitta had not been recovered, 2008 TC3 would have been assumed to be a carbonaceous chondrite-type asteroid. However, AhS turned out to be a unique and com-plex meteorite. The AhS stones are diverse, with ~70-80% of those studied so far being various types of ureilites (achondrites from the mantle of a differenti-ated asteroid), and 20-30% being various types of chondrites (mainly enstatite-, ordinary- and Rumuruti-types) [12-14]. Based on the apparent dominance of ureilites, AhS was classified as an anomalous polymict ureilite [4]. It has been inferred that 2008 TC3 was a loosely-consolidated, heterogeneous breccia that disin-tegrated in the atmosphere, with its clasts landing on Earth separately and most of its mass lost [4,13]. De-termining its structure and composition has been hin-dered so far because none of the studied AhS stones showed contacts between ureilitic and chondritic lithologies. We have now discovered AhS 91A and AhS 671, the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC3. Combined petrologic, geochemical, physical, and spectroscopic studies of these stones [14] provide clues and cautions to distinguishing differentated dark aster-oids from primitive dark asteroids in remote spectros-copy.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
