Centerpiece - Cat Mountain
By Russell W. Kempton

Our Solar System is a place of violence. From tiny little dust grains bumping into one another to the unimaginable energy released through asteroidal and cometary impact. Even Earth is the collisional product of a wide range of impacts. The cratering event at Chicxulub loses it's significance when compared to the Mars-sized impactor that tilted Earth's axis and formed the Moon. Similarly, Mercury suffered an impact that stripped off its outer layers and Venus was slammed so hard that it now spins in the opposite direction! (Verschuur, 1996).

Impacts are destructive. They turn big pieces into little ones. Their very nature tends to confuse or obliterate any record of prior events.

While the end-product of these shattering collisions occasionally fall to Earth as meteorites, the likelihood of finding a study sample displaying a sequential visual record of many impacts within the meteorite was considered remote at best. "Remote" occurred in 1993.

Tucson, Arizona

William Goldups walked his dog along the same path near his house daily. One evening, sometime in 1980 or 1981, Goldups, who lived in the Drexel Heights area of Tucson, Arizona USA, noticed a rock on the path that was darker, larger, and denser than any other rocks in the desert alluvium. Goldups brought it home, where it stayed until after his death in 1990. George Bakutis, a friend, inherited the rock, and in 1993 brought it to the University of Arizona's Lunar and Planetary Laboratory, where it was confirmed to be a rare type of stone meteorite (Kring 1993).

Goldups, in his daily walk, had found a rock that was originally from the bottom of an asteroid crater. Known as an impact melt breccia, Goldups' rock formed through a series of impacts as the asteroid collided with other objects. In one of the impacts, the asteroidal surface temperature went from around -150° C to over 1000° C as the impactor's kinetic energy was instantly converted to heat. The crater bottom became lined with melted rock and fragments awaiting additional impacts to jettison chunks of this melted breccia off the asteroid.

Meteorite impact melt breccias are rare. They usually display clasts of one type of material embedded into a partial-melt of a similar or different material. But Goldups rock contained intact chondrule bearing clasts that appeared to have been immersed into a total melt of similar material and displayed flow lines in the melt! However unlikely it was to have melted material from the crater floor of an asteroid lying upon a path in Tucson, it is even more incredible to find a rock displaying a visual record of its many violent formative stages.

Cat Mountain

Goldups lived at the south end of the Tucson Mountains. The most prominent feature in the geographical area is the southern-most peak of the chain _ Cat Mountain _ roughly two miles northwest from where the meteorite was found. The 2.7 kg meteorite was classified as an L5 impact melt breccia (the clasts contained type 5 chondrules) and the name "Cat Mountain" was submitted and accepted by the Meteorite Nomenclature Committee in 1993.

Visual examination and the circumstances of the find suggested that Cat Mountain was a very fresh fall. Because he walked the same path in his neighborhood each evening and had not seen the "rock" on the previous day, Goldups surmised that he had found a meteorite that had fallen within the previous 24 hours.

Some reflectance spectra tests can be used to measure terrestrial weathering effects. Two specific wavelengths, 0.5µm and 0.6µm, are very sensitive to terrestrial weathering because the oxidation of abundant iron increases the absorption in this wavelength range. By measuring the ratio of 0.5µm to 0.6µm, Goldups theory that Cat Mountain was recovered within 24 hours of falling was confirmed (Wlotzka, 1993).

Observations and Questions

In 1993, I had the opportunity to examine Cat Mountain prior to it's classification. The exterior simply didn't look like any meteorite I'd seen. It was an unusual dark olive/brown color with the surface knobby and very smooth resembling a washed river stone. Exposed on the interior sawn half were two large, 4 cm x 6 cm, dark gray elliptical clasts. They were chondrule-rich and contained, what appeared to be, shock veining. The clasts were surrounded by a lighter gray, completely melted, igneous textured matrix that seemed to "flow" around the elliptical clasts. Remarkably, the surrounding matrix contained an elongated and aligned stream of metal particles and vesicles (similar to vugs) concentrated near the boundary interface between the melt and the clasts.

Somehow, as this meteorite formed, intact chondrule-bearing pieces of clastic L type material were pressed into an already existing melt of similar material deforming the matrix boundary, and then cooled, before the clasts were absorbed or the chondrules metamorphosed to mush. Additionally, at a later time, one or several impact events would have been needed to blast this brecciated material off the surface of the asteroid without destroying the evolutionary record visible within its interior.

The observations were simple enough but they indicated a very complex impact, heating, and cooling history for this little meteorite. The presence of chondrules in the clasts starts the clock running for parent body accretion at 4.55 billion years. Shock veins within these clasts indicates another impact, after accretion, on the Cat Mountain parent body creating a regolith. A later, more violent impact, would have been needed to produce the melt into which these clasts were mixed, and then cooled, at some unknown rate. Finally, there had to be another impact/ejection event to launch this material away from the parent body and, ultimately, its impact with Earth. Cat Mountain seems to be the product of too many impact events to have survived intact.

Sorting it Out

Over the past five years, about a dozen professional papers have been written about Cat Mountain, each contributing either theory or fact, but in 1996, David Kring at the University of Arizona's Lunar and Planetary Laboratory published a comprehensive paper that appears to confirm the unlikely number of events and added another two impacts to its history! (Kring et al., 1996).

Kring's evidence indicates that, after accretion of the parent body, the unmelted clasts in the meteorite were affected by an impact producing the shock veining. An additional impact may have metamorphosed the chondrules to type 5. Then, noble gas isotope measurements indicated that a major cratering impact occurred 880 million years ago. The energy of this event produced a 1 km or larger diameter crater and created a melt breccia lens at the bottom. The chondrule bearing clasts were buried within this mix and cooled over a period of a few thousand years. An impact at around 500 million years ago may have been the one that ejected the material off the surface and then, 20 million years ago one more impact reduced the meteoroid to a meter-sized object prior to impacting Earth.

The Cat Mountain meteorite is a product of at least six impact events. It provides us with one of the most complete records of a complex history of collisions occurring on a primitive chondritic body. But there's one more lesson to be learned from this meteorite. It's one of contribution. Cat Mountain demonstrates the diversity of contributions that can be made in meteoritics. Sometimes it's as complex as the work of David Kring, and other times, as in the case of the observant Mr. Goldups, it's as simple as walking a dog.

References

Verschuur, Gerrit L., 1996. Impact!: The Threat of Comets and Asteroids, New York, Oxford University Press.

Kring, D. A., Cat Mountain: A meteoritic sample of an impact-melted chondritic asteroid (abstract), Lunar Planet. Sci., XXIV, 823-824, 1993.

Kring, D. A., Swindle, T. D., Britt, D. T., Grier, J. A., Cat Mountain: A meteoritic sample of an impact-melted asteroid regolith, J. Geophys Res., Vol.101, No. E12, 29,353-29,371, December 25, 1996

Wlotzka, F., The Meteoritical Bulletin, No. 75, 1993 December, Meteoritics, 28, 692-703, 1993.

Russell Kempton is the Director of New England Meteoritical Services based in Mendon, Massachusetts, USA.