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: [email protected] 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 23˚ a way to answer this question visited equator ecliptic the Moon. Along with several other in- plane 1.5˚ struments, the Lunar Prospector space- craft carried a neutron spectrometer, which detected a sharp decrease in the flux of medium-energy neutrons ema- 5.2˚ nating from polar regions. This was a Earth key observation, one that merits a full Moon 6.7˚ 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.