Things that go bump in the night

Monday, December 24, 2007
As Earth scientists we are aware that objects (sometimes really big objects) have impacted Earth in the past. As keen natural observers, often working in pristine, non-light-polluted parts of our world we have a better than average chance of seeing celestial objects flash across the sky but few of us have had the opportunity to get up close and personal with a recent impact site, However, this is exactly what happened last Fall in a remote part of Peru near the border of Bolivia. 
 
At 11:45 on the morning of September 15th 2007 a meteorite landed about 5 km from the Bolivian – Peruvian border near the village of Carancas in Peru. The site location is shown in Figure 1 at 16º 39’ 32” S; 69º 02’ 38” W and at an elevation of 3,824m. The impact created a moderately large crater about 13 - 14m (~ 45 feet) in diameter. The rim of the crater is between one and two metres in height and the crater is at least 4 m deep. The impact took place on the floodplain of a large river system. There were reports of people smelling sulphur and stories of people being ill.
 
 Location of the Carancas, Peru, impact site near the south end of Lake Titicaca.
 

Figure 1: Location of the Carancas, Peru, impact site near the south end of Lake Titicaca.

Some have theorised that noxious gases were released from the meteorite or that the impact released gases with arsenic from the water table. However, I think a simpler explanation might be that hydrogen sulphide was released from organic debris trapped in the sediments around the impact site. Looking at the detailed images available on Google Earth of the area west of the impact site it appears that this is a region that may have permafrost, and/or has been recently glaciated since there are a number of depressions that resemble kettle holes or thermo-karst melt holes. Michael Farmer, a meteorite collector who visited the site suggested that the meteorite likely weighed about 10 tons on impact. His site is interesting and has some excellent images of the impact site. 
 
 The Carancas impact site (top) with a fragment (lower left). Note that the “crust” is a resin coating used to bind the specimen for thin-sectioning. It is not an ablation rind frequently found in meteorites. Thin sections illustrating the largely pyroxene/olivine nature of the chondrite are shown on the lower right.
 

Figure 2: The Carancas impact site (top) with a fragment (lower left). Note that the “crust” is a resin coating used to bind the specimen for thin-sectioning. It is not an ablation rind frequently found in meteorites. Thin sections illustrating the largely pyroxene/olivine nature of the chondrite are shown on the lower right.

Material recovered by Peruvian authorities show that the meteorite is a chondrite (stony meteorite) with a composition of about 50% pyroxene, 20% olivine and 10% feldspar. The remainder is made up of 15% kamacite (a 90:10 alloy of iron and nickel) and 5% troilite (an end member of the iron sulphide mineral, pyrrhotite). The troilite end member is rare in crustal rocks but is relatively common in meteorites. There were also traces of chromite and native copper present. Specimens of the Carancas meteorite are now being sold for about $ US 10/gram. The full scientific results of the examination of this relatively unusual meteorite have not yet been released.    

 
Alan V. Morgan 
 
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