Meteor Crater in Arizona — The origin of this classic, simple meteorite impact crater was long the subject of controversy. The discovery of fragments of the Canyon Diablo meteorite, including fragments within the breccia deposits that partially fill the structure, and the presence of a range of shock-metamorphic features in the target sandstone, confirmed its impact origin. Target rocks include Paleozoic carbonates and sandstones; these rocks have been overturned just outside the rim during ejection. The hummocky deposits just beyond the rim are remnants of the ejecta blanket. This aerial view shows the dramatic expression of the crater in the arid landscape. The rim diameter is 1.2 kilometers, age is 49,000 ± 3000 years. Location is 35°02’N, 111°01’W. Aerial image courtesy of D. Roddy, residing at www.lpi.usra.edu.
Manicouagan, Canada — The moderately eroded, central part of the structure (the plateau surrounded by the lake) is partly covered by impact melts and contains shattered rocks and several uplifted peaks about 5 kilometers north of the center. The quantity of data obtained on the melt sheet and the underlying target rocks make Manicouagan one of the most intensely studied large complex impact structures in the world, and it is an important source of ground-truth data for understanding the cratering process. The radiometrically determined age of the structure is close to (but not quite identical with) the biostratigraphically derived age of the Triassic-Jurassic boundary. The original rim diameter was ~100 kilometers; age is 214 ± 1 million years. Locatio is 51°23’N, 68°42’W. Courtesy of Space Shuttle, image STS42-207-14.
Kebira Crater in Egypt’s Western Desert’s outer rim is 31 km in diameter, as indicated by the dashed circular curve superimposed on the image. Landsat color composite image is courtesy of Boston University Center for Remote Sensing.
Kamil Crater is located at Djebel Kamil, south of Gilf Kebir near the Sudanese border in Egypt. This crater is 45 meters in diameter and is considered one of the best-preserved found on Earth to date. Photograph courtesy Museo Nazionale dell’Antartide Università di Siena.
Known Impact Structures (Craters Map). This image shows the geographic distribution of about 160 structures that have been positively identified as impact structures based on the presence of shock-metamorphic effects and/or the presence of a meteoritic component or fragments at the structure, as of 2000.
Zhamanshin crater in Kazakhstan. This image combines 90-meter resolution Shuttle Radar Topography Mission (SRTM) topography with Spaceborne Imaging Radar (SIR-C) polarimetric synthetic aperture radar to show the subtle crater that formed only 870,000 years ago. Courtesy: James Garvin/NASA.
Call them trouble-makers of the heavens. Asteroids and periodic comets can wander into the Earth’s neighborhood, and on occasion, crash into our planet. Earth has the scars to prove it.
Over geologic time, these malicious cosmic interlopers have left their mark on our world. But obtaining a true inventory of Earth’s impact craters is made difficult by erosional processes over eons of time, as well as vegetation overgrowth – along with politics and even a touch of secrecy.
In the past, most images of craters were aerial or from the Space Shuttle. See Figures 1-2. Today, in the 21st century, thanks to an armada of Earth-orbiting satellites that provide worldwide coverage, including use of high-resolution imaging technology, the prospect for eyeing new sites has been boosted. Moreover, special processing can reveal insights about a crater imaged by a digital system.
“Impact-minded searchers have discovered at least ten new impact structures through satellite remote sensing. Before Landsat, and to some extent since 1973, observations from airplanes, and in particular aerial photos, were a source of information about possible craters,” notes Nicholas Short, formerly with NASA’s Goddard Space Flight Center in Greenbelt, Maryland. He explains in a web-based remote sensing tutorial that in large parts of the world the more obvious craters have been found by conventional techniques.
Author's Note: The author would like to thank Bevan French of the Smithsonian Institution and David Kring of the Lunar and Planetary Institute for their guidance in writing this article.
“But in parts that were poorly mapped or explored, evidence from satellite imagery may be the first overview-type looking that discloses heretofore unnoticed impact shapes or scars,” Short reports. “Previously known impact craters are being re-examined in the context of their surroundings. But, the scientific ‘fun’ has been to search for new ones, especially in isolated regions not easily accessed or populated. The basic strategy in the hunt is to look for distinct circular structures or features, the hallmark of craters, and then to find confirmatory evidence that impact was involved.”
Crater-spotting is exemplified by the work of researchers at Boston University’s Center for Remote Sensing. There, Farouk El-Baz and his colleague Eman Ghoneim discovered the remnants of the largest crater in the Sahara. El-Baz, the Center’s director, named the find “Kebira,” meaning “large” in Arabic and also relating to the crater’s physical location on the northern tip of the Gilf Kebir region in southwestern Egypt. See Figure 3.
One of the data sources that assisted in mapping of the feature is the Shuttle Radar Topography Mission, which allows measurement of elevations in three dimensions. Landsat Enhanced Thematic Mapper Plus (ETM+) images and RadarSat-1 data were also tasked.
But why hadn’t this large feature been seen before?
“Kebira may have escaped recognition because it is so large – equivalent to the total expanse of the Cairo urban region from its airport in the northeast to the Pyramids of Giza in the southwest,” said El-Baz in a 2006 university press statement. “Also, the search for craters typically concentrates on small features, especially those that can be identified on the ground. The advantage of a view from space is that it allows us to see regional patterns and the big picture.”
In another case, the so-named “Kamil Crater” was located a few years ago during a Google Earth “low flight charting mission”– some 1,000 meters above ground level. Scientists think the impact crater was created within the past couple thousand years.
Located at Djebel Kamil, south of Gilf Kebir near the Sudanese border in Egypt, the crater is 45 meters in diameter and is considered one of the best-preserved craters found on Earth to date. Subsequent ground truth visits by teams found thousands of meteorite fragments scattered within the crater and surrounding area. See Figure 4.
The need for satellite remote sensing to hand off to in-the-field confirmation of a crater was recently illustrated. A circular depression, called the Luizi structure, deep in the Democratic Republic of the Congo, was pinpointed a few years ago as a possible crater by Philippe Claeys of the Department of Geology and head of Research Unit Earth System Sciences at Vrije Universiteit Brussel in Brussels, Belgium.
Looking at the world distribution map of impact structures, it was clear to Claeys and his colleagues that many craters remain to be discovered on the old shield of Central Africa. See Figure 5 for a World Map of Craters from the year 2000. In 1990, based on the circular morphology, an impact origin was proposed for a structure. Unfortunately, the feature was located in a rather remote region, difficult to visit in a country in tumult and strife for more than 25 years.
In the Claeys-headed research, a digital elevation model of the promising crater was generated from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imaging instrument flying on NASA’s Terra satellite. That remote sensing study of the Luizi structure supported a possible impact crater origin.
At the 42nd Lunar and Planetary Science Conference in March of this year, study leader Ludovic Ferrière, curator of the rock collection at the Natural History Museum of Vienna in Austria, reported that the feature was definitely an impact crater.
A detailed analysis of the Luizi structure combined a remote sensing study with geological field observations and examination of rock samples during a 2010 field campaign. The researchers confirmed Luizi to be a complex and huge impact crater in the remote Congo – the first known impact crater in central Africa, bringing the number of known meteor craters on Earth to 182.
Creating algorithms for automatic identification of craters on Earth is a non-starter, said Tom Stepinski, the Thomas Jefferson Chair Professor in the Department of Geography at the University of Cincinnati. Stepinski has been a leader in this arena – but for use on beyond-Earth targets.
Craters on planets that either lack atmosphere or have very tenuous atmosphere are well preserved. Not so for Earth, he continued. For one, craters erode very fast (on geological time scales) due to fast scale of erosion on Earth, so terrestrial topography preserves only a very fresh crater (like the one in Arizona). Older impact sites on Earth are heavily degraded and cannot be detected remotely.
Additionally, even for fresh craters, images cannot be used because vegetation masks the sites of impacts to the degree that makes automatic detection of them from satellite images impossible. “Thus, on Earth we need to rely on manual finds. Yes, we can use satellite data Digital Elevation Models and images to help us in this process, but the process cannot be automatic…algorithmic,” Stepinski explained.
Use of radar imagery taken from space has been a huge help in interpreting prospective impact craters, said Richard Grieve at the Earth Sciences Sector, Natural Resources Canada in Ottawa. “One of the problems with optical imagery is that what you’re getting back is basically a reflection of vegetation. And that vegetation doesn’t necessarily follow the structure of the crater,” he told Imaging Notes, “so I think radar is one of the ways to go.” See Figure 6 of the Zhamanshin crater in Kazakhstan.
Grieve said that the cratering rate on Earth is considered to be twice as high as on the Moon. “So there are still craters on our planet to be found, particularly in places like Africa, which has not been well explored.” Also, with the Earth covered mostly with water, Grieve suggested that the U.S. Navy is holding tight what undersea crater sites they have charted, “but they are not telling everybody.”
Evolution of Our Planet and Its Life
While being on the lookout for impact sites on Earth may be a daunting challenge, there are several rationales for identifying new craters, explained David Kring at the Center for Lunar Science & Exploration at the Lunar and Planetary Institute in Houston, Texas.
“My team’s discovery of the Chicxulub impact crater and its link to the K-T boundary mass extinction event illustrate how impact cratering can affect both the geologic and biologic evolution of Earth,” Kring told Imaging Notes. Discovery of new craters, he added, will help researchers explore how other events may have punctuated the evolution of our planet and its life.
Kring also flagged the fact that discovering new craters can help measure better the environmental effects of impact events in the geologic history, and thus provide the foundation needed to better assess future impact hazards. “Sometimes saving the planet means looking at its past,” he said.
There’s another plus, Kring suggested, in identifying new impact craters on Earth. “They are true natural wonders and provide any host country with an economic attraction and an opportunity to enhance science education of the public.”
Discerning Circles, the Eye-brain Connection
“Satellite images from orbit are useful for recognizing candidate impact craters. Of course, natural geological processes besides impact cratering can create circular features on the Earth’s surface. And the human eye-brain connection loves to discern simple shapes like circles, whether they are really there or not,” said Clark Chapman, a leading asteroid expert at the Southwest Research Institute in Boulder, Colorado.
Certainly the earth must have been as heavily cratered as the moon, Chapman said. “But most of the moon’s craters formed billions of years ago and remain today because of the moon’s minimal geological activity. The earth is a geologically active planet, with continents forming and eroding away and seafloors in constant motion.”
That being the case, Chapman added, virtually all of the ancient craters on Earth were rendered invisible long ago. Those remaining that geologists can decipher from on-the-ground studies are often not recognizable from space. Most known terrestrial craters are of rather recent origin and haven’t been around long enough to have been eroded away, he told Imaging Notes.
“Understanding the earth’s impact history can help us understand the histories of other planets, since comets and asteroids like those that have struck the earth strike other planets, as well,” Chapman said. “And craters have a practical importance. They can focus geological processes that collect valuable resources, like oil and nickel, for extraction.”
The impact cratering record on Earth is extremely “under-served,” said James Garvin, Chief Scientist at NASA’s Goddard Space Flight Center.
“The ability to preserve the scars of major collisional events is extremely challenged by the incredible dynamism of Earth’s geological processes. Indeed, the current record of ‘proven’ impact sites is measured in the range of 150-200, rather than in the thousands, as one might presume,” Garvin said.
Part of the issue with discovering the largely missing terrestrial cratering record, Garvin said, is a combination of preservation (how well an impact landform or signature can be preserved over time in a given geologic setting) and detection.
“We have about 150 million square kilometers of land area today to search, with much of it vegetated, overprinted by very recent geological events, such as shifting sands, and some of it covered with ice,” Garvin advised. “Methods of detecting impact features – craters, signatures in rocks, geophysical expressions – while improving, remain somewhat limited,” he told Imaging Notes.
For Garvin, the bottom line: “This is a geological forensics problem!”
Bolstering the topographic reconnaissance of Earth as a whole is possible over the next decade via planned or recommended missions that NASA’s Earth Science Program may implement.
Given increased thinking of how best to thwart an incoming object from stirring up a bad day in our world, plus given new awareness of the impact flux at the Moon and Mars, “we need a ‘mission to planet Earth’ approach to finding our own planet’s impact record.”
Looking ahead, Garvin spotlights the emerging global satellite remote sensing datasets, including those from NASA’s Earth Observing System (EOS) spacecraft, the Shuttle Radar Topography Mission (SRTM), Canada’s RadarSat missions, and commercial remote sensing satellites, such as GeoEye. “These datasets could revolutionize preliminary detection of candidate impact features – or signatures – over the next decade.”
Garvin pointed out that the key to realistic detection will be establishment of a set of definitive criteria for recognition tied to an existing benchmark set of signatures calibrated in the new remote sensing datasets. New orbital assets such as Canada’s RadarSat-2 and Astrium and DLR’s TerraSAR-X could contribute to this criteria-shaping, as would NASA’s upcoming ICE-Sat-2 and missions of the Deformation, Ecosystem Structure and Dynamics of Ice (DESDynI) that involve Earth-imaging radar and Lidar technologies.
Ultimately, however, field expeditions involving geophysics (gravity, seismic surveys) and sample analysis will be essential, Garvin said. New airborne geophysical methods, in which topography and micro-gravity can be measured at scales fine enough to investigate kilometer-scale features, are particularly interesting.
Garvin also senses that declassification of certain datasets would augment the search for candidate craters. For example, a bonus would be making available full resolution Shuttle Radar Topography Mission digital elevation models at 30-meter horizontal scales for Africa, South America and Asia. Furthermore, and another plus, would be release of U.S. Department of Defense imaging of regions of Africa, Asia, and South America, including their specialized radar images of known craters, to understand better their signatures.
“The impact record of Earth is a key to understanding the history of life,” Garvin observed. That objects impact Earth and affect climate is not in question, he said; rather, what is in question “is a more complete history of how impact events have shaped the details of the environmental-biological-geological history of our planet for the past 4.6 billion years.”
Garvin concluded that the next decade or two “could see an explosion in the recognition of as-yet undiscovered aspects of the Earth’s impact record.”