{ P L A N E T ary S CI E NC E } The biggest hole in the A gigantic impact basin holds the keys to the solar system’s past — and may also provide a base for human exploration. ⁄ ⁄ ⁄ BY JAMES OBERG MOON ixteen-hundred miles wide and Shadows and ice nearly 8 miles deep, the South Pole- The notion of lunar ice sounds outlandish. Aitken basin sprawls across the Why should the Moon — a world with no Moon’s southern farside. It’s the larg- atmosphere and whose rocks contain almost est, deepest, and probably the oldest, no volatile materials, especially water — have impact basin in the known solar system. any ice at all? SFor scientists, investigating the basin would Here’s the secret. By a fortuitous coinci- provide a look into what may be the Moon’s dence, the Moon’s rotation axis tilts only 1.5° mantle, conveniently exposed at the surface. to the ecliptic plane, the path of the Earth- Also, dating the impact that created the feature Moon system around the Sun. If astronomers’ will nail down a major point in the history of calculations are correct, it has been that way the Earth and Moon system and, by extension, for at least 2 billion years, and perhaps twice as the entire solar system. long — right back to the last major impacts For mission planners looking ahead to that shaped the lunar surface. humankind’s return to the Moon and eventual At the Moon’s north and south poles, no missions to Mars, the South Pole-Aitken basin portion of the half-degree-wide Sun ever rises provides a natural target — one that has a “cli- higher than about 2° above the horizon. This mate” as benign as any the Moon has to offer. means some craters close to the poles have Additional reasons include the possibility of floors that are eternally in shadow. These siting radio telescopes near the Moon’s south should become very cold, with temperatures of pole, which stands on one edge of the basin. –280° Fahrenheit (–173° Celsius) or so. Finally, there’s the possibility of finding The cold floors of these craters will trap water ice in the soil of nearby craters. Besides random molecules of water. The molecules its scientific attraction, ice could be mined to might come from the tails of passing comets, produce air and fuel for a Moon base. or even comet impacts, which would give the Moon a transient atmosphere loaded with James Oberg, a former NASA space engineer spe- water vapor. Other sources include impacting cializing in shuttle orbit operations, has been follow- meteorites with water-rich rock, or even occa- ing everyone’s space programs for decades. sional lunar outgassing. A MEDIUM-SIZE ASTEROID struck the Moon billions of years ago, creating the South Pole-Aitken basin. Knowing when the impact occurred is one of planetary scientists’ most-wished-for dates. RON MILLer 50 astronomy ⁄ ⁄ ⁄ november 05 North polar region ⁄ ⁄ ⁄ ROBO TIC M O O N M I SS I O N S Lunar topography South Pole-Aitken basin Nearside Farside ASA N LAVA FLOWS FILLED many basins on the Moon’s nearside. But a 50-percent-thicker crust on the farside made it more difficult for magma to reach the surface. Note the “hole” in the southern CLEMENTINE IMAGES stitched together farside: the South Pole-Aitken basin. yield a view of the lunar north pole. The amount of permanently shaded terrain is less in the north than the south. NASA South Pole-Aitken basin South polar region JAXA AFTER ORBITING THE MOON for a year, the Selene probe will detach its propulsion module, which will make an automated soft landing on the Moon. MooNWatCHERS • Chief focus: mineral and chemical survey The renewal of scientific interest in the of polar regions Moon has spurred several countries’ devel- • May include hard-landing impact probe Iron Titanium ASA ASTRONAUTS CAUGHT GLIMPSES of the South Pole-Aitken basin N from orbit, yet remained unaware of what they were seeing. opment of lunar spacecraft. IT’S AN ANOMALY, ALL RIGHT. The basin stands out in elemental Finally, altitude measurements from the Clementine probe Chang’e 1 • China National Space Administration maps that show abundances of iron and titanium. revealed the basin’s true size. NASA SMART-1 • European Space Agency (ESA) • Launch: late 2007 • Launched: September 23, 2003 • Reach Moon: few days after launch Impacts elsewhere on the Moon would Shackleton Crater, which is not visible to depth, but the new data from Arecibo • Reached Moon: November 15, 2004 • Thruster propulsion redistribute rock and soil and other surface Earth-based observers. makes that unlikely,” he continues. “There • Ion-drive propulsion • Main instruments: imager, altimeter, materials, so any ice would be mixed with Two years later, neutron spectrometer are no places that we have looked at with • Main instruments: imager, spectrometers spectrometers • Chief focus: testing technology, lots of dirt. And impacts within the ice field data from the Lunar Prospector spacecraft any of these wavelengths where you see that • Chief focus: testing ion drive, studying SHADOWS GATHER in south polar craters, studying lunar environment itself would gouge out older deposits and suggested the presence of hydrogen (and by kind of signature.” Moon, looking for polar ice many of whose floors never see sunlight. throw them atop younger ones. inference, water) about a yard underground Lunar-A Icy or rough? Dropping to temperatures of hundreds Selene Until about 10 years ago, lunar ice was all at both poles. The observations correlated of degrees below zero, such cold terrain • Japanese Space Agency (JAXA) • Japanese Space Agency (JAXA) speculation. From Earth, and from the mostly directly with passes over permanently Instead, Campbell’s team concludes the can trap passing molecules of hydrogen • Launch: 2006 • Launch: TBA equatorial orbits of the Apollo missions, the shaded areas. backscatter it observed is caused by and water. NASA • Reaches Moon: 5 days after launch • Thruster propulsion Moon’s polar regions are literally out of sight. But radar astronomers using the Are- surface roughness, not water ice. But Paul • Thruster propulsion • Main instrument: mapping camera The ice-cap theory couldn’t be tested until cibo radio telescope at 12-centimeter wave- Spudis of the Johns Hopkins University’s • Main instruments: multi-wavelength • Chief focus: studying Moon’s interior specially equipped polar-orbiting spacecraft lengths were skeptical, and remain so. Applied Physics Laboratory disputes Camp- directly and melt a small volume around the imagers, spectrometers with seismology reached the Moon during the 1990s. In 2003, the Arecibo radar probed the bell’s interpretation. “In fact, they do detect dust and look for water with a mass spec- • Chief focus: mapping elemental • Carries two surface penetrators with floors of two craters in permanent shadow zones of high backscatter,” he says. “They trometer.” Spudis agrees that more orbital abundances, mineral composition seismometers Slippery target at the lunar south pole, Shoemaker and claim it’s roughness, not ice. We claim that and lander missions are called for. • Carries two relay and radio astronomy satellites Lunar Reconnaissance Orbiter Hints of lunar polar ice came in 1996, when Faustini. It also examined the floors of Her- it’s consistent with the idea that it is ice.” If there is any water at all near the lunar • NASA radio data from the Clementine probe indi- mite and several small craters within the The latest radar results, Spudis contin- poles, Campbell and his colleagues con- Chandrayaan-1 • Launch: October 1, 2008 cated the wall of a south polar crater was large crater Peary at the north pole. ues, show the same as earlier Earth-based tinue, “It is widely scattered and perma- • Indian Space Research Organization • Reaches Moon: 4 days later more reflective than expected — and ice “Lunar Prospector found significant con- radar studies, and they are just as suscep- nently frozen inside the dust layers.” (ISRO) • Thruster propulsion was the most likely explanation. The space- centrations of hydrogen at the lunar poles tible to different interpretations. “We have From the resources point of view, says • Launch: September 1, 2007 • Main instruments: high-resolution craft took data on four passes over the equivalent to water ice at concentrations of a basically three separate data sets,” he Spudis, “It doesn’t matter whether there’s • Reaches Moon: 5 days after launch imager, spectrometers south pole, and one pass showed an few percent of the lunar soil,” says Bruce explains. “Two show positive for ice and ice there or not — there is a hydrogen • Thruster propulsion • Chief focus: looking for water ice, enhanced, tightly beamed backscatter of Campbell of the Center for Earth and Plan- one is ambiguous. Judge for yourself what enrichment at the poles.” • Main instruments: terrain-mapping mapping surface in detail radio signals, indicating ice. That pass was etary Studies at the Smithsonian Institution. the answer is.” The form of the hydrogen — either as camera, spectrometers the only one where the beam included large “There have been suggestions that it may be Campbell replies, “If there is ice at the molecules of H2 or H2O — affects only the amounts of permanent darkness, including in the form of thick deposits of ice at some poles, the only way left to test it is to go there kind of processing required; it doesn’t 52 astronomy ⁄ ⁄ ⁄ november 05 www.astronomy.com 53 MOON MINING Pole-Aitken. “Sample return from the basin is one of the highest-priority science mis- sions,” wrote a team of lunar scientists in 2003. The mission, they added, comes “sec- ond only to Kuiper Belt-Pluto exploration.” Samples should help scientists answer the most fundamental question about the South Pole-Aitken basin: How old is it? The many craters it bears indicate the basin is quite old.