9.1.1 General Properties of the Moon

The Moon has only one-eightieth the mass of Earth and about one-sixth Earth’s surface gravity—too low to retain an atmosphere (Figure 9.1). Moving molecules of a gas can escape from a planet just the way a rocket does, and the lower the gravity, the easier it is for the gas to leak away into space. While the Moon can acquire a temporary atmosphere from impacting comets, this atmosphere is quickly lost by freezing onto the surface or by escape to surrounding space. The Moon today is dramatically deficient in a wide range of volatiles, those elements and compounds that evaporate at relatively low temperatures. Some of the Moon’s properties are summarized in Table 9.1, along with comparative values for Mercury.

9.1.2 Composition and Structure of the Moon

The composition of the Moon is not the same as that of Earth. With an average density of only 3.3 g/cm³, the Moon must be made almost entirely of silicate rock. Compared to Earth, it is depleted in iron and other metals. It is as if the Moon were composed of the same silicates as Earth’s mantle and crust, with the metals and the volatiles selectively removed. These differences in composition between Earth and Moon provide important clues about the origin of the Moon, a topic we will cover in detail later in this chapter.

Studies of the Moon’s interior carried out with seismometers taken to the Moon as part of the Apollo program confirm the absence of a large metal core. The twin GRAIL spacecraft launched into lunar orbit in 2011 provided even more precise tracking of the interior structure. We also know from the study of lunar samples that water and other volatiles have been depleted from the lunar crust. The tiny amounts of water detected in these samples were originally attributed to small leaks in the container seal that admitted water vapor from Earth’s atmosphere. However, scientists have now concluded that some chemically bound water is present in the lunar rocks.

Most dramatically, water ice has been detected in permanently shadowed craters near the lunar poles. In 2009, NASA crashed a small spacecraft called the Lunar Crater Observation and Sensing Satellite (LCROSS) into the crater Cabeus near the Moon’s south pole. The impact at 9,000 kilometers per hour released energy equivalent to 2 tons of dynamite, blasting a plume of water vapor and other chemicals high above the surface. This plume was visible to telescopes in orbit around the Moon, and the LCROSS spacecraft itself made measurements as it flew through the plume. A NASA spacecraft called the Lunar Reconnaissance Orbiter (LRO) also measured the very low temperatures inside several lunar craters, and its sensitive cameras were even able to image crater interiors by starlight.

The total quantity of water ice in the Moon’s polar craters is estimated to be hundreds of billions of tons. As liquid, this would only be enough water to fill a lake 100 miles across, but compared with the rest of the dry lunar crust, so much water is remarkable. Presumably, this polar water was carried to the Moon by comets and asteroids that hit its surface. Some small fraction of the water froze in a few extremely cold regions (cold traps) where the Sun never shines, such as the bottom of deep craters at the Moon’s poles. One reason this discovery could be important is that it raises the possibility of future human habitation near the lunar poles, or even of a lunar base as a way-station on routes to Mars and the rest of the solar system. If the ice could be mined, it would yield both water and oxygen for human support, and it could be broken down into hydrogen and oxygen, a potent rocket fuel.

9.1.3 The Lunar Surface: General Appearance

If you look at the Moon through a telescope, you can see that it is covered by impact craters of all sizes. The most conspicuous of the Moon’s surface features—those that can be seen with the unaided eye and that make up the feature often called “the man in the Moon”—are vast splotches of darker lava flows.

Centuries ago, early lunar observers thought that the Moon had continents and oceans and that it was a possible abode of life. They called the dark areas “seas” (maria in Latin, or mare in the singular, pronounced “mah ray”). Their names, Mare Nubium (Sea of Clouds), Mare Tranquillitatis (Sea of Tranquility), and so on, are still in use today. In contrast, the “land” areas between the seas are not named. Thousands of individual craters have been named, however, mostly for great scientists and philosophers (Figure 9.2). Among the most prominent craters are those named for Plato, Copernicus, Tycho, and Kepler. Galileo only has a small crater, however, reflecting his low standing among the Vatican scientists who made some of the first lunar maps.

We know today that the resemblance of lunar features to terrestrial ones is superficial. Even when they look somewhat similar, the origins of lunar features such as craters and mountains are very different from their terrestrial counterparts. The Moon’s relative lack of internal activity, together with the absence of air and water, make most of its geological history unlike anything we know on Earth.

9.1.4 Lunar History

To trace the detailed history of the Moon or of any planet, we must be able to estimate the ages of individual rocks. Once lunar samples were brought back by the Apollo astronauts, the radioactive dating techniques that had been developed for Earth were applied to them. The solidification ages of the samples ranged from about 3.3 to 4.4 billion years old, substantially older than most of the rocks on Earth. For comparison, both Earth and the Moon were formed between 4.5 and 4.6 billion years ago.

Most of the crust of the Moon (83%) consists of silicate rocks called anorthosites; these regions are known as the lunar highlands. They are made of relatively low-density rock that solidified on the cooling Moon like slag floating on the top of a smelter. Because they formed so early in lunar history (between 4.1 and 4.4 billion years ago), the highlands are also extremely heavily cratered, bearing the scars of all those billions of years of impacts by interplanetary debris (Figure 9.3).

Unlike the mountains on Earth, the Moon’s highlands do not have any sharp folds in their ranges. The highlands have low, rounded profiles that resemble the oldest, most eroded mountains on Earth (Figure 9.4). Because there is no atmosphere or water on the Moon, there has been no wind, water, or ice to carve them into cliffs and sharp peaks, the way we have seen them shaped on Earth. Their smooth features are attributed to gradual erosion, mostly due to impact cratering.

The maria are much less cratered than the highlands, and cover just 17% of the lunar surface, mostly on the side of the Moon that faces Earth (Figure 9.5).

Today, we know that the maria consist mostly of dark-colored basalt (volcanic lava) laid down in volcanic eruptions billions of years ago. Eventually, these lava flows partly filled the huge depressions called impact basins, which had been produced by collisions of large chunks of material with the Moon relatively early in its history. The basalt on the Moon (Figure 9.6) is very similar in composition to the crust under the oceans of Earth or to the lavas erupted by many terrestrial volcanoes. The youngest of the lunar impact basins is Mare Orientale, shown in Figure 9.7.

Volcanic activity may have begun very early in the Moon’s history, although most evidence of the first half billion years is lost. What we do know is that the major mare volcanism, which involved the release of lava from hundreds of kilometers below the surface, ended about 3.3 billion years ago. After that, the Moon’s interior cooled, and volcanic activity was limited to a very few small areas. The primary forces altering the surface come from the outside, not the interior.

9.1.5 On the Lunar Surface

“The surface is fine and powdery. I can pick it up loosely with my toe. But I can see the footprints of my boots and the treads in the fine sandy particles.” —Neil Armstrong, Apollo 11 astronaut, immediately after stepping onto the Moon for the first time.

The surface of the Moon is buried under a fine-grained soil of tiny, shattered rock fragments. The dark basaltic dust of the lunar maria was kicked up by every astronaut footstep, and thus eventually worked its way into all of the astronauts’ equipment. The upper layers of the surface are porous, consisting of loosely packed dust into which their boots sank several centimeters (Figure 9.8). This lunar dust, like so much else on the Moon, is the product of impacts. Each cratering event, large or small, breaks up the rock of the lunar surface and scatters the fragments. Ultimately, billions of years of impacts have reduced much of the surface layer to particles about the size of dust or sand.

In the absence of any air, the lunar surface experiences much greater temperature extremes than the surface of Earth, even though Earth is virtually the same distance from the Sun. Near local noon, when the Sun is highest in the sky, the temperature of the dark lunar soil rises above the boiling point of water. During the long lunar night (which, like the lunar day, lasts two Earth weeks¹), the temperature drops to about 100 K (–173 °C). The extreme cooling is a result not only of the absence of air but also of the porous nature of the Moon’s dusty soil, which cools more rapidly than solid rock would.

Learn how the moon’s craters and maria were formed by watching the below video, which was produced by NASA’s Lunar Reconnaissance Orbiter (LRO) team, about the evolution of the Moon, tracing it from its origin about 4.5 billion years ago to the Moon we see today. See a simulation of how the Moon’s craters and maria were formed through periods of impact, volcanic activity, and heavy bombardment.

9.1.6 The Ways Moons can Form

It is characteristic of modern science to ask how things originated. According to planetary scientists, in general, there are three ways we think any moon can form:

1. Co-accretion: This is when a moon forms at the same time as its central planet, accreting from the same cloud of swirling gas and dust.
2. Capture: This is when a moon forms elsewhere in the solar system but is captured by the gravitational pull of its soon-to-be central planet.
3. Giant impact: This i when a moon forms from the accretion of debris that is blasted off a planet due to a large impact/collision event.

The trick then becomes figuring out which of these origin stories are most likely true for each moon.

9.1.7 Origin of the Moon

While the giant impact hypothesis does not account for all of the evidence, it does align with the Moon’s orbital characteristics (see lecture) and offers potential solutions to most of the major problems raised by the chemistry of the Moon. First, since much of the Moon’s raw material was derived from the Earth’s stony mantle, the Moon’s lack of metals, lower density, and small core is easily understood. Second, in a giant impact scenario, most of the volatile elements in the debris would have been lost due to intense heating following the impact; when compared to Earth rocks, Apollo lunar samples are depleted in volatiles, which is consistent with the giant impact expectation. Yet, by such as identical abundances of various oxygen isotopes. Third, the Moon and Earth have nearly identical isotope^[1] ratios (like oxygen), which suggests they formed from a similar source material; if the Moon was mad primarily of terrestrial mantle material ejected from a big impact, this isotopic similarity makes sense. Other bodies in the solar system and meteorites do not share the same degree of similarity and show much higher variability. If the Moon and Earth formed together, this would explain why they are so chemically similar. Fourth, the lunar crust is made of a mineral called anorthosite, which arises from the slow cooling of a magma ocean. To form a magma ocean, a lot of heat is needed to melt all the material, and a giant impact could have provided that massive amount of heat. Although there are still outstanding questions, all of the above reasons have made the giant impact hypothesis remains the prevailing formation mechanism for the Moon.

Text Attributions

This text of this chapter is adapted from:

• Sections 9.1, 9.2, 9.4 of OpenStax’s Astronomy 2e (2022) by Andrew Fraknoi, David Morrison, and Sidney Wolff. Licensed under CC BY 4.0. Access full book for free at this link.
• Chapter 8 of An Introduction to Geology (2017) by Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, and Cam Mosher. Licensed under CC BY-NC-SA 4.0.

Media Attributions

• “How were the Moon’s Craters & Maria Formed?.” YouTube, uploaded by The Mad Scientist, 10 Mar 2014, https://www.youtube.com/watch?v=mIRPeYGKfic.