Monahans (1998)

standby for monahans (1998) photo
Fell March 22, 1998
102° 53.5′ W., 31° 36.5′ N. On a Sunday evening at 6:37 P.M., a fireball that was observed for over 150 miles, accompanied by sonic booms and a dust train, dropped two stones on the town of Monahans, Texas. The first stone, weighing 1,243 g, created a 4-inch-deep crater in the ground about 25 feet from where seven boys were playing basketball. This warm, black, fusion-crusted stone was recovered immediately. The second stone, which weighed 1,344 g, created a 2-inch-deep crater in the asphalt of Allen Street, and it was recovered the following morning. The cratered section of the road was also preserved.

Within 50 hours of the fall, these meteorites were subjected to short-lived radionuclide analysis at NASA–JSC labs. Petrographic studies have identified two lithologies, one dark and the other light. Based on Ar–Ar age calculations, the light lithology has a very old maximum age of 4.53 b.y. while the dark lithology has a slightly younger maximum age of 4.50 b.y.; this difference is probably due to Ar loss during time spent within a thin regolith. The dark lithology also contains high concentrations of implanted solar noble gases, consistent with a regolith origin. Beyond that, the dark lithology has experienced a higher equilibration and shock history (S4 compared to S2). From studies of cosmogenic nuclides in the light lithology, a cosmic-ray exposure age of 6.0 (±0.5) m.y. was calculated. This CRE age falls within the 5–9 m.y. range of the exposure ages of many other H chondrites, and it forms the latest of three distinct collisional events which occurred ~7.0, 22, and 33 m.y. ago. Also of importance is the higher concentration of solar rare gases present in the dark lithology, which suggests that it spent an additional 13–18 m.y. in a regolith setting at shallow depth. This early immature regolith was subsequently deeply buried by impact ejecta, preserving its cosmogenic history until it was ejected from the asteroid.

Salt crystals measuring 0.5–5 mm were discovered in the matrix lithology and identified as crystals of purple and blue halite (NaCl) and sylvite (KCl), the first identification of these minerals in ordinary chondrites. They are thought to have acquired the purple and blue colors during the long transit to Earth through the radioactive decay of elements in close proximity. The Monahans halide has an I–Xe age of ~4.559 b.y., which is consistent with other isotopic dating systems (Busfield et al., 2004). These minerals probably formed through the evaporation of asteroidal brines on a large, partially liquid, carbonaceous-type parent body (Ceres and cometary origins have been considered; e.g., Fries et al., 2013; Kebukawa et al., 2016). This material became mixed with surface constituents by later impact processing, and was ultimately ejected by low-energy (low-temperature) impacts, ultimately being incorporated within the regolith of the H-chondrite asteroid located in relatively close proximity.

Within these xenolithic halite crystals, a variety of silicate and carbonaceous inclusions have been found, representing a number of chondritic parent body sources (Kebukawa et al., 2014). Analyses of these solid inclusions revealed macromolecular carbon similar to that of matrix carbon in CM and CV chondrites, short-chain aliphatic compounds, graphite and diamond phases, silicates such as olivine, pyroxene, and feldspar, Fe-oxide/hydroxide, sulfide, and phosphate. More importantly, remnant fluid and vapor bubbles are present in the halite crystals. Studies have demonstrated that even modest shock pressures would likely have destroyed any trapped fluid inclusions (Madden et al., 2003), and therefore it has been suggested that geysering might be the actual ejection mechanism from the parent body (Fries et al., 2011). Fries et al. (2012) have reported finding the first meteoritic occurrence of methane dissolved in the halite. They consider it likely that the relatively abundant methane was exsolved from the carbonaceous inclusions which originated on the halite source parent body (different from the H-chondrite asteroid), and they submit that similar light organics could have been delivered to Earth in a similar manner. Further scenarios for the origination and transference of halite crystals to the asteroid 6 Hebe can be found on the Zag page.

The thermal history of the H-chondrite parent body was calculated by Harrison and Grimm (2010) based on cooling rate data and closure times. They found that the object accreted over a short time period of 2.2 m.y. In an alternative viewpoint, Monnereau et al. (2012) determined a more rapid accretion time period of 0.1–0.2 m.y. while 26Al was still extant. Moreover, Sokol et al. (2007) concluded that accretion of the ~150 km H-chondrite parent body occurred relatively late after most radioactive 26Al had decayed, at least 2 m.y. after CAI formation; it was probably heated by continuing impacts. Consistent with this scenario, John T. Wasson (2016) presented evidence that the slow heating generated entirely by the decay of 26Al is insufficient to melt asteroids, and that an additional heat source would have been required; e.g., the rapid heating incurred from major impact events. He determined that the canonical 26Al/27Al ratio of 0.000052 is much too low to cause any significant melting, and that a minimum ratio of 0.00001 would be required to produce a 20% melt fraction on a well-insulated body having a significant concentration of 26Al. For example, the initial ratio of 0.0000004–0.0000005 calculated for the angrites Sah 99555 and D’Orbigny based on their 26Al–26Mg isochrons is too low to have generated any significant melting without an additional heat source.

It is generally considered that the H-chondrite parent body eventually formed an insulated ‘onion-shell’ structure with a diameter of 150–260 km. The asteroid was composed approximately (by volume) of 84%, 10%, and 6% of type 6, type 4/5, and type 3 material, respectively. Amelin et al. (2005) employed thermal models to calculate the progressive increase in petrologic types from the core to the surface as follows: 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. Peak temperatures were determined to be 865–1000°C, 675–865°C, and <675°C for type 6, type 4/5, and type 3 material, respectively. The higher petrologic types were excavated at depth by impact, forming craters measuring tens of km wide and reaching depths of 5.6–11.2 km on their 200 km-diameter model. Fission track thermochronometry indicates that type 7 chondrites cooled more slowly at greater depths than did those of lower petrologic types (Trieloff et al., 2003). Consequently, type 7 chondrites experienced a longer period of thermal metamorphism within this interior layer, and now they exhibit extensively recrystallized textures that are transitional to an achondrite classification.

Importantly, a complex cooling history for the higher petrologic type H chondrites (5/6) was suggested from thermometric studies conducted by Ganguly et al. (2012). They reconciled data from calculations of two-pyroxene thermometers with the Ar–Ar, Pb–Pb, and Hf–W closure temperatures of select minerals to determine a cooling history consistent with very rapid cooling between ~800°C and 450°C, followed by a very slow cooling stage, and then another rapid cooling stage. By contrast, those H chondrites with lower petrologic types experienced a steady state of very rapid cooling. It was proposed that this scenario was more consistent with a collisional disruption and re-accretion of the parent body as opposed to a smoothly transitional ‘onion shell’ model. Thermal modeling of the H and L chondrite parent bodies was conducted and reported by Blackburn et al. (2017) and Edwards et al. (2017). Based in large part on combined metallographic cooling rates and Pb-phosphate age data, they ascertained that both asteroids accreted 2.05–2.35 m.y. after CAI formation and reached diameters >275 km forming concentrically zoned ‘onion shell’ structures. Thereafter, both asteroids experienced a catastrophic disruption at 60 (±10) m.y. after CAIs (attested by ubiquitous quenching of type 6 material) and reaccreted into smaller unsorted rubble pile bodies.

The S(IV)-type asteroid 6 Hebe is thought by many to be the probable parent body of the H-type ordinary chondrites and possibly the IIE iron meteorites as well. Hebe is a 187-km-diameter asteroid located next to both the 3:1 and ν6 resonances providing an efficient and rapid transfer mechanism into Earth-crossing orbit and a significant source of meteorites to Earth. It has been estimated that 6 Hebe could contribute ~10% of the meteorite flux to Earth and that it may be the source of one of the major ordinary chondrite groups. Models show that by mixing a component of 40% FeNi-metal with 60% H5 chondrite, an exact match to the spectra of 6 Hebe is produced. The IIE irons could be created through impact-melting on the metal-rich H-chondrite parent body to produce melt sheets or pods near the surface. Read more about the formation of IIE irons on the Miles page.

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. Current studies by Rubin and Bottke (2009) have led to the conclusion that family-forming events resulting in large meteoroid reservoirs having homogenous compositions which are located near dynamical resonances such as the Jupiter 3:1 mean motion resonance are the likely source of the most prevalent falls, including the H chondrites. See further details on this topic on the Abbott page.

After being studied at NASA’s JSC, the Monahans (1998) stone that fell on the road was returned to the City of Monahans. There it will be displayed with a specimen of the IIF iron meteorite that was found there in 1938. The other stone, which fell on private property, was subsequently sold at auction, and only a small portion was distributed to collectors. The specimen of Monahans (1998) shown above is a tiny fragment weighing 7 mg. The photo below shows the complete mass as found.

standby for monahans (1998) main mass photo

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