Fell March 22, 1998
102° 53.5′ W., 31° 36.5′ N. On a Sunday evening at 6:37 P.M., a fireballA fireball is another term for a very bright meteor, generally brighter than magnitude -4, which is about the same magnitude of the planet Venus as seen in the morning or evening sky. A bolide is a special type of fireball which explodes in a bright terminal flash at its end, often with visible fragmentation. Click on Term to Read More 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 craterBowl-like depression ("crater" means "cup" in Latin) on the surface of a planet, moon, or asteroid. Craters range in size from a few centimeters to over 1,000 km across, and are mostly caused by impact or by volcanic activity, though some are due to cryovolcanism. Click on Term to Read More 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 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
, these meteorites were subjected to short-lived radionuclideRadioactive isotope - Atomic nuclide that decays radioactively . Click on Term to Read More
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 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
. The dark lithology also contains high concentrations of implanted solar noble gasesElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. Click on Term to Read More
, 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 ageTime interval that a meteoroid was an independent body in space. In other words, the time between when a meteoroid was broken off its parent body and its arrival on Earth as a meteorite - also known simply as the "exposure age." It can be estimated from the observed effects Click on Term to Read More
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 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
, 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 ejectaFractured and/or molten rocky debris thrown out of a crater during a meteorite impact event, or, alternatively, material, including ash, lapilli, and bombs, erupted from a volcano. Click on Term to Read More
, preserving its cosmogenic history until it was ejected from the asteroid.
Salt crystals measuring 0.5–5 mm were discovered in the 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
lithology and identified as crystals of purple
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 transitWhen a small celestial body moves in front of a much larger one (as when Mercury or Venus appears in silhouette against the solar disk or when a satellite passes in front of Jupiter or Saturn). The shadow of a satellite may also transit the disk of its primary.
to Earth through the radioactive decayProcess in which an isotope's nucleus changes ('decays') to produce another isotope. The original atom is called the 'parent' and the resulting atom, the 'daughter'. There are three modes of radioactive decay: • Emission of a particle (He nucleus) that decreases the atomic number (Z) by 2 and the atomic Click on Term to Read More
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 evaporationProcess in which atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state. Click on Term to Read More
of asteroidal brines on a large, partially liquid, carbonaceous-type 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
(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 silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the
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 carbonElement commonly found in meteorites, it occurs in several structural forms (polymorphs). All polymorphs are shown to the left with * indicating that it been found in meteorites and impact structures: a. diamond*; b. graphite*; c. lonsdalite*; d. buckminsterfullerene* (C60); e. C540; f. C70; g. amorphous carbon; h. carbon nanotube*. Click on Term to Read More
similar to that of matrix carbon in CM and CV chondritesMeteorite class named after the Vigarano meteorite that fell in Italy in 1910. They have abundant large, well-defined rimless (?) chondrules of magnesium-rich olivine (~0.7 mm diameter; 40-65 vol. %), often surrounded by iron sulfide. They also contain 7-20 vol. % CAIs. The often dark-gray matrix is dominated by Fe-rich Click on Term to Read More
, short-chain aliphatic compounds, graphiteOpaque form of carbon (C) found in some iron and ordinary chondrites and in ureilite meteorites. Each C atom is bonded to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons. The two known forms of graphite, α (hexagonal) and β (rhombohedral), have Click on Term to Read More
and diamondOne of the naturally occurring forms of carbon found in meteorites. Each C atom is bonded through covalent sp3 hydrid orbitals to four others. The strength of the C-C bonds makes diamond the hardest naturally occurring substance (according to the Mohs scale) in terms of resistance to scratching. There are Click on Term to Read More
phases, silicates such as 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
, pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification Click on Term to Read More
, and feldsparAn alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars and due to their abundance, feldspars are used in the classification of igneous rocks. A more complete explanation can be found on the feldspar group page. Click on Term to Read More
, 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
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 accretionAccumulation of smaller objects into progressively larger bodies in the solar nebula leading to the eventual formation of asteroids, planetesimals and planets. The earliest accretion of the smallest particles was due to Van der Waals and electromagnetic forces. Further accretion continued by relatively low-velocity collisions of smaller bodies in the Click on Term to Read More
time period of 0.1–0.2 m.y. while 26
Al 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 26
Al had decayed, at least 2 m.y. after CAISub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they Click on Term to Read More
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 26
Al 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 26
Al 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 26
Al. For example, the initial ratio of 0.0000004–0.0000005 calculated for the angrites Sah 99555 and D’Orbigny based on their 26
Mg 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 coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. Click on Term to Read More
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. FissionBreaking apart of a body into smaller fragments. In nuclear physics, fission refers to splitting of a heavy atomic nucleus into two or more lighter nuclei with an associated release of energy. The mass of the nucleus before fission is greater than the combined masses of the resulting fragments; the Click on Term to Read More
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 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
Importantly, a complex cooling history for the higher petrologic typeMeasure of the degree of aqueous alteration (Types 1 and 2) and thermal metamorphism (Types 3-6) experienced by a chondritic meteorite. Type 3 chondrites are further subdivided into 3.0 through 3.9 subtypes.
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 chondriteOrdinary chondrites low in free Ni-Fe metal (4 to 10 vol. %), containing olivine (Fa22-26) and the orthopyroxene hypersthene (Fs19-22). Average chondrule diameters (0.7 mm) are larger than those in H chondrites. The asteroid 433 Eros is suspected as a parent body, based on reflectance spectra, but most L chondrites Click on Term to Read More
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 CAIsSub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they Click on Term to Read More
(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 orbitThe elliptical path of one body around another, typically the path of a small body around a much larger body. However, depending on the mass distribution of the objects, they may rotate around an empty spot in space • The Moon orbits around the Earth. • The Earth orbits around Click on Term to Read More
and a significant source of meteorites to Earth. It has been estimated that 6 Hebe could contribute ~10% of the 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
flux to Earth and that it may be the source of one of the major ordinary chondriteWork in Progress Ordinary chondrites (OCs) are the largest meteorite clan, comprising approximately 87% of the global collection and 78% of all falls (Meteoritical Society database 2018)1. Meteorites & the Early Solar System: page 581 section 6.1 OC of type 5 or 6 with an apparent shock stage of S1, Click on Term to Read More
groups. Models show that by mixing a component of 40% FeNi-metal with 60% H5 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
, 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
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 chondriteOrdinary chondrites with a high content of free Ni-Fe metal (15-19 vol. %) and attracted easily to a magnet. Their main minerals are olivine (Fa16-20) and the orthopyroxene bronzite (Fs14.5-18.5), earning them their older name of bronzite chondrites. Chondrules average ~0.3 mm in diameter. Comparison of the reflectance spectra of Click on Term to Read More
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 meteoroidSmall rocky or metallic object in orbit around the Sun (or another star).
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
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 meteoriteMeteorite composed mainly of iron (Fe) and nickel (Ni) in the form of two alloys, kamacite and taenite. Due to their metallic makeup and extraordinary weight, iron meteorites are easily distinguished from ordinary rocks. Also, because they rarely break up in the air and suffer much less from the effects Click on Term to Read More
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.