Department of Geology, University of the Witwatersrand
Private Bag 3, WITS 2050, South Africa
Dedicated to: Eugene M Shoemaker - A Pioneer in
Space Exploration and Impact Catering
Meteorites are `rocks from heaven' - the only samples available to us for direct study of the solid matter `out there in Space,' besides a few kilograms of Lunar rocks returned by astronauts and space probes from the Moon, as well as tiny cosmic dust particles collected on high-flying aircraft and by spacecraft. Right up to the end of the eighteenth century it was unthinkable that any rocks might come from anywhere else than from the Earth itself. It took the personal observation of a meteorite fall in northern France at the end of that century by eminent members of the French Academy of Science to correct this opinion. By the mid-19th century, several scientists had already begun to establish major meteorite collections, and with the improvement of petrographic microscopes, diligent microstudy of meteorites was soon taken up. From about 1960 onwards, and in particular after the revolution in the microanalytical disciplines which made mass spectrometry and electron microprobe analysis standard analytical techniques, it has become clear how much precious information is contained in meteorites with regard to the evolution of the solar nebula and many bodies in the Solar System.
In recent years it has also been shown that Earth has `collected' a few meteorites that originated from Mars. Until the first chemical data was obtained in situ on Mars by the rover `Sojourner' of the Pathfinder spacecraft, commencing in July 1997, Martian meteorites provided the only samples from this planet available for study on Earth.
There are today thousands of meteorites in the collections of museums, private collectors, and research institutions throughout the world. Figure 1 is a compilation of documented meteorite falls and finds on the African continent - and this does not even include the more than 1400 meteorites collected by a German research team in recent years in the Libyan Desert.
The Hoba iron meteorite (front cover), exhibited where it fell several thousand years ago, near the town of Grootfontein in northern Namibia, is the largest known meteorite on Earth. A vast amount of extraterrestrial material has, for decades, been subjected to sophisticated mineralogical, chemical, and physical analysis, resulting in a wealth of information: relating, for example, to the formation of primary material in the early phase of solar nebula development, to the complex processes that took place on the parent bodies from which meteorites had been derived, to the geological processes that affected them upon impact on Earth and during residence on the Earth's crust.
Meteorites are classified as stony, stony iron, and iron meteorites. The stony meteorites are further divided into those with chondrules (chondrites) and those without chondrules (achondrites), which are roundish particles that show that their fillings crystallized from a melt. Chondrites are further subdivided into several classes according to their mineralogical and chemical composition. Different types of meteorites have been defined: for example, the primitive carbonaceous chondrites, which provide us with information about the processes that led to the formation of solid material in the earliest stages of the development of the Solar System around 4.6 billion years ago. Other chrondite types are believed to represent material formed in different regimes in the Solar Nebula. Achondrites (which are very similar to the rocks derived from the Earth's mantle), stony irons (composed of silicate minerals as well as a significant proportion of metallic phases), and the iron meteorites are considered to represent different zones on differentiated parent bodies (presumably zoned like the earth into silicate-rich outer and metal-rich inner domains). Finally, the degree of deformation (for example, brecciation) is used to further subclassify meteorites.
Individual groups of meteorites have common characteristics. On the basis of spectral characteristics, some groups have been assigned to specific parent bodies. For example, the group known as eucrites (a type of chondrite) is believed to originate from the asteroid Vesta. Overall, most meteorites are probably derived from the asteroid belt between Mars and Jupiter. Only study of meteorites can tell us about the nature of the bodies from which they originated and about the surface processes that have been active on them.
The study of meteorites (besides several decades of space exploration) has confirmed how important the process of meteorite impact cratering has been for the surface evolution on all solid planetary bodies in the solar system - including Earth's surface! Most meteorites show, at least to some degree, mineral deformation phenomena - termed shock or impact metamorphism - that were acquired as a consequence of hyper-velocity impact events on the parent body. There are a number of shock metamorphic deformation effects, which are characteristic of deformation at the very high pressures and temperatures which can only be achieved upon the impact of a large projectile travelling at cosmic velocities (ca. 11-72 km/s). These deformation effects have never been observed in terrestrial rocks that were subjected to normal geological processes and are thus indicative of impact-related deformation. The same deformation effects were found in many samples returned from the heavily cratered Highland terranes on the Moon, which are more than 4 billion years old. While most researchers, right into the middle of the 20th century, believed that lunar craters were the result of volcanic activity, the manned and unmanned exploration of the Moon in the Sixties and Seventies demonstrated that impact cratering has played a much more important role in the development of the lunar surface than any other process.
In contrast to the Moon, where geological activity essentially stopped a long time ago, the Earth has remained geologically active until this day. Thus, much of the earlier formed and heavily impact-cratered crust of the Earth has been recycled throughout geological time, but remnants of a historical impact record have been found in abundance on this planet as well (Figure 2).
In the last 15 years much discussion has resulted from the recognition that the evolution of life on Earth has also been severely affected by the impact of large extraterrestrial projectiles (asteroids and comets). There is no longer any doubt that a major impact event took place some 65 million years ago on the Yucatàn peninsula in Mexico, resulting in a ca. 200-km wide impact structure (Chicxulub) and in an environmental catastrophe that left its mark, a mass extinction of life forms, in the biological record of this planet. It is estimated that the projectile that so tragically contributed to the demise of the dinosaurs measured about 10 kilometres in diameter. Currently there is much debate about the actual danger of such a large, catastrophic impact event to the human race. That large impact events do still occur in the Solar System, was vividly demonstrated to all of us in July 1994 with a series of impacts of large fragments of the Shoemaker-Levy 9 comet into the atmosphere of planet Jupiter.
Dr Gene Shoemaker, co-discoverer of the Shoemaker-Levy 9 train of comet fragments, died tragically on 18 July 1997 while travelling in the Australian outback during one of the Shoemakers' annual impact crater expeditions to this continent. Gene must be regarded as one of the true pioneers of impact cratering studies, but he was also involved with, and renowned for, his contribution to a series of space exploration projects, including the Apollo project which was so successful regarding sample return from the Moon. As of late he had also been instrumental in organizing NASA's comet and asteroid spotting project - directed at providing better data of those projectiles that could potentially prove fatal for Earth, and at the same time obtaining early warning if a body on an Earth-orbit crossing path were discovered. In recognition of Gene's contribution to Astrogeology, this article is dedicated to him.
The study of terrestrial impact structures has only been pursued by a relatively small, but dedicated, group of planetologists and earth scientists since the 1960s. Few more than 150 impact structures, ranging in age from under one million years to more than 2000 million years, and in size from more just a few tens of metres to over 250 kilometres, have been identified so far on Earth. Two to four new ones are added to this list every year. But comparing this record with the number of impact structures that even a small telescope can reveal on the Moon demonstrates that we have only detected a small proportion of the total number. And only a better understanding of the terrestrial impact cratering record and the environmental effects resulting from large impacts, coupled with knowledge about the current population of potentially Earth-orbit crossing asteroids and comets, can lead to a realistic assessment of the impact danger for mankind.
Figure 3 is a compilation of all known impact craters and structures in Africa. At the last published count in 1994, fifteen known or suspected impact structures were listed; since then another 2 have been added. Most recently, a large impact structure, the Morokweng Structure, was discovered in South Africa. Morokweng was initially identified by its strong near-circular geophysical (aeromagnetic) anomaly. It was formed at 145 million years ago, but, since then, has been heavily eroded. This is also the reason why it has not been possible to determine the true diameter of the original structure. Current geological knowledge favours a diameter in excess of 100 kilometres, which would make Morokweng one of the ``big 4'' (the others being Chicxulub, Sudbury in Canada, and Vredefort in South Africa). Interestingly, the impact time at 145 million years ago coincides with a major break in the global geological record - the boundary between the Jurassic and Cretaceous eras - which has been tentatively related to an at least partial mass extinction. As the 65 million year old Chicxulub impact marks a major extinction at the Cretaceous-Tertiary Boundary, it is suggested that further detailed work should be carried out on J-C boundary sites around the world.
Three suspected candidates for impact structures have been confirmed in Zimbabwe (Highbury and Sinamwenda) and discovered in Chad (Gweni-Fada), while the impact origins of the Kalkkop and Vredefort structures in South Africa are now firmly established. Possible impact structures in the rain forest of Equatorial Guinea and in West Africa have been suggested on the basis of satellite imaging, but still need to be confirmed by ground-based geological studies. And yet, despite these advances, the African impact crater record is still far behind those of North America, Europe, and Australia. A large number of structures still remain to be discovered on this continent!
In the past, impact structures in Africa were mainly discovered from aerial photography related to oil exploration activity in Northern Africa, and because several impact researchers from the northern hemisphere extended their interests into Africa in the 1960s and early 1970s. They proposed that a few structures on this continent were the result of impact cratering (e.g., Vredefort; Pretoria Saltpan/Tswaing; Roter Kamm). Over the past decade a small, but active, group of geologists and mineralogists at the University of the Witwatersrand, in close collaboration with some colleagues abroad, have carried out detailed impact structure investigations in several African countries. This group is keen to extend their work in Africa - which would only be possible through interaction with institutions from other African countries.
For this reason, the author would like to appeal to all researchers in Africa who have knowledge of possible impact structures (for example, because a circular or near-circular structure has been identified on aerial or satellite imagery, or a circular geophysical anomaly has been detected), to contact him (contact numbers and address are given above). As only geological ground studies, followed by mineralogical and chemical analysis of rock samples, can provide unequivocal proof of the existence of an impact structure, plans would, then, have to be made for geological fieldwork. Existing connections with overseas workers may facilitate such work.
As with impact structures, studies of African meteorites, as well as expeditions dedicated to the search for meteorites, have in the past generally been directed by non-African institutions. Obviously this has a lot to do with availability of funding for such work. It is, however, strongly felt that the widely noted lack of knowledge about the importance of the study of meteorites, of how to identify them, and of impact structures also contribute to this one-sided research situation. Meteorites, especially the iron and stony-iron meteorites, can be recognized by their metallic or semi-metallic appearance, frequently grooved surface structure, and strong magnetism. It is important that, when such material is discovered, all information, for example about its position, orientation on the surface, time of fall (if the fall was observed), and any visual observations made or sound heard, be meticulously recorded. Contamination from metal tools and chemical reagents must be avoided. A small number of scientists, for example at the Universities of Cape Town, Cairo, and of the Witwatersrand in Johannesburg, have in the past actively studied meteorites. Should a meteorite be recognized and a sample recovered, a report should be made to the Meteoritical Society, for establishing contacts with experienced meteoriticists and to ensure proper classification, cataloguing, further processing, and wide dissemination of research results. The secretary of the Society is Dr Monica Grady, Natural History Museum, Cromwell Road, London SW7 5BD, UK, Fax No. +44 71 9389268, E-mail firstname.lastname@example.org; alternatively the author of this article could be contacted.
It would be appreciated if members of the Working Group on Space Sciences in Africa, who receive this publication, could pass copies of this article on to all other institutions in their country, who might be involved in planetological or geological studies (e.g., Geological Surveys).
Another reason for drawing attention to African Meteorites and meteorite craters is that in July 1999 the 62nd Annual Meeting of the Meteoritical Society will be held in Johannesburg at the University of the Witwatersrand. This will mark the first time that this prestigious scientific organization will hold its annual conference on the African continent - and only the second time that it will take place in the southern hemisphere. It is hoped that African scientists will be able to participate with a large number of scientific contributions. To this effect, and in order to promote meteoritical and planetary science in Africa, the Organizing Committee (chaired by the author and to be contacted through him) is actively involved in organising sponsorship for travel support of scientists from other African countries who intend to present a contribution in July 1999. Should there be colleagues who might, already at this stage, be interested in contacting this committee, as they might have an interesting object (specimen or structure) or a special interest related to the general subject of the conference, please do not hesitate to contact us now. We may also be able to support your research to facilitate its completion prior to the conference.
However, the most important aim of the Organizing Committee for the 1999 Annual Meeting is to distribute knowledge. One way of doing this will be through presentations from invited lecturers, who will present public lectures on a wide variety of planetary science topics. Many eminent earth scientists, astronomers, chemists, and physicists will attend the 1999 conference; many of them have already agreed to share their formidable knowledge. Should you be interested in having your institution host such a lecturer, please contact us as soon as possible. The Meteoritical Society and other organizations have already pledged their active and financial support for this plan. The only way that this plan could fail would be through lack of interest on this continent. We are relying on your support in making this lecture tour a success!
The time has come to strengthen planetological studies, including those of meteorites and impact structures, in Africa. If we pool our resources and collaborate we undoubtedly have the potential to carry out such research at the highest level!