eso9106 — Science Release
Seventy-seven Dead Cows in the Desert
Astronomers Study Gigantic Stony-Iron Meteorite
16 July 1991
The lonely Atacama desert is a perfect place to study distant celestial bodies in the space around us. The following story shows how this may be done, not only through powerful astronomical telescopes at isolated mountain-top observatories , but also down on the barren desert plain in a much more direct way.
Two astronomers from the European Southern Observatory and a Chilean geologist have just completed a detailed study of a gigantic, but little known meteoritic impact in a remote region of the Atacama Desert. Over a period of four years they carefully searched a large area in the middle of nowhere and collected seventy-seven specimens of the Vaca Muerta ("Dead Cow") meteorite with a total mass of more than 3400 kg. This meteorite is of the rare stony¬iron type (mesosiderite) and the new finds have more than tripled the available material of this type which is of great importance for the study of the early history of our solar system.
ESO astronomers Holger Pedersen (now at the Copenhagen University Observatory, Denmark) and Harri Lindgren (ESO La Silla) worked with Claudio Canut de Bon (Director of the "Museo Mineralogico Ignacio Domeyko", Universidad de La Serena, Chile). The astronomers did the work in their spare time and have expressed great appreciation for the excellent collaboration with their Chilean colleagues as well as with the authorities in this country.
"We are used to observe remote objects in space", says Holger Pedersen, "but it was really great fun for once to do down-to-Earth astronomy!" In addition to the large planets and their moons, there are many smaller solid bodies which move in elliptical orbits in the solar system. They come in all sizes, from minor planets with diameters above a few hundred meters, to meter-sized meteoroids (boulders) and down to microscopic dust.
From time to time, a small dust grain from interplanetary space enters the earth's atmosphere with a very high velocity, often of the order of 10 km/sec or more. It is immediately heated by the friction with the air and begins to glow; this is what we call a meteor (a "shooting star"). Such events are very frequent and can be seen on every cloudfree night. More rarely a larger object, even a small boulder, may enter and will then be seen as a bright bolide. It leaves a luminous trail across the sky which can sometimes be seen in full daylight. If the boulder is big enough, a part of it will survive the descent through the atmosphere and will hit the ground, where it may be found as a meteorite.
About 3500 years ago, a large meteorite with a mass of several tons and measuring at least one meter across fell from the sky over the central part of the Atacama Desert in northern Chile. During its rapid passage through the Earth's atmosphere the big stone disintegrated into numerous smaller pieces which impacted in the desert sand over an area of some 20 km . Here they remained in well preserved condition, due the extremely dry conditions in the desert.
The fall-zone lies in a very remote part of the desert and most of the meteorite fragments escaped notice until recently. However, some of them were collected already in the 1860's, when prospectors first travelled through this inhospitable region in search of precious minerals. When they found some heavy masses which became shiny when polished, they thought they had hit upon a silver-mine. They collected some stones and brought an unknown number to the mining town of Copiapo, perhaps more than 1000 kg altogether.
Most of this material was probably discarded, but some stones (in total about 45 kg) found their way into mineral collections and were recognized as meteoritic. A few years later this meteorite fall was given the name "Vaca Muerta" (the Dead Cow) after a nearby dry riverbed (Quebrada Vaca Muerta), but soon after the exact location was completely forgotten.
For more than 100 years, nobody knew where the Vaca Muerta meteorite had fallen. However, in 1985 the site was rediscovered by Edmundo Martinez, following a lengthy study of the old accounts. At that time Martinez was a student of geology at Universidad del Norte, Antofagasta; he now runs a travel agency in San Pedro de Atacama, a small town in the middle of the Atacama desert.
Searching for the pieces
In addition to a few fragments which had been collected and worked on by miners in the last century (in order to extract the valuable iron-nickel clumps within them), Martinez found one big body which had not been molested. His brother spoke of this discovery to Canut de Bon, who soon thereafter decided to embark upon a scientific study of the area together with his friends, the ESO astronomers.
This involved a painstaking on-foot search of the fall area, that is now known to measure about 11 x 2 km. The distribution of the recovered pieces indicates that the meteorite entered from East-South-East, i.e. it flew over the high Andes mountains before the impact. One of the largest fragments hit the ground with such a force that a 10 meter crater was excavated to a central depth of almost 2 meters.
In total, 77 specimens were located during ten expeditions to the area between February 1987 and January 1991. Twenty of these pieces had already been deplaced and partly worked on by the miners (see the photo); this was also indicated by some artifacts from last century which were found nearby, including mining tools, cooking utensils, cans, beer and cognac capsules, corks, horse shoe nails, parts of boot soles and a coin from 1871. But 57 specimens, ranging from a few grams to one piece weighing no less than 309 kg, were found in "virgin" condition, i.e. undisturbed since the fall, except for some erosion. Such pieces are particularly valuable for meteoritic studies.
Pedersen, Lindgren and Canut de Bon have now prepared a very detailed account of their work with the title "Vaca Muerta Mesosiderite Strewnfield". It will appear in the international journal Meteoritics of the Meteoritical Society, the world's foremost authority in this scientific field.
The scientific study begins
All meteoritic material has now been recovered and is safely kept in Chilean collections, in particular at the Universidad de La Serena and also at the Museo Nacional de Historia Natural, Santiago de Chile. The combined mass exceeds 3400 kg and the meteorite is therefore by far the largest known in its class; this type of meteorite is much more rare than the common stony and iron meteorites.
Detailed laboratory analysis of the Vaca Muerta meteorite has begun and it is slowly unveiling its dramatic story. Its age has been dated by radiochemical methods and mineralogical studies are made of its composition and internal structure. The time of the fall was determined as 3500 ± 1300 years before present by means of Carbon-14 dating2 by A.J.T. Jull of the University of Arizona, Tucson, U.S.A.
Some of the minor planets, along with the comets, are thought to consist of material that dates back to the very beginning of the solar system. The minor planet from which the Vaca Muerta meteorite derives is about 4500 million years old and therefore nearly as old as the solar system itself.
The early life of this minor planet was obviously very violent. At some time a partially molten, volcanically active body moving at high speed through the solar system collided catastrophically with a metallic-core minor planet. When the finely intertwined materials cooled and solidified, they formed a cosmic breccia (mixture of minerals) which was half stony and half metallic. Later, after an unknown period of time, this minor planet split into a swarm of smaller fragments, some of which now fall to the Earth at rare moments. One of them was the Vaca Muerta meteorite.
This particular kind of stony-iron meteorite is known from about 30 other locations only. The amount recovered at Vaca Muerta has tripled the material available to laboratory study. When fully analyzed, the seventy-seven "dead cows" from Atacama will undoubtedly provide us with much new insight into the enigmatic history of the early solar system.
 This technique is based on the fact that while the meteorite is still in spaccI it is continuously bombarded by cosmic rays, leading to a particular internal proportion of Carbon-12 and -14 atoms. As soon as it passes through the earth '8 atmosphere, it is shielded from cosmic rays and the proportion begins to change as the radioactive Carbon-14 atoms decay. A measurement of this proportion will therefore indicate the time since the fall.
ESO EPR Dept