The Moon has been studied more closely than any planetary body beyond Earth, having been examined for several centuries by telescope from Earth. Despite this, one of the most obvious features of the Moon was unknown before the era of spaceflight. In 1959 the Soviet spacecraft Luna 3 took pictures as it flew past the far side, making us aware, for the first time, that the near side has an unusual characteristic: the large dark maria, or "seas," that we can see with the naked eye from Earth, occur almost exclusively on the Earth-facing hemisphere. The far side is composed almost totally of the high, bright regions called terrae. These highland terrae are much more heavily cratered than the maria, and so are older. The nearside maria are thin lava flows filling large impact basins that were blasted out of the terrae. The lava flows that filled the maria were very fluid basalt's (some are over 1000 km long, yet only about 10 meters thick). In contrast, the highland rocks that compose the terrae represent mostly plagioclase-rich crust formed early in the Moon's history.The Moon is the only satellite on which we have done hands-on geologic field work: 842 pounds of rocks, drill cores, and loose surface material samples were brought back, and a network of geophysical instruments were deployed by astronauts over a brief 3 year period from 1969 to 1971. The seismometers they set up there picked up waves generated from moonquakes, meteorite impacts, and man-caused seismic waves. Moonquakes are relatively mild in comparison to earthquakes, and occurrences are usually related to tidal effects caused by changes in the Earth-Moon distance (unlike those of Earth which are the result of shifting plates moving past one another). From analysis of these seismic waves, some information about the interior was obtained. The Moon's crust was determined to vary in thickness, being quite thin-a few tens of kilometers under some basins-to more than 100 km under some highland regions. Sampling rocks thrown out by impacts has allowed the mineralogy of the materials from beneath the crust to be analyzed in the laboratory. Knowledge of the deeper core is more limited, though, due to its depth of more than 1,000 km below the surface. Currently, our knowledge of the core is based on theory, grounded in what is known about the magnetic field and comparison with what we know of our own core. The magnetic field is less than 1/1000 of Earth's, hence we believe that, unlike Earth's core, the Moon's core is relatively free of liquid metal. Rocks returned from the Moon, however, indicate an ancient magnetic field, suggesting a different past.
Rocks returned from the Apollo missions have allowed specific ages to be determined for some units. The highlands formed about 4 b.y. ago, and the maria about 2.5 to 3.8 b.y. ago. Thus, most of the youngest lava's we see today on the Moon's surface flowed before many of the oldest rocks found on Earth were even formed, indicating that the Moon has been largely geologically inactive for the last 2.5 billion years, except for occasional impact events (large impacts still occur sporadically: the 80-km-diameter crater Tycho was formed about 105 million years ago, not long (geologically) before a larger impact on Earth closed out the Mesozoic era and presumably killed off the dinosaurs). Because of this geologic stability, and the lack of water, weathering on the Moon is limited to impact "gardening" and darkening by solar radiation. As a result, Tycho looks very fresh. Geological processes on Earth, in contrast, have buried the younger Mesozoic-ending impact beneath thousands of feet of sedimentary deposits, making it visible only by geophysical prospecting methods.
Despite our having visited the Moon, the origins of some of its features still remain controversial, and much is yet to be learned. This is not surprising, considering that less than 80 hours of geologic field work have been conducted on the Moon. Though no manned missions are currently planned to go back to the Moon, recent robotics missions are giving our Moon a second look and are providing interesting discoveries such as the possible discovery of water ice at the south pole. An affordable way to map geologic variations of the lunar surface is with multispectral sensors-the Galileo spacecraft obtained a few images in seven different spectral bands as it flew past Moon in 1990 and again in 1992 on its way to Jupiter. The Clementine spacecraft mapped nearly the entire surface in seven bands as it orbited the Moon in 1994. These data are currently being processed to reveal compositional variations. Global altimetry data were also collected for the first time by Clementine. One of the significant discoveries from these data so far is the verification of a previously suspected impact basin near the south pole that is the largest known impact basin in the Solar System.