The Moon

In this guide, you will learn about the Moon, its physical properties, its effects on Earth, and its geological history.

The Moon is a natural satellite of Earth. It’s the fifth largest satellite in the solar system, and the largest satellite relative to the planet it orbits. Aside from lighting up the night sky, it serves many functional purposes that make life on Earth possible.

Fast Facts

The Moon’s Formation

The Moon was born in a relatively early, violent period of the solar system’s history, when enormous amounts of matter were colliding regularly and planets were being formed.

As humanity wasn’t around at the time of the Moon’s formation, it is impossible to fully understand how the Earth-Moon system was created. There are several different theories.

The Giant-Impact Theory

The giant-impact theory is a prevailing theory explaining the Moon’s creation. Scientists theorize that an ancient planet called Theia, roughly the size of Mars, collided with the Earth. This impact vaporized Theia, and possibly most of the proto Earth. It left a massive ring of molten debris floating in space¹Asphaug, E. (2014). Impact Origin of the Moon? Annu. Rev. Earth Planet. Sci, 2014(42).

Over time, gravitational forces pulled the debris together, forming the moon.

Computer simulations of the giant-impact event suggest that this theory is both possible and likely, compared to other posited explanations. However, one point of contention with the giant-impact theory is that samples analyzed from the Moon show that the moon and the Earth have very similar geological compositions.

If the debris from Theia created the Moon, most of the material would have had to come from Theia itself. We would expect the Moon’s composition to differ more from Earth’s composition than it does.

The giant-impact theory is also supported because the Moon is very “dry” – the materials that easily vaporize are not present on the Moon’s surface. This suggests that a high energy impact event must have occurred.

Finally, the energy behind a giant impact would explain both the Earth’s rotation and the fact that the Moon has enough momentum to be slowly drifting away from the Earth.

Possible Explanations of Similar Composition

If the giant-impact theory is correct, the impacting planet Theia may have been formed similarly to Earth, perhaps from the same accretion disk (an enormous structure of materials orbiting a central body). If the two bodies were very similar to begin with, this explains the geological similarities.

Alternatively, the impact may have been strong enough to completely vaporize much of Earth and the outside body. This could result in the formation of a synestia, a doughnut shaped disk or ring of similar materials. The Earth was re-formed, and the Moon could have been formed from the similar materials in this synestia.

Although the exact mechanisms are unknown, given our current level of understanding, some type of giant-impact event between Theia and the Earth is the most likely explanation for our Moon’s formation.

More Recent Development (Two Impacts?)

More recent models (2021 and later) suggest that Theia may have hit the Earth twice. The first time, a “hit and run” in which Theia hit Earth at a rapid speed, and continued off into space. Then, Theia may have returned hundreds of thousands of years later to directly impact Earth at a much slower speed, relatively speaking. The two-impact theory may be more likely, as it explains how Theia could slow down enough to form the Moon on the second impact, which better fits simulations²Wall, M. (2021). Ancient impact that formed Earth’s moon… Space.com.

Unlikely Scenarios

There are many other possibilities explaining how the Moon was created. Below are some theories which are regarded to be incorrect or outdated.

Co-accretion Theory

The co-accretion theory suggests that the Moon and the Earth were formed in tandem from the same cloud of materials. Eventually, Earth’s dominant gravity allowed it to capture the Moon, and the Moon orbited the Earth.

While this explains the similarities in composition between the Earth and Moon, it does not hold up in modeling. The Moon has significant angular momentum; it is moving away from the Earth. A significant amount of force must have been placed upon the Moon to create this momentum. This theory does not explain where this momentum could have come from, nor does it explain why the Earth rotates as it does.

Capture Theory

The capture theory posits that the Moon was created away from the Earth, and that it eventually came close enough for the Earth’s gravitational forces to “capture” it. This theory is virtually impossible, as the Moon is far too large for Earth to capture it. Even asteroids, which are many times smaller than the Moon, rarely remain in Earth’s orbit for more than a few months at a time before being flung back out into space. If an object the size of our Moon came flying past Earth, it would just keep going.

This theory also doesn’t explain the similarities in geological composition.

The Fission Theory

The fission theory suggests that the Earth was spinning incredibly fast, throwing molten rock out into space, which eventually combined to form the Moon. While this explains the similar compositions, the theory fails because we don’t believe the Earth could have been spinning fast enough to achieve this. Even if it had, the material would have eventually fallen back to Earth, rather than forming a stable Moon.

The Moon’s Geology

Geology is the study of the physical structure, composition, and inner workings of celestial bodies. Like the Earth, the Moon is mostly made of rock.

Surface Craters

During the Moon’s first 600 million years of existence, it was bombarded with large meteors and comets from the chaotic growing solar system. After that point, things settled down. Now, the collisions that occur are much smaller.

Whenever an object crashes into the moon, a crater is formed. The craters may be microscopic, or they may be miles wide. The resulting crater size depends on the size and speed of the impacting object, which can travel upwards of 102,000 kph (64,000 mph).

Thousands of meteorites impact the Moon’s surface each year. Most meteorites are very small, with the larger ones being around the size of a ping-pong ball. The bulk of impacts are so small that they go undetected. Since the Moon has an extremely thin atmosphere, meteorites don’t burn up the way they would in Earth’s atmosphere. They make it to the surface.

Occasionally, larger impacts occur, which can be recorded and measured. The largest recorded meteorite impact on the Moon in modern history was estimated to be about 400kg (881lbs). It was recorded as a white flash on Sept 11th, 2013 with Spanish telescopes and left a crater 40 meters wide⁶Gannon, M. (2014). Record Breaking Meteorite… Space.com. This is still extremely small compared to the massive craters formed in the Moon’s earlier days.

The largest crater, The South Pole–Aitken basin, is 2,500km (1,600 mi) wide⁷G. Jeffrey Taylor. (1998). The biggest hole in the solar system. Planetary Science Research Discoveries Hawaii.edu. For comparison, that’s roughly the same distance across the United States from the southernmost point of Texas to the Canadian border, or the distance from the southernmost point of Spain to the northmost point of Scotland. The meteor that created this crater may have been as large as 200km (124mi) wide and must have been traveling very fast. 

If the impact was large enough, the borders of the crater form lunar mountain ranges. The highest mountain on the Moon, Mons Huygens, is over half the height (62% height) of Mt. Everest at 5.5km (3.4mi). It was formed from an impact event.

When meteorites, asteroids, or comets hit the moon, they can leave behind trace amounts of the minerals they contain, spread out across a wide area⁸Campbell, B. (2013) A meteorite explodes on the Moon: Q&A… Smithsonian Insider.

Layers of the Moon

The Moon is a rocky body made of layers, compacted on top of one another⁹Inside the Moon. Nasa.gov (Accessed Jun-26-2022). If you’re familiar with the layers of the Earth, the layers of the Moon are somewhat similar.

Crust

The exterior surface layer of the Moon is called the crust. It’s covered in a dusty soil-like material known as regolith. The regolith was formed from years of lunar impacts, big and small. The thickness of the crust varies from place to place. It’s estimated to be about 50km (31mi) thick on average. The far side of the moon has a thicker crust than the side that faces the Earth.

Lunar regolith is contaminated when it enters the Earth’s atmosphere, and is extremely limited in quantity, so synthetic materials called lunar regolith simulants are created to mimic the properties of lunar regolith in labs. Researchers can use it to study the properties and potential uses of lunar regolith without using lunar actual regolith itself.

Mantle

The second layer under the crust is the mantle. It’s the thickest layer of the Moon at roughly 1350km (840mi) in depth.

By analyzing basalt (volcanic) rocks that erupted from deep within the Moon, scientists have determined that the mantle is composed of olivine and pyroxene, which are common minerals also found in Earth’s mantle.

Partial Melt

Since the mantle is made of many minerals, some melt at lower temperatures than others. Partial melting occurs when a portion of a solid is melted. The bottom portion of the mantle contains partial melt.

The Core

The core is at the center of the Moon. It’s thought to be relatively small, about 660km in total diameter. It’s much smaller than the Earth’s core. Earth’s core is about 33% of Earth’s total mass, and the Moon’s core is only about 1% to 3% of the Moon’s total mass. It can be further divided into two parts: the outer and the inner core.

Moonquakes have been detected, which may indicate flowing magma deep inside the Moon. The outer core is fluid molten liquid.

The inner core is solid metal, mostly iron and some nickel. It’s estimated to be about 480km or 300mi in diameter. 

Despite the Moon’s active core, there is no present volcanic activity on the Moon’s surface. This may be because the magma is too dense to rise to the surface. 

Due to the low gravity, volcanoes created wide plateaus on the Moon, rather than the cone-like mountains of Earth. Mons Rümker is one example of a plateau that formed as a result of volcanic activity. Its top altitude is about 1.1km (0.68mi).

Water on the Moon

Researchers have long speculated that there may be water on the Moon as ice. The first direct evidence of water’s presence was detected through spectrometry during the Apollo 14 mission in 1971, when water ions were found.

In 2008, instruments on India’s Chandrayaan-1 spacecraft detected water ice at the lunar poles. An impact probe also detected minimal amounts of water in the Moon’s thin atmosphere. This was the first time proof of water’s existence was directly detected.

In 2020, Chang’e 5, a Chinese spacecraft which landed on the Moon, directly found water on the Moon’s surface in physically collected samples.

We now understand definitively that water exists on the Moon. Because of the Moon’s incredibly thin atmosphere, most water is lost quickly to vaporization, leaving only trace amounts in the atmosphere. The remaining water exists as ice, mostly found at the poles or in heavily shadowed regions of craters.

It’s currently estimated that about 600 million metric tons of water exist on the Moon¹⁰Nasa. Nasa radar finds ice deposits…. With upcoming missions to the Moon, both manned and unmanned, researchers expect more water to be found.

Lunar Phases & Events

You’re likely aware that the Moon has different phases, such as a full Moon, when the Moon is fully illuminated, and a new Moon, when it’s completely dark. Lunar phases are used to describe what portion of the Moon is lit.¹¹USRA.edu. Moon Phases.

Just as the Earth always has a light and a dark side (day and night), the Moon is also always half illuminated by the Sun. As the Moon revolves around the Earth, the amount of sunlight we can see from the Earth’s surface changes. The Moon’s phases are not the result of the Earth casting a shadow on the Moon (that would be a lunar eclipse).

It takes the Moon a little over 27 days to complete one revolution around the Earth. This period is known as the sidereal month. However, it takes an average of about 29.5 for the Moon to go through all of its phases, the lunar or synodic month. The reason a synodic month is longer than a sidereal month is because the Earth-Moon system is also revolving around the sun. The relative angles are always changing as the distance around the Sun changes, so it takes slightly longer to complete one full lunar phase cycle than it does for the Moon to complete one revolution around the Earth. The actual length of a lunar/synodic month varies throughout the year.

The orientation or rotation of the Moon in the sky will appear to change depending on where you’re viewing it from on Earth.

As the Moon goes through its phases, it appears to be shrinking or growing in the night sky. When it’s growing, it’s said to be waxing. When it’s shrinking, it’s said to be waning.

The Moon has four principal phases and four intermediate phases. The four principal phases are the new moon, first quarter, full moon, and third or last quarter.

The Moon has four intermediate phases in between the principal phases. They are waxing crescent, waxing gibbous, waning gibbous, and waning crescent.

The full order of phases looks like this:

1. New Moon – Moon is dark, completely in shadow
2. Waxing Crescent – A growing crescent shape
3. First Quarter – About half the moon is lit
4. Waxing Gibbous – More than half the moon is lit, still growing
5. Full Moon – The moon is fully lit
6. Waning Gibbous – More than half the moon is lit, shrinking now
7. Last Quarter – Half the moon is lit, still shrinking
8. Waning Crescent – Getting increasingly tiny, crescent shape
9. New Moon – Start over from the beginning

*Moon phases were rendered and may not be 100% true to life

Lunar Eclipses

When the Earth, Moon, and Sun are in the proper position, a lunar eclipse can occur. A lunar eclipse in an event in which the surface of the Moon is blocked out by Earth’s shadow. There are, on average, three lunar eclipses per year. The Moon does not appear completely black during a lunar eclipse, but reddish, because of the Sunlight refracting around Earth’s atmosphere. Moon is still lit, just not directly.

A total lunar eclipse happens when the entirety of a full moon is covered by Earth’s innermost shadow. Slightly less than a third (~29%) of lunar eclipses are total lunar eclipses.

More common are partial lunar eclipses, which happen when only part of the Moon is covered by Earth’s shadow. During partial lunar eclipses, only part of the Moon becomes red.

This diagram depicts a total lunar eclipse, not to scale. The Moon is exactly opposite the Sun from the Earth. The umbra is Earth’s innermost shadow, the darkest part of the shadow, where light is completely blocked by the Earth. The penumbra is the shadow region where only part of the sun’s light is obscured by the Earth.

Solar Eclipses

Solar eclipses occur when the shadow of the Moon completely blocks out the sun. These events are rarer than lunar eclipses. Just as there are partial and total lunar eclipses, there are partial and total solar eclipses. Since the Moon is significantly smaller than the Earth, the shadow it casts is also smaller.

Even when solar eclipses occur, they can only be seen from specific areas of the Earth at a time. So even if a solar eclipse is happening, you might not see it from where you are on Earth. A total solar eclipse happens once every 18 months, but only once roughly every 375 years at the same location.

As with lunar eclipses, solar eclipses involve an umbra and penumbra shadow region. The umbra cast by the Moon during a solar eclipse is much smaller than the umbra cast by the Earth in a lunar eclipse.

Orbit, Tides, & Gravity

A phenomenon known as tidal locking occurs when a body takes the same time to rotate as it does to complete one orbit around its partner. The Moon is tidally locked to the Earth. Therefore, one side of the Moon is always facing the Earth, and the opposite side is always facing away. So the physical features of the Moon look the same regardless of where and when you view it from Earth.

Unless you possess a spaceship, you’ve never seen the far side of the Moon outside photographs. The only people who have seen the far side of the Moon in person were the astronauts of some of the Apollo missions during orbit. Some refer to the far side of the Moon as the “dark side” of the Moon, but in reality it gets as much light as the near side of the Moon. We just can’t see it.

Orbit

The Moon follows an elliptical orbit around the Earth. It’s not a perfect circle, so sometimes the Moon is farther away, and at other times it is closer. The average distance is 384,467 km (238,896 mi) from the Earth’s center to the Moon’s center. That distance is about thirty times the diameter (width) of the Earth.

The Moon is at perigee when it’s closest to the Earth. At perigee, the Moon is 363,104 km (225,623 mi) from center to center.

When the Moon is farthest away, it’s at apogee. The apogee distance is 405,696 km (252,088 mi) away.

The radius of the Moon’s orbit gets larger by about 3.8 centimeters per year. The Moon is drifting farther away from us. When the Moon was first created 4.5 billion years ago, the Moon was much closer to the Earth. If it was possible to view the Moon from the Earth back then (it wasn’t), the Moon would have appeared many times larger in the sky than it does today.

Tides

If you’ve ever been to the beach, you’ve likely seen tides in action. At high tide, the ocean level rises, the waves come in farther, and more of the beach is covered in water. At low tide, the ocean recedes, and appears farther out. More of the beach is exposed. We have the Moon to thank for much of this phenomenon.

The Earth’s gravity pulls on the Moon, but the Moon’s gravity also pulls on the Earth. Tidal forces are effects resulting from the gravitational pull of one body on another. As the Moon orbits the Earth, the Moon’s gravity pulls on certain parts of the Earth more than others. There is a differential in gravitational force from one location to another.

The tidal forces cause the Earth, and its oceans, to swell or stretch towards the Moon on whatever side of the Earth the Moon currently is closest to. The Moon is pulling harder on the oceans than it is the Earth below. This creates a high tide at the side nearest the Moon. The sea level rises during high tide on the side of Earth facing the Moon.

On the side of Earth opposite the Moon, another high tide is created. Here, the high tide is created as a result of the Moon pulling on the Earth itself more than it pulls on the water.

The remaining water, perpendicular to the Moon’s position from Earth, experiences a low tide.

The Sun also exerts tidal forces on the Earth, but to a lesser extent than the Moon, since the Sun is so much farther. During a total solar eclipse, the combined pull from the Sun and Moon together can result in a slightly higher tide than usual at the location the eclipse is occurring.

Since the Earth rotates nearly once every 24 hours, any given area of the ocean will experience two high and two low tides per day.

The Moon Stabilizes Earth’s Axis

The Earth’s axis, from the south to the north pole, tilts between 22.1 and 24.5 degrees throughout the year. It “wobbles” on its axis. The change in angle gives us the different seasons in the northern and southern hemispheres.

The gravity from the Moon stabilizes this wobble and prevents the axis from tilting further. Without the Moon, the Earth’s axis would tilt much more unpredictably. The effects on climate would be catastrophic. While life may have still been possible without the Moon, it would be a much, much different world than what we know today.

History of the Moon

As you know, the Moon has been around for a long time. It looks a lot different today than it did billions of years ago. The timeline of the Moon’s history can be roughly divided into different periods on the lunar geological timescale. The periods are based on significant lunar events.

Pre-Nectarian Period (4.53-3.92 Billion Years)

The Pre-Nectarian period is the first on the timescale. It runs from 4.53 to 3.92 billion years ago.

Researchers believe the Moon was created after a Mars-sized object called Theia collided with the Earth. This made the Moon extremely hot. It took billions of years to cool down, and its core is still cooling to this day.

The Magma Ocean (About 4.5-4.3 Billion Years Ago)

The impact energy from the creation of the Moon heated the materials to extremely high temperatures, vaporizing nearly all of Theia and likely a huge part of the proto Earth. After a short time on the cosmic timeline, perhaps under a hundred years, the remaining molten rock and debris around the Earth accreted to form the Moon¹²URSA.edu. Moon Formation.. At this point, the Moon is thought to have been fully covered in an ocean made of magma.

This ocean of magma had a depth of 1000km¹³Elkins-Tanton L. (2011). The lunar magma ocean… Earth and Planetary Science Letters. and temperatures above 1900K (1626°C or 2960°F). The denser minerals, like heavy liquid iron, would have sunk to the bottom of the magma ocean. The high rotational energy of the Moon resulted in a slow cooling process. The magma ocean remained for tens, possibly hundreds of millions of years before finally solidifying. A 2020 study¹⁴M. Maurice. A long-lived magma ocean on a young Moon. Sci Adv. 2020 Jul suggests it took between 150 and 200 million years, in total.

The presence of a magma ocean is evidenced by the fact that researchers found high purity minerals consistent with a magma ocean after analyzing rock samples from lunar expeditions.

The Moon was still being hit with large meteors throughout this period.

Bombardment (4.51-3.8 Billion Years Ago)

Throughout the early history of the solar system, during and after the magma ocean, the Moon was bombarded with large asteroids. These impacts gave rise to the craters and mountain ranges across the Moon’s surface.

The oldest crater, the South Pole-Aitken basin, is estimated to have been formed about 4.2 to 4.3 billion years ago, relatively soon after the magma ocean solidified.

Late Heavy Bombardment (4.1-3.8 Billion Years Ago)

It’s theorized that during this time period, the inner solar system was littered with giant objects and high-energy impact events were increasingly common¹⁵G. Jeffrey Taylor. Wandering Gas Giants and Lunar Bombardment. Planetary Science Research Discoveries Hawaii.edu.

Mare Nectaris, a lunar lava plane, was created during the late heavy bombardment period of the solar system. It was formed by volcanic activity induced by a lunar impact on the opposite side of the Moon 3.8–3.9 billions year ago. The start of this event roughly marks the end of the pre-Nectarian period.

Nectarian Period (3.92 to 3.85 Billion Years Ago)

The Nectarian Period is the time during which the Nectaris basin (Mare Nectaris, the “sea of nectar”) and other significant basins were formed by impact events. This time still coincides with late heavy bombardment.

Imbrian Period (3.85 to 3.20 Billion Years Ago)

The Imbrian period is the time during which the Imbrium basin was created. The Imbrium Basin is 1,146km (712 mi) in diameter, making it the Moon’s second largest basin, and the largest created by an impact event. It occupies much of the visible side of the Moon. This time period is further divided into the Early Imbrian Period (3.85 to 3.8 billion years ago) and the Late Imbrian Period (3.8 to 3.2 billion years ago).

The Early Imbrian Period overlaps with the end of Late Heavy Bombardment. This period began the moment a huge impactor collided with the Moon and created the Imbrium Basin¹⁶D. Wilhelms, J. McCauley, N. Trask. (1987) The geologic history of the Moon. The impactor may have been another proto planet. Not as large as Theia, but still enormous.

The start of this period was determined by analyzing well preserved samples taken from the Apollo 14 mission, which landed at the Fra Mauro Formation. The Fra Mauro Formation was created from ejecta of the Imbrian impact event.

The Early Imbrian Period ended when the Hevelius Formation came to rest.

The Late Imbrian Period is marked as a time when many of the Moon’s basins partially melted and were filled with basalt (volcanic rocks). It started when the Hevelius Formation came to rest and ended when the Imbrian System finished solidifying¹⁷D. Wilhelms, J. McCauley, N. Trask. (1987) The geologic history of the Moon.. The exact cause of this is unknown, but may be due to either bombardment thinning out the outer layer of the Moon, or the mantle rising and bringing molten material up with it.

Eratosthenian Period (3.2 to 1.1 Billion Years Ago)

The Eratosthenian period is the longest on the lunar geological timescale and is named after the Eratosthenes crater. During this period, the Eratosthenes crater and similar craters were formed, and volcanism on the Moon slowly ended. The start of this time period roughly coincides with the impact event that created the Eratosthenes crater, about 3.2 billion years ago, but this event does not definitively define the start of this period. Rather, this period is defined as the time when craters similar to the Eratosthenes crater were formed.

These craters do not have visible ray systems, ejecta thrown out from the crater’s center in ray or web-like patterns.

Copernican Period (1.1 Billion Years Ago to Today)

The current period on the lunar geological timescale is the Copernican Period. This period is defined by the newest craters formed with presently visible ray systems. Ray systems can be spread out from the impact site as little as a few meters to as far as hundreds of kilometers. They slowly erode or disappear over time.

The Giordano Bruno crater is one example of a newer crater with a visible ray system. Do you see all the light white rays coming out of the sides?

The ray system is even more clear when you view the crater from top down.

The Future

The Moon will continue moving away from the Earth at a rate of 3.78cm per year. It will never fully leave us, or be flung off into space. At the current rate, the Moon would eventually settle into a fixed orbit and stop moving away in about 15 billion years. However, the Sun is expected to swell into a red giant and destroy both the Moon and the Earth in about 7.5 billion years. So the Moon will never reach this final orbiting position.

For the foreseeable future, the Moon will continue to be hit with small meteorites. The overall surface of the Moon shouldn’t change much. Slightly larger impacts may create new craters, but none on the scale of that during the Moon’s early history.

Humanity continues to explore the Moon with robots and orbiting satellites. Manned missions to the Moon may resume within the next decade. NASA has started the Artemis missions, which will see more humans visiting the Moon relatively soon. Hopefully, as quickly as 2026, though the dates keep getting pushed back.

Certainly within the next decade, NASA or other space programs will see that humanity returns to the Moon. China’s space program may also be working on manned lunar missions.

With more boots on the Moon, our knowledge of this celestial body will continue to grow. With the incredible ongoing pace of technological development, it’s quite an exciting time to be alive!

Footnotes & References

• 1
  Asphaug, E. (2014). Impact Origin of the Moon? Annu. Rev. Earth Planet. Sci, 2014(42).
• 2
  Wall, M. (2021). Ancient impact that formed Earth’s moon… Space.com
• 3
  For more information on specific sample compositions, see this article on The Chemical Composition of Lunar Soil from Washington University in St. Louis
• 4
  Moon facts (2021). Space.com
• 5
  Turkevich, A. (1973). The Surface of the Moon, p.64
• 6
  Gannon, M. (2014). Record Breaking Meteorite… Space.com
• 7
  G. Jeffrey Taylor. (1998). The biggest hole in the solar system. Planetary Science Research Discoveries Hawaii.edu
• 8
  Campbell, B. (2013) A meteorite explodes on the Moon: Q&A… Smithsonian Insider
• 9
  Inside the Moon. Nasa.gov (Accessed Jun-26-2022)
• 10
  Nasa. Nasa radar finds ice deposits…
• 11
  USRA.edu. Moon Phases.
• 12
  URSA.edu. Moon Formation.
• 13
  Elkins-Tanton L. (2011). The lunar magma ocean… Earth and Planetary Science Letters.
• 14
  M. Maurice. A long-lived magma ocean on a young Moon. Sci Adv. 2020 Jul
• 15
  G. Jeffrey Taylor. Wandering Gas Giants and Lunar Bombardment. Planetary Science Research Discoveries Hawaii.edu
• 16
  D. Wilhelms, J. McCauley, N. Trask. (1987) The geologic history of the Moon.
• 17
  D. Wilhelms, J. McCauley, N. Trask. (1987) The geologic history of the Moon.