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 oc- cur 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- 88 SCIENTIFIC AMERICAN DECEMBER 2003 COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. Light and Dark 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.