CM2.0/2.1 chondrite (Yamakawa and Yin, 2013)
Genomict Regolith Breccia
(with thermally metamorphosed (dehydrated) and Tagish Lake-like components)

standby for sutter's mill photo
Photo taken by Lisa Warren in Reno, Nevada
standby for sutter's mill photo

Fell April 22, 2012
38° 48' 14" N., 120° 54' 29" W.

On Earth Day 2012, April 22 at 7:51 A.M., a fireball accompanied by a sonic boom was seen, heard, and even smelled by local residents as it streaked over California and Nevada moving in a generally western direction. As the approximately 70-ton, 3-m-sized object reached an altitude of 48 km at a speed of 28.6 (±0.7) km/second, it exploded with the release of energy equivalent to an ~4-kiloton explosion (Jenniskens et al., 2012). Fragments of "black gold" fell within a strewn field encompassing the towns of Lotus and Coloma, including the location of the first discovery of California gold in 1848 at Sutter's Mill.

Two days after the fall, the first charcoal-colored stone weighing 5.5 grams was recovered by meteorite hunter Robert Ward. Utilizing NEXRAD high-resolution Doppler weather radar data, along with seismic data, Marc Fries of the Planetary Science Institute constructed a more accurate map of the inferred strewn field. Because of rather strong winds aloft blowing towards the ENE, the approximately 4 × 2 mile strewn field has been depicted curving slightly to the north reflecting the drift of smaller, lighter fragments. Models predict that the largest fragments weighing perhaps 10–20 kg would have landed ~19 miles farther west of the known strewn field (Fries et al., 2012). A helium airship was employed by scientists from NASA and the SETI Institute to search for possible impact features, but none were reported.

Over the next few weeks, meteorite hunters and locals together spent thousands of manhours searching the rattlesnake and poison oak infested strewn field collecting numerous small fragments. The largest single find weighing in at 205.2 g was made by Jeffrey Grant. Although many remain unofficially recorded, nearly 100 Sutter's Mill fragments have been recovered having a combined weight of over 1,000 grams. A map was constructed by Jim Wooddell showing the recovery locations of many reported fragments.

A consortium of investigators led by Dr. Peter Jenniskens of the SETI Institute has begun the long process of analysis and classification. Initial characterization of Sutter's Mill conducted at Johnson Space Center by M. Zolensky (2012) indicated that this is a carbonaceous chondrite breccia showing many petrological similarities to CM chondrites. Sutter's Mill was described as a highly comminuted regolith breccia by Kebukawa et al. (2013), consistent with the wide variety of components present in the matrix and the presence of solar-wind-implanted noble gases. Matrix components include chondrules similar in size to those in CM2.5 Murchison, isolated lithic fragments, aggregates of forsteritic olivine and low-Ca pyroxene, abundant CAIs, grains of the sulfides pyrrhotite, pentlandite, and oldhamite (CaS), and rare FeNi-metal; the low abundance of the latter being attributed to seismically-driven gravitational sorting/settling by Zolensky et al. (2013). The presence of oldhamite and Fe-Ni-Cr phosphides in Sutter's Mill attests to the impact fragmentation of an E chondrite or aubrite (A. Rubin, UCLA), or alternatively, the oldhamite could have been formed during the dehydration process at temperatures of at least ~750°C as outlined by Haberle et al. (2013), or during impact-generated heating to >300°C as suggested by Beck et al. (2013).

Ott et al. (2013) found that diamond was present in an abundance of ~471 to ~1460 ppm, indicating the possible admixture of a ureilite component, while a lower limit was calculated for a presolar SiC content in Sutter's mill of 3.8 (±0.4) ppm. Analyses of specimen SM2-5 by Kebukawa et al. (2014) led to their discovery of two relatively large diamond grains, considered to be xenolithic in origin, and to have likely formed through a chemical vapor deposition (CVD) process on a large parent body. In addition, Haberle et al. (2013) reported finding bluish-white grains within the matrix that have been identified as the first occurrence of portlandite (Ca(OH)2), thought to be a product of reduction of CaSO4 catalyzed by CO and CO2. Utilizing X-ray micro-tomography, Tsuchiyama et al. (2014) have found ubiquitous µm-scale solid inclusions present in all calcite grains studied, and they identified one calcite grain that likely harbors an ~2 µm-sized remnant spherical fluid inclusion incorporating a bubble with a solid particle inside. However, due to the small size of the bubble it could not be ascertained if it contains an aqueous fluid.

Advanced analyses of numerous Sutter's Mill samples were conducted at the Center for Meteorite Studies (L. Garvie, 2013), leading to the conclusion that there are two distinct mineralogical classes present—one is rich in olivine and the other is rich in amorphous clays. The olivine-rich (75–80 wt%) material exhibits characteristics akin to the Belgica-group of thermally metamorphosed CM chondrites, while the clay-rich material is considered to be a strong match to the C2-ungrouped Tagish Lake. It was proposed that both of these disparate classes of chondritic material were independently incorporated into the Sutter's Mill parent object, itself characterized as a rubble pile.

A detailed study of Sutter's Mill by Beauford et al. (2012, 2013) revealed that it is a complex regolith breccia consisting of a primary accretional matrix containing two dominant clast lithologies present in approximately equal abundances in variable combinations and in breccia-in-breccia clasts, attesting to a history of impact mixing and regolith recycling. One clast type is a dark-colored chondrule-rich lithology (CRD) and the other is a light-colored chondrule-poor lithology (CPL), with the components of each expressing a different degree of aqueous alteration. Other minor lithologies reported include sub-mm-sized dark inclusions (DI), carbonate-rich clasts, and a xenolithic component consisting of enstatite, oldhamite, and phosphides likely derived from E chondrites or aubrites (Zolensky et al., 2014).

Similar to the dust mantles prevalent around other CM components, dark, fine-grained rims are present on many of the coarse-grained objects in Sutter's Mill. These rims are considered likely to have formed and hardened during impact compaction processes, but they might be accretionary rims developed in the nebula, or possibly the result of a combination of both of these formation processes (Haack et al., 2012). These fine-grained rims were investigated by Beauford and Sears and it was found that their presence is limited to those primary CRD lithologies that experienced only limited aqueous alteration, and that their formation was restricted to the period prior to comminution and evolution of the CM regolith.

Nagashima et al. (2012) found that O-isotopic compositions of olivine from type-I and type-II chondrules and AOAs plot along the CCAM line, and they identified abundant coarse dolomite and calcite grains, the latter having O-isotopic compositions nearly identical to calcites in CM chondrites. Major and trace element analyses of three separate samples were consistent with those of CM chondrites (Yin et al., 2012; Friedrich et al., 2012). Grady et al. (2012) studied the abundance and isotopic composition of carbon and argon by stepped combustion in a Sutter's Mill sample and found close similarities to carbonaceous chondrites, with the closest match demonstrated for C2-ung Tagish Lake. O-isotopic measurements conducted by Kohl et al. (2013) of acid-washed Sutter's Mill material, thus eliminating carbonate mineral influence, led them to conclude that aqueous alteration increased the water/rock ratio and shifted the three-isotope plot away from the CCAM line towards the TFL.

Like CM chondrites, Sutter's Mill is a breccia containing features indicative of both weak aqueous alteration and thermal metamorphism to 500–750°C affecting the chondrule mesostasis. Some clasts have been heavily aqueously altered to subtype 2.0, resulting in the replacement of some chondrule constituents with the phyllosilicates Fe-cronstedtite/tochilinite + Mg-serpentine (A. Rubin, UCLA). Notably, Howard et al. (2009) have argued that the phyllosilicate abundances among CM chondrites are within a few percent of each other, and thus reflect similar aqueous alteration processes. Other clasts (e.g., SM2-5, thought to represent a comminuted regolith breccia) exhibit secondary heating features consistent with Stage III, based on the scale of Nakamura (2005) (Zolensky et al., 2014). In these clasts, phyllosilicates have been converted to fine-grained olivine, tochilinite has been converted to troilite, and carbonates have been destroyed. See the Murchison page for further details on classification based on thermal metamorphism.

Cooper and Jenniskens (2012) and Dillon et al. (2013) measured soluble organic compounds in Sutter's Mill and identified mono-carboxylic acids (e.g., formic acid and acetic acid) typically present in significant abundances in many CM chondrites; they were present in much lower abundances than in Murchison. In addition, large abundances of soluble inorganic compounds were found, particularly sulfate, which is common to CM chondrites. Using advanced methods to characterize soluble organic compounds, Schmitt-Kopplin et al. (2012) found that they were present in comparatively low abundances, comprising highly oxygenated species or organometallic compounds. An organic C component was determined to reside in both hollow and filled, 15N-rich nanoglobules that likely formed in the cold solar or presolar nebula (Nakamura-Messenger et al., 2013). They also observed that the constituents in the relatively anhydrous matrix component of Sutter's Mill were mineralogically similar to the matrix of Acfer 094, a unique carbonaceous chondrite tentatively classified as a subgroup of the CO chondrites; Simon and Grossman, 2015). Utilizing X-ray spectroscopy in a study of specimen SM2-5, Kebukawa et al. (2014) determined that the matrix organic matter has a lower N/C ratio compared to other carbonaceous chondrites.

Pizzarello et al. (2012) determined that amino acids were scarce in Sutter's Mill, and that they contain low-complexity hydrocarbons, mainly naphthalene. Analyses by Glavin et al. (2012, 2013) of both pre- and post-rain samples also revealed lower C2–C5 amino acid abundances (~660–9,500 ppb) compared to those in Murchison (~14,000 ppb). Similarly, analyses of one of the most pristine Sutter's Mill specimens (SM2) by Burton et al. (2012) found a 20 × lower abundance of amino acids than measured in Murchison. These low levels are considered likely the result of significant parent body aqueous alteration and/or thermal (>150°C) metamorphism. Advanced infrared analyses by Flynn et al. (2013) indicated the presence of carbonates and associated organic matter. This organic matter consists in large part of aliphatic hydrocarbons, and it was determined to be compositionally different from organic matter identified in Murchison, but consistent with the type identified in Tagish Lake.

CT scans conducted at AMNH (Ebel et al.) provided density and porosity data for two Sutter's Mill samples. From these it was determined that Sutter's Mill has a bulk density of 2.23 g/cm³. Similarly, bulk density and grain density measurements were made by Britt et al. (2012) using conventional methods. The bulk density for one sample was determined to be 2.31 g/cm³; all sample density values are within the range of those for CM chondrites. A measurement of porosity showed that it is relatively high at 31 (±1.4) %, also similar to typical values for CM chondrites. Likewise, the magnetic susceptibility value is within the range for CM chondrites. Results of reflectance spectroscopy performed by Grady et al. (2013) was consistent with a CM classification.

An analysis of Mn–Cr systematics in Sutter's Mill calcite and dolomite revealed a resetting of this chronometer ~4.563 b.y. ago, while a more recent resetting event within 1 b.y. ago was evident in the Re–Os system (Walker et al., 2013 and references therein). The ε54Cr value for Sutter's Mill calculated by Yamakawa and Yin (2013) is identical to that of Murchison, indicating an origin for both meteorites from the same precursor. Similarly, the secondary carbonate mineral dolomite was utilized by Jilly et al. (2014) for Mn–Cr radiometric dating in Sutter’s Mill. This short-lived (half-life = 3.74 m.y.) chronometer is well suited for that purpose in that Mn becomes sequestered in the precipitating carbonate, while Cr remains with the percolating aqueous fluid. This creates a measurable excess of 53Cr through the in situ decay of radioactive 53Mn, a value which is temporally related to the onset of secondary carbonate formation during aqueous alteration. This age for carbonate formation was determined to be 4.5637 (+0.001.1/–0.001.5) b.y., or 2.34–5.26 m.y. after CV3 CAIs, which is an absolute age anchored to the U-corrected Pb–Pb age of the D’Orbigny angrite. Their study showed that carbonate formation occurred relatively early in Sutter's Mill, as well as in other carbonaceous chondrite groups—a result of aqueous processing sustained by radiogenic heating of accreted ices.

Noble gas studies conducted by Hamajima et al. (2012) and Ott et al. (2013) established a very young CRE age for Sutter's Mill that defines the low end of the range for the CM2 chondrite group (previously exhibiting two major peaks at 0.2 and 2.0 m.y.), reflecting a relatively recent ejection from its parent body ~19–51 t.y. ago. Cosmogenic radionuclide studies conducted by Nishiizumi et al. (2014) provide a similar very young CRE age of 82 (±8) t.y. Furthermore, in a broad study of cosmogenic radionuclides in Sutter's Mill and a large number of other CM group members, Nishiizumi et al. (2013) detected several major collisional clusters representing a mixture of both petrologic types 1 and 2. The pre-atmospheric size of the Sutter's Mill meteorioid was also calculated by Nishiizumi et al. (2013, 2014), and it was demonstrated to have been a minimum of ~1 m in diameter based on a bulk density of 2.3 g/cm³, which is consistent with the estimate of 1–2 m based on other parameters.

Current studies suggest that both cometary dust and meteorites should be produced from the disruption of Jupiter-family comets which originate in the Kuiper belt. Studies have shown that Antarctic micrometeorites have a similar carbonaceous chondrite:ordinary chondrite ratio (~7:1) as the composition of zodiacal dust (M.M.M. Meier, 2014). Based on observational evidence and current modeling, it is thought that comets should be dark in color and have a low density and strength, a high porosity, a solar ratio of elements, an elevated ratio of C, H, O, and N, a high interstellar grain content, anhydrous and highly unequilibrated silicates, few to no chondrules, and a low cosmic-ray exposure age (<10 m.y.). Both the CI and CM groups of meteorites exhibit characteristics which are consistent with the above descriptions.

Orbital data obtained from several carbonaceous chondrites (e.g., the CI chondrite Orgueil [eyewitness plotting] and the CM chondrites Maribo and Sutter's Mill [instrument recording] are a good match to the orbits expected from the disruption of Jupiter-family comets, but are unlike the orbits of ordinary chondrites and most other asteroidal objects (M.M.M. Meier, 2014). Both the orbital eccentricity and semimajor axis for Maribo is nearly identical to those of Comet Encke and the associated Taurid swarm of objects (Haack et al., 2011). On the other hand, a CRE age study of CM chondrites conducted by Meier et al. (2016) shows a possible relationship exists to the asteroid breakup event ~8.3 m.y. ago that formed the Ch/C/Cg-type members of the Veritas family. In addition to the large abundance of 3He-enriched interplanetary dust discovered in 8.2 m.y.-old deep-sea drill cores, ~1/6 of all CM meteorites have 21Ne-based CRE ages that are consistent with derivation from this catastrophic breakup, while others with significantly younger CRE ages could represent secondary collisions among the Veritas fragments.

Fragments from many Sutter's Mill samples were generously donated to the University of Arizona and other institutions where studies will be conducted in preparation for the upcoming carbonaceous asteroid sample return mission, OSIRIS-REx. The 0.053 g specimen shown above is a portion from stone #SM14, as listed in Jeniskens' official NASA database, a stone that impacted the garage door of Suzanne Matin and broke into fragments weighing together 11.5 g. A BBC News video captured the actual recovery of portions of this stone, and also features meteorite hunter Mike Farmer searching the strewn field with fellow hunters as he discusses the significance of this rare fall.

standby for sutter's mill photo
The photo above shows the broken fragments of a stone that impacted a parking lot, which were immediately recovered by Dr. Peter Jenniskens.

Video of the fall of the Sutter's Mill meteorite, April 22, 2012
A brilliant fireball plunges through the sky of the Western United States, accompanied by several detonations.
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An X-ray computed tomography video of Sutter's Mill 18. The image resolution is 14 micron/voxel and the field of view is ~2 × 3 cm².
Courtesy of Yin Lab at UC Davis.
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