Highlands and lowlands

The highlands comprise countless overlapping craters, ranging in size from the smallest visible in photographs (1 meter on the best Apollo photographs) to more than 1000 km.

The maria cover 16% of the lunar surface and are composed of lava flows that filled relatively low places, mostly inside immense impact basins.

the mare must be considerably younger than the basins in which they reside. For example, the impact that formed the large Imbrium basin (the Man-in-the-Moon's right eye) hurled material outwards and sculpted the mountains surrounding the Serenitatis basin (the left eye); thus, Serenitatis must be older. The Serenitatis basin is also home to Mare Serenitatis. If the lava in Mare Serentatis formed when the basin did, they ought to show the effects of the giant impact that formed Imbrium. They show no signs of it.

Furthermore, the maria contain far fewer craters than do basin deposits, hence have been around a shorter time (the older the surface, the greater the number of craters).

One other type of deposit associated with the maria, though it blankets highlands areas as well, is known as dark mantle deposits. They cannot be seen except with telescopes or from spacecraft near the Moon, but are important nonetheless. Before Apollo, most scientists believed that the dark mantle deposits were formed by explosive volcanic eruptions known as pyroclastic eruptions (literally, 'pieces of fire'). Some deposits seemed to be associated with low, broad, dark cinder cones, consistent with the idea that they were formed by pyroclastic eruptions—this is how cinder cones form on Earth. This bit of geologic deduction was proven by the Apollo 17 mission and its sampling of the 'orange soil', a collection of tiny glass droplets like those found in terrestrial pyroclastic eruptions.

Some mysteries persist about the maria. For one, why are volcanoes missing except for the cinder cones associated with dark mantle deposits? Second, if no obvious volcanoes exist, where did the lavas erupt from? In some cases, we can see that lava emerged from the margins of enormous impact basins, perhaps along cracks concentric to the basin. But in most cases, we cannot see the places

where the lava erupted. Another curious feature is that almost all the maria occur on the Earth-facing side of the Moon. Most scientists guess that this asymmetry is caused by the highlands crust being thicker on the lunar farside, making it difficult for basalts to make it all the way through to the surface.

MOON ROCKS

the first highland rocks were collected during the first lunar landing, the Apollo 11 mission, which landed on a mare, Mare Tranquillitatis. Although most of the rocks collected were, indeed, basalts, some millimeter-sized rock fragments were quite different. They were composed chiefly of the mineral plagioclase feldspar; some fragments were composed of nothing but plagioclase. Such

rocks are called anorthosites. Some scientists suggested that these fragments were blasted to the Apollo 11 landing site by distant impacts on highland terrain. Thus, they argued, the highlands are loaded with plagioclase. This was a bold extrapolation confirmed by subsequent Apollo missions to highland sites.

How did they get that way? One way is to accumulate it by flotation in a magma (molten rock). This happens in thick subterranean magma bodies on Earth. So, plagioclase floated in a magma. But if ALL the lunar highlands are enriched in plagioclase, then the magma must have been all over the Moon. The early Moon must have been covered by a global ocean of magma, now commonly referred to as the lunar magma ocean.

Although some scientists still remain unconvinced about the veracity of the magma ocean hypothesis, nothing we have learned since has contradicted the idea that 4.5 billion years ago the Moon was covered by a layer of magma hundreds of kilometers thick.

An interesting thing about highland breccias, especially those we call impact melt breccias (rocks

partly melted by an impact event), is that most of them fall into a relatively narrow span of ages, from about 3.85 to 4.0 billion years.

Many highland breccias and a few igneous rocks are enriched compared to other lunar samples in a set of elements not familiar to most of us. The elements are those that tend not to enter the abundant minerals in rocks. The result is that as a magma crystallizes the part that is still liquid becomes progressively richer in these special elements. The rocks that contain them are called KREEP, for potassium (chemical symbol K), rare-earth elements (abbreviated REE), and phosphorus (P). Most Moon specialists believe that KREEP represents the last dregs from the crystallization of the magma ocean. Huge impacts dug down to the lower crust of the Moon and excavated it, mixing it with other debris to form KREEPy breccias.

THE DUSTY LUNAR SURFACE

The lunar surface is charcoal gray and sandy, with a sizable supply of fine sediment. Meteorite impacts over billions of years have ground up the formerly fresh surfaces into a powder we call the lunar regolith. the regolith is thin, ranging from about two meters on the youngest maria to perhaps 20 meters in the oldest surfaces in the highlands.

MOONQUAKES, THE MOON'S INTERIOR, AND THE MYSTERIOUS MAGNETIC FIELD

We know about moonquakes from four seismometers set up by the Apollo missions. Besides telling us how many and how strong moonquakes are, the data acquired by the Apollo seismic network help us figure out something about the nature of the Moon's interior. On Earth, seismology has allowed us to know that the planet has a thin crust (20-60 km over continents, 8-10 km over ocean basins), a thick silicate mantle (down to 2900 km), and a large metallic iron core (2900 km to the center at 6370 km). The Moon is quite different. The crust is thicker than Earth's continental crust, ranging from 70 km on the Earth-facing side to perhaps 150 km on the farside. The mare basalts represent a thin veneer on this mostly plagioclase-rich crust, averaging only about 1 km in thickness (inferred mostly from photogeological studies). Evidence from samples collected on the rims of the large basins Imbrium and Serentatis and from remote sensing instruments carried onboard two Apollo missions, the Clementine Mission, and the forthcoming Lunar Prospector Mission suggest that the lower crust may not contain as much plagioclase as does the upper half of the crust. Beneath the crust is the lunar mantle, which is the largest part of the Moon. There might be a difference in rock types above and below a depth of 500 km, perhaps representing the depth of the lunar magma ocean. Beneath the mantle lies a small lunar core made of metallic iron. The size of the core is highly uncertain, with estimates ranging from about 100 km to 400 km.

That little core is important, though. The Moon does not have much of a magnetic field, so the Lunar core is not generating magnetism the way Earth's core is. Nevertheless, it did in the past. Lunar rocks are magnetized, and the strength of the magnetic field has been measured by special techniques. Also, older rocks have stronger magnetism, suggesting that the Moon's magnetic field was stronger in the distant past, and then decreased to its weak present state. Why this happened is unknown. What is known is this: you cannot navigate around the Moon using a compass!

There are other mysteries about the Moon's magnetism. Although the field was always weak and is extremely weak now, there are small areas on the Moon that have magnetic fields much stronger than the surrounding regions. These magnetic anomalies have not been figured out. Some scientists have associated them with the effects of large, basin-forming impacts. Others have suggested that the ionized gases produced when comets impact the Moon might give rise to strong magnetic anomalies in the crater ejecta. The jury is still out. The Lunar Prospector Mission will thoroughly map the distribution of magnetic anomalies, perhaps helping to solve this mystery.

THE MOON'S ORIGIN: A BIG WHACK ON THE GROWING EARTH

One would think that an impact between an almost Earth-sized planet and a Mars-sized planet would be catastrophic. The energy involved is incomprehensible. Much more than a trillion trillion tons of material vaporized and melted. In some places in the cloud around the Earth, temperatures exceeded 10,000°C. A fledgling planet the size of Mars was incorporated into Earth, its metallic core and all, never to be seen again. Yes, this sounds catastrophic. But out of it all, the Moon was created and Earth grew to almost its final size. Without this violent event early in the Solar System's history, there would be no Moon in Earth's sky, and Earth would not be rotating as fast as it is because the big impact spun it up. Days might even last a year. But then, maybe we would not be here to notice.

A BRIEF HISTORY OF THE MOON

We know the general outlines of what happened to the Moon after it was formed by a giant impact. The first notable event, which may have been a consequence of the giant impact, was the formation and crystallization of the magma ocean. Nobody knows how deep it was, but the best guess is that it was at least 500 km deep. The first minerals to form in this mind-boggling magmatic system were the iron and magnesium silicates olivine and pyroxene. They were denser than the magma, so they sank, like rocks in a pond, though not as fast. Eventually, plagioclase feldspar formed, and because it was less dense than the magma, began to float to the top, like bubbles in cola. It accumulated and produced mountains of anorthosite, producing the first lunar crust. The magma ocean phase ended by about 4.4 billion years ago.

Almost as soon as the crust had formed, perhaps while it was still forming, other types of magmas that would form the norites and troctolites in the highlands crust began to form deep in the Moon. A great mystery is where inside the Moon and how deep. Many lunar specialists believe the magmas derived from unmelted Moon stuff beneath the magma ocean. In any case, these magmas rose and infiltrated the anorthosite crust, forming large and small rock bodies, and perhaps even erupting onto the surface. Some of the magmas reacted chemically with the dregs of the magma ocean (KREEP) and others may have dissolved some of the anorthosite. This period of lunar history ended about 4.0 billion years ago.

All during these first epochs, left-over projectiles continued to bombard the Moon, modifying the rocks soon after they formed. The crust was mixed to a depth of at least a few kilometers, perhaps as much as 20 km, as if a gigantic tractor had plowed the lunar crust. Though not yet proven, the rate of impact may have declined between 4.5 and 4.0 billion years ago, but then grew dramatically, producing most of the large basins visible on the Moon. This cataclysmic bombardment is postulated to have lasted from 4.0 to 3.85 billion years ago.

Once the bombardment rate had settled down, the maria could form. Basalts like those making up the dark mare surfaces formed before 3.85 billion years ago, but not as voluminously as later, and the enormous bombardment rate demolished whatever lava plains formed. However, between 3.7 and about 2.5 billion years ago (the lower limit is highly uncertain), lavas flowed across the lunar surface, forming the maria and decorating the Moon's face. Along with the basalts came pyroclastic eruptions, high fountains of fre that launched glowing droplets of molten basalt on flights up to a few

hundred kilometers.

Since mare volcanism ceased, impact has been the only geological force at work on the Moon. Some

impressive craters have been made, such as Copernicus (90 km across) and Tycho (85 km). These flung bright rays of material across the dark lunar landscape, adding more decoration. In fact, some of the material blasted from Tycho caused a debris slide at what would become the Apollo 17 landing site. Samples from this site indicate that the landslide and some associated craters formed about 110 million years ago. This, therefore, is the age of the crater Tycho. It is a triumph of geological savvy that we were able to date an impact crater that lies over 2000 km from the place

we landed! The impacts during the past billions of years also have mixed the upper several meters of crust to make the powdery lunar regolith. The Sun has continued to implant a tiny amount of itself into the regolith, giving us its cryptic record and providing resources for future explorers. And recently, only seconds ago in geologic time, a few interplanetary travellers left their footprints here and there on the dusty ground.

THE MOON AND EARTH: INEXORABLY INTERTWINED

The Moon ought to be especially alluring to people curious about Earth. The two bodies formed near each other, formed mantles and crusts early, shared the same post-formational bombardment, and have been bathed in the same flux of sunlight and solar particles for the past 4.5 billion years. Here are a few examples of the surprising ways in which lunar science can contribute to understanding how Earth works and to unravelling its geological history.

Origin of the Earth-Moon System: No matter how the Moon formed, its creation must have had dramatic

effects on Earth. Although most scientists have concluded that the Moon formed as a result of an

enormous impact onto the growing Earth, we do not know much about the details of that stupendous event. We do not know if the Moon was made mostly from Earth materials or mostly projectile, the kinds of chemical reactions that would have taken place in the melt-vapor cloud, and precisely how the Moon was assembled from this cloud.

Magma oceans: The concept that the Moon had a magma ocean has been a central tenet of lunar science since it sprung from fertile minds after the return of the first lunar samples in 1969. It is now being applied to Earth, Mars, and asteroids. This view of the early stages of planet development is vastly different from the view in the 1950s and 1960s. Back then, most (not all) scientists believed the planets assembled cold, and then heated up. The realization that the Moon had a magma ocean changed all that and has led to a whole new way of looking at Earth's earliest history.

Early bombardment history of Earth and Moon: The thousands of craters on the Moon's surface chronicle the impact record of Earth. Most of the craters formed before 3.9 billion years ago. Some scientists argue that the Moon suffered a cataclysmic bombardment (a drastic increase in the number of impacting projectiles) between 3.85 and 4.0 billion years ago. If this happened and Earth was subjected to the blitzkrieg as well, then development of Earth's earliest crust would have been affected. The intense bombardment could also have influenced the development of life, perhaps delaying its appearance.

Impacts, extinctions, and the evolution of life on Earth: The mechanisms of evolution and mass

extinctions are not understood. One possibility is that some mass-extinction events were caused by

periodic increases in the rate of impact on Earth. For example, the mass extinctions, which included the demise of the dinosaurs, at the end of the Cretaceous period (65 million years ago), may have been caused by a large impact event. Attempts to test the idea by dating impact craters on Earth are doomed because there are too few of them. But the Moon has plenty of craters formed during the past 600 million years (the period for which we have a rich fossil record). These could be dated and the reality of spikes in the impact record could be tested.

How geologic processes operate: The Moon is a natural laboratory for the study of some of the geologic processes that have shaped Earth. It is a great place to study the details of how impact craters form because there are so many well-preserved craters in an enormous range of sizes. It is also one of the places where volcanism has operated, but at lower gravity than on either Earth or Mars.

LIFE AND WORK AT A MOON BASE