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Natural Resources Canada in Ottawa.“One of the problems with optical imageryis that what you’re getting back is basicallya reflection of vegetation. And thatvegetation doesn’t necessarily follow thestructure of the crater,” he told ImagingNotes, “so I think radar is one of the waysto go.” See Figure 6 of the Zhamanshincrater in Kazakhstan.Grieve said that the cratering rate onEarth is considered to be twice as high ason the Moon. “So there are still craterson our planet to be found, particularly inplaces like Africa, which has not been wellexplored.” Also, with the Earth coveredmostly with water, Grieve suggestedthat the U.S. Navy is holding tight whatundersea crater sites they have charted,“but they are not telling everybody.”Evolution of Our Planet and Its LifeWhile being on the lookout forimpact sites on Earth may be a dauntingchallenge, there are several rationalesfor identifying new craters, explainedDavid Kring at the Center for LunarScience & Exploration at the Lunar andPlanetary Institute in Houston, Texas.“My team’s discovery of the Chicxulubimpact crater and its link to theK-T boundary mass extinction eventillustrate how impact cratering canaffect both the geologic and biologicevolution of Earth,” Kring told ImagingNotes. Discovery of new craters, headded, will help researchers explorehow other events may have punctuatedthe evolution of our planet and its life.Kring also flagged the fact thatdiscovering new craters can helpmeasure better the environmentaleffects of impact events in the geologichistory, and thus provide the foundationneeded to better assess future impacthazards. “Sometimes saving the planetmeans looking at its past,” he said.There’s another plus, Kring suggested,in identifying new impact craters onEarth. “They are true natural wondersand provide any host country with aneconomic attraction and an opportunity toenhance science education of the public.”Discerning Circles, the Eye-brainConnection“Satellite images from orbit are usefulfor recognizing candidate impact craters.Of course, natural geological processesbesides impact cratering can createcircular features on the Earth’s surface.And the human eye-brain connectionloves to discern simple shapes like circles,whether they are really there or not,”said Clark Chapman, a leading asteroidexpert at the Southwest Research Institutein Boulder, Colorado.Certainly the earth must havebeen as heavily cratered as the moon,Chapman said. “But most of the moon’scraters formed billions of years ago andremain today because of the moon’sminimal geological activity. The earthis a geologically active planet, withcontinents forming and eroding awayand seafloors in constant motion.”That being the case, Chapmanadded, virtually all of the ancientcraters on Earth were rendered invisiblelong ago. Those remaining that geologistscan decipher from on-the-groundstudies are often not recognizable fromspace. Most known terrestrial cratersare of rather recent origin and haven’tbeen around long enough to have beeneroded away, he told Imaging Notes.“Understanding the earth’s impacthistory can help us understand thehistories of other planets, since cometsand asteroids like those that havestruck the earth strike other planets,as well,” Chapman said. “And cratershave a practical importance. They canfocus geological processes that collectvaluable resources, like oil and nickel,for extraction.”Geological ForensicsThe impact cratering record onEarth is extremely “under-served,” saidJames Garvin, Chief Scientist at NASA’sGoddard Space Flight Center.“The ability to preserve the scarsof major collisional events is extremelychallenged by the incredible dynamismof Earth’s geological processes.Indeed, the current record of ‘proven’impact sites is measured in the range of150-200, rather than in the thousands,as one might presume,” Garvin said.Part of the issue with discovering thelargely missing terrestrial cratering record,Garvin said, is a combination of preservation(how well an impact landform orsignature can be preserved over time in agiven geologic setting) and detection.“We have about 150 million squarekilometers of land area today to search,with much of it vegetated, overprintedby very recent geological events, such asshifting sands, and some of it coveredwith ice,” Garvin advised. “Methodsof detecting impact features – craters,signatures in rocks, geophysical expressions– while improving, remain somewhatlimited,” he told Imaging Notes.For Garvin, the bottom line: “Thisis a geological forensics problem!”4SS Figure 4. Kamil Crater is located at Djebel Kamil, southof Gilf Kebir near the Sudanese border in Egypt. This crateris 45 meters in diameter and is considered one of thebest-preserved craters found on Earth to date. Photographcourtesy Museo Nazionale dell’Antartide Università di Siena.Looking AheadBolstering the topographic reconnaissanceof Earth as a whole is possible overthe next decade via planned or recommendedmissions that NASA’s EarthScience Program may implement.Given increased thinking of how bestto thwart an incoming object from stirringup a bad day in our world, plus givennew awareness of the impact flux at theimaging notes // Summer 2011 // www.imagingnotes.com25