{ p l a n e t ary s c i e n c 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 t i c 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. Graham Ryder of the Lunar and Planetary Institute said in 2000

that it may be as young as 3.92 billion years ES T

SCOUTING FOR ICE-RICH RUBBLE, A CI a robotic mining rover scavenges old — still older than all the nearside basins the floor of a shadowed crater. — or as old as 4.3 billion years. O & ASSO

Knowing where the basin’s birth falls NIT within that range will help scientists better

understand the period called the Late J. FRASSA Heavy Bombardment. Debate has raged for EXPLORING THE MOON will help scientists learn more about the solar system’s history decades whether this period belonged to and our place within it. It’s an essential step in the journey to Mars. ASA N the final stage of planetary formation — or THE NEXT HUMANS TO GO to the Moon will likely establish manned bases in the polar if it was a separate shower of asteroids and regions. Such bases could provide access to volatile resources (for example, water ice or giant meteorites caused by some unknown something else. Yes, you can grab samples farside and Earth; the lander would have hydrogen) trapped in shadowed craters. disruptive event. and probably get pieces of the basin floor. needed to act quickly and then blast off But saying this or that piece comes from the with whatever it had collected. change the rationale for going to the poles thick floods of dark lava that fill most basins Probing the rubble impact melt sheet or is from the lower crust In 2003, another robotic mission called to establish bases. on the nearside, some to overflowing. Thus, Scientists expect to find the basin full of — well, that’s more problematical.” Moonrise, proposed by Colorado School of THE ROVER-MINER DELIVERS the scooped-up mixture of ice and lunar The long-term value of such a resource South Pole-Aitken appears indistinct when well-mixed rocks of a type called melt brec- He laughs. “We make a landing, we Mines’ Michael Duke, also failed to get the soil to an automated refinery. would be profound. Using solar power, water viewed by human eyes. Sheer size also con- cia. This results from “gardening” by ran- obtain some samples — and then we spend nod under a different NASA program. could be broken down into oxygen and tributes to the vague impression because it’s dom impacts over the ages. The samples the next 30 years trying to figure out what As Spudis observes, however, “With a hydrogen and stored cryogenically for use as hard to see all of the basin at once. they’re telling us.” robotic mission, you have a large chance of propellant. Water and oxygen are also key The existence of a “Big Backside Basin” Within the basin are some unique fea- getting an ambiguous answer. The older the components for life-support systems. was suspected as early as 1962, when Wil- The existence of a tures, and geologists want to sample them event on the Moon, the less certain you are But however abundant polar hydrogen liam K. Hartmann and Gerard Kuiper all — either with rovers or multiple fixed- that a sample you pick up is related to that. is, building Moon-based propellant refiner- noted tall mountains at the lunar south “Big Backside Basin” lander missions. Central peaks of large cra- And going to the oldest feature on the ies will require hauling massive amounts of pole and thought these might form part of ters should contain samples from below the Moon just magnifies the problem many apparatus from Earth to the lunar surface. its rim. Other sections of the basin rim was suspected as basin debris. A feature dubbed Olivine Hill times over. You can’t be sure what you’ve Therefore, for the first human return to the were photographed from lunar orbit by the in the central region may be a deposit of got — even after you have it.” Moon — and even for early expeditions to Apollo 8 astronauts in 1968, but the overall early as 1962. the deepest rocks. NASA’s renewed emphasis on returning Mars — hauling the propellant from Earth picture remained murky. Northwest of there, according to Carle to the Moon may remove some of the hur- may still cost less than first establishing a A decade ago, however, images from the Pieters of Brown University, is a thorium dles scientists and mission planners have AT THE REFINERY, liquid oxygen and hydrogen are loaded into a lunar-based “fuel mine.” Clementine probe, assembled into mosaics will include fragments from deep within anomaly she describes as “an enigma.” It faced. And the possibility of having trained robot tanker (left) for delivery to to cover large areas, clearly showed the basin the Moon’s crust, possibly from the upper may be debris from the Imbrium impact on crews on the Moon opens many possibili- crews for use as fuel, air, and water. The big hole floor is distinct from surrounding terrain — mantle. Orbital measurements indicate the the exact opposite side of the Moon. Scien- ties for pathbreaking science. Looking beyond the immediate south polar in particular, it appears darker. Laser altim- basin holds minerals not found elsewhere tists have already mapped and sampled a Says Spudis, “If you go back to the region, the big basin remains at the center etry also put figures on the basin’s gross on the Moon’s surface. Although lunar region adjoining Imbrium that is rich in Moon permanently and can revisit sites to of attention for new missions. Even with no properties: 1,600 miles (2,600 kilometers) geologists estimate about 20 percent of the thorium and other radioactive minerals. understand things in their true field con- possibility of polar ice, it would draw much wide and 7.5 miles (12 km) deep. basin’s material comes from elsewhere on text, I think that’ll be a whole different ball scientific interest. In Clementine surface-composition the Moon, they are confident they can Dating the cataclysm game.” Follow-up work, he notes, is The region’s name, given by Don Wilhelms maps, the basin stands out prominently. identify the special local minerals. The first proposed sample-return mission extremely important in terrestrial geology. ES T A

CI of the USGS, is a little misleading. “Neither the Observations with a later generation of sen- Spudis describes the process: “You take a to this area was developed in the late 1990s “If people go to live at the south pole as south pole nor Aitken Crater are actually sors on Lunar Prospector (1998–1999) lot of remote-sensing data and learn the for NASA’s low-cost Discovery program, part of a human return to the Moon,” he

O & ASSO inside the basin,” says Michael Duke of the showed the soil is more iron-rich and properties of the basin floor in a regional but it was not selected. The mission would says, “I think they’ll have a very good NIT Colorado School of Mines. “They are simply slightly more radioactive than the rest of sense. That would let you interpret the sam- have used a stationary lander with a robotic chance of unraveling a lot of the South the two most identifiable surface features that the Moon’s surface. It also generally lacks ples you get as belonging to that unit or not.” arm and screening device to collect a Pole-Aitken mystery.” J. J. FRASSA mark the boundary of the structure.” the typical anorthosite minerals retrieved The long impact history introduces a lot pound or two of small rocks and dust. THE HYDROGEN TANKER fills a scout- For more on dating the Moon’s ing ship’s fuel pods in preparation for The South Pole-Aitken basin lies just from the nearside by Apollo astronauts. of room for error. Cautions Spudis, “With a A relay satellite placed in lunar orbit ONLINE over the limb on the Moon’s farside. Heavily Scientists want to get their hands — and sample from the Moon, you never know if would have provided less than a week of EXTRA early history, go to www.astronomy. an exploration trip. com/toc cratered by later impacts, it lacks the large, instruments — on rock samples from South you’re really sampling a regional unit or communications between the lander on the

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