L5 fragmental breccia
Fell in 2003, March 26 at 11:50 P.M.
41° 29′ 05′ N., 87° 40′ 45′ W.
As midnight neared, ending the last Wednesday of March, 2003, a terrific fireball passed overhead on its long approach path. Following a SSW to NNE trajectory, the fireball put on a fiery display for residents of several states including Missouri, Illinois, Indiana, Michigan, and Ohio, ultimately delivering sonic detonations as multiple fragmentation events occurred at altitudes of 37, 29, and 22 km. Hundreds of fragments ended their journey in the southern suburbs of Chicago, showering the towns of Crete, Park Forest, and Olympia Fields with stones weighing from a few grams to 5.26 kg. Many of these fragments impacted cars, homes, lawns, sidewalks, streets, and even the local Fire Station, giving many unsuspecting residents an opportunity to recover a fresh meteorite—and others an opportunity to repair their damaged windows, roofs, and other structures.
Joining these resident ‘meteorite hunters’ were some cosmochemists from local institutions, including the University of Chicago, the Chicago Field Museum, and the Argonne National Laboratory. Among the many meteorite collectors and dealers who traveled to the fall location was Steve Arnold (International Meteorite Brokerage; ‘Meteorite Men’), along with his wife Qynne and two young daughters Kelsey and Lauren. All of the Arnold’s found meteorites during their trip—the first ever meteorite finds for Qynne, Kelsey and Lauren. The Park Forest meteorite shown at the top of the page was one of several found by Lauren, and it was subsequently put up for sale via eBay auction. The story behind her find, written by Lauren herself, inspired my own bid—but in the end, I lost out to fellow collector Mike Fowler. Remarkably, three years later, this 3.5 g specimen is in the Weir Collection thanks to Mike’s generosity. Lauren’s personal account of the successful Arnold Family meteorite expedition follows:
We made a deal that we had to keep our first meteorite we found, but we could sell the others. After getting to the motel the first day, we went to Burger King and both my sister and I got a Kid’s Meal, and guess what was the toy in them? A huge meteor! It came with a Superman Doll and a comic book. My Dad was so excited, and he read the comic book to us and it was about Superman finding a bunch of meteors after lunch. Dad told us ‘You know what that means don’t you? It is a sign that we are going to find a bunch of meteorites today.’
Well I found two that day, my sister found one and so did my Mom. The next couple of days we each found several more. We are all keeping our first ones, but we are selling the rest. Well, my Dad is selling them for us. But we get to keep the money! So bid real high because these are special. Oh, and we went back to Burger King that night and bought some more of the Kid’s Meal toys, so you also get one free included with each of our special meteorites we are selling if you are lucky enough to be the high bidder. I hope the toy brings you as much luck as it did us.’
‘Hi, my name is Lauren Arnold, I’m 11 years old. My Dad has been in the meteorite business my whole life, but until now I have never had a chance to hunt meteorites with him. Several weeks ago a big meteorite broke up and landed all over Chicago and my Dad went up to find some. He found a lot of them. So on his third trip to Chicago, my sister, my Mom and I talked him into taking us with him.
Burger King Superman-Meteor Kid’s Meal toy
Park Forest was analyzed and classified at the University of Chicago (S. Simon), the Field Museum of Natural History (M. Wadhwa), and the Planetary Studies Foundation (P. Sipiera). Park Forest has olivine Fa and pyroxene Fs contents consistent with the L-group ordinary chondrites; O-isotopic ratios also confirm this classification (Simon et al., 2004). In accord with the scheme of Van Schmus and Wood, Park Forest is classified as a petrologic type 5. Park Forest is a strongly shocked (S5) monomict breccia consisting of light gray clasts within a darker matrix. It contains scattered FeNi-metal flakes throughout, while FeS veins occur only in the dark lithology, which is considered to be derived from the light lithology through impact shock processes that mobilized sulfides.
Park Forest stone exhibiting both light and dark lithologies
Photo courtesy of The University of Chicago News Office
Orbital, velocity, trajectory and pre-atmospheric mass data of the Park Forest bolide were obtained from U.S. Department of Energy and Department of Defense satellites, with additional information supplied by ground-based video camera and seismic and acoustic instruments (Brown et al., 2004). Satellite data indicate that the meteoroid’s entry velocity fell from an initial maximum of ~20 km/sec above 60 km, to ~14 km/sec at lower altitudes. Seismic and acoustic measurements infer a total energy equivalent to ~0.4–0.5 kilotonnes of TNT. This is an Apollo-type asteroid with a heliocentric orbit and an aphelion that likely coincides with the 3:1 mean-motion resonance with Jupiter at 2.50 AU. Cosmogenic radionuclide measurements indicate that the pre-atmospheric mass of Park Forest was between 900 and 7,000 kg, with the higher value being consistent with a model based on porosity. Based on a gross fragmentation model, it could be argued that Park Forest was initially a meteoroid ~2 m in diameter with a pre-atmospheric mass of ~14,000 kg. In their compilation of measurement techniques, Brown et al. (2004) derived an initial pre-atmospheric mass estimate of 11,000 (±3,000) kg and a diameter of 1.8 m. Meier et al. (2017) conducted a re-analysis of cosmogenic radionuclides and noble gas abundances for Park Forest, and their new data, consistent with that of two established models, provided the following results:
- a pre-atmospheric mass of 1,730–5,870 kg
- a pre-atmospheric diameter of 1.06–1.76 m
- a porosity of 0–40%
- a density of 2.0–3.4 g cm–3
In addition, Meier et al. (2017) determined that the radiogenic gas retention ages ([U, Th]–He and K–Ar) for Park Forest are consistent with that of the catastrophic disruption of the L chondrite parent body, an event dated at 470 (±6) m.y. ago as attested by the abundance of fossil L chondrites and associated chromite grains recovered from Mid-Ordovician limestone quarries in Sweden and other countries (see the Beenham page for more information). Meier and Welten (2014) determined that Park Forest lacks solar wind noble gases, which indicates it is not a regolith breccia but is instead a fragmental breccia. They determined a 21Ne-based CRE age of ~17 m.y., and found that various radiometric chronometers had been reset in the relatively recent past. Meier et al. (2017) conducted a re-analysis of the cosmogenic noble gases for Park Forest, and their results gave a CRE age of 12 (±1) m.y. based on 3He, and 14 (±2) m.y. based on 21Ne and 38Ar.Park Forest is one of a growing number of meteorites with a precisely calculated orbit (e.g., Pribram, Innisfree, Lost City, Peekskill, Neuschwanstein, Morávka, Tagish Lake, Novato). Meier et al. (2017) assert that the location of the L chondrite parent body at the time of breakup was near the Jupiter 5:2 mean motion resonance (signifying the ratio between the orbital period of Jupiter and that of an asteroid) from which rapid delivery from the inner asteroid belt to an Earth-crossing orbit could be attained, consistent with the CRE age calculated for the fossil L chondrites recovered from mid-Ordovician limestone quarries. Based on spectrographic and mineralogical data for more than 1,000 near-Earth asteroids, Binzel et al. (2016) determined the probable main belt source region for each of the ordinary chondrite groups (see diagram below).
Diagram credit: Binzel et al., 47th LPSC, #1352 (2016) Thereafter, L chondrite material would have been delivered to the more efficient, albeit slower, 3:1 resonance (and the less important ν6 secular resonance) where transfer to Earth occurred over an extended period of millions of years. The S-type asteroids of the Gefion family have been historically considered a probable source for the L chondrites. However, in consideration of such factors as pre-breakup size, breakup age, and shock-darkening history of the parent asteroid, Meier et al. (2017) contend that the Ino asteroid family may be a more plausible source (see the Beenham page for further details). Utilizing surface thermal conductivity estimates for the Park Forest meteoroid, they calculated its possible orbital evolution following its ejection ~14 m.y. ago. They conclude that its delivery to Earth was either directly through the 5:2 resonance or that it migrated to the 3:1 resonance buried at a significant shielding depth within a larger meteoroid.
Diagram credit: Meier et al., MAPS, vol. 52, p. 1563 (2017)
‘Park Forest (L5) and the asteroidal source of shocked L chondrites’
(http://dx.doi.org/10.1111/maps.12874) The total recovered weight of the Park Forest meteorite is only ~30 kg out of a predicted ground fall mass of up to 150 kg. The specimen shown at the top is a 3.53 g half stone which was broken upon impact; dark shock veins can be seen traversing the light gray lithology. The photo below is an excellent petrographic thin section micrograph of Park Forest shown courtesy of Peter Marmet.
click on image for a magnified view
Photo courtesy of Peter Marmet
For additional information on the Park Forest fall, read the PSRD article by Linda M.V. Martel—’Meteorite Shower in Park Forest, Illinois‘, Aug 2004.