This 9.3 kg stone was found in Grady, New Mexico and determined to be distinct from Grady (1933), which is classified as an L chondrite. While this meteorite contains a total iron content (26.23 wt%) within the range of H-chondrite falls, it has Fa and Fs values well below those of normal H chondrites (Rubin and Krot, 1993). It was likely reduced during parent body metamorphism. Recent studies have provided evidence of reduction of unequilibrated ordinary chondrites corresponding to increased metamorphism, perhaps through the progressive dehydration of phyllosilicates. Beginning with type 4 equilibrated chondrites, the trend is reversed, and an increase in metamorphism corresponds to progressive oxidation of the chondrite.
It was determined that the H-chondrite parent body suffered three distinct collisional events at ~7.0, 22, and 33 m.y. ago (Marti and Graf, 1992; Eugster et al., 2006, 2007). These ejection events produced only weak shock effects (S1S2) and radiogenic gas loss, but injected abundant fragments into Earth-crossing resonances. The H chondrites are a good spectrographic match with the S(IV)-type asteroids 6 Hebe, 3 Juno, and 7 Iris, with 6 Hebe being the favored parent asteroid up to this time. However, hydrocode models show inconsistencies exist between expected and observed CRE ages based on the scenario of direct injection into resonances. The steady delivery of H chondrite material from 6 Hebe to Earth also remains unexplained. Studies by Rubin and Bottke (2009) led them to conclude that family-forming events resulting in large meteoroid reservoirs, which have homogenous compositions and locations near dynamical resonances such as the Jupiter 3:1 mean motion resonance, are the likely source of the most prevalent falls including H chondrites and HED achondrites (especially howardites). As a matter of fact, a number of asteroids having H-like mineralogies have been observed near the 3:1 (2.50 AU) and 5:2 (2.82 AU) resonances (Burbine et al., 2015 and references therein). See further details on the Abbott page.
Based on models comparing PbPb age data with closure temperatures for chondrules and phosphates in H chondrites, it was estimated that accretion of the H chondrite parent body began about 1.7 m.y. after formation of CAIs and continued for 3.5 m.y. (Amelin et al., 2005). These thermal models also permitted a calculation to be made reflecting the progressive increase in petrologic types from the core to the surface: from the core outward to a distance of 44.9 km is type 6 material; between 44.9 km and 48.9 km is type 5 material; between 48.9 km and 56.9 km is type 4 material; and from 56.9 km to the surface at 92.5 km is type 3 material.
In their modeling of the accretion and impact history of ordinary chondrites, Blackburn et al. (2017) calculated the timing of the catastrophic disruption of the H- and L-chondrite parent bodies to be ~60 m.y. after CAIs. This timing is consistent with two competing dating techniquesUPb (and HfW) chronometry and metallographic cooling rates (Ni diffusion profiles in Fe-metal)which record cooling associated with both an onion shell structure prior to disruption and a rubble pile after disruption, respectively. Utilizing Pbphosphate age data, Edwards et al. (2017) determined that the H and L chondrites of petrologic type 6 (i.e., those located at the greatest depths in a concentrically zoned body) show a similar timing for closure of the Pb-phosphate system of ~60 m.y. after CAIs; this age reflects the occurrence of ubiquitous quenching during parent body disruption. Employing thermal models, they constrained the timing of accretion for the two parent bodies to 2.02.35 m.y. after CAIs, and they derived an estimate for the minimum size of the two parent bodies of ~275 km in diameter.
Another S-class asteroid, 433 Eros, recently played host to the NEAR-Shoemaker spacecraft (see NEAR's final image below). A successful landing was followed by an unprecedented multiple spectrographic analysis of the surface. Results from this indicate that Eros has primitive Mg/Si, Al/Si, Ca/Si and Fe/Si ratios, consistent with ordinary chondrite mineralogy, closely resembling H-group chondrites. Contrary to this data, the magnetometer data exclude any relationship between 433 Eros and H- or L-group chondrites, although the LL-group chondrites could not be excluded. The specimen of Grady (1937) shown above is a 3.25 g partially crusted fragment.
The surface of Eros from NEAR prior to loss of transmission.
Image measures 20 feet across. NASA photo.