Many organizations perform scientific studies using remote sensing that embrace most of the disciplines already considered in this tutorial. Section 14 on Meteorology is a cogent illustration of a field within the Earth sciences that has a strong scientific-research bent and a broad set of applications. Some applications are direct spin-offs from satellite programs that were initially proof-of-concept efforts. Because of the professional geological background of the writer, there seems to be an overabundance of examples in this field, resulting in part from remote sensors that create imagery dominated by geologic phenomena. In keeping with this experience, Basic Science II presents a geological subject that also was once a specialty of the writerimpact cratering. For years, impact cratering was esoteric. Now, impacts are hot topics, especially since someone identified them as a cause of dinosaur extinction. More to the point, scientists and the general public now realize that impacts are genuine dangers to mankind, capable of causing catastrophes greater than any other natural process known to affect the Earth's surface. Plus, they are likely to occur (as they already have) as huge events sometime in the future of civilization. This subject of impacts is pertinent and interesting, and impact craters are often detectable by remote sensing.
(An exceptional summary of impact cratering, and especially the record of these events imposed on the target rocks, is a 120 page treatise written by a colleague, Dr. Bevan M. French, and entitled Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures, published by the Lunar and Planetary Institute [Houston, TX]; LPI Contribution No. 954, 1998.)
One of the payoffs of the space program is that many people realize that fast-moving asteroids, comets, and large meteorites collide with growing or stable planets during their formative period. Moreover, impacts persist in modifying such bodies throughout their history. The surfaces of some terrestrial-type planets and many of their satellites display craters that totally pockmark their surfaces: Mercury (left) and Callisto (fourth satellite out from Jupiter) are prime examples.
Earth's Moon has conspicuous craters with generally circular depressions, ranging in size from less than an inch to more than 1,200 km (746 mi) in diameter. Many of the larger ones are visible at full Moon through a pair of binoculars. The most conspicuous lunar impact structure is Tycho (below, on the left) located on the Moon's southern hemisphere. We can readily see that it is the source of great streaks (rays) almost visible to the naked eye that result from deposits of ejecta hurled across the Moon.
This crater is the classic exemplar of a large impact structure, with these hallmarks: circular raised rim; concentric nest of slumped walls inside this rim; central (uplift) peak; rough, irregular crater floor (here a mix of fragmental ejecta and lava extrusion); and exterior ejecta in hummocky deposits. Typical of farside craters is Goclenius (55 km, 34 mi wide) and several smaller ones (right) as they appeared to Apollo 8 astronauts circling our lunar neighbor. Note their flat interiors filled with mare lavas.
Less than 100 years ago, scientists considered the concept of craters forming by impacts from meteorites and other extraterrestrial bodies ("bolides"), such as comets, unrealistic and highly improbable. Several scientists had, by then, suggested that such craters covered the Moon, but the bulk opinion attributed these to volcanic processes. And, indeed, there were depressions in terrestrial volcanic fields that bore sometimes strong resemblance to lunar craters.
One example shown next is the elongate caldera on the summit of the raised shieldlike basaltic volcano on Isle Fernandina in the Galapagos Islands. Note the irregularities in its shape and the numerous lava flows emanating from the side of the volcano's slopes.
Another volcanic type that leads to near circularity of some of the resulting craters is the maar crater exemplified below by the Crater Elegante (1.5 km [0.9 mile] diameter) in the Pinacate basaltic field in northwest Mexico (below the Arizona border). This kind of volcanic crater often has a subdued rim. It is formed when lava encounters near surface water which flashes into steam causing rock overhead to be pushed out explosively leaving a depression that is somewhat backfilled by volcanic fragments (and later debris washed in). The gashlike depressions outside the rim are caused by water erosion.
18-1: Until the space program in the 1960s, debate over the nature of lunar crater raged as a controversy for more than a century. But with close exploration of the Moon and then planets like Mercury and satellites like Callisto disclosed that most planetary surfaces were heavily cratered. The weight of opinion shifted drastically in favor of impact as the dominant process creating the myriads of circular depressions spread widely over these surfaces. Can you develop (deduce) some arguments that support this impact hypothesis? ANSWER
Impact as a lunar and terrestrial process was first suggested by European geoscientists in the early 1800s. The proposal that Meteor Crater in Arizona had an impact origin, opened the possibility of impact as the cause of other, similar, circular features. G.K. Gilbert conceived the idea and then supported it by finding iron meteorites around the crater. Work by R. Baldwin and others kept the impact process alive as an alternative but the vast majority of astronomers and geologists were highly skeptical. All this changed in 1960 with the classic study of cratering mechanics at Meteor Crater and several nuclear explosion craters by Eugene M. Shoemaker, which opened up the possibility of impact as the cause of similar circular features, of which more than 50 were then known on Earth. The critical proof emerged in the 1960s from the study of the effects of the intense pressure (shock) waves generated during impact on the target rocks at terrestrial craters, with key studies by E. Chao, W. von Engelhardt, B.M. French and others. At that time, the writer (NMS), while working on rocks associated with underground nuclear explosions which generate pressures of the same magnitudes, noted that shock metamorphic features in these rocks were identical to corresponding features in impactites (rocks from natural craters); this tie-in led to the argument that only impacts could cause high pressures on the order of those in underground nuclear explosions, as the features produced are never found in rocks associated with volcanism, even the types called "explosive". With the return of the first lunar samples from Apollo 11, shock effects in moon rocks were observed, supporting the conclusion that the bulk of lunar craters are the result of impact.
Gradually, the idea that impact is one of the fundamental formative processes acting on planets won broad acceptance. In fact, scientists have now proven that planets grow by accretion of infalling materials, with the craters representing the last stages of buildup, as the planets reach their full sizes. The nature of impact cratering is important, yet introductory geology textbooks still treat the subject poorly. So, this summary comes from the authors experience in the field.
Heavily cratered planetary bodies, such as the above, share the impact markings that appear on their ancient surfaces but have not been fully demolished or masked by erosion, lava outflow, deposition, or obliterating mountain activity. Cratering on their surfaces was most intense from the last stages of planetary growth, very early in solar system time (beginning about 4.6 billion years ago), through a later period of about one billion years, after which, the flux of objects striking those surfaces dropped off notably. Earth, Venus, and parts of Mars, also profusely cratered at the outset, by contrast now show far fewer craters because of the subsequent destructive processes that erased or covered most of the impact evidence left on the primitive surfaces.
Distribution of Craters on Earth
Earth today, despite its many recyclings of continental and oceanic crust, retains signs of huge impacts imposed in the last two billion years, as well as smaller ones that took place in historic times. Scientists have found about 160 surviving craters of definite or probable impact origin. This is well below the estimates of tens of thousands that we would expect, if Earth's surface and crust had not experienced such dynamic ruin from plate tectonic action and atmospheric-driven erosion. Also, protection by deep oceanic waters (more than 70% of the planets surface) and burial by sediments further account for this deficiency in anticipated numbers. Nevertheless, more craters remain to be discovered, and satellite imagery should be an effective means for conducting a systematic search, as we shall see near the end of this survey.
We extracted the world map below that locates nearly all known craters on Earth's land surfaces from the Home Page of a Web Site on Terrestrial Impact Cratering (http://gdcinfo.agg.emr.ca/crater/world_craters.html), put together by the Geological Survey of Canada (GSC). The distribution of craters seems non-uniform, a fact explained by variations in ages of surficial rocks, distribution of mountain systems, and differences in the extent of exploration.
We can also display the map by individual continent, after accessing this site. For each continent, the craters have names and you can click on these for a description and, often, an aerial view, along with their precise geographic coordinates, size, and estimated time of formation.
18-2: Inspection of the above distribution map shows large areas on Earth in which confirmed impact craters are absent or sparse. Offer several explanations for this scarcity. ANSWER
The same group at Goddard Spaceflight Center has also put online an extended narrative (http://gdcinfo.agg.emr.ca/crater/paper/cratering_e.html) that discusses crater formation and morphology, identification, and possible hazards in the future, which nicely supplements the review in this subsection. Calvin Hamilton prepared a similar treatment, supported by selected images of terrestrial and planetary craters, at (http://bang.lanl.gov/solarsyst/tercrate.htm). A simple listing of craters, with location, size, and age, can be brought up from the Internet simply by pressing here.
For anyone seeking a comprehensive and technical treatment of crater mechanics, we recommend the book "Impact Cratering: A Geologic Process," by H.J. Melosh, 1989, Oxford Monographs on Geology and Geophysics No. 11, Oxford University Press.
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