Ice at the Lunar Poles

A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society

This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders Ice at the Lunar Poles

That the Moon harbors ice at high latitudes is well known. The source of that water, however, may come as something of a surprise

Richard R. Vondrak and Dana H. Crider

n March 6, 1998, The New York This same story, plus or minus some This orientation, which results in the OTimes presented its readers with details, appeared that spring in every- absence of seasons on the Moon, has some intriguing news: “An American thing from USA Today to NASA press endured for billions of years. As a con- spacecraft has found evidence that rel- releases. In the five years that have sequence, there are places near the atively large amounts of frozen water since elapsed, much has been learned poles, such as the interiors of some exist on the Moon, scattered in craters from the spacecraft involved in this steep-walled craters, that are perma- over vast spans of the north and south discovery, and scientists like ourselves nently shaded from the Sun and thus poles....” This article explained that have had a chance to ponder and ex- remain at frigid temperatures—about “[r]ather than being entirely a dry tend these results. So we are in a good –200 degrees Celsius—throughout the wasteland ... the Moon may have position now to review the chain of long lunar day. Any gases that reach enough water, in the form of small ice events leading up to the dramatic 1998 such surfaces would quickly freeze in a crystals mixed in loose dirt, to sustain announcement—and to explain why manner reminiscent of the “cold traps” lunar colonies and provide fuel for some of the statements made at that used in laboratory vacuum systems to rockets exploring the solar system,” a time may have to be modified in light collect stray vapors. reference to the fact that oxygen and of recent scientific progress. The notion that the Moon might har- hydrogen, the constituents of water, In particular, we would like to de- bor ice in its own natural cold traps make excellent rocket propellants. scribe here why we and others believe languished for almost two decades, de- Continuing to read on about the that the source of the polar water is spite a great deal of scientific scrutiny Moon, one would have learned that more complicated than these news sto- focused on the Moon during the Apol- “any water present would have come ries indicated. Interestingly, an appre- lo space program. Indeed, study of the from its bombardment by comets” and ciation for the details of where the wa- rocks brought back by the Apollo as- that “the only water that would re- ter came from also calls into question tronauts showed no evidence that wa- main on the airless Moon would have the assertion that the deposits of ice at ter has ever been present in substantial to be hidden in craters in places shield- the lunar poles represent a unique re- quantities on the Moon. Yet the con- ed from sunlight, which would evapo- source for future colonization or space cept of cold traps on the Moon re- rate any moisture.” exploration. A fuller understanding of emerged in 1979, when James R. the origins of this lunar water also Arnold of the University of California, Richard R. Vondrak obtained a doctorate in highlights what, in our view, may be San Diego used the information gar- physics and astronomy from Rice University in the greatest value of these ice deposits: nered during the Apollo years to esti- 1970. After postdoctoral appointments at the as a scientific resource for understand- mate that substantial amounts of water Royal Institute of Technology in Stockholm and at ing the evolution of both the Moon might be generated within the lunar Rice, he took a position at Stanford Research In- stitute. He later moved to the Lockheed Palo Alto and the Earth. soil (which, because it lacks organic Research Laboratory, where he served as Director components, is more properly called of Space Physics, before joining NASA in 1995 to Hot Bodies, Cold Traps regolith). That water, according to head the Laboratory for Extraterrestrial Physics at The history of this subject really starts Arnold, may migrate to the lunar poles Goddard Space Flight Center. Dana H. Crider re- in 1961, when Kenneth Watson, Bruce and be deposited there as ice. But be- ceived her Ph.D. in 1999 from Rice University. C. Murray and Harrison Brown of the cause none of the Apollo landings or She spent the next three years at Goddard Space California Institute of Technology put orbiters visited the lunar poles, there Flight Center as a National Research Council forth the first serious scientific specula- was really no direct evidence for or postdoctoral associate and as a research affiliate tion about the existence of ice at the lu- against Arnold’s supposition. through Catholic University of America. She is nar poles. In an article in the prestig- Curiously enough, the next ad- currently studying the interaction of the solar wind with Mars and with the Moon as a research ious Journal of Geophysical Research, they vance came from studies not of the faculty member with the Catholic University pointed out that the equatorial plane Moon but of the planet Mercury. In Physics Department. Address for Vondrak: of the Moon is canted only 1.5 degrees the early 1990s maps made with radio NASA/GSFC, Code 690, Greenbelt, MD 20771. from the ecliptic (the orbital plane of telescopes revealed bright radar re- Internet: richard.vondrak@nasa.gov the Earth and Moon about the Sun). flections bouncing back from both po-

322 American Scientist, Volume 91 © 2003 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. Figure 1. Seeing the ever-changing phases of the Moon reminds the viewer that half of Earth’s companion is illuminated—or rather that almost half of it is. Some areas near the poles are permanently in shadow because the Sun remains quite low in the sky throughout the lunar day. Space scientists have long pondered whether such places on the Moon might retain frozen volatiles such as water ice, and, as of five years ago, most were convinced that they do. The authors used numerical models to investigate the probable source of that water and its fate after it freezes out. This image of the Moon above the Earth’s horizon and airglow was recorded by the astronauts of Space Shuttle Columbia, who died tragical- ly during re-entry in February. (Image courtesy of NASA.) lar caps of Mercury. Those reflections probe that the U.S. Department of De- kilometers in diameter and more than had a particular signature consistent fense and NASA launched in 1994 12 kilometers deep in places (relative with water ice on the surface. Because with the intent of visiting a nearby as- to the rim)—that is, deeper than the the spin axis of Mercury is nearly per- teroid. To test its instruments, con- Marianas Trench in the Pacific Ocean. fectly perpendicular to its orbital trollers first sent Clementine into a po- The existence of this giant basin further plane, that planet too has permanent- lar orbit around the Moon. A technical revived interest in the possibility of po- ly shaded regions at its poles. So the glitch forced the planned asteroidal en- lar ice on the Moon. discovery that such spots hold ice on counter to be scrubbed, but the scien- Lacking a source of illumination, the an otherwise searingly hot planet tific payoff from its travels around the optical camera on Clementine could not boosted the credibility of those who Moon was considerable. For example, obtain images from within permanently predicted that volatiles like water con- Clementine discovered a deep gash on shaded polar regions. But recognizing dense and survive in high-latitude the far side of the Moon that extends to the possibility that some of these places cold traps on the Moon. the South Pole. That structure, now might harbor ice, the scientists running The first spacecraft to observe the lu- known as South Pole–Aitken Basin, is the mission attempted an impromptu nar poles in detail was Clementine, a an old impact crater, more than 2,500 experiment, one that mimicked www.americanscientist.org © 2003 Sigma Xi, The Scientific Research Society. Reproduction 2003 July–August 323 with permission only. Contact [email protected]. plane of N At last, in 1998, a probe that offered N the Earth’s 23û a way to answer this question visited equator ecliptic the Moon. Along with several other in- plane 1.5û struments, the Lunar Prospector spacecraft carried a neutron spectrometer, which detected a sharp decrease in the flux of medium-energy neutrons ema- 5.2û nating from polar regions. This was a Earth key observation, one that merits a full Moon 6.7û explanation of its significance. The surface of the Moon gives off Sun plane of neutrons not because it is itself partic- S the Moon's ularly radioactive. Rather, these neu- equator trons result from the impact of galactic S cosmic rays, which rain down from deep space and hit the lunar surface, Figure 2. The Moon’s equatorial plane is tilted by 6.7 degrees from the plane of its orbit around knocking neutrons out of the regolith. Earth, which is itself tilted by 5.2 degrees in the opposite direction from the ecliptic (the plane The neutrons that are kicked loose of rotation of the Earth and Moon around the Sun). The result is that the rotation axis of the travel quite fast initially, but they then Moon is very nearly perpendicular to the ecliptic. Consequently, the Moon lacks seasons, and some polar depressions can remain in permanent shade. (Adapted from Spudis 1996.) lose their kinetic energy through colli- sions with the various atoms present until, finally, the neutrons attain the the radar technique used earlier to map those conclusions. In particular, a same temperature as the surrounding the ice caps on Mercury. The con- group of astronomers using the Areci- material. Midway during their trans- trollers directed a radio transmitter on bo radio telescope were able to detect formation from fast to slow (or ther- Clementine toward the lunar surface in similar radar reflections from sunlit ar- mal) neutrons, they are considered the polar regions and detected the re- eas near the lunar poles, suggesting “warm” or epithermal neutrons. Re- flected signals on Earth. The Clemen- that these signals merely indicated markably, these physical goings-on tine scientists interpreted the results of rough topography. So as late as 1997, (which take place within the top meter this test as being consistent with the people still did not know whether or or so of lunar regolith) can be moni- presence of ice, but others questioned not there is ice at the lunar poles. tored from orbit, because some of the neutrons are scattered into space. In particular, one can measure how the flux of epithermal neutrons changes from place to place. Seeing a relatively large number of epithermal neutrons being given off indicates that the fast neutrons initially formed must be taking their time shed- ding kinetic energy and becoming thermal neutrons. That is, a lot of neutrons must be lingering at epithermal energies for a long while. Conversely, detecting that only a scant number of epithermal neutrons are cast into space implies that the transformation of neutrons from fast to thermal energy levels is happening quickly. What might account for that? The presence of hydrogen. Why? Because an atom of hydrogen has roughly the same mass as a neutron. So when a neutron collides with a hydrogen atom, the neutron can lose most of its kinetic energy instantly, just as a collision between a speeding cue ball and another billiard ball often leaves the cue ball standing still. By measuring the fluxes of neutrons Figure 3. Astronauts landed on the Moon during the 1960s and ’70s, and they brought back at several energies, mission scientists many rocks—all barren of water. But because none of these missions had visited the lunar poles, scientists could not know whether water ice might be found there. In this Apollo 17 photo- could use the neutron counters on Lu- graph, Harrison Schmitt (the only geologist-astronaut to visit the Moon) is shown next to a large nar Prospector to estimate the amount boulder on the southeastern edge of the Sea of Serenity. Shallow depressions at waist level of hydrogen in the regolith—but only mark where regolith on the flank of the boulder was sampled. (Photograph courtesy of NASA.) hydrogen, not specifically the hydro-

324 American Scientist, Volume 91 © 2003 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. ferred to be in the form of frozen wa- make better measurements until the ter, in part because the large concen- craft eventually struck a mountain. But trations found are difficult to explain in the end, the high-risk experiment otherwise. The richest deposits discov- was adopted. ered correspond to the floors of polar On July 31, 1999 Lunar Prospector craters in permanent shadow. This as- hit the Moon, presumably where it was sociation, too, strongly suggests that aimed: the floor of a permanently ice is present. shadowed crater near the south pole. Still, the neutron surveys could not But even with the most powerful tele- show definitively whether the hydro- scopes, not a puff was seen. Although gen detected was in ice. So at the end this negative result proved a disap- of its mission, Lunar Prospector was pointment to those of us with a keen 100 kilometers targeted to strike the Moon in an area interest in the Moon, it came as little of a cold trap in the hope that the im- surprise: That final experiment was, in Figure 4. Radar image of the surface of Mer- pact might throw out material that more ways than one, a long shot. cury shows distinctive reflections (light ar- could then be definitively identified. eas) within many high-latitude craters, sug- Lunar Prospector had a mass of just The Source gesting that water ice is present within what 160 kilograms and was destined to Lunar Prospector demonstrated that are presumably permanently shaded areas. strike the surface at a shallow angle there is abundant hydrogen at the lu- The north pole of the planet is marked by a (6.3 degrees), so the crash was expected nar poles, but the data collected shed cross. Each pixel corresponds to 1.5 kilome- to vaporize only about 18 kilograms of very little light on its origins. The hy- ters. (Image from Harmon et al. 2001, cour- water. Calculations showed that steam drogen could come from impacts of icy tesy of John Harmon, National Ionosphere would be emitted for only about 10 comets and asteroids. But another pos- and Atmospheric Center Arecibo Observato- ry, and Academic Press.) seconds. Careful measurements from sible source, neglected in most ac- Earth of the resulting plume, it was counts, is the protons (hydrogen nu- gen inside water. The lead investiga- thought, might just be able to discrimi- clei) that make up the bulk of the solar tor working with these instruments, nate between hydrogen bound to min- wind, the continuous flow of ionized William C. Feldman of Los Alamos erals and true water products. This gas emanating from the Sun. National Laboratory, and his cowork- plan was controversial because of its This ambiguity inspired us to devel- ers on the Lunar Prospector team expected low probability of success. In op a numerical model that, we hoped, found a good bit of hydrogen near the particular, some scientists would have might reveal the source of the newly poles, at least some of which they in- preferred slowly reducing altitude to discovered hydrogen. One of us (Von- SPA Basin SPA

Ð10 0 10 near side far side elevation (kilometers)

Figure 5. Lunar near side is familiar to any sky watcher. But detailed analysis of the far side had to await the 1994 Clementine mission, which obtained the measurements of surface albedo (the fraction of incident sunlight reflected) used to construct these mosaics. A prominent finding of Clementine was the huge South Pole–Aitkin Basin (the largely dark patch seen in the lower portion of the far-side image), which displays some 13 kilometers of relief in records obtained from Clementine’s laser altimeter (far right). This surprising discovery helped to rekindle interest in lunar ice deposits. The low angle of illumination near the poles casts long shadows, accentuating the rugged topography. (Mosaics courtesy of the U.S. Geological Survey Astrogeology Research Program.) www.americanscientist.org © 2003 Sigma Xi, The Scientific Research Society. Reproduction 2003 July–August 325 with permission only. Contact [email protected]. north pole south pole electrons and are lost into space as neutral hydrogen or diatomic hydrogen. Some of these solar-wind particles, however, bury themselves in grains of lunar regolith. Later, micrometeorites strike the surface, melting bits of regolith and causing the implanted hydrogen to bond with the oxygen atoms present in various iron oxide minerals. The reaction releases both neutral OH and H2O, and leaves metallic iron em- bedded in glass. Indeed, iron in glass, the chemical signature of this ongoing process, was seen in rocks that the Apollo missions returned. Some of the water molecules created in this way reach the polar cold traps. Figure 6. Clementine, being placed in a polar orbit, obtained clear views of both lunar poles. By How do they get there? Once released combining images obtained over many weeks, mission scientists could judge where the areas of from the regolith, a particle follows a permanent shadow lay. These Clementine mosaics of the polar regions (above 70 degrees latitude) ballistic trajectory and, because there is show that constant darkness envelops a total area of about 10,000 square kilomters, with more essentially no atmosphere, will not col- near the south pole than at the north. (Courtesy of the USGS Astrogeology Research Program.) lide with anything until it lands. After it sets down, the particle accommo- drak) had worked as a postdoctoral in- ultimate fate of the solar wind that con- dates itself to the local surface temper- vestigator on the Apollo program, ana- tinuously strikes the Moon to see ature and is eventually released again lyzing measurements of ions from the whether this influx might help account for another hop. Water is too heavy to thin lunar atmosphere, including the for the findings of Lunar Prospector. escape the weak lunar gravity, but effects of the temporary contamination Although it is very tenuous, with a atomic and diatomic hydrogen are not. produced by the gas exhaust of the lu- density of about five atoms per cubic The way most of the water is lost nar landers. These studies were very centimeter, the solar wind flows very before reaching the poles is through useful in identifying the various fast, about 500 kilometers per second photodisassociation—sunlight breaks processes whereby gases are lost or re- (roughly a million miles per hour), them apart. In the end, a tiny fraction, tained in lunar materials. The other which brings about 40 grams of materi- about 0.04 percent of the solar-wind (Crider) had studied the interaction be- al to the Moon each second. Over time, protons that hit the Moon in the first tween the solar wind and the upper at- this inflow adds up to a considerable place, arrives at the cold traps in the mosphere of Mars. With this back- supply. When solar-wind protons strike form of water vapor, which presum- ground, we were eager to examine the the surface, most immediately pick up ably freezes out. We reached this conclusion with the north pole south pole help of a computer model that simu- lates the random migration of water molecules from lower latitudes to the polar regions. Our model shows that although the process is inefficient, it can nevertheless deliver large amounts of water to the lunar poles, about four tons per year. If the resulting deposits were durable, they would be able to build up to the quantity that Lunar Prospector indicated in only 100 million years, which, considering that the surface of the Moon has remained largely unchanged over the past few billion years, repre- lowcorrected epithermal neutron counts high sents a comparatively short span. In addition to the steady supply from solar-wind hydrogen and oxy- highinferred hydrogen concentration low gen-bearing minerals, the episodic impact of comets presumably provides an Figure 7. Neutron-spectrometer measurements from Lunar Prospector indicated concentrations additional source of water, which of hydrogen—presumably present as part of water ice—near both poles, suggesting that the total amount of ice in the upper meter of regolith exceeds 430 million tons. (The region shown is could conceivably be of considerable the same as in Figure 6.) Low levels of epithermal neutrons, shown after a subtle correction to importance. For example, completely the raw data, correspond to high concentrations of hydrogen, the maximum on this scale being covering all the cold traps with an ice roughly 250 parts per million. Shading indicates the general surface topography. (Images cour- layer 10 centimeters thick would re- tesy of William C. Feldman.) quire about 1015 grams of water, which

326 American Scientist, Volume 91 © 2003 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. wider area with ejecta, the stuff kicked OH H2O H H2 up in the collision. Thus a meteorite fall either adds or removes material, grain boundary depending on position with respect to the impact point. To study how a continual barrage of meteorites affects these ice deposits, we formulated another numerical model. For this one, we assumed a flat surface, in which the area of the ejecta blanket laid down after a meteorite impact is about four times the area of the crater that is creat- iron oxygen ed. So a typical point on the surface Figure 8. Most commentators attribute the source of the Moon’s ice to incoming comets, but an- will experience more burials than ex- other mechanism may well account for the bulk of it: the combination of solar-wind protons with cavations, thereby increasing its eleva- oxygen atoms in lunar materials. When these protons (hydrogen nuclei) strike a grain of lunar re- tion with time. The net effect is a golith, they implant themselves under the surface (straight blue arrows, left). Some of these pro- process called “gardening,” because it tons escape as neutral hydrogen atoms (H) or as diatomic hydrogen molecules (H2), both of is reminiscent of the purposeful over- which are rapidly lost to space. But the constant rain of micrometeorites often heats regolith turning of soil. grains enough to cause the implanted protons to combine with the oxygen found in various lu- Incoming meteorites with mass nar minerals (shown here schematically as an array of iron and oxygen atoms). The released greater than one milligram are so infre- products, OH and H2O, are sufficiently heavy that they cannot escape the tug of the Moon’s grav- quent on the Moon that we could treat ity. Instead, they travel over the surface in a series of hops, which vary in height depending on them as individual events in our simu- the ambient temperature. Such a particle may thus migrate to the poles with its jumps generally decreasing in size along the way (right). The evolution of a particle spawned by an incoming so- lation. Smaller bodies are, however, so lar-wind proton (orange arrow) is shown here for simplicity as taking a direct path from low to numerous that we had to consider high latitudes, whereas an actual particle would follow a much more random route. their arrivals as a continuous process. Our model demonstrates that the bom- could be carried in a comet with a di- temperatures of the lunar cold traps, bardment from meteorites and other ameter of only 1 kilometer and a water sublimation would take billions of loss mechanisms removes most of the content of 10 percent. Of course, when years. One could also imagine that ice derived continuously from the solar such a body blasts into the Moon, some of the ice would erode away be- wind, leaving only about 6 percent of things get quite hot, and much of the cause photons coming from distant the original amount. Our modeling of water present would be immediately stars hit it from time to time, as do pro- these space-weather processes also lost to space. We have not yet modeled tons coming from the Sun. But these shows that the water freezing out con- this complex process, so we cannot processes do not prove very effective, tinuously on the surface gets spread predict the consequences. Still, one can and they become totally irrelevant if the rather uniformly through the upper surmise that the residual gases re- ice is covered with even a thin layer of meter or two of the lunar regolith in leased might form a temporary atmos- dust. The main agents that weather ice about a billion years. Interestingly, we phere around the Moon, which would deposits, it turns out, are meteorites. found that over longer periods the ice then deliver some of this water vapor The impact of a meteorite produces does not get any more concentrated—it to the polar cold traps. a crater, but it also blankets a much merely reaches to greater depths.

Staying Power S N Whether the water comes from solar wind or from comets or from a combination of these two routes remains a matter of some speculation. But what is clear is that the Moon has ample sources of water and ways for it to reach the poles. How long would the ice formed in cold traps survive? Al- though no rain falls on it, no rivers flow through it and no glaciers advance over it (actions that quickly erode the land here on Earth), the lunar surface is ex- Figure 9. Water molecules reaching a zone of permanent darkness near one pole would freeze posed to various subtle assaults that out in this “cold trap,” forming, over time, an ice deposit (blue layer). Such accumulations in slowly alter its makeup. And the ice in permanently shadowed craters would be expected to form preferentially on the southern side, cold traps is in no way immune from because the Sun reaches higher in the sky to the south, illuminating more of the northern crater wall. In theory, this ice might still feel the effect of photons (yellow dots) arriving from such “space weathering.” distant stars or ones that are reflected or reradiated from the crater rim. Solar-wind protons (or- One might guess that the ice would ange dots) traveling along magnetic-field lines could also reach these ices. But even a thin dust- slowly disappear in any case, through ing of regolith would be enough to shield the ice from such influences. The mechanism most sublimation (transformation from solid likely to remove ice is bombardment by meteorites (red), a process that continually heats and directly to gas). In fact, at the frigid overturns the lunar surface. www.americanscientist.org © 2003 Sigma Xi, The Scientific Research Society. Reproduction 2003 July–August 327 with permission only. Contact [email protected]. Our estimate for the amount of water ner strata, the comet-borne ice just gets retained within a cold trap is close to mixed in with what was delivered con- the amount that Feldman and his col- tinuously. That is not to say that some leagues estimate to be present on the ba- subtle chemical or isotopic signature of sis of their neutron surveys (around 1.5 the original ice layer would not re- percent ice by weight). Given the uncer- main, but we suspect that it would tainties—both in our modeling and in take a careful analysis to uncover it. the Lunar Prospector measurements— our results indicate the solar wind is an Polar Riches? ample source for the hydrogen detected Some people speculate that the ice at the in the polar cold traps. And our com- lunar poles might one day supply water puter model suggests that there is more for a lunar base or be used as a rocket hydrogen at depths greater than one fuel, thus enabling easier exploration of meter, something Lunar Prospector the solar system. Although these de- Figure 10. Layer of ice deposited in a lunar cold could not sense. posits might be tapped for such ends, trap after the impact of a comet would be sub- But what of the many comets that one should remember that a great deal ject to incoming meteorites (red), which would excavate some spots, depositing ejecta in adja- have hit the moon; surely they deliv- of hydrogen and oxygen is available at cent areas (top). This continual process would ered some water to the cold traps too, lower lunar latitudes too, where it erode the ice layer and mix it into the upper did they not? Perhaps. But we suspect would probably be more accessible to several centimeters of the regolith (middle). The that their additional contribution is mi- lunar colonists than at the poles. For ex- result of this meteorite-induced mixing (called nor. After all, when a comet suddenly ample, the concentration of hydrogen in “gardening”) would be to spread the ice into deposits a layer of ice on the surface of the regolith at the Apollo landing sites is the upper portion of the regolith (bottom). a cold trap, gardening will prevent roughly 50 parts per million. It could be most of it from being preserved. What released by simply heating these mate- A more complex pattern emerges is more, gardening will slowly mix rials. Oxygen, too, is plentiful. Indeed, when we allow that larger meteorites what remains into the underlying re- typical Moon rocks contain almost 50 also arrive from time to time. A nearby golith. With our model, we find that percent oxygen by weight within vari- strike would, typically, dump ejecta on even an ice layer deposited suddenly ous oxide minerals. As many workers the surface of a cold trap, burying the must be about 10 centimeters thick to have shown, some of this oxygen could site under a layer of material that con- be noticeable over the background of be readily extracted by heating lunar re- tains very little hydrogen. (Even if the ice that accumulates gradually from golith in the presence of hydrogen. So it meteorite scooped up material rich in the action of the solar wind. For thin- is unclear whether future lunar ice, the heat of impact would cause most of it to evaporate.) Such an ejecta blanket slowly accumulates ice at the hydrogen concentration (parts per million) top, a result of the continuous genera- 101 103 105 101 103 105 101 103 105 101 103 105 101 103 105 101 103 105 tion and transport of water from the solar wind. The ongoing gardening mixes some of that ice downward. So 1 given enough time, a thin ejecta blanket, which was initially depleted of hy- original ejecta

(meters) (meters) blanket drogen, could regain the same concen- elevation surface tration as in the regolith below. Alternatively, this stratum might become so thick initially or so quickly buried by other layers of ejecta that it

gets preserved in something like its depth original state. (meters) 1 The pattern of ice concentration as a 0 (before)0 (after) 0.2 0.4 0.6 0.8 function of depth that one would find at a particular spot would thus de- time (billions of years) pend on the history of nearby impacts and whether these events excavated Figure 11. Numerical modeling of the evolution of a polar ice deposit over time shows what material or deposited it at the site in would happen if a large impact nearby laid down a thick blanket of ejecta over the site. Before question. But our numerical model, the impact (first panel at left), the hydrogen concentration (a proxy for the amount of ice) is which we put through some 100,000 roughly uniform to a depth that reflects how long the ice has been accumulating. The ejecta randomly arranged runs, allowed us blanket initially has little hydrogen in it, because the impact would have driven off most of the to calculate the overall outcome. We ice in the area hit or because it scooped out material from places that lacked ice (second panel). Subsequent “gardening” of the surface regolith adds hydrogen (that is, ice) to the upper part of found that on average only about 6 the ejecta blanket, which over time is buried deeply enough to become immune to this influ- percent of the ice originally delivered ence. An ice core from such a site would thus show a vestigial layer of low hydrogen concen- to the cold traps remains. Still, this tration, indicating a substantial impact had taken place nearby. The hydrogen concentration leaves them with about 4 percent wa- left in thin ejecta blankets after gardening would not, however, be very different from that ter by weight. found in adjacent layers.

328 American Scientist, Volume 91 © 2003 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. 0 establish whether there have been times Whether or not Polar Night is fund- 1 when the solar wind changed in com- ed, it is a sure bet that, eventually, position or when large comets struck some new mission will visit the polar 2 the Earth-Moon system. Such measure- cold traps in an effort to answer the ments might also show whether dense many outstanding questions. So we are 3 clouds of interstellar gas ever penetrat- optimistic that one day soon investiga-

depth (meters) ed into the inner solar system. tors will know for certain how much 4 Interpreting such lunar cores will water exists at the lunar poles, what form it takes and whether it comes 5 probably be difficult, given that the effects of gardening and sporadic mete- from comets or, as we believe, primari- 101 102 103 104 average hydrogen concentration orite impacts may produce signals that ly from the action of the solar wind. (parts per million) are muted, inconsistent from place to And one can begin now to envision place and otherwise confusing. How- how planetary scientists will analyze Figure 12. Determination of the overall hydro- ever, if planetary scientists are able to lunar ice cores to decode the past few gen concentration as a function of depth in a sort all this out, the results might pro- billion years of history of the Earth- polar cold trap required the authors to aver- vide scientific insights into the past that Moon system, just as glaciologists to- age thousands of runs of their computer mod- cannot be obtained in any other way. day study ice cores to elucidate more el. The end result of a simulation of 1 billion Obviously, the next step is to measure recent events in our planet’s past. years of ice accumulation and reworking more precisely the polar regions of the shows high concentrations to a depth of a meter or two, tapering to much lower values be- Moon and eventually to sample them. Bibliography low. Because many of the lunar cold traps Soon the European Space Agency will Arnold, J. R. 1979. Ice in the lunar polar re- would, however, be able to accumulate ice for place into lunar orbit SMART-1, the first gions. Journal of Geophysical Research 84, B10:5659–5668. longer than a billion years, the authors’ mod- Small Mission for Advanced Research eling predicts that some polar ice deposits may and Technology. Although the SMART- Crider, D. H., and R. R. Vondrak. 2000. The solar wind as a possible source of lunar polar be considerably thicker than the one shown. 1 sensors are not specifically designed hydrogen deposits. Journal of Geophysical Re- for detecting polar ice, they will search search 105, E11:26773–26782. colonists would need to mine polar ice for the infrared signature of ice in the Crider, D. H., and R. R. Vondrak. 2002. Hydro- deposits to get their water and rocket permanently shadowed craters. gen migration to the lunar poles by solar fuel. But these deposits will have a Japan, too, will soon launch a probe wind bombardment of the Moon. Advances unique scientific value, whether or not destined for the Moon: the Lunar-A in Space Research 30:1869–1874. Crider, D. H., and R. R. Vondrak. In press. they ever offer any commercial payoff mission, which will carry two penetra- Space weathering effects on lunar cold trap as an exploitable resource. tors. These devices, targeted to strike deposits. Journal of Geophysical Research. Cores retrieved from lunar cold the Moon at mid-latitudes on both the Feldman, W. C., S. Maurice, A. B. Binder, B. L. traps might provide a frozen record near side and far side, will measure the Barraclough, R. C. Elphic and D. J. of the past, analogous to the ice cores flow of heat from the lunar interior to Lawrence. 1998. Fluxes of fast and epither- retrieved from Greenland and Antarc- the surface, heat flow being an impor- mal neutrons from Lunar Prospector: Evi- dence for water ice at the lunar poles. Sci- tica, which chronicle the history of the tant parameter in the calculation of the ence 281:1496–1500. Earth’s climate over hundreds of temperature in the polar cold traps. In Feldman, W. C., S. Maurice, D. J. Lawrence, R. thousands of years with ice that is 2005 Japan plans to fly a highly capable C. Little, S. L. Lawson, O. Gasnault, R. C. thousands of feet thick. On Earth, sea- remote-sensing mission, known as Wiens, B. L. Barraclough, R. C. Elphic, T. H. sonal changes in snowfall often leave SELENE (short for SELenological and Prettyman, J. T. Steinberg and A. B. Binder. 2001. Evidence for water ice near the lunar bands of varying thickness and com- ENgineering Explorer). poles. Journal of Geophysical Research position in ice cores, which reveal an- U.S. scientists have conceived of var- 106:23231–23251. nual variation in temperature and ious missions targeted specifically at Harmon, J. K., P. J. Perillat and M. A. Slade. precipitation that took place during studying ice deposits at the lunar poles, 2001. High-resolution radar imaging of the distant past. The lunar record but NASA has not yet allocated funds Mercury’s north pole. Icarus 149:1–15. would be considerably more com- for any of them. A team led by Paul Nozette, S., C. L. Lichtenberg, P. Spudis, R. Bon- pact—each millimeter corresponding Lucey of the University of Hawaii re- ner, W. Ort, E. Malaret, M. Robinson and E. M. Shoemaker. 1996. The Clementine bistatic to perhaps a million years of time. But cently proposed a particularly ambi- radar experiment. Science 274:1495–1498. if scientists prove able to decode the tious mission called Polar Night, which Spudis, P. D. 1996. Ice on the bone dry Moon. information contained in lunar polar consists of a remote-sensing orbiter and Planetary Science Research Discoveries ice cores, we could learn something several instrumented probes that can http://www.psrd.hawaii.edu/Dec96/Iceon about the ancient changes in the solar penetrate the lunar surface. The land- Moon.html. wind and perhaps even about dra- ing sites for the penetrators will be se- Watson, K., B. C. Murray and H. Brown. 1961. The behavior of volatiles on the lunar sur- matic events in the Earth-Moon sys- lected from the orbital mapping of the face. Journal of Geophysical Research tem, such as cometary and asteroidal temperature, hydrogen abundance and 66:3033–3045. impacts, which may have left traces radar signature of the candidate polar in the lunar ices. regions. After six months of mapping, For relevant Web links, consult this issue of For example, the ratio of deuterium the probes will be targeted to land in American Scientist Online: (a heavy isotope of hydrogen) to nor- the cold traps where they will measure mal hydrogen varies between the Sun, directly the hydrogen and water con- http://www.americanscientist.org/ comets and the interstellar medium. tent. If selected by NASA next year, Po- template/IssueTOC/issue/394 Records of this ratio in lunar ices may lar Night could be launched in 2007. www.americanscientist.org © 2003 Sigma Xi, The Scientific Research Society. Reproduction 2003 July–August 329 with permission only. Contact [email protected].