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Dhofar 961 wasnt like the other Moonrocks. Looking at its freshly cut face,geochemist Randy Korotev noticedimmediately how dark it was

almost purple and that it contained big

metallic grains. It was so different from any-thing hed seen before that he began to wonder.Was it from the big one?

Korotev, of Washington University inSt Louis, Missouri, already knew that Dhofar961 was a piece of the Moon, chipped off bysome anonymous impact so that it escapedthe Moons feeble gravitational grasp and suc-cumbed to Earths. Tens of thousands of yearsago, Dhofar 961 fell into the Oman desert.A few years ago, it fell into the hands of col-lectors eager to make a buck.After fruitless searches oneBay, Korotev found a repu-

table dealer online selling a6-gram piece of Dhofar 961 forUS$1,000 per gram 30 timesmore expensive than gold.Korotev bought a sliver, andsacrificed a third of it for a chemical analysisthat confirmed his suspicions.

Unlike the other 59 known lunar meteorites,which have chemical compositions that traceback to three specific regions on the Moon,Dhofar 961 probably hails from a fourth:a deep, dark hole at the bottom of the lunarbackside, known as the South PoleAitkenbasin. It marks the site of the biggest-known

blast the Moon has seen. And trapped withinDhofar 961 might be a record of that event,

which would make it a clueto whether, and when, theinner Solar System endured acatastrophic pummelling in itsyouth.

Really large impacts such asthis one leave not just craters butbasins, deep and complex, their shockwaves frozen into concentric rings like abullseye. Even by basin standards, though,South PoleAitken is a doozy. Within the SolarSystem it is second in size only to Marss 10,000-kilometre-long Borealis basin, which as scien-tists report in this issue (see page 1212) wasmade by an impact so large that it seems to havesliced the top off Marss northern hemisphere.

South PoleAitken itself is morethan 2,600 kilometres across and12 kilometres deep, big enough

to blot out half of China andhide the highest mountains ofTibet.

South PoleAitken is notonly the biggest basin on the

Moon, but also the oldest, based on the rela-tive chronology that geologists piece togetherby mapping the way craters overlap each other.An absolute date for it is, however, unknown.

Just after the Solar System formed4.6 billion years ago, leftover planetesimalsregularly blasted the newborn planets. The bar-rage even knocked off enough of Earth to createthe Moon in the first place. By 3.8 billion years

ago, impact rates had tailed off to a level nottoo different from those of today (see graphic).

The question is what happened in between: didthe impacts decrease smoothly, or was there, asmany scientists suspect, a big spike 3.9 billionyears ago? Given South PoleAitkens promi-nence at the bottom of the cratering heap, itsage provides a crucial constraint on this lateheavy bombardment or lunar cataclysm.An early date for South PoleAitken meansa broader peak in the bombardment rate, orpossibly a steady rate throughout the period.A later date speaks to cataclysm.

This is why the US National Academieslast year called dating South PoleAitken the

most important goal in lunar science. Date thebasin, and you test the idea of a cataclysm, with

A giant crater on the lunar farside holds

the key to a catastrophic bombardment

that reshaped the Moon, Earth and other

planets. Eric Hand reports.

The Moon is a

witness plate for

what happened onthe Earth.

David Kring

The south pole

of the Moon, with a

relief map of South Pole

Aitken (purple area on the inset),

mapped from the Clementine orbiter.

The hole atthe bottomof the Moon

NASA

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The meteorite Dhofar 961

could pin down the timing

of a lunar cataclysm.

Time (billions of years)

Formation of Earth

and Moon

Cataclysmic bombardment

of Earth and Moon

Fluxofimpactingasteroids

and/orcomets

4.5 3.9 Today

THE LUNAR CATACLYSM

Its a major unsolved problem, saysJeffrey Taylor, a geologist at the Uni-

versity of Hawaii. And the Moon is

the only place we can address it.

Dating the impactsLook at the Moon for even

a moment, and its clearthat the place has beenbrutalized. Yet for manyyears, scientists thoughtthat its cratered surfaceresulted from inner tur-moil rather than outer.Impact craters weremistakenly identifiedas volcanic calderas, the

remnants of explosiveeruptions. You have noidea how pervasive thisidea was that volcan-ics were responsible foreverything on the Moon,

says Don Wilhelms, aretired geologist who was

the first to map the SouthPoleAitken basin in the

early 1970s, when workingat the US Geological Survey

in Menlo Park, California. WithSteve Squyres, Wilhelms was also

the first to propose the existence ofthe Borealis basin on Mars1.As the space race took off, rich new

Moon maps were produced, and pio-neering astrogeologists buried the volcanic

theories (see page 1164). The big basins, theydeduced, were all caused by impacts. Then, bythe early 70s, the Apollo astronauts hadbrought back another surprise: Moonrocks that showed that the impacts wereall roughly the same age. The hugeImbrium basin came in at an age of3.85 billion years; nearby Nectaris,separated in the relative chronology

by hundreds of substantial craters,

was just 50 million years younger. Nothing wasolder than 4 billion years.

In 1973, Fouad Tera and his colleagues

at the California Institute of Technology inPasadena first used the term cataclysm toexplain the extreme pace of the impacts. Itmust in any event have been quite a show fromthe Earth, assuming you had a really goodbunker to watch from, they wrote in an abstractto that years Lunar and Planetary ScienceConference.

It wasnt just a show from the Earth, though;it was the greatest show the Earth itself has everexperienced the sort of show youre lucky tocome through intact. The Moon and Earth areso close to each other that whatever happenedon the Moon also happened on Earth and

then some. The record has been lost on Earthbecause most impact craters are erased throughweathering, erosion and the continuous churnof plate tectonics. That makes the Moon awitness plate for what happened on the Earth,says David Kring, a geologist at the Lunar andPlanetary Institute in Houston, Texas.

And what a bad time it was. To model whatEarth went through, Kring scaled up what hap-pened to the Moon by a factor of 13 to accountfor the fact that Earth is a much larger target2.Given Wilhelms estimate of 15 major lunar-impact basins in the 50 million years betweenNectaris and Imbrium, this meant to Krings

team that a projectile big enough to form a 20-kilometre crater hit Earth every few thousandyears. Every million years, something wouldcome along big enough to make a 1,000-kilo-metre basin. Such impacts would have vapor-ized Earths oceans and steam-sterilized thesurface; Kring says an atmosphere of rock

vapour could linger for thousandsof years after the impact.

Heres the crazy part: Krings esti-mate is, in fact, very conservative.Earths strong gravity could attract

impactors at a frequency asmuch as 500 times higher than

the Moon would. Moreover,Kring does not includein his calculations the 30other huge basins that,according to Wilhelms,were formed after SouthPoleAitken and beforeNectaris. If South PoleAitken turns out not to besignificantly older than

Nectaris, then the frequencyof doomsday rocks hittingEarth rises yet higher.

Whats more, Wilhelms

basin count a baseline formany studies is now old,

all its ensuing implications. It tells you whatwas happening on the Moon early on and, byinference, what was going on in the rest of theSolar System. It tells you whether the innerplanets got smacked suddenly in an atmos-phere-annihilating blast of impacts. And thathas implications for the origins of life. Was thegreat bombardment so severe that it sterilizedany life that had got started before then? Didit create the hellish conditions that many ofthe earliest life-forms seem to have endured?Could it even have moved life from oneplanet to the next, throwing travellers such as

Dhofar 961 from surface to surface, complete withbacterial hitchhikers?

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and is conservative itself. Herbert Frey, at theGoddard Space Flight Center in Greenbelt,Maryland, recently finished a new hunt for

basins, based on topographical data collectedby the Clementine Moon orbiter. At the 2008Lunar and Planetary Science Conference, Freysteam reported 92 basins bigger than 300 kilo-metres across twice as many as Wilhelms.Weve grossly underestimated the actual fluxof objects that hit the Moon,Frey says. It means that theEarth was probably not avery good place to be 4 billionyears ago.

Yet some astrobiologistssay that a cataclysm may havecatalysed the origin of life

rather than snuffed it out. A stream of com-ets or asteroids hitting the planet would havebrought foreign organic material to Earth. Thebombardment might have pierced the crust,stirring up deep convective currents in themantle in such a way as to establish early con-tinental crust.And, although surface oceansmight have been stripped away, subsurfacewater and heat could have nourished heat-loving organisms. Its probably no coincidencethat in phylogenetic trees of life, the roots ofthe three major branches bacteria, archaeaand eukaryotes tend to be heat-loving3.

Ultimate causesOne thing the lunar rocks make clear is thatthe bombardment dropped off pretty quicklyabout 3.85 billion years ago. But what wasdoing the pummelling in the first place?Some have claimed it was simply the expectedcollisions of things left over from the formationof the Solar System, but theres no obvious waythere would have been enough projectiles at the

beginning to last as long as would be needed forthat. Others say the cataclysm was a fresh spikeof new bombardments, but what would have

caused such an influx of new impactors? Somepeople didnt like it because they couldnt thinkof a mechanism, says Wilhelms.

A few years ago, one possible explanationsurfaced from researchers based in Brazil, theUnited States and in Nice, France. The team

developed a dynamical model forthe Solar System that explainedwhy Uranus and Neptune circlethe Sun farther out and moreeccentrically than expected4. Intheir model (sometimes calledthe Nice model), they startedthe infant Solar System with

Neptunes orbit inside that of Uranus, and let theclock run. Some 700 million years later, Jupiterand Saturn fell into an orbital pattern, and theresulting gravitational pull caused Uranus andNeptune to be kicked farther away from theSun. That in turn disrupted a massive disk of icycomets in the Kuiper belt beyond Pluto, and sentthem hurtling into the inner Solar System.

Theres one problem with all this. The chemicalcomposition of material within lunar craters, aswell as their size distribution, matches nicely withasteroids, not comets suggesting that asteroidswere the main, or most recent, impactors dur-ing the bombardment. The Nice modellers have

an answer for that: the changes in Jupiters andSaturns orbits may have also disrupted the aster-oid belt between Mars and Jupiter. That couldhave been enough to send asteroids smashinginto Earth, the Moon and more.

Korotev says he is now a believer in the lunarcataclysm, thanks in part to the Nice model.Other work is resolving his other long-stand-ing problem with the cataclysm hypothesis:

his belief that the dating of most Moon rocksto around 3.9 billion years ago is the result of

an artificial selection bias. He and some otherresearchers have argued that the astronautsmight have simply kept picking up rocks fromthe Imbrium impact over and over again, andthat scientists interpreted them as being fromdifferent impacts.

Marc Norman, a cosmochemist at the Aus-tralian National University in Canberra, con-tinues to wring new dates from the Apollocollection that may counter this challenge.Norman has been looking at impact melts, therecrystallized remains that contain an isotopicrecord of a rock being melted by an impact.In one Apollo rock, Norman dated 21 impact

melts to within a 200-million-year window5

.And he found that the melts fell into a numberof age clusters, which he interprets as represent-ing four different impact events. If his interpre-tation is correct, that would be more evidencethat multiple big basins were formed within thenarrow time period of the bombardment.

Other evidence is coming from meteorites,which unlike the geographically constrainedApollo rock collection are thought to havebeen hacked from all over theMoon. Work-ing with Kring, Barbara Cohen, now at theMarshall Space Flight Center in Huntsville,Alabama, analysed four meteorites contain-

ing impact melts representing seven to nineimpact events. None of the melts, she found,

It means that the

Earth was probably

not a very good place

to be 4 billion years

ago. Herbert Frey

Astronauts like John YoungAstronauts like John Young

shown here on the Apollo 16(shown here on the Apollo 16

mission in 1972) had geologicalmission in 1972) had geological

training to choose the mosttraining to choose the most

promising rocks for analysis.promising rocks for analysis.

Time capsule: impact-melt Moon rocks, such as the slice shown here, all date from around the same time.

R.ZEIGLER

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was older than 3.92 billion years6. Even thenotorious martian meteorite Allan Hills

84001 famous a decade ago for claims thatit contained evidence of life offers supportfor the lunar cataclysm theory: parts of the 4.5-billion-year-old rock were altered in some sortof major event 3.9 billion years ago.

Reaching for the MoonStill, some lunar scientists are not satisfiedwith Moon rocks fetched by astronauts orfallen from the sky. The best way to date SouthPoleAitken, they say, is to go there, get a rock,and date it.

Sending a simple robotic lander would be rel-atively cheap, but the robots dating capabilities

would not be good enough. Radioisotope dat-ing requires a mass spectrometer, and one smallenough to fly on a lander would have uncertain-ties of 10%. On a 4-billion-year-old rock, thats400 million years exactly the sort of error thata mission travelling to date South PoleAitken issupposed to dispose of, not create. You want tobe able to say that your 4.2-billion-year-old age isdifferent from a 4.0, explains Taylor. It requiresmore accuracy than we have at present.

And so a group of lunar scientists is pushingfor a South PoleAitken sample return mission,which would be the first lunar sample returnsince the last Soviet Luna spacecraft returned

170 grams of soil in 1976. The group, led by BradJolliff of Washington University in St Louis, plan

to propose a mission for the next NASA NewFrontiers competition, a mission class capped at

$650 million. NASA intends to start acceptingproposals in December, with eventual selectionin 2010 and a launch date no earlier than 2015.

In the most recent New Frontiers contestin 2005, a South PoleAitkenmission called Moonrise madeit to the final round but was ulti-mately bested by Juno, which isset to launch towards Jupiterin 2011. At the time, the riskof doing a sample return mis-sion, with its many stages andcomponents, was consideredriskier than a simple orbiter

such as Juno, says Jolliff, whowas the deputy principal investigator on thatproposal.

The old Moonrise project proposed two sep-arate landers: one to go near the basin rim andthe other, the centre, where impact melts areapt to be concentrated. Jolliff is leaving openthe option to send just one lander to the basincentre. By 2020, if NASAs plans to return peo-ple to the Moon are realized, astronauts couldalready be encamped at the nearby Shackletoncrater, placing them near the rim of the SouthPoleAitken basin. The Moonrise lander wouldcollect rock and soil, and return to Earth with

about a kilogram of material, Jolliff says.Samples from a revamped Moonrise

mission would allow lunar scientists to datemany impact-melt crystals. The oldest, andmost frequent, dates should correspond to the

South PoleAitken impact. But other impact-melt dates would undoubtedly pollute the pic-ture, as there are half a dozen other large basinswithin South PoleAitken. And debate wouldcontinue. Landing in the middle of a field andgetting a scoop of dirt is not going to give youthe answer you need, says Norman.

In fact, Norman advocates both robotic andmanned missions to the Moon, saying both areneeded for a balanced exploration programme.My feeling about sample return may be a lit-tle more nuanced than simply humans versusrobots, he says. Both can do the job providedwe do the geologic homework, and neither will

do an adequate job if we dont. Others, how-ever, argue that the mystery of the great back-side basin wont be solved until a human goesto the source and plucks a rock from within itsblast shadow.

But returning people to the Moon will costat least $230 billion over two decades (accord-ing to the US Government AccountabilityOffice), compared with the New Frontiers$650-million cut-off. And Korotev thinks hecan solve some of the mystery simply by datingthe half-gram piece of the Moon he bought for$542 (plus $12 shipping and insurance). Laterthis year, he will share his precious sliver of

South PoleAitken with Cohen. In Huntsville,she plans to sink a diamond-tipped drill bitinto Dhofar 961 and extract several cores, eachas fine as a human hair.

After vaporizing the sampleswith a laser, she will measureargon gas that has been trappedinside the rock crystal latticefor billions of years. Countingthose atoms might allow her tocount back in time to the blis-tering crucible of the bombard-ment. If shes successful, she willextract a date desired by lunar

scientists for many moons abig message from a little bottle. Eric Hand covers physical sciences for Nature

from the Washington DC office.

1. Wilhelms, D. E. & Squyres, S. W. Nature309, 138140(1989).

2. Kring, D. & Cohen, B.J. Geophys. Res.107, 4146 (2002).3. Zahnle, K. & Sleep, N. H. in Comets and the Origin of Life (eds

Thomas, P., Chyba, C. & McKay, C., eds) 175208 (Springer,1997).

4. Gomes, R. et al. Nature435, 466469 (2005).5. Norman, M. D., Duncan, R. A. & Huard J. J.Geochim.

Cosmochim. Acta70, 60326049 (2006).6. Cohen, B. A., Swindle, T. D. & Kring, D. A. et al.Science290,

17541756 (2000).

See Editorial, page 1143, and News Feature,

page 1164.

Landing in the

middle of a field and

getting a scoop of dirt

is not going to give

you the answer you

need.

Marc Norman

N A S A

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