Thenew by PAUL D

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TheNew BY PAUL D. SPUDIS Moon Recent lunar missions have shown that there is still much to learn about Earth’s closest neighbor

THE MOON does not yield her secrets easily. Although Earth’s airless satellite was the ﬁrst maria—Latin for “seas”—because of their planetary object to be explored by spacecraft smooth, dark appearance. One of the biggest and the only body ever visited by astronauts, surprises of the space age came in 1959, scientists still have many unanswered ques- when the Soviet spacecraft Luna 3 pho- tions about its history, composition and in- tographed the moon’s far side, which had ternal structure. In recent years, researchers never been seen before because it is always have called for renewed exploration of the turned away from Earth. The photographs moon; the European Space Agency and showed that it almost completely lacks the Japan are planning to send probes into lunar dark maria that are so dominant on the near orbit, and NASA is considering landing an side. Although scientists now have some the- unmanned spacecraft on the moon’s far side. ories that could explain this dichotomy of By studying the moon, these missions may terrain, it remains an unsolved puzzle. also illuminate the history of all the rocky Analysis of the lunar rocks and soil planets in the inner solar system: Mercury, brought back to Earth by the Apollo astro- Venus, Mars and especially Earth. Because nauts and by unmanned Luna landers al- the moon’s surface has remained relatively lowed researchers to get a glimpse of the unchanged for the past three billion years, it moon’s evolution. The evidence suggests that may hold the key to understanding how the the moon was created about 4.5 billion years inner planets formed and evolved. ago when a Mars-size body hit the early When astronomers ﬁrst gazed at the Earth. This collision sent a spray of vapor- moon through telescopes 400 years ago, they ized rock into orbit around Earth, and these found that its surface consists of two princi- small bodies rapidly coalesced into the pal types of terrain: bright, rugged, heavily moon. They accumulated so quickly that the cratered highlands and dark, more sparsely heat generated by the process melted the out- cratered lowlands. Galileo Galilei, the 17th- er portion of the nascent moon and formed

century astronomer, called the lowlands a global ocean of liquid rock, or magma. The NASA

DECEMBER 2003 COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. MOON’S SOUTH POLE is shown in this mosaic of 1,500 images taken by the Clementine spacecraft’s ultraviolet/visible camera in 1994. The pole is at the center of the mosaic; the lunar latitude of 70 degrees south is at the edge. Both Clementine and the Lunar Prospector orbiter found evidence of water ice in the permanently shadowed areas near the moon’s poles.

www.sciam.com SCIENTIFIC AMERICAN 87 COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. lunar crust then formed from low-densi- map the moon globally with a variety of Lunar Prospector mapped the moon’s ty minerals that floated to the surface of remote-sensing instruments. A hint of the surface composition using gamma-ray this magma ocean. fascinating discoveries awaiting global re- and neutron spectroscopy. It confirmed This early phase was followed by a vi- connaissance came from two flybys of the Clementine’s detection of ice near the olent pelting of the moon’s surface by Earth-moon system in the early 1990s by south pole and discovered additional de- comets, asteroids and meteoroids. Some the Jupiter-bound Galileo spacecraft. In posits at the north pole. An alpha-parti- of the larger objects blasted out enormous the southern hemisphere of the moon’s cle spectrometer measured gas emissions basins more than 2,000 kilometers in di- far side, mission scientists saw an unusu- from the lunar interior while a magne- ameter. Most craters and basins, at least al signature of high-iron rocks in the floor tometer mapped the distribution of local on the near side, were filled with iron-rich of the South Pole–Aitken (SPA) basin, the surface magnetic anomalies. Additional basaltic lava over the next 300 million to largest basin on the moon. Galileo also radio tracking of the spacecraft improved 400 million years, forming the dark maria mapped some of the maria using spectral our knowledge of the moon’s gravity seen today. As time went on, the bom- filters that provided information on sur- field. Finally, ground controllers deliber- bardment eased, with impacts becoming face composition; the results suggested ately crashed it into the moon in an at- less frequent and less powerful. This fact that researchers could use remote space- tempt to induce a release of water vapor explains why the maria, which are craft data to delineate the sequence of from the surface. Telescopes on Earth younger than the highlands, have fewer lava flows in the maria. and in space were trained on the crash and smaller craters. Little has occurred on site to observe a vapor plume, but none the moon since about three billion years Maria and Highlands was detected. ago; after the volcanic fires died, the only IN 1994 the U.S. Department of De- By putting the Apollo discoveries in a activity has been the occasional formation fense launched the Clementine spacecraft. global context, the Clementine and Lunar of an impact crater, the constant rain of Its goal was to test lightweight sensors de- Prospector measurements prompted sci- micrometeorites and the six blink-of-an- veloped for national ballistic-missile de- entists to revise their understanding of the eye visits by a dozen astronauts more than fense while traveling in a polar orbit of the moon and its history. For example, in the 30 years ago. moon. Clementine successfully orbited Oceanus Procellarum, a huge depression Because the moon has experienced the moon for 71 days. It obtained a com- in the western part of the moon’s near impact, volcanism and tectonic activity, it plete global map of the lunar surface in 11 side, the astronauts of Apollo 12 and can serve as a touchstone for understand- wavelengths in the visible and near- Apollo 14 found anomalous basaltic ing those processes. In particular, the infrared parts of the spectrum. The space- rocks that were rich in trace elements col- moon’s companionship to Earth makes it craft also carried a laser ranger that al- lectively known as KREEP (“K” for po- an ideal place for studying the extraplan- lowed researchers to make a topograph- tassium, “REE” for rare-earth elements etary events that occurred in this part of ic map of the entire moon for the first and “P” for phosphorus). Geologists re- the solar system during its early history. time. In addition, radio tracking of the fer to these trace elements as incompati- Nearly all traces of the asteroids and spacecraft’s orbit provided better infor- ble—that is, they do not fit well into the comets that struck Earth billions of years mation on the moon’s gravity field. And crystal structures of common rock-form- ago have been erased from our planet’s an improvised radar experiment uncov- ing minerals. The presence of KREEP-rich geologically active surface. Yet this record ered tantalizing hints that water ice exists rocks indicates that the early moon un- is preserved on the moon, where it can be in the permanently shadowed areas near derwent intense melting and differentia- recovered and read. the lunar south pole. tion, a process in which the incompatible Scientists learned much from the Following up on Clementine, NASA elements were concentrated in the molten Apollo explorations, but many mysteries sent the Lunar Prospector spacecraft part of an increasingly solid, crystallized remained after that program ended. Re- into a polar orbit of the moon in 1998. system. Lunar Prospector revealed that searchers realized that they needed to One of NASA’s Discovery-class missions, the highest concentrations of KREEP occur in the Oceanus Procellarum, although the reason for this unusual distribution is Overview/The Moon’s Mysteries not clear. ■ In the 1990s the Clementine and Lunar Prospector spacecraft provided Furthermore, the lunar orbiters con- scientists with global maps of the moon’s topography, surface composition, firmed that the highlands of the moon are gravitational variations and magnetic anomalies. dominated by anorthosite, an igneous ■ The findings gave context to the discoveries made by the Apollo missions but rock composed primarily of the mineral also raised new questions. In particular, researchers want to know more about feldspar and rich in calcium and alu- the violent bombardment of the moon that occurred about four billion years ago. minum. These rocks were created early in ■ The European Space Agency, Japan and the U.S. plan to send more unmanned lunar history, when the outer portion of probes to the moon to solve some of the lingering lunar mysteries. the moon was completely molten; the low-density anorthosite floated to the sur-

NEAR SIDE FAR SIDE

Mare Imbrium 15 Mare Serenitatis 17

Oceanus Procellarum Mare Tranquillitatis 11 12 14 16

South Pole–Aitken Basin

CLEMENTINE’S IMAGES of the moon’s near side show the two almost completely lacks maria. Six Apollo missions visited the near principal types of terrain: bright, heavily cratered highlands and side (the yellow circles show the landing sites and mission dark, smooth lowlands called maria. In contrast, the far side numbers). Now NASA wants to send a robotic lander to the far side.

face of the magma ocean. Although sci- Tranquility. Lunar researchers learned a ages, the bulk erupted between 3.8 billion entists had postulated this phase of lunar valuable lesson: samples from a single lo- and three billion years ago. history based on the Apollo samples, the cation on the moon are not necessarily Although maria are recognized by proof came from the Clementine and Lu- representative of large regions. their dark color, certain areas of the high- nar Prospector data, which indicated the Because lava flows typically have uni- lands appear to be intermediate in re- global distribution and large abundance form and distinct compositions, the data flectance and contain relatively high of anorthosite. Because the only source of from Clementine and Lunar Prospector amounts of iron. Some of these surfaces heat that could melt the entire moon can be used to map the flows that have oc- are mare deposits that have been covered would be a very rapid accumulation of curred in the maria. The age of each flow by blankets of highland debris—layers of small bodies, the presence of large quan- can then be determined by measuring its ejected rock spread by the impacts that tities of anorthosite in the lunar crust sup- density of impact craters. Older mare created the moon’s basins. Because these ports the theory that the moon coalesced flows have been exposed to bombard- mare lavas predate the highland debris from the debris of a planetary collision. ment longer than younger flows, so they layers, which were laid down during The lunar orbiters also explained one have higher crater densities. Scientists al- basin formation 3.8 billion years ago, of the more puzzling discoveries of the ready know the ages of the mare flows at they indicate that the eruption of lava Apollo missions: the unusually high con- the Apollo sites from analyzing the ra- onto the moon began well before the ages tent of titanium in the mare basalts col- dioisotopes in the rock samples, so they of the oldest mare flows sampled by Apol- lected by the Apollo 11 astronauts during can estimate the ages of other flows by lo. Global mapping has shown that these the first moon landing. Lunar geologists comparing their crater densities with ancient mare flows are very widespread had been hard-pressed to explain how those of the landing-site flows. The results across the far side of the moon and the very high density, titanium-rich magmas show that although the moon has mare limb regions (the border between the near could have ascended through the moon’s lavas of widely varying compositions and and far sides). low-density anorthosite crust. Clementine and Lunar Prospector showed that the PAUL D. SPUDIS is a senior professional staff member at the Applied Physics Laboratory of high-titanium lavas found by Apollo 11 Johns Hopkins University. Since 1982 he has been a principal investigator in the Planetary are actually quite rare on the moon. Al- Geology Program of the NASA Office of Space Science, specializing in research on the pro- though mare basalts have a range of tita- cesses of impact and volcanism on the planets. From 1980 to 1990 he was a geologist with nium concentrations, only a small frac- the U.S. Geological Survey’s Branch of Astrogeology, and from 1990 to 2002 he was a staff

tion have the extreme compositions ob- THE AUTHOR scientist at the Lunar and Planetary Institute in Houston. He was deputy leader of the sci-

LUNAR AND PLANETARY INSTITUTE served at the ﬁrst landing site in the Sea of ence team for the U.S. Department of Defense’s Clementine mission to the moon in 1994.

ACCELERATION ABUNDANCE OF THORIUM ABUNDANCE OF FERROUS M evidence ofwatericeatthemoon’spoles. distance tothesurfaceonceasecond spacecraft enabledscientiststodraw with hydrogenatomsinwatermolecules. of medium-energyneutronsbouncing off (millimeters per second (parts per million) OXIDE (percent) moon’s surface.Clementinecarrieda the firstdetailedglobalmapsof showed theenormousextentof SCIENTIFIC AMERICAN 16 during eachpolarorbit.Theresults per second) <0.5 –2.5 8 0 Clementine andLunarProspector oon Maps LUNAR PROSPECTOR >6 an impactfeaturethatstretches detection oficein thesedarkareas. THE OBSERVATIONS South Pole–Aitkenbasin( The resultsconfirmed Clementine’s 3 1 laser rangerthatmeasuredthe 0 1 5 splotch onmoon’sfarside 2,600 kilometersacross. made bythe also discovered THORIUM GRAVITY purple IRON purple ). Ice ), COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. 8– 4 –4 –8 NORTH POLE KREEP thorium, whichbehavesmuchlikethetraceelementscollectivelyknownas abundance of10elementsinthemoon’scrust.Onethosewas far side(abovetheSouthPole–Aitkenbasin). are highestinthemariaonnearsideandlowestcentralpartof showing theconcentrationofironinsoilslunarsurface.Ironlevels wavelengths (750and950nanometers),researcherscreatedamap and near-infraredpartsofthespectrum.Usingdatafromtwothose mantle mayhaverisentowardthesurfaceofbasinsafterimpact. basins. Onepossibleexplanationisthatplugsofdenserockfrom thelunar than expectedgravity( moon’s gravity.Carefultrackingofthespacecraft’sorbitrevealed stronger surface, LunarProspectorwasabletopreciselymeasurevariationsinthe the nearside,butreasonforthisunusualdistributionisnotclear. minerals. ThehighestlevelsofthoriumoccurintheOceanusProcellarumon CLEMENTINE’S CAMERAS TRAVELING INANORBIT GAMMA-RAY SPECTROMETER ERSD FARSIDE NEAR SIDE — it doesnotﬁtwellintothecrystalstructuresofcommonrock-forming WATER ICE TOPOGRAPHY red areas KILOMETERS that cameasclosesevenkilometerstothelunar captured imagesin11wavelengthsthevisible was usedbyLunarProspectortomeasurethe 08 ) abovesomeoftheyoungestimpact SOUTH POLE centimeter persecond) (neutrons persquare DECEMBER 2003 MEDIUM-ENERGY 0.212–0.214 0.217–0.219 0.223–0.225 0.228–0.230 NEUTRONS FLUX OF

PAUL D. SPUDIS AND LUNAR AND PLANETARY INSTITUTE (topography, iron and thorium maps); REPRINTED BY PERMISSION FROM A. S. KONOPLIV ET AL. CREDITIN SCIENCE, VOL. 281, PAGES 1476–1480, 1998 (gravity map), AND W. C. FELDMAN ET AL. IN SCIENCE, VOL. 281, PAGES 1496–1500; 1998 (water ice map); © 1998 AAAS Bumpy and Lumpy the near side is relatively thinner, and rocky bodies that formed from the solar THE MOON IS a very rough world. The therefore ascending magmas can reach nebula—were gradually cast out of the in- difference in altitude from its lowest point and break through the surface more eas- ner solar system or absorbed by the out- (within the SPA basin) to its highest (on ily there than on the far side. The enor- er planets. If researchers can confirm that the rim of the Korolev basin on the far mous SPA basin contains most of the the lunar cataclysm did indeed occur, the side) is more than 16 kilometers. On mare lavas on the far side, yet even these discovery would have profound implica- Earth, where the maximum altitude dif- deposits are very thin and of limited ex- tions for the history of all the inner plan- ferential is about 20 kilometers, surface tent. The SPA basin as a whole is largely ets. It is possible, for example, that a very topography is the result of tectonic activ- unfilled by mare lavas, whereas even the large body in the asteroid belt broke apart ity that creates high mountain belts and smallest basin on the near side tends to about 3.9 billion years ago and that the deep ocean trenches. The moon, in con- have copious lava fill. debris was swept toward the Earth-moon trast, has a static outer shell; the lunar The topographic map produced by system. If that is the case, it could mean crust has been cold and rigid for at least Clementine’s laser ranger revealed the as- that lunar cratering history is unique and the past four billion years. Topographic tounding dimensions of the SPA basin, cannot be used as a guide for dating fea- relief on the moon is entirely related to which stretches 2,600 kilometers across, tures on other planets besides Earth. impact craters and basins. It is no accident making it the largest impact crater in the One way to tell whether the lunar cat- that the largest basin on the moon is also entire solar system. Clementine also doc- aclysm actually occurred would be to de- the locale for the greatest extremes in al- umented the presence of numerous addi- termine the absolute age of the SPA basin. The moon may hold the key to understanding how the INNER PLANETS FORMED AND EVOLVED.

titude, although it is somewhat surprising tional basins, some of which were un- Scientists know that SPA must be older that such a large and old feature still re- known before the orbiter’s flight. Re- than any other lunar basin, and the oldest tains most of its original relief. searchers now estimate that the moon has of the basins that can be reliably dated Internally, the moon also appears to more than 45 basins (defined as impact from impact-melt samples is Mare Seren- be quite lumpy. Radio tracking of the tra- features with diameters greater than 300 itatis, which is estimated to be 3.87 billion jectory of Lunar Prospector, which trav- kilometers). Based on crater densities years old. The impact that created SPA eled in a low orbit that came as close as within the basins, SPA appears to be the clearly happened after the lunar crust so- seven kilometers to the lunar surface, oldest and Orientale the youngest. lidified, which occurred about 4.3 billion showed stronger than expected gravity Scientists, however, know the absolute years ago. The age of SPA must therefore above some of the youngest impact ages of only the basins that were visited fall between these dates, but nearer to basins. Scientists do not think the mare by the Apollo and Luna missions. Ra- which end? basalts in the basins are the source of the dioisotope dating of the impact-melt sam- If SPA is found to be close in age to the gravity anomalies; individual lava flows ples—rocks that melted when an asteroid other basins, then scientists would have a appear to be quite thin—from a few me- or comet struck the moon, therefore re- strong argument for the lunar cataclysm. ters to a few tens of meters—and total ac- vealing when an impact occurred—shows On the other hand, if SPA’s age is deter- cumulations are typically 200 meters or that all these basins formed in a narrow mined to be close to the solidification age less. Rather researchers believe the mass interval between 3.9 billion and 3.8 bil- of the lunar crust, there would be no need concentrations are plugs of dense rock lion years ago. This small range of basin to postulate a lunar cataclysm. The moon’s from the lunar mantle that rose toward ages has been interpreted to mean that the cratering history could be seen as evi- the surface of the basins after impact. moon experienced a very high impact rate dence of an exponentially declining fre- The moon’s unusual dichotomy of for a short period, which has been dubbed quency of impacts. In this case, the lunar terrain, with the near side dominated by the lunar cataclysm. record could indeed serve as a guide to the dark maria and the far side by bright But how could such a deluge have oc- interpretation of cratering on inner plan- highlands, may also be explained by curred? Models of the solar system’s ear- ets such as Mars. To date the SPA basin, structural differences under the surface. ly history posit that the frequency of im- however, researchers would need to ob- Although scientists have not definitively pacts should have tapered off between 4.5 tain samples of its impact melt. resolved this problem, the most likely rea- billion and four billion years ago because Perhaps the most exciting result of the

NASA son for the dichotomy is that the crust on most of the planetesimals—the small Clementine and Lunar Prospector mis-

www.sciam.com SCIENTIFIC AMERICAN 91 COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. sion was the evidence of water ice at the because the ice would never be vaporized searchers do not know, however, the lunar poles. Because the moon’s spin axis by sunlight. physical state of this material, its exact is inclined just 1.5 degrees—that is, its Although Clementine did not carry composition, purity or accessibility. This axis is almost perpendicular to the plane any instruments specifically designed to knowledge can be acquired only by future of Earth’s orbit around the sun—the sun search for polar ice, the mission’s science missions to the moon. is always at or near the horizon when team was able to improvise an experi- The images from Clementine also viewed from the lunar poles. (In contrast, ment using the radio transmitter onboard showed that some regions near the Earth’s axis is tilted about 23 degrees.) If the spacecraft. Whereas rocky surfaces moon’s poles appear to be in near-con- a place near the lunar pole is about 600 scatter radio waves randomly, ice partly stant sunlight. An area near the rim of meters above the average surface eleva- absorbs the waves and reflects some of Shackleton crater, for example, is illumi- tion, it is in permanent sunlight; if it is at them coherently. When Clementine di- nated for more than 75 percent of the lu- least 600 meters below the surface, it is in rected radio waves at permanently shad- nar rotation period. These areas have a perpetual shadow. In the latter areas, the owed regions near the moon’s south pole, relatively benign thermal environment, only sources of heat would be the meager the reflected signals were characteristic of with surface temperatures ranging from amount of radioactive decay from the lu- an icy surface. Four years later the neu- –60 to –40 degrees C. (In contrast, tem- nar interior and the feeble cosmic radia- tron spectrometer carried by Lunar Pros- peratures near the lunar equator swing tion. Scientists estimate that these per- pector showed large amounts of hydro- from –150 to 100 degrees C.) Locating an manently dark regions, which have ex- gen in the dark regions of both poles; the unmanned or manned outpost in one of isted for two billion to three billion years, most likely explanation is that the craft the sunlit areas near the poles would are extremely cold—on the order of –223 detected the hydrogen in water ice. Cur- greatly ease the challenge of designing to –203 degrees Celsius. These cold traps rent estimates indicate that more than 10 equipment to survive the temperature ex- could accumulate water ice derived from billion tons of ice exist within the upper tremes of the lunar surface. And if ice comets and meteorites hitting the moon, foot or so of the surface at both poles. Re- could be retrieved from a permanently

Back to the Moon The resurgence of scientific interest in the moon has inspired space agencies to plan new lunar missions. Spacecraft Country Launch Date Mass without Fuel Lunar Research (kilograms)

PAST AND PRESENT MISSIONS Clementine U.S. Jan. 25, 1994 227 Used cameras and laser ranger to map surface composition and topography. Radar experiment found first evidence of water ice at the lunar poles. Lunar Prospector U.S. Jan. 7, 1998 158 Spectrometers revealed abundance of elements in the crust and detected further evidence of ice. Magnetometer and electron reflectometer measured magnetic fields. SMART-1 European Sept. 27, 2003 280 On arrival at the moon in early 2005, camera and spectrometers Space will chart the moon’s minerals and peer into dark craters Agency to search for ice.

FUTURE MISSIONS Lunar A Japan Aug.–Sept. 2004 520 The orbiter will drop two penetrators that will burrow into the surface on opposite sides of the moon. Seismometers and heat-flow sensors will probe the lunar interior. SELENE Japan 2005 1,600 Large array of cameras, spectrometers and other instruments will map the moon’s surface composition, topography, gravity and magnetic fields in even greater detail. South Pole– U.S. Before 2010 To be determined Robotic lander will collect samples of rock and soil from Aitken Basin the basin floor and rocket them to Earth for analysis of age

Sample Return and composition. LUNAR AND PLANETARY INSTITUTE

92 SCIENTIFIC AMERICAN DECEMBER 2003 COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. dark area nearby, the base would have a or comet, studying it could reveal the moon and put it on a course back to source of water that could be used for life composition and structure of the lunar Earth. After using Earth’s atmosphere to support as well as for rocket fuel (by crust in the basin target site. Some re- decelerate, the vehicle would land in a re- breaking the water into liquid hydrogen searchers suspect that the colliding object mote area and turn on a radio beacon to and oxygen, the most powerful chemical may well have penetrated the crust and attract the recovery team. All these ele- propellants). exposed parts of the upper mantle, pos- ments make the mission ambitious and sibly from depths as great as 120 kilome- technically challenging, but it is well with- Return to the Moon ters. If the impact melt contains some ma- in our capabilities. AS A RESULT of the successes of Clem- terial from the mantle, scientists may be NASA has already requested proposals entine and Lunar Prospector, a spate of able to characterize in some detail the for a sample-return mission to the SPA new lunar missions are in various stages composition of the deep lunar interior. basin, which could be launched before of preparation. In September the Euro- A sample-return mission to the SPA 2010. But when will astronauts return to pean Space Agency launched the SMART- basin is simple in concept but difﬁcult to the moon? There are numerous scientiﬁc 1 spacecraft, whose primary mission is to execute. Mission planners must select a reasons for human exploration. A manned test an ion-propulsion engine during a 16- landing site that would yield the proper mission would provide excellent opportu- The return of astronauts to the moon requires a political rationale, NOT A SCIENTIFIC JUSTIFICATION.

month journey to the moon. After enter- samples to solve the scientific issues con- nities for a whole range of studies, from ing lunar orbit, SMART-1 will use a cam- cerning the SPA’s age and composition. planetary exploration to astronomy. And era and x-ray sensor to map the moon’s Researchers can use existing remote-sens- the existence of water ice at the lunar poles surface. In 2004 Japan is scheduled to ing information to identify areas that, by could make it much easier to establish a launch Lunar A, an orbiter that will drop virtue of their composition and geologic permanent human presence. NASA has re- two hard-landing probes, called penetra- setting, are good candidates to yield the cently sketched out proposals that would tors, to the moon’s surface. Equipped desired rocks. Because the sites would be permit new human missions to the moon with seismometers and heat-flow sensors, on the moon’s far side, the lander would using existing launch and space trans- the probes will gather information on the have to either operate autonomously or portation infrastructure, thereby saving moon’s interior and possibly map its communicate with ground controllers billions of dollars in development costs. core. And in 2005 Japan plans to follow through a relay satellite. But the return of astronauts to the up the mission with a larger orbiter called What is more, the spacecraft must ob- moon requires a political rationale, not a SELENE. This craft will map the moon in tain both rocks and soil from the landing scientific justification. It will never be un- even greater detail using x-ray and gam- site. Rocks are needed to analyze the min- dertaken solely for scientific purposes, nor ma-ray spectrometers, a terrain camera, a eralogy and date the samples, whereas the should it. The missions must address a laser altimeter and a radar sounder. soil enables scientists to determine wide range of national concerns. Once we In addition, the newly appreciated sig- whether the collected rocks are actually do return, though, new vistas of scientif- nificance of the SPA basin has revived the representative of the area. (The soil may ic possibilities will beckon. We have read idea of landing a robotic probe there to also contain small fragments of rare or ex- part of the moon’s story, but much is still collect samples and rocket them to Earth otic rock types.) The samples must be murky. Future exploration will most like- for analysis. A 2002 report by a panel of packed into a small Earth-return vehicle ly show us that the history of our closest scientists sponsored by the National carried by the lander. This vehicle would neighbor is more complicated and inter- Academy of Sciences advocated such a then use a rocket engine to lift it off the esting than we ever imagined. mission. The primary goal of the effort would be to obtain samples of the SPA MORE TO EXPLORE basin’s impact melt. By revealing when The Once and Future Moon. Paul D. Spudis. Smithsonian Institution Press, 1996. the basin formed, these rocks could set- A New Moon for the Twenty-First Century. G. Jeffrey Taylor in Planetary Science Research tle the question of whether there was a lu- Discoveries, August 2000. Available online at www.psrd.hawaii.edu/Aug00/newMoon.html Lunar Meteorites and the Lunar Cataclysm. Barbara A. Cohen in Planetary Science Research nar cataclysm. Moreover, because the im- Discoveries, January 2001. Available online at www.psrd.hawaii.edu/Jan01/lunarCataclysm.html pact melt is a composite of all the rocks The Clementine Atlas of the Moon. D. Ben J. Bussey and Paul D. Spudis. Cambridge University

NASA that were struck by the colliding asteroid Press, 2004.