THE MOONGATEWAY TO THE SOLAR SYSTEM

G. Jeffrey Taylor, PhD

WHEN ASTRONAUTS dug into the Moon’s surface during the Apolloprogram, they were doing more than digging up dry, dark sediment. They weretime travelers. The rocks and sediment returned by Apollo contain vital cluesto how Earth and the Moon formed, the nature and timing of early melting, theintensity of impact bombardment and its variation with time, and even thehistory of the Sun. Most of this information, crucial parts of the story of planetEarth, cannot be learned by studying rocks on Earth because our planet is sogeologi-cally active that it has erased much of the record. The clues have been

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geologi-cally active that it has erased much of the record. The clues have beenlost in billions of years of mountain building, volcanism, weath-ering, anderosion. Colliding tectonic plates

The Moon, front and back

The top right photograph is a telescopic image of the Earth-facing half of the Moon

obtained at Lick Observatory in California. The one below right was taken during the

Apollo 16 mission and shows mostly the farside, except for the dark areas on the left,

which can be seen, though barely, from Earth. The two major types of terrain are

clearly visible. The highlands, which are especially well displayed in the photograph of

the farside, are light-colored and heavily cratered. The maria are darker and smoother;

they formed when lava flows filled depressions. One of the mysteries about the Moon is

why fewer maria occur on the farside than nearside. Note how many craters stand

shoulder to shoulder on the farside. These immense holes chronicle the Moon’s early

bombardment, an intense barrage that probably affected the early Earth as well. 1

and falling rain have erased much of Earth history,especially the early years before four billionyears ago.

The Moon was geologically active in its heyday, producing a fascinatingarray of products, but its geologic engine was not vigorous and all records ofearly events were not lost. Its secrets are recorded in its craters, plains, androcks. This guide reveals the secrets that lunar scientists have uncovered sincethe Apollo missions returned 382 kilograms (843 pounds) of rock andsediment from the lovely body that graces the night sky.

The emphasis here is on geology. The samples returned by Apollo arethe stars of the show. [See the “Lunar Disk” activity on Pages 39–42 andthe “Apollo Landing Sites” activity on Pages 43–46.] Understanding theMoon, however, requires other geological approaches, such as geologicalmapping from high-quality photographs, the study of analo-gous featureson Earth (for instance, impact craters), and experiments in laboratories.

THE LUNAR LANDSCAPE

TheMoonisnot likeEarth. It doesnothaveoceans, lakes, rivers, or streams. Itdoes not have wind-blown ice fields at its poles. Roses and morning glories do notsprout from its charcoal gray, dusty surface. Red-woods do not towerabove its cratered ground. Dino-saur foot prints cannot be found. Paramecium

scopes were invented. Although we now know they are not seas (the Moonnever had any water), we still use the term maria, and its singular form, mare.

The highlands and craters

Closer inspection shows that the highlands com-prise countless overlappingcraters, ranging in size from the smallest visible in photographs (1 meter on thebest Apollo photographs) to more than 1000 km. Essentially all of these cratersformed when meteor-ites crashed into the Moon. Before either robotic orpiloted spacecraft went to the Moon, many scientists thought that most lunarcraters were volcanic in origin. But as we found out more about the nature oflunar craters and studied impact craters on Earth, it became clear that the Moonhas been bombarded by cosmic projectiles. The samples returned by the Apollomissions confirmed the pervasive role impact processes play in shaping thelunar landscape.

The term “meteorite impact” is used to describe the process of surfacebombardment by cosmic ob-ject. The objects themselves are variously referredto as “impactors” or “projectiles.”

The impact process is explosive. A large impac-tor does not simply bore itsway into a planet’s surface. When it hits, it is moving extremely fast, more than

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above its cratered ground. Dino-saur foot prints cannot be found. Parameciumnever conjugated, amoeba never split, and dogs never barked. The windnever blew. People never lived there—but they have wondered about itfor centu-ries, and a few lucky ones have even visited it.

Highlands and lowlands

The major features of the Moon’s surface can be seen by just looking up atit. It has lighter and darker areas. These distinctive terrains are the bright lunarhighlands (also known as the lunar terrae, which is Latin for “land”) andthe darker plains called the lunar maria, Latin for “seas,” which theyresembled to Thomas Hariot and Galileo Galilei, the first scien-tists toexamine the Moon with telescopes. The names terrae and maria weregiven to lunar terrains by Hariot and Galileo’s contemporary, JohannesKepler. In fact, the idea that the highlands and maria correspond to lands andseas appears to have been popular among ancient Greeks long beforetele-

way into a planet’s surface. When it hits, it is moving extremely fast, more than20 km/sec (70,000 km/hour). This meet-ing is not tender. High-pressure wavesare sent back into the impactor and into the target planet. The impactor is sooverwhelmed by the passage of the shock wave that almost all of it vaporizes,never to be seen again. The target material is compressed strongly, thendecompressed. A little is vaporized, some melted, but most (a mass of about10,000 times the mass of the impactor) is tossed out of the target area, piling uparound the hole so produced. The bottom of the crater is lower than the originalground surface, the piled up material on the rim is higher. This is thecharacteristic shape of an impact crater and is different from volcanic calderas(no piled up materials) or cinder cones (the central pit is above the originalground surface). A small amount of the target is also tossed great distancesalong arcuate paths called rays.

Real impacts cannot be readily simulated in a classroom. In fact, there are veryfew facilities where we can simulate high-velocity impacts. Nevertheless,classroom experiments using marbles, ball bearings, or other objects can stillillustrate many important points about the impact process. For ex-ample,objects impacting at a variety of velocities (hence kinetic energies) producecraters with a variety of sizes; the more energy, the larger the crater.

The maria cover 16% of the lunar sur-face and are composed of lava flows thatfilled relatively low places, mostly inside immense impact basins. So, although theMoon does not have many volcanic cra-ters, it did experience volcanic activity.Close examination of the relationships be-tween the highlands and the maria showsthat this activity took place after the high-lands formed and after most of the crateringtook place. Thus, the maria are younger than the highlands. [See the “Clay Lava Flows”activity onPages 71–76 and the “Lava Lay-ering” activity on Pages 77–82.]

How do we know that the dark plains are covered with lava flows? Why not someother kind of rock? Even before the Apollo missions brought back samples from themaria, there were strong suspicions that the plains were volcanic. They contain somefeatures that look very much like lava flows. Other features resemble lava channels,which form in some types of lava flows on Earth. Still other features resemble col-lapses along underground volcanic fea-tures called lava tubes. These and otherfeatures convinced most lunar scientists before the Apollo missions that the mariawere lava plains. This insight was con-

firmed by samples collected from the maria: they are a type of volcanic rockcalled basalt.

The maria fill many of the gigantic impact basins that decorate the Moon’s nearside.(The Moon keeps the same hemisphere towards Earth because Earth’s gravity has

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Rivers or lava channels?

The origin of river-like valleys, called rilles, was debated before the Apollo 15 mission visited one of them, Hadley Rille,

shown here from orbit (white arrow shows the landing site) and from the lunar surface. Some scientists argued that rilles

were river valleys, implying that the Moon had flowing water at one time. Others thought that rilles resembled the channels

that form in lava flows. Observations by the Apollo 15 crew and samples returned settled the argument: rilles are volcanic

features.

(The Moon keeps the same hemisphere towards Earth because Earth’s gravity haslocked in the Moon’s rotation.) Some scientists contended during the 1 960s that thisdem-onstrated a cause and effect: impact caused not only the formation of a large craterbut led to melting of the lunar interior as well. Thus, it was argued, the im-pactstriggered the volcanism. However, careful geologic mapping using high-qualitytelescopic images, showed that the mare must be considerably younger than the basinsin which they reside. For example, the impact that formed the large Imbrium basin(the Man-in-the-Moon’s right eye) hurled material out-wards and sculpted themountains surrounding the Serenitatis basin (the left eye); thus, Serenitatis must beolder. The Serenitatis basin is also home to Mare Serenitatis. If the lavas in MareSerentatis formed when the basin did, they ought to show the effects of the giantimpact that formed Imbrium. They show no

Lava flows dribbling across Mare Imbrium

Long lava flows (lower left to upper right through center of photo) in Mare Imbrium. The flows are up

to 400 kilometers long (the entire extent is not shown in this Apollo photograph) and have 30-meter

high cliffs at their margins. The ridges running roughly perpendicular to the flows are called wrinkle

ridges. The ridges formed after the flows because they did not impede the advance of the lava flow.

Maria mysteries

Some mysteries persist about the maria. For one, why are volcanoesmissing except for the cinder cones associated with dark mantle deposits?Sec-ond, if no obvious volcanoes exist, where did the lavas erupt from? Insome cases, we can see that lava emerged from the margins of enormousimpact ba-sins, perhaps along cracks concentric to the basin. But in mostcases, we cannot see the places where the lava erupted. Another curiousfeature is that almost all the maria occur on the Earth-facing side of theMoon. Most scientists guess that this asymmetry is caused by thehighlands crust being thicker on the lunar farside, making it difficult forbasalts to make it all the way through to the surface. [See the “MoonAnomalies” activity on Pages 9 1–98.]

THE DUSTY LUNAR SURFACE

Some visitors to Kilauea Volcano, Hawai‘i, have been overheard to say, uponseeing a vast landscape covered with fresh lava, “It looks just like the Moon.”Well, it doesn’t. The fresh lava flows of Kilauea and other active volcanoes areusually dark grayish and barren like the Moon, but the resemblance ends there.

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ridges. The ridges formed after the flows because they did not impede the advance of the lava flow.

The smooth plains surrounding the prominent lava flows are also lava, but older, so the margins are

harder to see. The craters on the right along the ridge are about 5 kilometers in diameter.

signs of it. Furthermore, the maria contain far fewer craters than do basindeposits, hence have been around a shorter time (the older the surface, thegreater the number of craters). The Apollo samples, of course, confirmedthese astute geological observa-tions and showed that the maria fillingsome basins formed a billion years after the basin formed.

One other type of deposit associated with the maria, though it blanketshighlands areas as well, is known as dark mantle deposits. They cannot beseen except with telescopes or from spacecraft near the Moon, but areimportant nonetheless. Before Apollo, most scientists believed that thedark mantle deposits were formed by explosive volcanic eruptions knownas pyroclastic eruptions (literally, “pieces of fire”). Some deposits seemedto be associated with low, broad, dark cinder cones, consistent with theidea that they were formed by pyroclastic eruptions—this is how cindercones form on Earth. This bit of geologic deduction was proven by theApollo 17 mission and its sampling of the “orange soil,” a collection oftiny glass droplets like those found in terrestrial pyro-clastic eruptions.

usually dark grayish and barren like the Moon, but the resemblance ends there.The lunar surface is charcoal gray and sandy, with a sizable supply of finesediment. Meteorite impacts over billions of years have ground up the formerlyfresh surfaces into powder. Because the Moon has virtually no atmosphere, eventhe tiniest meteorite strikes a defenseless surface at its full cosmic velocity, atleast 20 km/sec. Some rocks lie strewn about the surface, resembling bouldersstick-ing up through fresh snow on the slopes of Aspen or Vail. Even theseboulders won’t last long, maybe a few hundred million years, before they areground up into powder by the relentless rain of high-speed projectiles. Of course,an occasional larger impactor arrives, say the size of a car, and excavates freshrock from beneath the blanket of powdery sediment. The meteoritic rain thenbegins to grind the fresh boulders down, slowly but inevitably.

The powdery blanket that covers the Moon is called the lunar regolith, a term formechanically produced debris layers on planetary surfaces. Many scientists alsocall it the “lunar soil,” but it contains none of the organic matter that occurs insoils on Earth. Some people use the term “sediment” for

regolith. Be forewarned that the regolith samples in the Lunar Sample Disk are labeled “soil.”Although it is everywhere, the regolith is thin, ranging from about two meters on the youngestmaria to perhaps 20 meters in the oldest surfaces in the highlands. [See the “Regolith For-mation”activity on Pages 47–52.]

Lunar regolith is a mixed blessing. On the one hand, it has mixed local material so that ashovelful contains most of the rock types that occur in an area. It even contains some rock fragmentstossed in by impacts in remote regions. Thus, the regolith is a great rock collection. It also containsthe record of impacts dur-ing the past several hundred million to a billion years, crucialinformation for understanding the rate of impact on Earth during that time. On the otherhand, this impact record is not written very clearly and we have not come close to figuring it out asyet. The blanket of regolith also greatly obscures the details of the bedrock geology. This made fieldwork during Apollo difficult and hinders our understanding of lunar history.

The regolith consists of what you’d expect from an impact-generated pile of debris. Itcontains rock and mineral fragments derived from the original bed-rock. It also contains glassyparticles formed by the impacts. In many lunar regoliths, half of the particles are com-posed ofmineral fragments that are bound together by impact glass; scientists call these objects agglutinates.The chemi-cal composition of the regolith reflects the composition of the bedrock under-neath.Regolith in the highlands is rich in aluminum, as are highland rocks. Regolith in the maria isrich in iron and magnesium, major constituents of ba-salt. A little bit of mixing from beneathbasalt layers or from distant highland locales occurs, but not enough to ob-scure the basic Raking moon dirt

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basalt layers or from distant highland locales occurs, but not enough to ob-scure the basicdifference between the highlands and the maria.

Raking moon dirt

One of the most useful ways of obtaining samples of

Moon rocks was to drag a rake through the regolith. This

allowed rock fragments larger than about one centimeter

to remain on tines of the rake, while smaller fragments

fell through. Note the large range in the sizes of rock

fragments. One large boulder lies near the rake, a

medium-sized one is visible between the astronaut’s

feet, along with countless other pebbles. Most of the

regolith is smaller than fine sand. The astronaut’s

footprints are distinct because the regolith is composed of

a large percentage of tiny particles (about 20% is smaller

than 0.02 millimeters).

One of the great potential bits of information stored in thecomplex pile atop the lunar surface is the history of the Sun. Thenearest star puts out pro-digious amounts of particles called thesolar wind. Composed mostly of hydrogen, helium, neon, car-bon,and nitrogen, the solar wind particles strike the lunar surface andare implanted into mineral grains. The amounts build up with time.In principle, we can determine if conditions inside the Sun havechanged over time by analyzing these solar wind products,especially the isotopic composition of them.

The same solar wind gases may prove useful when peopleestablish permanent settlements on the Moon. Life support systemsrequire the life-giving elements: hydrogen and oxygen (for water),carbon, and nitrogen. Plenty of oxygen is bound in the silicate,minerals of lunar rocks (about 50% by vol-ume) and the solar windprovided the rest. So, when the astronauts were digging up lunarregolith for return to Earth, they were not merely sampling— theywere prospecting!

MOON ROCKS

Geologists learn an amazing amount about a planet by examiningphotographs and using other types of remotely sensed data, buteventually they need to collect some samples. For example, althoughgeologists determined unambigously from photo-graphs that the mariaare younger than the highlands, they did not know their absolute age,the age in years. Rocks also provide key tests to hypotheses. Forinstance, the maria were thought to be covered with lava flows, but wedid not know for sure until we collected samples from them. Also, nomethod can accurately determine the chemical and mineralogi-calcomposition of a rock except laboratory analysis. Most important,samples provide surprises, telling us things we never expected. Thehighlands provide the best example of a geological surprise, and onewith great consequences for our understanding of what Earth was like4.5 billion years ago.

A fist-sized piece of the original lunar crust

This rock sample was collected during the Apollo 15 mission.

It is an anorthosite, a rock composed of little else but the

mineral feldspar. Anorthosites formed from the enormous

magma system, the lunar magma ocean, that surrounded the

newly formed Moon. Because of its importance in

understanding the origin of the Moon’s crust, the rock was

nicknamed the “genesis rock.”

Smash and mix, mix and melt

This rock returned by the Apollo 16 mission attests to the

effects of impacts on a planet’s crust. It is a hodgepodge of

rock and mineral fragments, some of which themselves are

complicated mishmashes of rock debris. Geologists call these

complicated rocks breccias.

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Highland rocks, the lunar magma ocean, andmaybe a cataclysm

Strange as it may seem, the first highland rocks werecollected during the first lunar landing, the Apollo 11 mission,which landed on a mare, Mare Tranquillitatis. Although mostof the rocks collected were, indeed, basalts, some millimeter-sized rock fragments were quite different. They werecomposed chiefly of the mineral plagioclase feldspar; somefragments were composed of nothing but plagio-clase. [Seethe “Rock ABCs Fact Sheet” on Page 19.] Such rocks arecalled anorthosites. Some scientists suggested that thesefragments were blasted to the Apollo 11 landing site by distantimpacts on highland terrain. Thus, they argued, the highlandsare loaded with plagioclase. This was a bold extrapolationcon-firmed by subsequent Apollo missions to highland sites.

But this was not enough for some scientists. If the highlandsare enriched in plagioclase, how did they get that way? Oneway is to accumulate it by flotation in a magma (molten rock).This happens in thick subterranean magma bodies on Earth.So, plagioclase floated in a magma. But if ALL the

lunar highlands are enriched in plagioclase, then the magma

Hawai'i developed a method to determine the iron content of the lunar surface fromratioes of the intensity of light reflected in different wavelengths. The magma oceanhypothesis predicts that the lunar highlands should have low iron contents, less than about5 wt. % (when recorded as iron oxide, FeO). According to Clementine measurements, thehigh-lands average slightly under 5 wt. % FeO, consistent with the magma ocean idea.Further refinement of this test is underway using data from Clementine and theforthcoming U. S. Lunar Prospector Mission, scheduled for launch in early 1998.

The highlands also contain other types of igneous rocks. The most abundant are callednorites and troctolites, rocks composed of equal amounts of plagioclase and either olivineor pyroxene (both silicate minerals containing iron and magnesium). Age dating suggeststhat these rocks are slightly younger than the anorthosites and formed after the magmaocean had crystallized.Highland rocks are difficult to work with because all that cratering, so evident inphotographs of the highlands, has taken its toll on the rocks. Most highland rocks arecomplex mixtures of other rocks. The original igneous rocks have been melted, mixed,

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lunar highlands are enriched in plagioclase, then the magmamust have been all over the Moon. The early Moon must havebeen covered by a global ocean of magma, now commonlyreferred to as the lunar magma ocean. Although somescientists still remain unconvinced about the veracity of themagma ocean hypothesis,nothing we have learned since has contradicted the idea that 4.5billion years ago the Moon was covered by a layer of magmahundreds of kilometers thick. The idea has been extended to theyoung Earth as well, and even to Mars and some asteroids. Andall this sprung forth because creative and bold scientists sawspecial importance in a few dozen white frag-ments ofanorthosite strewn about in a pile of char-coal gray lunarregolith.

The magma ocean concept was tested by the 1994 U. S.Clementine Mission to the Moon. Clementine was in a pole-to-pole orbit for two months, during which it took thousands ofphotographs in several wavelengths. Scientists at the Universityof

smashed, and generally abused by impacts during the Moon’s first halfbillion years. We call these compli-cated rocks breccias. Some are somixed up that they contain breccias within breccias within breccias. Mostof the anorthosites, norites, and troctolites are actually rock fragmentsinside breccias. Separating them out is painstaking work.

An interesting thing about highland breccias, especially those we callimpact melt breccias (rocks partly melted by an impact event), is thatmost of them fall into a relatively narrow span of ages, from about 3.85to 4.0 billion years. This has led some scientists to propose (boldlyagain–lunar scientists don’t seem to be timid!) that the Moon was

bombarded with exceptional intensity during that narrow time interval. If ithappened, it probably affected Earth as well, perhaps leading to productionof the first sedimentary basins, and possibly inhibit-ing the formation ofthe first life on this planet or harming whatever life had developed by fourbillion years ago. This idea of a cataclysmic bombardment of the Moon isnot yet proven. It could be that the apparent clustering in rock ages reflectspoor sam-pling—we may only have obtained samples from one or twolarge impact basins. The idea can be tested by obtaining samples frommany more localities on the Moon.

potassium (chemical symbol K), rare-earth elements (abbreviatedREE), and phosphorus (P). Most Moon specialists believe thatKREEP represents the last dregs from the crystallization of themagma ocean. Huge impacts dug down to the lower crust of theMoon and excavated it, mixing it with other debris to formKREEPy breccias.

The maria:

lava flows and fountains of fire

The missions to mare areas brought back lots of samples of basalt.Basalts differ from the highlands rocks in having more olivine andpyroxene, and less

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Multi-ringed masterpieces

The Moon has about 35 multi-ringed, circular impact features larger than 300 km in diameter.

The one shown here, the Orientale basin, has three very prominent rings. The diameter of the

outer one is 930 km. These immense craters might all have formed in a narrow time interval

between 3.85 and 4.0 billion years ago. Scientists do not know for sure how the rings form

during the impact.

Many highland breccias and a few igneous rocks are enriched compared to other lunar samples in a setof elements not familiar to most of us. The elements are those that tend not to enter the abundantminerals in rocks. The result is that as a magma crystallizes the part that is still liquid becomesprogressively richer in these special elements. The rocks that contain them are called KREEP, for

A piece of a lava flow

This typical sample of a mare basalt is composed mostly of brown pyroxene

(grayish in the photo) and white feldspar. The holes in the sample are frozen

gas bubbles. Some basalt samples have many such bubbles (called vesicles

by geologists), whereas others have none.

plagioclase. Many of them also have surprisingly large amounts of an iron-titaniumoxide mineral called ilmenite. The first batch had so much ilmenite (and some otherrelated minerals) that they were called “high-titanium” mare basalts, in honor of theexceptional titanium contents compared to terrestrial basalts. The second mission,Apollo 12, returned basalts with lower titanium concentrations, so they were called“low-titanium” mare basalts. Subse-quent missions, including an automated sample-return mission sent by the Soviet Union, returned some mare basalts with even lowertitanium, so they were dubbed “very-low-titanium” basalts. Most scientists figurethat mare basalts have a complete range in titanium abundance. Data from the U. S.Clementine Mission confirm this, and show that the

high-titanium basalts are not really very common on the Moon.The shapes of the mineral grains and how they are intergrown in marebasalts indicate that these rocks formed in lava flows, some thin (perhaps ameter thick), others thicker (up to perhaps 30 meters). This is not unusualfor basalt flows on Earth. Many lunar mare basalts also contain holes,called vesicles, which were formed by gas bubbles trapped when the lavasolidified. Earth basalts also have them. On Earth, the abundant gasesescaping from the lava are carbon dioxide and water vapor, accompaniedby some sulfur and chlorine gases. We are not as sure what gases escapedfrom lunar lavas, although we know that water vapor was not one of thembecause there are no hints for the presence of water or water- bearingminerals in any Moon rock. The best bet is a mixture of carbon dioxide andcarbon monoxide, with some sulfur gases added for good measure.

Experiments conducted on mare basalts and pyroclastic glasses showthat they formed when the interior of the Moon partially melted. (Rocks donot have a single melting tempera-ture like pure substances. Instead theymelt over a range of tem-peratures: 1000–1200°C for

some basalts, for example.) The experiments also show that the meltingtook place at depths ranging from 100 to 500 km, and that the rocks thatpartially melted contained mostly olivine and pyroxene, with someilmenite in the regions that formed the high- titanium basalts. An involvedbut sensible chain of reasoning indicates that these deep rocks rich inolivine and pyroxene formed from the lunar magma ocean: whileplagioclase floated to form anorthosites in the highlands crust, the denserminerals olivine and pyroxene sank. So, although the anorthosites andmare basalts differ drastically in age and composi-tion, the origins areintimately connected.

What’s next?

Scientists are still working on the bounty returned by the Apollo missions.New analytical techniques and improved understanding of how geologicalpro-cesses work keep the field exciting and vibrant. Eventually we will

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carbon monoxide, with some sulfur gases added for good measure.

On Earth, when the amount of gas dissolved in magma (the name for lavastill underground) be-comes large, it escapes violently and causes anexplosive eruption. In places such as Hawai‘i, for example, the lava eruptsin large fountains up to several hundred meters high. The lava falls to theground in small pieces, producing a pyroclastic de-posit. This alsohappened on the Moon, producing the dark mantle deposits. One of thesewas sampled directly during the Apollo 17 mission. The sample, called the“orange soil,” consists of numerous small orange glass beads. They areglass because they cooled rapidly, so there was not enough time to formand grow crystals in them.

Small samples of pyroclastic glasses were also found at other sites. Someare green, others yellow, still others red. The differences in color reflect theamount of titanium they contain. The green have the least (about 1 weightpercent) and the red contain the most (14 weight percent), more than eventhe highest titanium basalt.

need additional samples and some extensive field work to fully understandthe Moon and how it came to be and continues to evolve. These samplingand field expeditions will probably be done by a combination of roboticand piloted spacecraft.

In the meantime, Nature has provided a bonus: samples from the Mooncome to us free of charge in the form of lunar meteorites. (See companionvol-ume Exploring Meteorite Mysteries.) Thirteen sepa-rate meteoriteshave been identified so far, one found in Australia and the rest inAntarctica. We are sure that they come from the Moon on the basis ofappearance and chemical and isotopic composition, but of course we donot know from where on the Moon they come. These samples have helpedsup-port the magma ocean idea. Most important, know-ing that meteoritescan be delivered to Earth by impacts on the Moon lends credence to theidea that we have some meteorites from Mars. The Martian

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meteorites are collectively called SNC meteorites. If wedid not know so much about the Moon we would neverhave been able to identify meteorites from the Moon,and, therefore, would not have been able to argue asconvincingly that some meteorites come from Mars.

mometers set up by the Apollo missions. Besides telling us howmany and how strong moonquakes are, the data acquired by theApollo seismic network help us figure out something about thenature of the Moon’s interior. On Earth, seismology has allowedus to know that the planet has a thin crust (20-60 km overcontinents, 8-10 km over ocean basins), a thick silicate mantle(down to 2900 km), and a large metallic iron core (2900 km to thecenter at 6370 km). The Moon is quite different. The crust isthicker than Earth’s continental crust, ranging from 70 km on theEarth-facing side to perhaps 150 km on the farside. The marebasalts represent a thin veneer on this mostly plagioclase-richcrust, averaging only about 1 km in thickness (inferred mostlyfrom photogeological studies). Evidence from samples collectedon the rims of the large basins Imbrium and Serentatis and fromremote sensing instruments carried onboard two Apollo missions,the Clementine Mission, and the forthcoming Lunar ProspectorMission suggest that the lower crust may not contain as muchplagio-clase as does the upper half of the crust. Beneath the crustis the lunar mantle, which is the largest part of the Moon. Theremight be a difference in rock types above and below a depth of500 km, perhaps repre-senting the depth of the lunar magmaocean. Beneath the mantle lies a small lunar core made of metallic

From the Moon, free of charge

The first lunar meteorite discovered in Antarctica hails from the lunar highlands and,

like most highlands rocks, it is an impact breccia. The lunar meteorites prove that

objects can be blasted off sizable objects without melting them, adding credence to

the idea that a group of twelve meteorites comes from Mars.

MOONQUAKES, THE MOON’S INTERIOR, AND THE MYSTERIOUS MAGNETICFIELD

The Moon does not shake, rattle, and roll as Earth does. Almost all moonquakes are smallerthan Earth’s constant grumblings. The largest quakes reach only about magnitude 5 (strongenough to cause dishes to fall out of cabinets), and these occur about once a year. This is clearevidence that the Moon is not at present geologically active. No internal motions drive crustalplates as on Earth, or initiate hot spots to give rise to volcanic provinces like Hawai‘i. Thisseismic inactivity is a wonderful virtue in the eyes of astronomers. Combined with the lack ofan atmosphere to cause stars to twinkle, the low moon- quake activity makes the Moon anideal place to install telescopes.

We know about moonquakes from four seis-

ocean. Beneath the mantle lies a small lunar core made of metalliciron. The size of the core is highly uncertain, with estimatesranging from about 100 km to 400 km.

That little core is important, though. The Moon does not havemuch of a magnetic field, so the lunar core is not generatingmagnetism the way Earth’s core is. Nevertheless, it did in the past.Lunar rocks are magnetized, and the strength of the magnetic fieldhas been measured by special techniques. Also, older rocks havestronger magnetism, suggesting that the Moon’s magnetic fieldwas stronger in the distant past, and then decreased to its weakpresent state. Why this happened is unknown. What is known isthis: you cannot navigate around the Moon using a compass!

There are other mysteries about the Moon’s magnetism. Althoughthe field was always weak and is extremely weak now, there aresmall areas on the Moon that have magnetic fields much strongerthan the surrounding regions. These magnetic anomalies have notbeen figured out. Some scientists have associated them with theeffects of large, basin- forming impacts. Others have suggestedthat the

Moon forming together, a two-body system from the start. This idea hastrouble explaining Earth’s rotation rate and how the moon-forming materialgot into orbit around Earth and stayed there, rather than falling to Earth.(These problems with to-tal amount of spinning involve both Earth’srotation and the Moon’s motion around Earth. The amount of rotation andrevolving is quantified by a physical property called angular momentum.)The problem was so frustrating that some scientists suggested that maybescience had proved that the Moon does not exist!

The annoying problems with the classical hypotheses of lunar origin ledscientists to consider al-ternatives. This search led to the seeminglyoutlandish idea that the Moon formed when a projectile the

size of the planet Mars (half Earth’s radius and one- tenth itsmass) smashed into Earth when it had grown to about 90% ofits present size. The resulting explo-sion sent vast quantities ofheated material into orbit around Earth, and the Moon formedfrom this debris. This new hypothesis, which blossomed in1984 from seeds planted in the mid-1970s, is called the giantimpact theory. It explains the way Earth spins and why Earthhas a larger metallic core than does the Moon. Furthermore,modern theories for how the planets are assembled from smallerbodies, which were assembled from still smaller ones, predictthat when Earth was almost done forming, there would have

Inside the Moon and Earth

Scientists have learned what Earth and the Moon are like inside by several techniques, the

most important of which is seismology, the study of earthquake (and, of course) moonquake

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ionized gases produced when comets impact the Moon might give rise to strongmagnetic anomalies in the crater ejecta. The jury is still out. The LunarProspector Mission will thoroughly map the distri-bution of magnetic anomalies,perhaps helping to solve this mystery.

THE MOON’S ORIGIN:A BIG WHACK ON THE GROWING EARTH

For a long time, the most elusive mystery about the Moon was how it formed. Theproblem baffled philosophers and scientists for hundreds of years. All of the hypothesesadvanced for lunar origin had fatal flaws, even though partisans tried tenaciously toexplain away the defects. The capture hypothesis, which depicts capture of a fullyformed Moon by Earth, suffered from improbability. Close encounter with Earth wouldeither result in a collision or fling the Moon into a different orbit around the Sun,probably never to meet up with Earth again. The fission hypothesis, in which theprimitive Earth spins so fast that a blob flies off, could not explain how Earth got to bespinning so fast (once every 2.5 hours) and why Earth and the Moon no longer spin that fast.The double-planet hypothesis pictures Earth and the

that when Earth was almost done forming, there would havebeen a body nearby with a mass about one-tenth that of Earth.Thus, the giant impact hypothesized to have formed the Moonis not an implausible event. The chances are so high, in fact,that it might have been unavoidable.

One would think that an impact between an almost Earth-sizedplanet and a Mars-sized planet would be catastrophic. Theenergy involved is in-comprehensible. Much more than atrillion trillion tons of material vaporized and melted. In someplaces in the cloud around the Earth, temperatures exceeded10,000°C. A fledgling planet the size of Mars was incorporatedinto Earth, its metallic core

most important of which is seismology, the study of earthquake (and, of course) moonquake

waves. Earth has a much larger metallic core than does the Moon.

and all, never to be seen again. Yes, this sounds catastrophic. But out ofit all, the Moon was created and Earth grew to almost its final size. Withoutthis violent event early in the Solar System’s history, there would be noMoon in Earth’s sky, and Earth would not be rotating as fast as it is becausethe big impact spun it up. Days might even last a year. But then, maybe wewould not be here to notice.

was at least 500 km deep. The first minerals to form in this mind-bogglingmagmatic system were the iron and magnesium silicates olivine andpyroxene. They were denser than the magma, so they sank, like rocks in apond, though not as fast. Eventually, plagioclase feldspar formed, andbecause it was less dense than the magma, began to float to the top, likebubbles in cola. It accumulated and produced moun-tains of anorthosite,producing the first lunar crust. The magma ocean phase ended by about 4.4billion years ago. [See the “Differentiation” activity on Pages 57–60.]

WHACK!

The Moon may have formed when an

object the size of the planet Mars smashed

into Earth when our future home was

about 90% constructed. This fierce event

made Earth larger and blasted off

vaporized and melted material into orbit.

The Moon formed from this debris. This

painting was created by William K.

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Hartmann, one of the scientists who

invented the giant impact hypothesis for

lunar origin.

A BRIEF HISTORY OF THE MOON

We know the general outlines ofwhat happened to the Moon after it wasformed by a giant impact. The firstnotable event, which may have been aconsequence of the giant impact, was theformation and crystallization of themagma ocean. Nobody knows how deepit was, but the best guess is that it

Sinking and floating in an ocean of magma

Soon after it formed, the Moon was surrounded by a huge shell of

molten rock called the lunar magma ocean. As crystals formed in it, the

denser ones such as olivine and pyroxene sank and the less dense

ones, such as feldspar, floated upwards, forming the original anorthosite

crust on the Moon. Dropping toothpicks and pennies into a glass of

water shows the process: the toothpicks (representing feldspar) float

and the pennies (olivine and pyroxene) sink.

Almost as soon as the crust had formed, perhaps while it was still forming,other types of magmas that would form the norites and troctolites in the high-lands crust began to form deep in the Moon. A great mystery is where insidethe Moon and how deep. Many lunar specialists believe the magmas derivedfrom unmelted Moon stuff beneath the magma ocean. In any case, thesemagmas rose and infiltrated the anorthosite crust, forming large and smallrock bod-ies, and perhaps even erupting onto the surface. Some of themagmas reacted chemically with the dregs of the magma ocean (KREEP) andothers may have dissolved some of the anorthosite. This period of lunarhistory ended about 4.0 billion years ago.

All during these first epochs, left-over projectiles continued to bombardthe Moon, modifying the rocks soon after they formed. The crust was mixedto a depth of at least a few kilometers, perhaps as much as 20 km, as if a gigantictractor had plowed the lunar crust. Though not yet proven, the rate of impact mayhave declined between 4.5 and 4.0 billion years ago, but then grew dramatically,producing most of the large basins visible on the Moon. This cataclysmic bom-bardment is postulated to have lasted from 4.0 to 3.85 billion years ago. [See the“Impact Craters” activity on Pages 61–70, and the “Regolith Formation”activity on Pages 47–52.]

Once the bombardment rate had settled down, the maria could form.Basalts like those making up the dark mare surfaces formed before 3.85

110 million years ago. This, therefore, is the age of the crater Tycho.It is a triumph of geological savvy that we were able to date animpact crater that lies over 2000 km from the place we landed!The impacts during the past billions of years also have mixed the

upper several meters of crust to make the powdery lunar regolith.The Sun has continued to implant a tiny amount of itself into theregolith, giving us its cryptic record and providing resources forfuture explorers. And recently, only seconds ago in geologic time, afew interplanetary travelers left their footprints here and there on thedusty ground.

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Basalts like those making up the dark mare surfaces formed before 3.85billion years ago, but not as voluminously as later, and the enormousbombardment rate demolished whatever lava plains formed. However,between 3.7 and about 2.5 billion years ago (the lower limit is highly uncer-tain),lavas flowed across the lunar surface, forming the maria and decorating theMoon’s face. Along with the basalts came pyroclastic eruptions, high foun-tains of fire that launched glowing droplets of molten basalt on flights up to a fewhundred kilometers.

Since mare volcanism ceased, impact has been the only geological force atwork on the Moon. Some impressive craters have been made, such asCopernicus (90 km across) and Tycho (85 km). These flung bright rays ofmaterial across the dark lunar landscape, adding more decoration. In fact,some of the material blasted from Tycho caused a debris slide at what wouldbecome the Apollo 17 landing site. Samples from this site indicate that thelandslide and some associated craters formed about

Anatomy of an impact crater

The crater Tycho in the southern highlands on the lunar nearside is 85 kilometers across. Its

terraced walls rise 3 to 4 kilometers above its floor. Its rim rises above the surrounding terrain,

and its floor sits below it. Smooth material on the floor of the crater consists of impact-melted rock

that flowed like lava across the growing floor in the later stages of excavation. In response to the

huge increase then decrease in pressure, mountains two kilometers high rose from its floor,

bringing up material from as much as ten kilometers in the crust. The blanket of material

surrounding the crater is called the ejecta blanket; this pile of debris becomes progressively

thinner away from the crater (especially evident on the left of the photo).

Although not visible on this photograph, large craters like Tycho also have rays emanating from

them. Rays form when materials thrown out of the crater land and excavate more of the lunar

landscape along arcuate paths. The Tycho event caused a landslide and several secondary

impacts at the Apollo 17 landing site, over 2000 kilometers away. Analysis of samples collected

during the Apollo 17 mission indicates that these events took place 110 million years ago. Thus,

Tycho formed 110 million years ago.

THE MOON AND EARTH: INEXORABLY INTERTWINED

The Moon ought to be especially alluring to people curious about Earth.The two bodies formed near each other, formed mantles and crusts early,shared the same post-formational bombardment, and have been bathed inthe same flux of sunlight and solar particles for the past 4.5 billion years.Here are a few examples of the surprising ways in which lunar sciencecan contribute to understanding how Earth works and to unraveling itsgeological history.

Origin of the Earth-Moon System. No matter how the Moon formed,its creation must have had dra-matic effects on Earth. Although mostscientists have concluded that the Moon formed as a result of anenormous impact onto the growing Earth, we do not know much about thedetails of that stupendous event. We do not know if the Moon was mademostly from Earth materials or mostly projectile, the kinds of chemicalreactions that would have taken place in the melt-vapor cloud, andprecisely how the Moon was assembled from this cloud.

Magma oceans. The concept that the Moon had a magma ocean hasbeen a central tenet of lunar sci-ence since it sprung from fertile mindsafter the return of the first lunar samples in 1969. It is now being appliedto Earth, Mars, and asteroids. This view of the early stages of planetdevelopment is vastly different from the view in the 1950s and 1 960s.Back then, most (not all) scientists believed the planets assembled cold,

extinctions are not understood. One possibility is that some mass-extinction events were caused by peri-odic increases in the rate ofimpact on Earth. For example, the mass extinctions, which includedthe demise of the dinosaurs, at the end of the Cretaceous period (65million years ago), may have been caused by a large impact event.Attempts to test the idea by dating impact craters on Earth are doomedbecause there are too few of them. But the Moon has plenty of cratersformed during the past 600 million years (the period for which wehave a rich fossil record). These could be dated and the reality ofspikes in the impact record could be tested.

How geologic processes operate. The Moon is a naturallaboratory for the study of some of the geo-logic processes that haveshaped Earth. It is a great place to study the details of how impactcraters form because there are so many well-preserved craters in anenormous range of sizes. It is also one of the places where volcanismhas operated, but at lower gravity than on either Earth or Mars.

LIFE AND WORK AT A MOON BASE

People will someday return to the Moon. When they do, it will be tostay. They will build a base on the Moon, the first settlement in thebeginning of an interplanetary migration that will eventually take them

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Back then, most (not all) scientists believed the planets assembled cold,and then heated up. The realization that the Moon had a magma oceanchanged all that and has led to a whole new way of looking at Earth’searliest history.

Early bombardment history of Earth and Moon.

The thousands of craters on the Moon’s surface chronicle the impactrecord of Earth. Most of the craters formed before 3.9 billion years ago.Some scientists argue that the Moon suffered a cataclysmic bombardment(a drastic increase in the number of impacting projectiles) between 3.85and 4.0 billion years ago. If this happened and Earth was subjected to theblitzkrieg as well, then development of Earth’s earliest crust would havebeen affected. The intense bombardment could also have influenced thedevel-opment of life, perhaps delaying its appearance.

Impacts, extinctions, and the evolution of life onEarth. The mechanisms of evolution and mass

beginning of an interplanetary migration that will eventually take themthroughout the Solar System.

There will be lots to do at a lunar base. Geologists will study the Moonwith the intensity and vigor they do on Earth, with emphasis on fieldstudies. Astronomers will make magnificent observations of theuniverse. Solar scientists will study the solar wind directly andinvestigate past activity trapped in layers of regolith. Writers andartists will be inspired by a landscape so different from Earth’s. Lifescien-tists will study how people adapt to a gravity field one-sixth asstrong as Earth’s, and figure out how to grow plants in lunargreenhouses. Engineers will investigate how to keep a complex facilityoperating continuously in a hostile environment. Mining and chemicalengineers will determine how to extract resources from Moon rocksand regolith. The seem-ingly dry lunar surface contains plenty of theingre-dients to support life at a Moon base (oxygen and hydrogen forwater, nitrogen and carbon for the growth of plants), including theconstruction

15Geology at a Moon Base

The Curatorial Facil-ity is one of thecleanest places you’ll ever see. To goinside, you must wear white suits, boots,hats, and gloves, outfits affection-atelyknown as “bunny suits.” Wipe a glovedhand on a stainless steel cabinet and youwill not find a trace of dust because the airis filtered to remove potentialcontaminating particles.

The samples are stored in a large vault,and only one at a time is moved to a glovebox. You can pick up the rocks byjamming your hands into a pair of the blackrubber gloves, allowing you to turn a rockover, to sense its mass and density, toconnect with it. A stereomicroscope al-lows you to look at it closely. If you decideyou

need a sample, and of course you have beenapproved to obtain one, then expert lunar sampleprocessors take over. The sample is photographed

The best geology is done in the field. To understand rocks we must examine them up close, map their distributions, see the

structures in them, and chip off samples when necessary. The field geology done during the Apollo missions was hampered

by the lack of time the astronauts could devote to it. But that will change when people live permanently on the Moon.

Geologists will be able to spend as much time as they need to decipher the story recorded by lunar rock formations. This

painting shows three astronauts (one in the distance) examining the outside of a lava tube, an underground conduit that

carried red-hot lava to an eruption site perhaps hundreds of kilometers away.

materials to build and maintain the base (regolith can be molded into bricks; iron, titanium,and alumi-num can be smelted and forged into tools and build-ing materials). It will be anexciting, high-tech, faraway place inhabited by adventurous souls. [See the Unit 3 activitiesbeginning on Page 99.]

WHERE MOON ROCKS HANG OUT

Since arrival on Earth, lunar samples have been treated with the respect they deserve. Most of thetreasure of Apollo is stored at the Lunar Curatorial Facility at the Johnson Space Center, Houston,Texas. A small percentage is stored in an auxiliary facility at Brooks Air Force Base near SanAntonio, Texas, placed there in case a disaster, such as a hurricane, befalls Houston and the samplesare destroyed. Many small samples are also in the laboratories of investi-gators around the world,where enthusiastic scien-tists keep trying to wring out the Moon’s secrets.

processors take over. The sample is photographedbefore and after the new sample is chipped off. Thisis time consuming, but valuable to be sure we knowthe relationships of all samples to each other. Inmany cases, we can determine the orientation aspecific part of a rock was in on the surface of theMoon before collection.

A select small number of pieces of the Moon are ondisplay in public museums, and only three piecescan actually be touched. These so-called lunar"touchstones" were all cut from the same Apollo 17basaltic rock. One touchstone is housed at theSmithsonian Air and Space Museum in Washington,D.C. Another touchstone is at the Space CenterHouston facility adjacent to the Johnson Space Cen-ter. A third touchstone is on long-term loan to theMuseo de Las Ciencias at the Universidad NacionalAutonoma de Mexico. Visitors to these exhibitsmarvel at the unique experience of touching a pieceof the Moon with their bare hands.

Safe haven for precious rocks

NASA stores the lunar sample collection in a specially constructed facility called the Lunar Curatorial

The Original Moon

Four and a half æons agoa dark, dusty cloud deformed.

Sun became star; Earth became large,and Moon, a new world, was born.

This Earth/Moon pair, once linked so close,would later be forced apart.

Images of young intimate tieswe only perceive in part.

Both Earth and Moon were strongly strippedof their mantle siderophiles.

But Moon alone was doomed to thirstfrom depletion of volatiles.

Moon holds secrets of ages pastwhen planets dueled for space.As primordial crust evolved

raw violence reworked Moon’s face.

After the first half billion yearsNASA stores the lunar sample collection in a specially constructed facility called the Lunar Curatorial

Facility at the Johnson Space Center in Houston, Texas. The priceless materials remain in a nitrogen

atmosphere, which is far less reactive chemically than normal oxygen-rich air. Scientists working in the

facility wear lint-free outfits affectionately known as “bunny suits,” and handle the samples in glove

boxes. In this photograph, Roberta Score is examining a piece of an Apollo 16 rock, while Andrea

Mosie(left) looks on.

SCIENTISTS AS POETS

Scientists do not view the world in purely objective ways. Each has biases and a unique way of looking atthe world. Science is not done solely with piles of data, hundreds of graphs, or pages of equations. It isdone with the heart and soul, too. Sometimes a scientist is moved to write about it in elegant prose likethat written by Loren Eisley or in poetry, like the poem written by Professor Carlé Pieters of BrownUniversity. Dr. Pieters holds her doctorate from MIT and is an expert in remote sensing of planetarysurfaces. She is especially well known for her tele-scopic observations of the Moon. The poem firstappeared in the frontispiece of Origin of the Moon, published by the Lunar and Planetary Institute.

After the first half billion yearshuge permanent scars appeared;ancient feldspathic crust survived

with a mafic mantle mirror.

But then there grew from half-lived depthsa new warmth set free inside.

Rivers and floods of partial meltresurfaced the low ‘frontside’.

Thus evolved the Original Moonin those turbulent times.

Now we paint from fragments of cluesthe reasons and the rhymes:

Sister planet;Modified clone;

Captured migrant;Big splash disowned?

The Truth in some or all of thesewill tickle, delight,temper, and tease.

16

Nearside of the Moon

Apollo Landing Sites

15* 17*

12** *

14

*

16

17

*

*

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Teacher Page

Background [also see “Teacher's Guide” Pages 1, 2, photo on 8, 12, and photo on 13]

The circular features so obvious on the Moon’s surface are impact craters formed when impactors smashed into the surface.The explosion and excavation of materials at the impacted site created piles of rock (called ejecta) aroundthe circular hole as well as bright streaks of target material (called rays) thrown for great distances.

Two basic methods forming craters in nature are:1) impact of a projectile on the surface and 2) collapse of the top of a volcano creating a crater termed caldera. By studying all

types of craters on Earth and by creating impact craters in experimental laboratories geologists concludedthat the Moon's craters are impact in origin.

Impact CratersPurposeTo determine the factors affecting the appearance ofimpact craters and ejecta.

that the Moon's craters are impact in origin.

The factors affecting the appearance of impact craters and ejecta are the size and velocity of the impactor, and the geology of thetarget surface.

By recording the number, size, and extent of erosion of craters, lunar geologists can determine the ages of different surface unitson the Moon and can piece together the geologic history. This technique works because older surfaces areexposed to impacting meteorites for a longer period of time than are younger surfaces.

Impact craters are not unique to the Moon. They are found on all the terrestrial planets and on many moons of the outer planets.

On Earth, impact craters are not as easily recognized because of weathering and erosion. Famous impact craters on Earth areMeteor Crater in Arizona, U.S.A.; Manicouagan in Quebec, Canada; Sudbury in Ontario, Canada; RiesCrater in Germany, and Chicxulub on the Yucatan coast in Mexico. Chicxulub is considered by mostscientists as the source crater of the catastrophe that led to the extinction of the dinosaurs at the end of theCreta-ceous period. An interesting fact about the Chicxulub crater is that you cannot see it. Its circularstructure is nearly a kilometer below the surface and was originally identified from magnetic and gravitydata.

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Teacher Page

Teacher Page Impact Craters

AristarchusTypical characteristics of a lunar impact crater are labeled onthis photograph of Aristarchus, 42 km in diameter, located Westof Mare Imbrium.

wallfloor

raysejecta centralraysejecta central

uplifts

raised

rim

raised rim - rock thrown out of the crater and deposited as a ring-shaped pile of debris at the crater’s edge during the explosion and excavcation of animpact event.

floor - bowl shaped or flat, characteristically below surrounding ground level unless filled in with lava.

central - mountains formed because of the huge increase and rapid decrease in pressure during theuplifts impact event. They occur only in the center of craters that are larger than 40 km diameter.

See Tycho crater for another example.

walls - characteristically steep and may have giant stairs called terraces.

ejecta - blanket of material surrounding the crater that was excavated during the impact event. Ejectabecomes thinner away from the crater. 19

Teacher PageLunar Roving Vehicle

Background

Purpose

To construct a model of a lunar roving vehicle.

The Apollo lunar roving vehicle was a battery-powered space buggy. The astro-nauts on Apollo 15, 16, and 17 used it toexplore their landing sites and to travel greater distances than astronauts on earlier missions. The lunar rover neatly folded upinside the lunar lander during trips to the Moon. Once on the Moon's surface, it unfolded with the help of springs. The lunarrover carried two astronauts and was manually driven. It was designed to climb steep slopes, to go over rocks, and to moveeasily over the Moon's regolith. It was able to carry more than twice its own weight in passengers, scientific instruments,rocks, and regolith samples. The wheels on the rover were made of wire mesh (piano wire) with titanium cleats for treads.Engineers did not use solid or air-filled rubber tires because they would have been much heavier than were the wire meshwheels. The Apollo spacecraft had a fixed amount of mass (payload) it could deliver to the surface, including the rover, roverbatteries, scientific instruments, sample collection de-vices, etc. Hence, the wire-mesh wheels were important to the overallpayload mass. This rover was not designed for prolonged use, and it is uncertain if future lunar explorers would use similardesigns and materials for their vehicles, use new, more durable compo-nents, or turn to robotic rovers.

If students are interested in constructing models that actually move, then refer to Page 38 for more information on rocket andmodel building.

20

Lunar Roving VehicleTeacher Page

Extensions

Hold competitions between student vehicles with these criteria for judging:1.Can the vehicle actually move -- by gravity; by some kind of propulsion system?2.Can the vehicle go over different surfaces -- smooth, flat, bumpy, or inclined?3.Is the vehicle sturdy?4.Can the vehicle carry a heavy load? Have the students decide the weight of the load.5.Could the vehicle withstand meteoritic bombardment?6.Would the vehicle work on the Moon?Discuss the pros and cons of manually driven vehicles versus remote-controlled robotic rovers on the Moon.

Diagram of the vehicle used by Apollo astronauts.

LunarRovingVehicle

21

Lunar Roving Vehicle

Purpose

To construct a model of a lunar roving vehicle.

Procedure

1.Describe the similarities and differences between the Apollo lunar roving vehicle and a typical family vehicle.

Lunar Roving Vehicle Discussion

1. What was special about the rover's wheels? Why weren't they made of rubber and filled with air?

1. Review the “Moon ABCs Fact Sheet.” Design a new lunar roving vehicle. Important design issues include size, weight, powersupply, number of passengers, controls, scientific instruments, tools, and storage compartments. Use the space provided on the nextpage to draw a picture of your design. Label the parts.

2. Construct a model of the lunar rover based on your design.

3. Give a name to the vehicle.

4. Write a descriptive essay about the special features and capabilitites of the vehicle and how you solved the design issues raised inQuestion 3.

22

•Meteorites, Clues to•Solar System History

•Afamilyona campingtripwatchesabright lightstreakacross theskyanddisappear.

Anexplorer comes uponacircular crater withrocks scattered aroundits rim.

Twoboys watch arock fall from thesky and landnear them.

Afarmer picks up anunusuallyheavy rock while plowing his field.

•Ascientistdiscovers therareelement iridiumina soil layer thatmarkstheendof theageofdinosaurs.theendof theageofdinosaurs.

•All of these people have discovered possible evidence of rocks from space that passed through theatmosphere and landed on Earth. Sometimes there is little or no evidence of the rock itself; it burned upin the atmosphere or broke up on impact. Other times the rock is all there is, with little evidence of itsfiery entry or crash landing. These events all involve the mysteries of meteorites: what they are, wherethey come from, how they got here, how they affect people, and what they tell us about the solarsystem. These are some of the questions that are investigated in Exploring Meteorite Mysteries.

•Meteorites are rocks from space that have survived their passage through the atmosphere to land onEarth’s surface. Some meteorites are seen or heard to fall and are picked up soon afterward, while mostare found much later. Some meteorites are large enough to produce impact craters or showers offragments, but others are small enough to hold in one hand, and still others are so small that you need touse a microscope to see them. Some meteorites are like igneous rocks on Earth, others are pieces ofmetal, and others are different from all known Earth rocks. Yet, despite their variety in size,appearance, and manner of discovery, all meteorites are pieces of other bodies in space that give usclues to the origin and history of the solar system.

•Noblesville meteorite. The 0.5 kg (fist-sized) meteorite found by the boys. Inside Noblesville is a gray stonymeteorite, but outside it is covered by a dark brown glassy crust.

•Brodie Spaulding (age 13) and Brian Kinzie(age 9). The boys are standing on the lawnwhere they observed the Noblesville, Indianameteorite fall on August 31, 1991. (Photo byM. Lipschutz)

23

•Meteorite Fall•and Recovery

•Meteors, Falls and Finds•Meteors, bright streaks of light moving rapidly across the sky, are•fairly common. On a clear night outside the city you could see an•average of three or four an hour. Sometimes many more meteors are•visible. These times are called meteor showers and many showers•return year after year on the same dates. These showers are associated with•comet dust left by long-passed comets. However, very few meteors, and none of the

•yearly meteor showers, yield meteorites on Earth’s surface. Most of the dust and ice particles burn up completely as they pass through the atmosphere.

•Only a few people each year actually see a meteorite fall. Meteorites that are recovered soon after they land on Earth are called falls. About 900meteorite falls have been recovered around the world, mostly in the last 200 years. The fall of a relatively small meteorite is exciting, but not dramaticunless it injures a person or damages property. When young Brodie Spaulding and Brian Kenzie observed the fall of the small Noblesville meteorite inAugust 1991 (see Lesson 1), they saw no bright light and heard only a whistling sound. The meteorite was slightly warm to touch and made a small holein the ground where it landed.

•Falls of large meteorites are rare, occurring only once every few decades, but are dramatic, beginning with the bright streak of light and thunderousnoise of a fireball. The falls of the Allende stony meteorite in rural Mexico and the Sikhote-Alin iron meteorite in Siberia, Russia, were two recent largefalls (see Lesson 15). Both meteorite falls began with bright light and explosions that were seen, heard and felt for great distances. The cover of thisfalls (see Lesson 15). Both meteorite falls began with bright light and explosions that were seen, heard and felt for great distances. The cover of thisbooklet shows the Sikhote-Alin fireball as depicted in an eyewitness painting. The fall sites for the two meteorites were soon found. Allende wasscattered over a 150 square kilometer area

Sikhote-Alinmeteorite. This is a fragment from the Sikhote-Alinshower that fell in Russia in February, 1947. It is an iron meteoritethat is covered by black fusion crust and indentations likethumbprints from melting during flight through the atmosphere.The Sikhote-Alin irons weighed a total of 23,000 kg, with thelargest piece weighing 300 kg.

Allende meteorite. This is a fragment from the Allende shower that fell in Mexico in February 1969. Itis a dark gray stony meteorite with black glassy fusion crust. The Allende stones weighed a total of2,000 kg, with the largest piece weighing 100 kg.

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•around the town of Pueblito del Allende. The Sikhote-Alin site was located from the air by its devastation of a forested area. On the ground scientistsfound over 100 craters of varying sizes. Both meteorites fell as thousands of fragments covering wide areas. The breakup and fall of a large meteoritelike Allende or Sikhote-Alin before impact is called a meteorite shower. (See Lessons 2 and 3)

•The impact of a huge meteorite has never been observed and recorded by people; however, many have been recorded as craters in the surfaces wherethey landed on the Earth or other planetary bodies. Me-teor Crater in Arizona is the best known meteorite impact crater on Earth. It is about 50,000years old and well preserved in the arid desert. Many small fragments of the Canyon Diablo meteorite have been found around the crater, but their totalmass is only a tiny fraction of the total mass of the incoming meteorite. The force of the impact is thought to have vaporized most of the meteorite.Imagine how powerful that explosion must have been if anyone were nearby to see and feel it!

Meteor Crater in Arizona. This 1.2 km wide, 150 m deep, crater was made by a 30m iron meteorite weighing about 1,000,000,000 kg. Thousands of fragmentstotaling 30,000 kg of the Canyon Diablo iron meteorite have been found, but mostof the meteorite was vaporized by the heat of the impact.

•Studies of numerous observed falls, combined with field and experimental studies of impact craters, give us a general picture of the fall process.Meteorites approaching Earth come in all sizes from microscopic to gigantic. The larger the size, the fewer the number of meteorites there are. Mostmeteorites approach Earth at speeds of about 20-3 0 km/sec. They are slowed down by friction with the air as they pass through the atmosphere. Theheat produced causes their outsides to melt to glass creating the fusion crust. The tiniest rocks and dust burn up as meteors without landing on Earth.Small meteorites like Noblesville are slowed to below the speed of sound. Larger meteorites like Allende and Sikhote-Alin don’t slow down much andmake sonic booms as they approach Earth at speeds greater than the speed of sound. Even larger meteorites, like Canyon Diablo that formed MeteorCrater, are hardly slowed at all by the Earth’s atmosphere and hit the Earth at very high speeds, making large impact craters. No meteorite this large hasfallen in recorded history. Most small to medium falls are stony meteorites and most of the larger showers and impact craters are produced by ironmeteorites. Iron meteorites are stronger than stony meteorites; therefore, they don’t break up as easily in space or as they pass through the atmosphere.

•Many meteorites fall to Earth each year, but are not observed. Few of these meteorites are ever found. From photographic records of fireballs andsmaller meteors, scientists have calculated that about 30,000 meteorites larger than 100 g fall on the Earth’s surface each year. Although this sounds likea huge number, there is very little chance of a meteorite falling on you. Most of these meteorites just go unnoticed because they fall quietly during thenight, in unpopulated areas, or in the ocean . However, some meteorites survive exposure at the Earth’s surface and are picked up hundreds or thousandsof years after they fall.

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•Meteorites that are collected with no visual evidence at the time of their fall are called finds andmake up the bulk of the world’s meteorite collections. Prior to 1970, about 1500 meteorite findshad been collected around the world. The discoveries of numerous meteorites in desert regionsin North America, Africa and especially Australia have added hundreds of new meteorites to thecollections in the last few years. But the best area in the world for collecting meteorites is the icydesert of Antarctica. In 1969, nine meteorites were found on Antarctic ice by a Japanese fieldteam. Since then about 17,000 meteorite fragments have been found by Japanese, European, andU.S. meteorite collection teams.

•Antarctic Meteorites

•Antarctica is a special place for collecting meteorites. More meteorite fragments have beenrecovered there than from the rest of the world combined. Yet because the continent is frozen,remote and uninhabited, not a single Antarctic meteorite fall has been observed. Several factorscombine to make Antarctica ideal for finding previously-fallen meteorites. The first is the ease offinding dark meteorites on ice. This aids in recovery of small and sometimes rare meteorites.The ice also helps to preserve the meteorites because they rust and weather away more slowly incold Antarctic temperatures than in warmer climates. The next factor is the movement of the icewhich concentrates meteorites that fell in different places at different times. The meteorites are

•Identifying•Meteorites

•Finding a meteorite on Antarctic ice when thereare no other rocks around is easy (if you can standthe cold!). Finding a meteorite on sand, a plowedfield, or a path or road isn’t hard. But finding ameteorite in a thick forest, or picking one out of apile of Earth rocks is challenging, even forexperts. There are many types of meteorites andthey are found in all sizes and shapes, but mostmeteorites have two things in common: Outsidethey have dark brown or black glassy crusts andinside they contain enough iron metal to attract amagnet. The outer glassy crust, of the

•meteorite, called its fusion crust, is produced asthe rock is heated by friction when it comes

•through the atmosphere. The outer part of therock melts and forms fusion crust that often hasflow marks or indentations like thumbprints. Theinside stays cool and is usually light gray to black which concentrates meteorites that fell in different places at different times. The meteorites are

enclosed in ice and move with a glacier until it comes to a rock barrier and stalls. The meteoritesare later exposed at the

•surface as the ice gradually erodes away. This concentration makes it difficult to tell whichmeteorites are parts of a meteorite shower, and which are individual falls. All Antarcticmeteorites are given separate names although some are later grouped as paired meteorites if datasuggest that they came from a single shower. It is estimated that the 17,000

•Antarctic meteorite fragments represent

•about 3,000 separate meteorites, or about the•same as the total for the rest of the world’s

•collection. (See Lesson 18)

•The concentration process and ease of finding meteorites in Antarctica led to national andinternational meteorite programs organized by the Japanese, Americans and Europeans, and toyearly expeditions to collect meteorites. The Japanese JARE (Japanese Antarctic ResearchExpedition)

inside stays cool and is usually light gray to blackin color, but some may be tan or, if weathered andrusted, brown.

•The three major types of meteor-ites are stony,iron, and stony- iron meteorites. These are easilydistinguished by their amounts of iron metal.Stony meteorites are mostly silicate mineralswith less than 25% metal, iron meteorites areessentially all metal, and stony-iron meteoritesare about half silicate minerals and half metal.Iron-rich meteorites can be easily identified bytheir

•density; they feel much heavier than Earth rocks.Most stony meteorites have shiny or rusty

•continued on next page

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•

•Collecting Antarctic meteorites. This scientist is collecting a meteorite on the ice in Antarctica. The Antarcticice aids meteorite collection by concentrating many meteorites in some areas, weathering them slowly, andmaking them easy to see. Scientists live and work in remote, hazard-ous conditions in order to recoverhundreds of meteorites per year.

•continued from previous page

•metal flecks visible inside: almost no Earth rocks haveiron metal. A few stony meteorites have no metal andare very similar to Earth rocks; these can be

•recognized by their glassy fusion crust. Stonymeteorites are the most abundant (94%) among fallsand irons are uncommon (5%). However, irons make upabout half of all finds, except in Antarctica. Stony-ironsare rare (1%) among both falls and finds.

•The only way to be sure if a rock is a meteorite is tohave it

•examined and analyzed by an expert. If you have asample that might be a meteorite, you should contact ameteoriticist, geologist or astronomer at a local sciencemuseum or university.

•Alternatively, you could contact a national meteoritecuration center at NASA Johnson Space Center inHouston or the Smithsonian

•National Museum of Natural•History in Washington, DC.

•program is run by the National Institute of Polar Research (NIPR) in Tokyo. TheEUROMET (European Meteorite) consortium is a cooperative program among manyEuropean countries with its headquarters at the Open University, Milton Keynes, England.The American ANSMET (Antarctic Search for Meteorites) program is a collaborationamong three government agencies: the National Science Foundation (NSF), NASA, and theSmithsonian Institution.

•In Antarctica, meteorites are concentrated on ice fields near mountains, especially theTransantarctic Mountains. The sites are far from the few coastal research stations or from the SouthPole station. The weather is extreme, with sub-zero temperatures and high winds to make lifehazardous. Teams of scientists spend

•one to two months in this frigid environment collecting meteorites. They travel to these sites byhelicopters or cargo planes, drive around in snowmobiles, and live in special polar tents. They musttake almost everything they need to survive because Antarctica provides only air, frozen water andrefrigeration. Despite these hazardous conditions, the teams have been highly successful incollecting meteorites. During approximately twenty years of collection, American expeditions havereturned over

•History in Washington, DC.

•Antarctic ice cave. A member of the U.S.meteorite collection team is standing outside anice cave in Antarctica.

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•Antarctic meteorite locations. Meteorites are found mostly along the 3,000 km Transantarctic

•Naming•Meteorites

•Noblesville, AllendeSikhote-Alin

Canyon DiabloGibeon, Brenham

ALH90411, EET83227

•Meteorites are named after the nearest town(Noblesville, IN) or post office so their names areoften picturesque. Because meteorites have beenfound the world over, the list of meteorite nameslooks like a geography lesson. When meteoritesare found far from towns, they may be namedafter their county of origin (Sioux County, NB),or after a nearby river (Calkalong Creek, Aus.),

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•Antarctic meteorite locations. Meteorites are found mostly along the 3,000 km TransantarcticMountains that diagonally cut the continent. These sites are remote from the U.S. research stationsSouth Pole and McMurdo (indicated with stars).

•8,000 meteorite fragments, and Japanese over 9,000. In only three expeditions Europeans found 530meteorites.

•Meteorite Curation•Scientistsin museums and universitiesaround the worldare responsible for the curation of non-Antarcticmeteorites. Curation includesclassifyingnew meteorites,storingthem, and distributingthemtoscientistsforstudy.WhenthethreeAntarcticmeteoritecollectionprograms began bringingback hundreds to thousands of meteoritefragments per year, each program set up its own facilities to do curation. Each of these facilitieshas

•specialclean labsbecauseAntarcticmeteoritesare less contami-natedby Earth’senvironmentand pollutionthanothermeteoritefinds. Meteorites are stored in clean cabinets, sometimes in a dry nitrogen gas, and handledand examined in glove box cabi-nets or lab benches with filtered air. The first task of the curators is to classifynew meteoritesand announce themto research scientists.Scientists then send requests for samples to study. Inresponse, the curators take small pieces of each requested meteorite and distribute them to the scientists.Finally, the curators store the meteorites in clean environments to preserve themfor futurestudies.

or after a nearby river (Calkalong Creek, Aus.),lake (Carlisle Lakes, Aus.) or other geographicfeature

•(Canyon Diablo, AZ). In deserts where manymeteorites are found in areas with few towns orgeographic names, meteorite names include both ageographic area and sample number. For example,Acfer 287 is from the Sahara Desert in Algeriaand Camel Donga 005 is from the Nullarborregion in Australia. In Antarctica, wherethousands of meteorites have been collected inyearly expeditions, the names include thegeographic area, year of collection and samplenumber. Geographic areas are often abbreviatedusing one to four letters. Thus ALH90411 standsfor sample 411 collected in 1990 in the AllanHills area of

•Antarctica. The names, locations and find datesof meteorites in the disks are given in theMeteorite ABC’s Fact Sheet on page 29.

•Micrometeorites•The smallest objects approaching Earth are cosmic spherules andinterplanetary dust particles (IDP). They are calledmicrometeorites because they are so small that a microscope is

•Meteorite curation. This is the meteorite curation facility atNASA‘s Johnson Space Center in Houston, Texas. It is operatedby the same group which curates the Apollo lunar samples. Acompanion facility is at the Smithsonian National Museum ofNatural History in Washington, DC.

micrometeorites because they are so small that a microscope isneeded to see them. Because micrometeorites are small and havevery large surface areas compared to their masses, they radiateheat rapidly and are not melted as they pass through theatmosphere. Cosmic spherules are droplets less than a millimeterin size that are found in deep sea sediments and Antarctic andGreenland ice. EUROMET has an active micrometeoritecollection program with a curation facility in Orsay, France.

•IDPs are micrometer-sized irregular aggregates that vary widelyin composition, mineralogy and structure. NASA collects IDP’s inthe upper atmosphere using military airplanes with collectorsattached under their wings. The collectors are opened uponreaching high altitudes and closed before returning to the ground.This ensures that only high altitude particles are collected. Some ofthese particles are man-made space debris, others are ash fromEarth’s volcanoes, but many are interplanetary dust. These IDP’sare curated at NASA Johnson Space Center in a lab adjacent to theAntarctic meteorite curation lab. NASA curators describe,announce and distribute the IDP’s which are studied by scientistsaround the world.

•Cosmic dust collection. NASA collects cosmic dust incollectors mounted on aircraft that fly in the stratosphere.

•Interplanetary dust particle. This fluffy aggregate ofgrains was collected by NASA high in the atmosphere. Itconsists of a variety of minerals loosely held together. Itis sitting on a metal surface with holes in it.29

•Crater cross sections. This diagram shows two views of a typical impact crater. The left view shows the circular crater with its rim and scattered ejecta. The rightview shows that the rim is above and the crater floor is below the original surface. The ejecta are thickest closest to the rim.

•crater diameter. These numbers vary with the speed, size, mass, and angle of approach of the impacting object, and with the nature ofthe target rocks.

•Finding a circular crater is not sufficient to identify it as an impact crater because there are also volcanic craters. Although their sizeranges overlap, impact craters tend to be larger than volcanic craters. Their structures also differ. A volcanic crater’s floor is often abovethe surrounding surface, while an impact crater’s floor is below the surrounding terrain. Thus a fresh impact crater is circular, with araised rim and a lowered floor. Impact craters are also surrounded by rocky material thrown from the crater, ejecta. The best proof of animpact crater is associated meteorite fragments; after that, the next best indicator is the nature of its rocks. They are broken, distorted oreven melted by the shock of the explosive impact. Much of the ejecta outside the crater is broken pieces of various rocks mixed togethereven melted by the shock of the explosive impact. Much of the ejecta outside the crater is broken pieces of various rocks mixed togetherto form a breccia. The rocks inside the crater are also breccias which are highly shocked and sometimes melted. The original bedrockbelow the crater is shocked and fractured. (See Lessons 6 and 7)

Catastrophic ImpactsLooking at the surface of the Moon we see craters ranging in sizefrom tiny to gigantic. The largest basins are the dark, roughlycircular mare that are filled with solidified basalt. Such large impactsmust have had a major affect on the whole Moon. Studies of lunarrocks returned by the Apollo missions showed that the giant impactshappened about 3.9 billion years ago (see companion volumeExploring the Moon). Studies also showed that the breccias formedby impact on the Moon are rich in some metals that are abundant inmeteorites, but rare in rocks on the surfaces of the Moon and Earth.Iridium is one such metal that is common in meteorites. Its discoveryin the K/T boundary soil offers an explanation of a catastrophicEarth event.

Aristarchus. The lunar crater Aristarchus is about40 km in diameter. It is one of the most studiedcraters on the Moon.

The K/T boundary is the layer of soil that marks the end of the Cretaceous (K) period and beginning of the Tertiary (T) period of geologic time. It occurred 65 millionyears ago when three-fourths of all species of life on Earth became extinct. Other time boundaries in earlier periods also mark extinctions of many species. Geologistshave tried to understand the causes of these mass extinctions, suggesting perhaps

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•major changes in climate. In 1980 geologists discovered that this layer is surprisingly rich in iridium. They suggested that the iridium was from a giantmeteorite that impacted the Earth throwing a tremendous volume of dust into the atmosphere. While the immediate effects of the impact would havebeen regional, the effect of the dust in the atmosphere could have been global. The climate might have been changed drastically for some time after theimpact. For years the impact hypothesis seemed plausible, but there were no terrestrial craters with the right age and size to have caused these changes.Recently, geologists found a 65 million year old buried crater that is over 200 km across on the Yucatan Peninsula in Mexico. It, possibly incombination with other craters the same age, might be the “smoking gun” of the K/T mass extinctions (see Lesson 14).

It is only natural to ask when the last large impact occurred onEarth and whether another one could occur soon. Meteor Craterwas made by the impact of a large meteorite 50,000 years ago.Although it is a relatively small crater, it would have caused majordestruction in a city, had there been any in existence at the time.Two medium-sized impacts occurred this century in Russia,Tunguska in 1908 and Sikhote-Alin in 1947 (see Lesson 15). TheTunguska explosion was large enough to have caused significantdestruction if it had happened near a city.

•Thethreatofglobaldevastationfrom

•a major impact led the U.S. govern-•ment and NASA to propose the

•The Tunguska Impact. In 1908 the biggestmeteor in recorded history shot across theTunguska River in Russia and exploded. •ment and NASA to propose the

•SpaceguardSurvey. It would have

•been an international network of automated telescopes that would scan the sky in search of all Earth-approaching asteroids or comets large enough (1km) to cause severe destruction.

•Once their orbits were determined, calculations would be done to predict whether any body would impact Earth. Although the full international programwas not approved, there are at least two smaller programs which search the sky for incoming asteroids and comets. A means of deflecting the asteroid orcomet out of its orbit would then be needed to avoid a catastrophe. Although the probability of a devastating impact is very low, the potential destructionof such an impact is so great that precautions are warranted.

•The impact of Comet Shoemaker-Levy 9 on Jupiter in July 1994 was the first time

•scientists predicted the impact of a small body on a planet. The comet was discov-ered in March 1993 after it broke apart into 22 fragments as it passedclose to Jupiter. The orbit of this “string of pearls” was

•determined by continued telescopic•observation. Calculations predicted that on its next pass through Jupiter’s orbit the fragments would impact the planet.

•Because of the predictions, the

•whole world watched and

•waited, while thousa