the day the sands caught fire' by jeffrey c. wynn and eugene m. shoemaker

8/13/2019 'The Day the Sands Caught Fire' by Jeffrey C. Wynn and Eugene M. Shoemaker

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The Day the Sands CaughtThe Me t eo r i t e H un t e rs : Pa r t I

A desert impact site demonstratesthe wrath of rocks from space

Copyright 1998 Scientific American, Inc.


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I

magine, for a moment, that you

are standing in the deep desert,

looking northwest in the eve-

ning twilight. The landscape is ab-

solutely desolate: vast, shifting

dunes of grayish sand stretch un-

interrupted in all directions. Not a

rock is to be seen, and the nearest

other human being is 250 kilome-

ters away. Although the sun has

set, the air is still rather warm50

degrees Celsiusand the remnantof the afternoon sandstorm is still

stinging your back. The prevailing

wind is blowing from the south, as

it always does in the early spring.

Suddenly, your attention is caught

by a bright light above the darken-

ing horizon. First a spark, it quick-

ly brightens and splits into at leastfour individual streaks. Within a

few seconds it has become a sear-

ing flash. Your clothes burst into

flames. The bright objects flit si-

lently over your head, followed a

moment later by a deafening crack.

The ground heaves, and a blast

wave flings you forward half thelength of a football field. Behind you,

sheets of incandescent fire erupt

into the evening sky and white boul-

ders come flying through the air.

Some crash into the surrounding

sand; others are engulfed by fire.

Glowing fluid has coated the

white boulders with a splatter that

Scientific American November 1998 65

by Jeffrey C. Wynn and Eugene M. ShoemakerFire

DONDIX

ON



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first looks like white paint but then turnsprogressively yellow, orange, red andfinally black as it solidifiesall withinthe few seconds it takes the rocks to hitthe ground. Some pieces of the whiterock are fully coated by this black stuff;they metamorphose into a frothy, glassymaterial so light that it could float on

water, if there were any water around.A fiery mushroom cloud drifts over younow, carried by the southerly breeze,blazing rainbow colors magnificently.As solid rocks become froth and red-dish-black molten glass rains down,you too become part of the spectacleand not in a happy way.

Deep in the legendary Empty Quarter

of Saudi Arabiathe Rub al-Khaliliesa strange area, half a square kilometer(over 100 acres) in size, covered withblack glass, white rock and iron shards.It was first described to the world in1932 by Harry St. John AbdullahPhilby, a British explorer perhaps betterknown as the father of the infamousSoviet double-agent Kim Philby. The

site he depicted had been known to sev-

eral generations of roving al-MurraBedouin as al-Hadida, the iron things.

There is a story in the Quran, the holybook of Islam, and in classical Arabicwritings about an idolatrous king namedAad who scoffed at a prophet of God.For his impiety, the city of Ubar and allits inhabitants were destroyed by a dark

cloud brought on the wings of a greatwind. When Philbys travels took himto the forbidding Empty Quarter, hisguides told him that they had actuallyseen the destroyed city and offered totake him there. Philby gladly acceptedthe offer to visit what he transliteratedin his reports as Wabar, the name thathas stuck ever since.

What he found was neither the lost cityof Ubar nor the basis for the Quranicstory. But it was certainly the setting ofa cataclysm that came out of the skies:the arrival of a meteorite. Judging fromthe traces left behind, the crash wouldhave been indistinguishable from a nu-clear blast of about 12 kilotons, com-parable to the Hiroshima bomb. It was

not the worst impact to have scarred

our planet over the ages. Yet Wabarholds a special place in meteor research.Nearly all known hits on the earth havetaken place on solid rock or on rock cov-ered by a thin veneer of soil or water.The Wabar impactor, in contrast, fell inthe middle of the largest contiguoussand sea in the world. A dry, isolated

place, it is perhaps the best-preservedand geologically simplest meteorite sitein the world. Moreover, it is one of only17 locationsout of a total of nearly160 known impact structuresthat stillcontain remains of the incoming body.

In three grueling expeditions to themiddle of the desert, we have recon-structed the sequence of events at Wabar.

The impact was an episodemuch repeated throughoutthe earths geologic and bio-logical history. And the so-

lar system has not ceased tobe a shooting gallery. Al-though the biggest meteorsget most of the attention, at

least from Hollywood, the more tangi-ble threat to our cities comes from small-er objects, such as the one that producedWabar. By studying Wabar and similar-ly unfortunate places, researchers canestimate how often such projectilesstrike the earth. If we are being shot at,there is some consolation in knowinghow often we are being shot at.

One has to wonder how Philbys Bed-

The Day the Sands Caught Fire66 Scientific American November 1998

A fiery mushroom clouddrifts over you now, carried by the southerly

breeze. As solid rocks become froth and reddish-black molten glass rains

down, you too become part of the spectacleand not in a happy way.

WABAR SITE consists of three craters and a sprinkling of two unusual types of rockblack glass and impactite.

Much of the site has been buried by the ever shifting sand dunes. It is located deep in the uninhabited Empty

Quarter of Saudi Arabia, or the Rub al-Khali; the authors expeditions there took two different routes (map).

The Wabar Meteorite Impact Site

MOVING DUNE

SEIF DUNE

COMPLEX DUNES

PHILBY A

11-METCRATE

ABHA

RIYADH

EMPTYQUARTER

WABAR

IRANIRAQ

KUWAIT

OMAN

YEMEN

UBAR

U.A.E.

QATAR

ETHIOPIA

ERITREA

SUDAN

EGYPT

JORDAN

REDSEA

ARAB

IAN

SEA

PERSIA

NGU

LF

ISRAEL

SAUDI ARABIA

JEDDAH

BAHRAIN

DJIBOUTI

BURIED DEBRIS



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The Day the Sands Caught Fire Scientific American November 1998 67

ouin guides knewabout Wabar,

which is found inthe midst of a colos-

sal dune field withoutany landmarks, in a landscape thatchanges almost daily. Even the famous-ly tough desert trackers shy away from

the dead core of the Empty Quarter. Ittook Philby almost a month to getthere. Several camels died en route, andthe rest were pushed to their limits.They were a sorry sight indeed on ar-rival at Mecca on the ninetieth day, thinand humpless and mangy, Philby tolda meeting of the Royal GeographicalSociety on his return to London in 1932.

Otherworldly

When he first laid eyes on the site,

he had become only the secondWesterner (after British explorer Ber-tram Thomas) to cross the Empty Quar-ter. He searched for human artifacts,for the remains of broken walls. Hisguides showed him black pearls litteringthe ground, which they said were thejewelry of the women of the destroyedcity. But Philby was confused and dis-appointed. He saw only black slag,chunks of white sandstone and two par-tially buried circular depressions thatsuggested to him a volcano. One of his

guides brought him a piece of iron the

size of a rabbit. The work of the OldPeople? It slowly dawned on Philbythat this rusty metal fragment was notfrom this world. Laboratory examina-tion later showed that it was more than90 percent iron, 3.5 to 5 percent nickeland four to six parts per million iridi-uma so-called sidereal element onlyrarely found on the earth but common

in meteorites.The actual site of the city of Ubar, insouthern Oman about 400 kilometers(250 miles) south of Philbys Wabar,was uncovered in 1992 with the help ofsatellite images [see Space Age Archae-ology, by Farouk El-Baz; ScientificAmerican, August 1997]. Wabar, mean-while, remained largely unexplored un-til our expeditions in May 1994, De-cember 1994 and March 1995. The sitehad been visited at least twice since1932 but never carefully surveyed.

It was not until our first trip that werealized why. One of us (Wynn) hadtagged along on an excursion organizedby Zahid Tractor Corporation, a Saudidealer of the Hummer vehicle, the civil-ian version of the military Humvee. Topromote sales of the vehicle, a group ofZahid managers, including Bill Chas-teen and Wafa Zawawi, vowed to crossthe Empty Quarter and invited the U.S.Geological Survey mission in Jeddah to

send a scientist along. This was no week-end drive through the countryside; it wasa major effort requiring special equip-ment and two months of planning. Noone had ever crossed the Empty Quar-ter in the summer. If something wentwrong, if a vehicle broke down, the car-avan would be on its own: the long dis-tance, high temperatures and irregular

dunes preclude the use of rescue heli-copters or fixed-wing aircraft.An ordinary four-wheel-drive vehicle

would take three to five days to navigatethe 750 kilometers from Riyadh to Wa-bar [see map on opposite page]. It wouldbog down in the sand every 10 minutesor so, requiring the use of sand laddersand winches. A Hummer has the ad-vantage of being able to change its tirepressure while running. Even so, the ex-pedition drivers needed several days tolearn how to get over dunes. With ex-perience, the journey to Wabar takes along 17 hours. The last several hoursare spent crossing the dunes and mustbe driven in the dark, so that bumper-mounted halogen beams can scan forthe unpredictable 15-meter sand cliffs.

Our first expedition stayed at the sitefor a scant four hours before movingon. By that time, only four of the six ve-hicles still had working air condition-ers. Outside, the temperature was 61

SAND-FILLED CRATER, 11 meters (36 feet) in diameter, was discovered by the authors on

their expedition to Wabar in December 1994. Under the sand the crater is lined with a

bizarre kind of rockimpactitethought to have formed when immense pressuresglued sand grains together. Around the crater rim are centimeter-size chips of iron and

nickel. From the size of the crater geologists estimate that it formed when a dense

metallic meteorite just one meter across smacked into the sand. This meteorite had split

off from the larger bodies responsible for the other two craters at Wabar.

JEFFREYC.W

YNN

2130.153'NORTH

50 28.445'EAST

IMPACTITE

BLACK GLASS

100METER

S

CAMP

MOVINGDUNE

PHILBY B

LAURIEGRACE(m

ap);SLIMFILMS(illustration)



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degrees C (142 degrees Fahrenheit)inthe shade under a tarpand the humid-ity was 2 percent, a tenth of what therest of the world calls dry. Wynn wentout to do a geomagnetic survey, and bythe time he returned he was staggering

and speaking an incoherent mixture ofArabic and English. Only some timelater, after water was poured on hishead and cool air was blasted in hisface, did his mind clear.

Zahid financed the second and thirdexpeditions as well. On our weeklong

third expedition, furious sandstorms de-stroyed our camp twice, and the temper-ature never dropped below 40 degreesC, even at night. We each kept a two-liter thermos by our beds; the burningin our throats awoke us every hour or so.

Shocking Rock

The Wabar site is about 500 by 1,000meters in size. There are at leastthree craters, two (116 and 64 meterswide) recorded by Philby and the other(11 meters wide) by Wynn on our sec-ond expedition. All are nearly complete-ly filled with sand. The rims we nowsee are composed of heaped-up sand,

anchored in place both by impactiterocka bleached, coarse sandstoneand by large quantities of black-glassslag and pellets. These sandy crater rimsare easily damaged by tire tracks. Thereare also occasional iron-nickel fragments.

Geologists can deduce that a craterwas produced by meteorite impactrather than by other processes such aserosion or volcanismby looking forsigns that shock waves have passedthrough rocks [see box above]. The im-pactite rocks at the Wabar site pass thetest. They are coarsely laminated, likeother sandstones, but these laminationsconsist of welded sand interspersedwith ribbonlike voids. Sometimes the


SPALLEDFRAGMENTS

DEFORMING METEOROID

IMPACTITE

UNDISTURBED SANDUNDISTURBED SANDDEFORMED SAND

MELT ZONE

SLIMFILMS

How would you recognize an impact crater if you fell intoone? It isnt easy. Although the moon is covered withcraters, it has no water, no weather, no continental driftso thecraters just stay where they formed, barely changed over theaeons. On the earth, however, all these factors have erasedwhat would otherwise have been an equally pockmarked sur-

face. To confuse matters further, more familiar processessuchas volcanism and erosionalso leave circular holes. Not untilearly this century did geologists first confirm that some cratersare caused by meteorites. Even today there are only about 160known impact structures.

Only about 2 percent of the asteroids floating around in theinner solar system are made of iron and nickel, whose frag-ments are fairly easy to recognize as foreign. But other types ofmeteorites blend in with the rest of the stones on the ground.The easiest place to pick them out is in Antarctica, because fewother rocks find their way to the middle of an ice field. Else-

where, recognizing a meteorite crater requires careful mappingand laboratory work. Geologists look for several distinctive fea-tures, which result from the enormous velocities and pressuresinvolved in an impact. Even a volcanic eruption does not sub-ject rocks to quite the same conditions. J.C.W.

Shatter cones.These impressions, foundin the rocksaround a cra-ter, look likecookie-cuttercones or chev-rons. Occasion-ally, you can

see them in rock outcroppings if thecones have fractured lengthwise. Noshatter cones appear at Wabar becausethe site formed in loose sand.

High-temperature rock types. Lami-nated and welded blocks of sand havebeen seen at Wabar and at nuclear testsites. In addition, tektitesglassy rocksthought to form when molten rock is

splattered intoorbit and thensolidifies on the

way back downappear aroundmany large im-pact sites.

Microscopic rock deformation.The crys-tal structure of someminerals is trans-formed by the shockwaves during an im-pact. Quartz, for ex-ample, develops stri-ations that are orient-

ed in more than one direction. It can alsorecrystallize into new minerals, such ascoesite and stishovite, detectable only inx-ray diffraction experiments.

Identifying Impact Craters

CAROLYNSHOEMAK

ER

CROSS SECTION of meteorite impact, as

reconstructed by computer simulations,

shows how the Wabar craters were creat-

ed within a matter of seconds. The mete-orite flattened as it hit the ground; a shock

wave traveled backward through the

body, causing part of it to spall off with lit-

tle damage; the rest of the meteorite melt-ed and amalgamated with sand directly

underneath; surrounding sand was com-

pressed into impactite. The whole mess

was then thrust into the air. Deeper layers

of sand were relatively unaffected.



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layers all bend and twist in unison, un-like those in any other sandstone wehave ever seen. The laminations areprobably perpendicular to the path tak-en by a shock wave. Moreover, the im-pactite contains coesite, a form ofshocked quartz found only at nuclearblast zones and meteorite sites. X-raydiffraction experiments show

that coesite has an unusualcrystal structure, symptomaticof having experienced enor-mous pressures.

The impactite is concentrat-ed on the southeastern rimsand is almost entirely absenton the north and west sides of the cra-ters. This asymmetry suggests that theimpact was oblique, with the incomingobjects arriving from the northwest atan angle between 22 and 45 degreesfrom the horizontal.

The two other types of rock found atWabar are also telltale signs of an im-pact. Iron-nickel fragments are practi-cally unknown elsewhere in the desert,so they are probably remnants of themeteorite itself. The fragments come intwo forms. When found beneath thesand, they are rusty, cracked balls up to10 centimeters in diameter that crum-ble in the hand. Daniel M. Barringer, anAmerican mining engineer who drilledfor iron at Meteor Crater in Arizonaearly this century, called such fragments,which occur at several iron-meteor

sites, shale balls.When the iron fragments are found at

the surface, they are generally smooth,covered with a thin patina of black des-ert varnish. The largest piece of iron andnickel is the so-called Camels Hump,recovered in a 1965 expedition andnow displayed at King Saud Universityin Riyadh. This flattened, cone-shapedchunk, weighing 2,200 kilograms (2.43tons), is probably a fragment that brokeoff the main meteoroid before impact.Because the surface area of an object is

proportional to its radius squared,whereas mass is proportional to the ra-dius cubed, a smaller object undergoesproportionately more air drag. There-fore, a splinter from the projectile slowsdown more than the main body; whenit lands, it may bounce rather than blastout a crater.

The other distinctive type of rock atWabar is the strange black glass. Glassyrock is often found at impact sites, whereit is thought to form from molten blobsof material splattered out from the cra-

ter. Near the rims of the Wabar craters,

the black glass looks superficially likeHawaiian pahoehoe, a ropy, wrinkledrock that develops as thickly flowinglava cools. Farther away, the glass pelletsbecome smaller and more droplike. Ata distance of 850 meters northwest ofthe nearest crater, the pellets are only afew millimeters across; if there are any

pellets beyond this distance, sand duneshave covered them. When chemicallyanalyzed, the glass is uniform in content:about 90 percent local sand and 10 per-cent iron and nickel. The iron and nick-el appear as microscopic globules in amatrix of melted sand. Some of the glassis remarkably fine. We have found fili-gree glass-splatter so fragile that it doesnot survive transport from the site, nomatter how well packaged.

The glass distribution indicates thatthe wind was blowing from the south-east at the time of impact. The wind di-rection in the northern Empty Quarteris seasonal. It blows from the north for10 months of the year, sculpting thehuge, horned barchan sand dunes. Butduring the early spring, the wind switch-

es direction to come from the southeast.Spring is the desert sandstorm seasonthat worried military planners duringthe Gulf War; it coincides with the mon-soon season in the Arabian Sea. All yearlong, the air is dead still when the sunrises, but it picks up in the early after-noon. By sunset it is blowing so hard

that sand stings your face as you walkabout; on our expeditions, we neededswim goggles to see well enough to setup our tents. Around midnight the winddrops off again.

Curtains

Black material and whitethe Wabarsite offers little else. This dichotomysuggests that a very uniform processcreated the rocks. The entire impact ap-parently took place in sand; there is noevidence that it penetrated down tobedrock. In fact, our reconnaissancefound no evidence of outcropping rock(bedrock that reaches the surface) any-where within 30 kilometers.

From the evidence we accumulated


At the point of impact, a conelike curtain of hot fluiderupted into

the air. The incandescent curtain of molten rock expanded rapidly

as more and more of the meteorite made contact with the ground.

SECOND-LARGEST CRATER at the Wabar site, Philby A, has been nearly buried by a

creeping seif (sword, in Arabic) dune. Only its southeastern rim, preserved by a gravel-ly mix of rock formed during impact, still pokes up above the sand. The 64-meter (210-

foot) crater marks the impact site of a five-meter meteorite, one of several pieces of the

original Wabar meteoroid (which broke apart in midair). The chunks hit the ground at

speeds of up to 25,000 kilometers per hour20 times as fast as a .45-caliber pistol bullet.

JEFFREYC.W

YNN



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during our expeditions, as well as fromthe modeling of impacts by H. Jay Me-losh and Elisabetta Pierazzo of the Uni-versity of Arizona, we have pieced to-gether the following sequence of eventsat Wabar.

The incoming object came from thenorthwest at a fairly shallow angle. Itmay have arrived in the late afternoon

or early evening, probably during theearly spring. Like most other meteor-oids, it entered the atmosphere at 11 to17 kilometers per second (24,600 to38,000 miles per hour). Because of theoblique angle of its path, the body tooklonger to pass through the atmospherethan if it had come straight down. Con-sequently, air resistance had a greatereffect on it. This drag force built up asthe projectile descended into ever dens-er air. For most meteoroids, the dragoverwhelms the rock strength by eightto 12 kilometers altitude, and the ob-ject explodes in midair. The Wabar im-pactor, made of iron, held together long-er. Nevertheless, it eventually broke upinto at least four pieces and slowed tohalf its initial speed. Calculations sug-gest a touchdown velocity of betweenfive and seven kilometers per second,about 20 times faster than a speeding.45-caliber pistol bullet.

The general relation among meteorite

size, crater size and impact velocity isknown from theoretical models, ballis-tics experiments and observations of nu-clear blasts. As a rule of thumb, cratersin rock are 20 times as large as the ob-jects that caused them; in sand, whichabsorbs the impact energy more effi-ciently, the factor is closer to 12. There-fore, the largest object that hit Wabar

was between 8.0 and 9.5 meters in di-ameter, assuming that the impact veloc-ity was seven or five kilometers per sec-ond, respectively. The aggregate mass ofthe original meteoroid was at least 3,500tons. Its original kinetic energy amount-ed to about 100 kilotons of explodingTNT. After the air braking, the largestpiece hit with an energy of between nineand 13 kilotons. Although the Hiroshi-ma bomb released a comparable amountof energy, it destroyed a larger area,mainly because it was an airburst rath-er than an explosion at ground level.

At the point of impact, a conelikecurtain of hot fluida mixture of theincoming projectile and local sanderupted into the air. This fluid becamethe black glass. The incandescent cur-tain of molten rock expanded rapidlyas more and more of the meteorite madecontact with the ground. The projectileitself was compressed and flattened dur-ing these first few milliseconds. A shock

wave swept back through the body;when it reached the rear, small pieceswere kicked offspalled off, in geolog-ic parlanceat gentle speeds. Some ofthese pieces were engulfed by the cur-tain, but most escaped and ploppeddown in the surrounding sand as far as200 meters away. They are pristine re-mains of the original meteorite. (Spalling

can also throw off pieces of the planetssurface without subjecting them to in-tense heat and pressure. The famousMartian meteorites, for example, pre-served their delicate microstructures de-spite being blasted into space.)

A shock wave also moved downward,heating and mixing nearby sand. Theratio of iron to sand in the glass pelletssuggests that the volume of sand meltedwas 10 times the size of the meteoriteimplying a hemisphere of sand about27 meters in diameter. Outside this vol-ume, the shock wave, weakened by itsprogress, did not melt the sand but in-stead compacted it into insta-rock:impactite.

The shock wave then caused an erup-tion of the surface. Some of the impac-tite was thrown up into the molten glassand was shocked again. In rock sam-ples this mixture appears as thick blackpaint splattered on the impactite. Otherchunks of impactite were completelyimmersed in glass at temperatures of10,000 to 20,000 degrees C. When thishappened, the sandstone underwent a

second transition into bubbly glass.The largest crater formed in a little

over two seconds, the smallest one inonly four fifths of a second. At first thecraters had a larger, transient shape, butwithin a few minutes material fell backout of the sky, slumped down the sidesof the craters and reduced their volume.The largest transient crater was proba-bly 120 meters in diameter. All the sandthat had been there was swept up in amushroom cloud that rose thousandsof meters, perhaps reaching the strato-

sphere. The evening breeze did not haveto be very strong to distribute moltenglass 850 meters away.

Fading Away

And when did all this take place? Thathas long been one of the greatestquestions about Wabar. The first dateassigned to the event, based on fission-track analysis in the early 1970s of glasssamples that found their way to theBritish Museum and the Smithsonian

Institution, placed it about 6,400 years


EJECTA BLANKET at the edge of the Philby A crater consists of three types of debris

from the impact: white impactite (a sandstonelike rock formed from compressed sand),

black glass (a lavalike rock formed from melted sand) and meteorite fragments (nearlypure iron, with a little nickel). The authors, dressed in special jumpsuits to protect them

from the harsh climate, are using magnetometers to search for the meteorite pieces. The

tall antenna on the white Hummer vehicle is for Global Positioning System tracking

essential in the middle of the desert, where it is easy to get lost in the protean landscape.

JOEPOLIMENIANDBILLCHASTEENZahidTractorCorporationandA.M.GeneralCorporation



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ago. Field evidence, however, hintsat a more recent event. The largestcrater was 12 meters deep in 1932,eight meters deep in 1961 andnearly filled with sand by 1982.The southeastern rim was onlyabout three meters high duringour visits in 1994 and 1995. Duneexperts believe it would be impos-

sible to empty a crater once filled.The Wabar site might have al-ready disappeared if impactiteand glass had not anchored thesand. At least two of the cratersare underlaid by impactite rocks,which represent the original bowlsurface before infilling by sand.We were able to collect severalsamples of sand beneath this im-pactite lining for thermolumines-cence dating. The results, preparedby John Prescott and Gillian Rob-ertson of the University of Ade-laide, suggest that the event took placeless than 450 years ago.

The most tantalizing evidence for arecent date is the Nejd meteorites, whichwere recovered after a fireball passedover Riyadh in either 1863 or 1891, de-pending on which report you believe.The fireball was said to be headed in thedirection of Wabar, and the Nejd mete-orites are identical in composition tosamples from Wabar. So it is likely thatthe Wabar calamity happened only 135years ago. Perhaps the grandfathers of

Philbys guides saw the explosion froma long way off.

The date is of more than passing in-terest. It gives us an idea of how oftensuch events occur. The rate of meteoritehits is fairly straightforward to under-stand: the bigger they come, the less fre-

quently they fall [see illustration above].The most recently published estimatessuggest that something the size of theWabar impactor strikes the earth aboutonce a decade.

There are similar iron-meteorite cra-ters in Odessa, Tex.; Henbury, Austra-lia; Sikhote-Alin, Siberia; and elsewhere.But 98 percent of Wabar-size events donot leave a crater, even a temporary one.They are caused by stony meteoroids,which lack the structural integrity ofmetal and break up in the atmosphere.

On the one hand, disintegration has thehappy consequence of protecting theground from direct hits. The earth hasrelatively few craters less than about fivekilometers in diameter; it seems thatstony asteroids smaller than 100 to 200meters are blocked by the atmosphere.

On the other hand, this shieldingis not as benevolent as it may seem.When objects detonate in the air,they spread their devastation overa wider area. The Tunguska ex-plosion over Siberia in 1908 isthought to have been caused by astony meteoroid. Although verylittle of the original object was

found on the ground, the airburstleveled 2,200 square kilometers offorest and set much of it on fire. Itis only a matter of time before an-other Hiroshima-size blast fromspace knocks out a city [see Col-lisions with Comets and Aster-oids, by Tom Gehrels; Scien-tific American, March 1996].

By the standards of known im-pacts, Wabar and Tunguska aremere dents. Many of the othercollision sites around the world,including the Manicouagan ring

structure in Quebec, and the Chicxulubsite in Mexicos northern Yucatn, arefar larger. But such apocalypses happenonly every 100 million years on aver-age. The 10-kilometer asteroid thatgouged out Chicxulub and snuffed thedinosaurs hit 65 million years ago, andalthough at least two comparable ob-jects (1627 Ivar and the recently discov-ered 1998 QS52) are already in earth-crossing orbits, no impact is predictedanytime soon. Wabar-size meteoroidsare much more commonand harder

for astronomers to spotthan the bigmonsters. Ironically, until the Wabarexpeditions, we knew the least aboutthe most frequent events. The slag andshocked rock in the deserts of Arabiahave shown us in remarkable detailwhat the smaller beasts can do.


The Authors

JEFFREY C. WYNN and EUGENE M. SHOEMAKER worked together at theU.S. Geological Survey (USGS) until Shoemakers death in a car accident in July

1997. Both geoscientists have something of an Indiana Jones reputation. Wynn,based in Reston, Va., has mapped the seafloor using electrical, gravitational, seismicand remote sensing; has analyzed mineral resources on land; and has studiedaquifers and archaeological sites around the world. He served as the USGS residentmission chief in Venezuela from 1987 to 1990 and in Saudi Arabia from 1991 to1995. His car has broken down in the remote deserts of the southwestern U.S., inthe western Sahara and in the deep forest in Amazonas, Venezuela; he has comeface-to-snout with rattlesnakes, pit vipers and camel spiders. Shoemaker, consideredthe father of astrogeology, was among the first scientists to recognize the geologicimportance of impacts. He founded the Flagstaff, Ariz., facility of the USGS, whichtrained the Apollo astronauts; searched for earth-orbit-crossing asteroids andcomets at Palomar Observatory, north of San Diego; and was a part-time professorat the California Institute of Technology. At the time of his death, he was mappingimpact structures in the Australian outback with his wife and scientific partner,Carolyn Shoemaker.

Further Reading

An Account of Exploration in the GreatSouth Desert of Arabia. Harry St. John B. Phil-

by in Geographical Journal, Vol. 81, No. 1, pages126; January 1933.Impact Cratering: A Geologic Process. H. J.Melosh. Oxford University Press, 1989.

Secret Impacts Revealed.J. Kelly Beatty in Sky& Telescope, Vol. 87, No. 2, pages 2627; Febru-ary 1994.

Hazards Due to Comets and Asteroids. Editedby Tom Gehrels. University of Arizona Press, 1995.

Rain of Iron and Ice: The Very Real Threat ofComet and Asteroid Bombardment. John S.Lewis. Addison-Wesley Publishing, 1996.

Additional information on impact structures can befound at http://bang.lanl.gov/solarsys/eng/tercrate.htm on the World Wide Web.

SA

1

102

104

106

108

0.01

4 20 90

ASTEROID SIZE (METERS)

TNT EQUIVALENT YIELD (MEGATONS)

400 2,000 9,000

1

WABAR-SIZE EVENT

TUNGUSKA

TSUNAMI

DANGER

CHICXULUB

THRESHOLD

OF GLOBAL

CATASTROPHE

102 104 106 108FREQUENCYOFIMPACT(YEARS)

AVERAGE FREQUENCY OF IMPACTS on the earth can

be estimated from the amount of scrap material zip-

ping around the solar system and the observed distri-

bution of craters on the moon. A two-kilometer rock,

capable of wreaking damage worldwide, falls onceevery million years on average. (In relating size to ex-

plosive energy, this graph assumes a stony asteroid at

20 kilometers per second.)

DAVIDMORRISONNASAAmesResearchCenter;LAURIEGRACE

Copyright 1998 Scientific American Inc

the day the sands caught fire' by jeffrey c. wynn and eugene m. shoemaker

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