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Our Planet's Surface Is Riddled with Craters Formed by High-Speed Cosmic

Our Planet's Surface Is Riddled with Craters Formed by High-Speed Cosmic

{ IMPACT SCIENCE }

Earth under fire Our planet’s surface is riddled with craters formed by high-speed cosmic impacts. ⁄⁄⁄ BY MIKE REYNOLDS

ost of us are familiar with the major features of the world — oceans, continents, mountain Mranges, valleys, deserts, rivers, lakes — the list goes on and on. One feature not likely to make this list is an . Craters appear few and far between on ’s surface, and most of the obvious ones formed THE in (above) is the from volcanic activity, not impacts. Yet, that doesn’t result of an impact 142 million years ago and has suffered mean impacts haven’t occurred. The evidence lies all the ravages of since. It measures 15 miles (24 km) around us — on Earth and on its neighbors. across and 3 miles (5 km) deep and covers most of this Take a look at the through a small telescope, image. The circular ring of hills at center spans 3 miles and and one thing stands out even to casual observers: Cra- represents the remains of the crater’s central uplift. NASA/USGS ters cover its surface. Time and time again, rocks have AOROUNGA CRATER (upper left) spans 10.5 miles (17 km) pummeled the Moon. Our satellite’s pockmarked face is in northern Chad. An or impacted this spot evidence that our was and still is an active several hundred million years ago. Sediments buried the place. Spacecraft views of our planetary neighbors pro- original crater, but later erosion has partially revealed it. It vide the same image of impacts and cratering. shows in full glory in this space-shuttle radar image. NASA/JPL

© 2011 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com  astronomy ⁄⁄⁄ august  (LPL/UA) (LPL/UA) Copernicus CONSOLIDATED LUNAR ATLAS LUNAR CONSOLIDATED ATLAS LUNAR CONSOLIDATED FROM FROM FROM NASA/JPL

THE CRATER Copernicus marks the spot THE MOON’S southern highlands are MERCURY looks a lot like the Moon, where an asteroid smashed into the Moon’s chock-a-block with impact craters dating with craters covering most of the visible surface 800 million years ago. Debris from more than 4 billion years ago. surface. The high crater density implies thrown out from the initial impact created Younger craters dot the floors and rims Mercury’s surface is among the oldest in many of the small craters surrounding it. of older, degraded craters. the solar system.

Planetary scientists now realize that rocks of various sizes have Because they don’t have solid surfaces, the gas-giant planets been colliding with solar system members since the whole system don’t show any signs of impacts. Still, we know they happen. formed some 4.65 billion years ago. Some of the impacting bodies We had front-row seats in July 1994, when fragments of Comet were quite small and hardly made a dent. Others were miles across Shoemaker-Levy 9 smashed into mighty Jupiter. The dark splotches and caused major changes on the impacted body’s surface. Our that resulted from these impacts lasted for months, dramatic proof Moon owes its very existence to a colossal impact; it formed from of what can happen when you release an energy equivalent to all debris created when a Mars-size object struck the early Earth. the nuclear arsenals on Earth. Even some of the small objects in our solar system — Even with our Moon providing evidence to anyone with a tele- and — show numerous impact scars. It would come as a scope, astronomers and geologists had a hard time believing Earth NEAR-EARTH asteroid Eros huge surprise if Earth were unique in the solar system as the only had suffered from major impacts. Most scientists thought terres- shows lots of craters, which rocky object without impact craters. No need to worry — telltale trial craters were volcanic in . After all, they have attributes proves even small objects signs of cratering in both the distant and recent past exist. The similar to known volcanic features. Some researchers even argued have been battered since evidence, however, is often subtle. could be volcanic. the solar system’s beginning. The large, saddle-shape fea- The solar system’s impact history can tell us The debate wasn’t resolved until the middle of the ture partially in shadow is about Earth’s past as well as give us a peek into 20th century. This was when geologists Eugene LAKE MANICOUAGAN in northern Quebec is the remnant of one of the largest impact features left on Himeros, a region with fewer the future. We know the early solar system was Shoemaker and Edward Chao discovered a Earth. The ring-shape lake surrounds the impact’s central uplift. The crater measures about 45 miles craters (suggesting relatively a far more dangerous place than it is today. MOST form of known as at several (70 km) across and resulted from an impact some 212 million years ago. NASA/USGS recent resurfacing). NASA/JHU/APL Objects with ancient surfaces, like the Moon crater sites. Coesite has a structure that can and Mercury, show impacts used to happen ASTEROIDS WERE be formed only quickly and under high How to make a crater impact scars have been eroded — in some cases entirely, in many much more frequently than they do now. pressure. This turned out to be the first Scientists understand the dynamics of crater formation better others, enough to make identification difficult. And impacts that Astronomers estimate as little as 5 percent blown to smithereens unambiguous signature of cosmic impacts. today. This is attributable in large part to sophisticated computer occurred in bodies of water — better than a 50-50 proposition — of the solar system’s original number of after losing a game Until a couple of decades ago, impacts simulations. Several variables play important roles in what kind of leave little traditional evidence. asteroids still remain. The rest were casual- by extraterrestrial bodies were considered crater an impact will create. The key factors include the size and So far, scientists have identified about 150 impact craters on ties, blown to smithereens after losing a of celestial interesting from a geological perspective, composition of the impacting body, the speed at which it is travel- Earth, with several new ones joining the list each year. For example, game of celestial chicken. but not of any major consequence. As astron- ing, and how its orbit intersects the object being struck. Another the in was thought to be volcanic in chicken. omers and geologists continued to examine the critical factor is whether the object being hit has an atmosphere origin just 20 years ago. After several meticulous expeditions, geol- Chicken Little evidence for impacts and bombardment through- and, if so, how dense it is. ogists confirmed Tswaing as an impact crater that formed about Perhaps the most surprising discovery to come from out the solar system, however, it became obvious that It all comes down to kinetic energy — the impactor’s mass and 220,000 years ago. Tswaing appears similar in size to its more- spacecraft reconnaissance of our solar system has been the collisions were a major force shaping the surfaces of these velocity at the moment of impact. (A planet’s atmosphere comes recent — and more-famous — cousin: in . degree of cratering found on its bodies. Although, in hindsight, worlds. Eventually, scientists realized evidence of early impacts into play because it will slow the impactor and may break it up.) In this shouldn’t have been a shock if we had accepted what the Moon should be visible on Earth as well. comparing two similar-size impacting bodies, one composed of Craters on Earth was trying to tell us. Most terrestrial planets, , asteroids, and Impact research gained a lot of momentum in 1980, when phys- and the other richer in silicate rock or ice, the iron impactor Meteor Crater has to be at the top of the list for any American who comets show evidence of a severe battering. Saturn’s moon Mimas icist Luis Alvarez and his geologist son, Walter, suggested a giant will prove much more devastating. wants to see what happens when a big rock hits Earth. This well- has a crater so big, it indicates the impact probably came close to impact caused the extinction of about 50 percent of all living spe- On worlds like the Moon and Mercury, the evidence of impacts preserved crater not only has a classic structure, but it’s also on a splitting the moon in half. The near-Earth asteroid Eros not only cies, including the dinosaurs, approximately 65 million years ago. has remained mostly undisturbed with the exception of additional scale you have to see in person to believe. The crater measures shows multiple craters but also one large impact site. Apparently, a comet or asteroid some 6 miles (10 kilometers) in impacts. This is obvious to anyone who examines the Moon closely: about 4,100 feet (1,250 meters) across and 570 feet (175m) deep. diameter struck near the tip of the Yucatán Peninsula and formed Craters appear within and on top of other craters. If you converted the crater into a football stadium, it would seat a Mike Reynolds is an astronomy professor and the Dean of Mathematics a crater called Chicxulub roughly 100 miles (160 km) across. The On Earth, signs of the earliest cratering have been erased. Plate million people. The impactor responsible for creating Meteor Cra- and Science at Florida Community College. He has written several astron- resulting firestorm sent Earth’s climate into a tailspin from which tectonics has resurfaced Earth in the past 2 to 3 billion years, leav- ter some 50,000 years ago was a classic iron octahedrite about 150 omy books, including Falling Stars: A Guide to Meteors & . it ultimately recovered — but not before life took a major hit. ing no evidence of the early, heavy bombardment. More recent feet (45m) in diameter. Scientists estimate the object was traveling

 astronomy ⁄⁄⁄ august  www.astronomy.com   astronomy  astronomy wealth exploring it. He searched for main the impactor mass interested Butte inCoon and spent much of hislife and personal Butte. Mining entrepreneur Daniel Moreau became meteorites from site. the craterthe wasvolcanic despite Foote’s recovery of numerous iron gist of the U.S. (USGS).Still, Survey Geological believed Gilbert caughtwhich attention the of Grove Karl chief the geolo- Gilbert, diamonds insome of He them. wrote apaper on hisresearch, ter. He carefully examined samples the and discovered microscopic with anumber of meteorites hepicked up at cra- the mineralogist A.E.Foote visited Butte. Coon Foote returned to it formed volcanically. settlers renamedreport. Local itButte Coon thought they because Hole, after scout the for George General Custer madethe who 1871 for someone to write about it. It initially wascalledFranklin’s Americans living region inthe crater, the knew but it took until Even so, it mostscientists wasvolcanic suspected innature. Native atmospheric pressure.normal impact created apressure that likely exceeded 20,000timesEarth’s 27,000andbetween 45,000mph (43,000and 72,000km/h).The nucleus, andgasspews from thesurface. the Sun’s heat sublimates ices from the When acomet approaches theSun, vive even theravages ofgaseruptions. COMET1 TEMPEL deep (175m)gouge. a4,100-foot-wide50,000 years (1,250m) and570-foot- agoandleft feature onEarth. The collision that created thiscrater occurred METEOR CRATER What happened nextchanged how scientists looked at Coon to change started The story chemist 1890s,when and inthe ofSome earliest crater the research took place at Meteor Crater. ⁄⁄⁄ in Arizona is probably the best-known impact inArizona isprobably impact thebest-known shows craters cansur- august  NASA/GSFC/METI/ERSDAC/JAROS, AND U.S./JAPAN ASTER SCIENCE TEAM

NASA/JPL/UMD Jupiter’s cloudtops, leaving darkspots. of Comet Shoemaker-Levy 9smashinto watchedfragments July1994,observers In solar system —even planets. ongas-giant IMPACTS HAPPEN everywhere inthe everywhere that most likely Odessa resulted from an impact. years, the Over Barringer’s son, D. Moreau Barringer, Jr., demonstrated in1926 excavatedbeen heavily, the three main craters are still recognizable. (170m) and exhibits aslightly Although raised rim. area the has ance than Meteor Crater. Themain crater spans about 550feet of impactseries structures somewhat different insizeand appear- craters,The Odessa located just southwest Texas, of Odessa, are a More subtleclues and David Levy, of Shoemaker-Levy Comet 9. comets. Thissearch ledto codiscovery, the with hiswife Carolyn adecade-longstarted search for asteroids Earth-crossing and Apollo incratering and science lunar geology. He also mechanicsthe of impacts and cratering. Shoemaker trained the ceived Astrogeology the Branch within USGSafter the researching American geologist Eugene Shoemaker. In early the 1960s,hecon- monds found at Crater. Barringer extreme pressure and temperature were neededto form dia- the ments conducted by F. P. Bundy and hiscolleagues concluded Australia craters and Odessa the inTexas. Andin1955,experi- other terrestrial impact scars, including Henbury the craters in Meteor Crater. With acceptance this came identification the of omers and geologists agreed that an asteroid impact created formation wasn’t for settled many years. But by 1930,mostastron- muchafter man the didso who to understand its secrets. research. Today, crater the name the bears Barringer also Crater, 1,405 pounds (639kg).Barringer wrote papers several about his tion to many meteorites. Thelargest recovered weighs rainbow, hedidrecover diamonds, platinum, and iridium inaddi- found never Barringer ameteoritic pot of gold at endof the his and jammed and broken drills than 1,000feet (300m)deep. Unfortunately, shaft drill aflooded holes crater. inthe ger several drilled of Some holes the went more impact and not avolcano. and to tried convince others that Butte Coon formed from an GEM WEB One of biggest the contributors of to science the impacts was Despite allof argument the evidence, the about crater’s the In order to reach main wasthe what mass,Barrin- hebelieved interactive web site www.lpl.arizona.edu/impacteffects. Calculate ofany theeffects approaching at the impactor

NASA AND THE HUBBLE COMET TEAM a prominent central peak. acrosssures andsports 88miles(140km) it. The hugecrater, named Herschel, mea- hit upwhenamassivebreaking object SATURN’S MOON MIMAS ended hisquest. Even though camecloseto

NASA/JPL/SSI These glasses typically where glasses arise These a significant amount of water gests crater this mostlikely formed acomet when Earth. struck diameter. of impact-melt Thediscovery calledsuevite sug- glass lion years old, measures more than 150miles(240 km)in Thiswell-eroded crater,glasses. approximately 1.8bil- impact with numerous shatter cones and impact Ontario, isan Canada, excellent example of an extreme heat and pressure of an impact. impact’s center. can also form Glasses inthe cones help also researchers identify an The locations and orientations of shatter pressurehigh and wave. shock high-velocity dence. Shatter cones result from an impact’s form another terrestrial piece of impact evi- — conical rock fragments with striations — together by afine-grained matrix. Shatter cones consisting of broken, angular fragments cemented chemical composition and brecciation: rocks cratersbury are of parts in1932.Other story the specific the — are one major clue. were They discovered first around Hen- the dence. —rocks formed from melting during an impact impact indicators obvious beyond the craters and meteoritic evi- removed from area. the tons meteorites, of Odessa acoarse iron have octahedrite, been The large feature Sudbury innorthern Astronomers and geologists have put together puzzle the of impacts, whichledusimpacts, Roter Kamm to comprehend the system shows the scars ofrepeated nature ofcosmic OUR SOLAR pinball. tain much water, acomet would have brought plenty. exists. Although original the terrestrial rock Sudbury didnot con- and how big object the willbe. —it’s Andit occurs. willoccur just amatter of when what could happen nextMeteor the when Crater or Chicxulub part of this space-shuttle radar image. ofthisspace-shuttle part acrossappears asabrightcircle inthelower 1.5miles(2.5km) southwest Namibiaandcreated crater. impact theRoter Kamm It JUST 5 MILLION YEARS AGO, also helpalso usunderstand dynamics the of such impacts, and Not allimpacts produce craters. Perhaps exam- best the comprehend the nature of cosmicpinball. The scars ple June isthe 30,1908,event inTunguska, Siberia. cover similar impact features but on to Earth, also Scientists think alargeScientists think object —either astony repeated impacts, ledusnot which only to dis- meteoroid or acomet —entered Earth’s atmos- planet. Oursolar system shows scars the of phere and above exploded surface. the teaches usalot about ourselves and our Although nocrater formed, sug- evidence gests acosmicevent: an initial fireball and nearly 1,000square miles. and trees felled inaradialpattern covering sound ofthe an explosion, dust the created, We often say that looking at cosmos the X ameteoroid in collided withEarth w.srnm.o  www.astronomy.com NASA/JPL

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