SCIENCE5 - Our Moon and its Mysteries

I still remember watching on TV in 1969 when Neil Armstrong became the first man to walk on the Moon.  I felt awe and wonder for that great achievement, and as an aerospace engineer, appreciated the science and engineering excellence behind the feat.  We’ve learned a lot about the Moon before and since, but many mysteries remain.  

 

I’m going to review the Moon’s history and science in terms of how it was formed, its physical characteristics, how the Earth - Moon relationship works, and Man’s observations and explorations of the Moon.  Then I’ll switch gears and talk about some little-known facts about the Moon, and finally, about the scientific questions that still remain to be answered.

My principal sources for this article include an outstanding, well researched Wikipedia article, “The Moon;” “Fact About the Moon,” Stacker.com; “List of Missions to the Moon,” Wikipedia; “10 Things You Didn’t Know About the Moon,” space.com; “Mysteries of the Moon:  What We Still Don’t Know After Apollo,” space.com; and “Lunar Mysteries That Science Still Needs to Solve,” wired.com; plus, numerous other online sources.

Introduction to the Moon

The Moon is Earth's only natural satellite at about one-quarter the diameter of Earth.  The Moon orbits the Earth once a month, on a slightly elliptical path at an average distance of 240,000 miles.  We only see one side of the Moon because it rotates around its axis in exactly the same time as it takes to orbit the Earth (more on this later).  The surface area of the Moon is slightly less than the areas of North and South America combined.  Gravity on the Moon’s surface is about one-sixth of Earth's.

The Moon has long been important in our culture and life.  Since prehistoric times, many cultures have personified the Moon as a deity.  The Moon's regular phases make it a convenient timepiece, and the periods of its waxing and waning form the basis of many of the oldest calendars.  Many festivities celebrate the Moon, particularly the “Full Moon” in the autumn called “Harvest Moon.”  The contrast between the Moon’s brighter and darker areas creates the patterns seen by different cultures as the “Man in the Moon.”  The Moon has long been associated with romance and is prominently featured in art and literature. The Moon also has been associated with insanity and irrationality; the words “lunacy” and “lunatic” (popular shortening “loony”) are derived from the Latin name for the Moon, “Luna.”

The Moon’s Formation 

Prevailing theories say that our universe was created from a single point with infinite density by the “Big Bang” explosion, roughly 13.8 billion years ago.  After an initial expansion, the Universe cooled sufficiently to allow the formation of subatomic particles, and later simple atoms.  Giant clouds of these earliest elements later coalesced through gravity to form galaxies, stars, and planets.  Earth formed about 4.54 billion years ago by accumulation from a cloud of dust and gas left over from the formation of the Sun.

Again, the prevailing theory is that the Moon formed after a giant impact of a Mars-sized body (called Theia) with the Earth about 4.425 billion years ago.  The collision tore away a chunk of the Earth and flung it into orbit, where it morphed into a massive ring of vaporized dust and rock that began to clump together.  Computer simulations have shown a range of 35-200 million years for the Moon to solidify into a spherical mass covered with a molten ocean, beginning its long cooling period.

 

The Moon was formed from the impact of a Mars-size body with Earth

Physical Characteristics of the Moon

The figure below will be useful in the discussion of the physical characteristics of the Moon.

 

For reference, here are photos of the near side (left) and far side (right) of the Moon.

Geology.  Today, after billions of years of cooling, the Moon has a distinct core, mantle, and crust.  The dense, solid iron-rich inner core has a radius of about 150 miles, surrounded by a molten outer core, primarily made of liquid iron, with a thickness of about 55 miles.  Above the core is the Moon’s mantle with a thickness of roughly 840 miles, with a partially melted layer of about 150 miles thickness below a solid layer of about 690 miles thickness.  Finally, above the mantle, is the crust, which has an average thickness of about 30 miles.

After the Moon’s formation, the molten ocean above the core started to cool and crystals began to form.  Denser mineral crystals sank down to the bottom of the ocean.  Lighter minerals crystalized and floated to the surface and began to form the upper mantle and the Moon’s crust.

 

Today, the inside of the Moon has a distinct core, mantle, and crust.

The far side of the lunar surface is on average about 1.2 miles higher than that of the near side, because the far side of the Moon has a much thicker crust.

Impact Craters.  A major geologic process that has affected the Moon's surface is impact cratering, with craters formed when asteroids and comets collided with the lunar surface.  There are estimated to be roughly 300,000 craters wider than 0.6-mile on the Moon's near side.  The lack of an atmosphere, weather, and recent volcanic activity, means that many of these craters are well-preserved.

There are more impact craters on the far side of the Moon than on the near side because, over hundreds of millions of years, the thinner crust on the near side has allowed volcanoes to erupt and fill in the ancient large impact craters and basins.  The greater thickness of the far side crust prevented fresh, molten rock from emerging from below to cover as many of its craters.

Volcanic Activity.  The Moon has been volcanically active throughout much of its history, with the first volcanic eruptions having occurred about 4.2 billion years ago.  Today, the Moon has no active volcanoes, even though a significant amount of magma persists under the lunar surface.

Topography.  The topography of the Moon has been measured with laser altimetry and stereo image analysis.  Its most extensive topographic feature is the giant far-side South Pole-Aitken basin, some 1,390 miles in diameter, the largest crater on the Moon.  Its floor is the lowest point on the surface of the Moon.  The highest elevations on the Moon's surface are located directly to the northeast, which might have been thickened by the oblique asteroid impact that produced the crater.  

 

The topography of the Moon, near side on left, far side on the right.

The dark and relatively featureless lunar plains, clearly seen with the naked eye, are called seas, as they were once believed to be filled with water.  They are now known to be vast solidified pools of ancient volcanic lava.  The majority of these lava deposits erupted or flowed into the depressions associated with impact basins.

Almost all the lunar seas are on the near side of the Moon, and cover 31% of the surface of the near side, compared with 2% of the far side.  As discussed above, this is due to the greater thickness of the Moon’s far side crust.

Lunar Soil.  Blanketed on top of the Moon's crust is a highly pulverized surface layer called “regolith,” formed by lunar impacts.  The finer regolith, the “lunar soil” of silicon dioxide glass, has a texture resembling snow.  The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 12.4 miles in the highlands to 1.9-3.1 miles in the seas.  Beneath the finely grained regolith layer is a layer of highly-fractured bedrock many miles thick.

Atmosphere.  Studies of Moon rock samples retrieved by the Apollo missions demonstrate that the Moon once possessed a relatively thick atmosphere for a period of 70 million years, between 3 and 4 billion years ago.  This atmosphere, came from gases ejected from lunar volcanic eruptions.  The ancient lunar atmosphere was eventually stripped away by solar wind (stream of charged particles released from the upper atmosphere of the Sun) and dissipated into space.

When sunlight hits the Moon's surface, the temperature can reach 260 degrees Fahrenheit.  When the sun goes down, temperatures can dip to minus 280 Fahrenheit.

Moon Dust.  A permanent “Moon dust” cloud exists around the Moon, generated by small particles from comet impacts. Estimates are that 5 tons of comet particles strike the Moon's surface every 24 hours, resulting in the ejection of dust particles.  The dust stays above the Moon (as high as 60 miles) approximately 10 minutes, taking 5 minutes to rise, and 5 minutes to fall.  Moon dust would be a problem for lunar explorers in terms of visibility, clogging mechanisms, and interfering with scientific instruments.

Water.  Liquid water cannot persist on the lunar surface.  When exposed to solar radiation, water quickly decomposes and is lost to space.  However, there is definitive evidence of water ice on the lunar surface, widely distributed in the darkest and coldest parts of its polar regions.  At the southern pole, most of the ice is concentrated at lunar craters, while the northern pole’s ice is more widely, but sparsely spread.  Also, astronomers have recently reported detecting molecular water on the sunlit surface of the Moon. The presence of usable quantities of water on the Moon could be an important factor in supporting future lunar habitation, and as a launch platform for future exploration of the solar system.

Moonquakes.  The Moon does not have active tectonic plates like Earth, but there are four different types of moonquakes that can occur: (1) Deep moonquakes happen extremely often, and occur 400-450 miles below the surface of the Moon in the solid mantle, thought to be caused by the gravitational pull of Earth on the Moon.  (2) Meteor Impacts can also cause moonquakes.  These impacts cause rippling moonquakes that can be detected by seismometers.  (3) Thermal moonquakes are caused by the expansion of the frigid crust when first illuminated by the morning sun after two weeks of deep freeze lunar night.  (4) Shallow moonquakes, only 12-19 miles deep in the Moon’s crust, are the most powerful and the most worrisome for researchers and those eager to colonize the moon.  Their cause is not certain.  

Earth - Moon System

This section will cover how the Moon interacts with the Earth.  I will be referencing the figure below.

 

This simplified diagram shows the geometry of the orbits of the Earth and the Moon.

Moon’s Orbit.  As shown in the figure, the Earth revolves around the Sun (period of a year) in the so-called “ecliptic” plane and rotates around its north-south axis (period a day).  Note that the Earth’s north-equatorial plane is offset from the ecliptic plane by 23.5 degrees.  (The tilted Earth's axis causes our seasons.  Throughout the year, different parts of Earth receive the Sun's most direct rays.  So, when the North Pole tilts toward the Sun, it's summer in the Northern Hemisphere. And when the South Pole tilts toward the Sun, it's winter in the Northern Hemisphere.)

The Moon revolves around the Earth (period of a month) in a plane that is offset from the ecliptic plane by 5 degrees (actually its about 5.145 degrees.  The Moon also rotates around its north-south axis (period of a month).  Since the Moon’s rotation period matches the time it takes the Moon to complete an orbit around the Earth, the Moon keeps the same face turned towards the Earth.  (See tidal locking below.)

The Moon’s orbit around the Earth is not perfectly circular.  Rather, it is slightly elliptical, with a maximum distance from Earth of around 252,700 miles, and a minimum distance of about 221,500 miles.  The non-circular orbit results from the perturbing gravitational pull of the much larger Sun (at an average distance of 93 million miles).  During the course of its orbit, the Moon also appears slightly larger and smaller in the Earth’s sky, because it is sometimes closer, or farther, than at other times.

Tidal Locking.  The Moon keeps the same side facing Earth because of a process called “tidal locking.”  The Moon originally rotated at a faster rate, but early in its history, its rotation slowed as a result of gravitational distortions on the Moon induced by the much larger Earth.  With time, the rotation of the Moon stabilized at its current rate.

However, because of a phenomenon called “libration,” over time, about 59% of the Moon's surface can actually be seen from Earth.  Libration is the “wagging” or “wavering” of the Moon perceived by Earth-bound observers, and is caused by changes in their perspective, mostly arising from the Moon’s non-circular and inclined orbit, and the orientation of the Moon in space. 

The gravitational attraction that masses have for one another decreases with increasing distance of those masses from each other.  As a result, the slightly greater attraction that the Moon has for the side of Earth closest to the Moon, as compared to the part of the Earth opposite the Moon, causes the Earth’s high and low ocean tides.

Lunar Phases.  The lunar phase, or Moon phase, is the shape of the Moon's directly-sunlit portion as viewed from Earth.  The lunar phases gradually change over a month, as the orbital positions of the Moon around Earth, and Earth around the Sun, shift.  The visible side of the Moon is variously sunlit, depending on the position of the Moon in its orbit.  Thus, this face's sunlit portion can vary from 0% (at new Moon) to 100% (at full Moon). 

In Western Culture, we divide the month into four primary and four intermediate Moon phases.  The primary lunar phases are: the new Moon, first quarter, full Moon, and last quarter (also known as third or final quarter).  These primary moon phases, along with the intermediate phases, are shown on the chart below.  Technically, the primary Moon phases occur at a specific moment in time, and the intermediate Moon phases take up the time in between.

 

The eight phases of the Moon.

Lunar Months.  There are several definitions of Earth’s “month” used by scientists and astronomers.  For our purposes, I will use the definition of the so-called “synodic” month, the length of time it takes the Moon to go through a full cycle of phases, i.e., from new Moon to new Moon: 29 days, 12 hours, 44 minutes, 2.7 seconds.  It is the synodic month that is the basis of many calendars today and is used to divide the year.

Lunar Eclipses.  Total lunar eclipses occur at full Moon, when Earth is between the Sun and Moon.  The Sun is much larger than the Moon, but it is the vastly greater distance of the Sun from the Moon that gives it the same apparent size - from the perspective of Earth.  A totally eclipsed Moon is sometimes called a “blood Moon” for its reddish color, which is caused by Earth completely blocking direct sunlight from reaching the Moon.  The only light reflected from the lunar surface, in a total lunar eclipse, has been refracted by Earth's atmosphere.  A total lunar eclipse can last up to nearly 2 hours.

 

Pat took this picture of an in-progress total lunar eclipse on September 28, 2015, from Flagstaff, Arizona.

Because the Moon's orbit around Earth is inclined by about 5.145 degrees to the orbit of Earth around the Sun, total lunar eclipses do not occur at every full Moon.  The recurrence of total eclipses of the Moon by Earth has a period of exactly 18 years, 10, 11, or 12 days (depending on the number of leap years), and 8 hours - making total lunar eclipses predictable.

Partial lunar eclipses occur when only a portion of the Moon enters Earth's shadow.  In most calendar years, there are two partial lunar eclipses; in some years one or three, or none occur.

Observation and Exploration

Observation:  Before Spaceflight.  According to archaeological evidence, the Egyptians and Babylonians began to measure time at least 5,000 years ago, based on observations of the cycles of the Sun and Moon.  Here are a few important milestones in the early knowledge of the Moon.

Understanding of the Moon's cycles was an early development of astronomy:  The ancient Greek philosopher Anaxagoras (d. 428 BC) reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former.  Elsewhere in the 5^th century BC to 4^th century BC, Babylonian astronomers recorded the 18-year cycle of total lunar eclipses.  The Chinese astronomer Shi Shen (4^th century BC) gave instructions for predicting solar and lunar eclipses.

The Greek mathematician Archimedes (287-212 BC) calculated the motions of the Moon and other objects in the solar system.  In the 2^nd century BC, Hellenistic astronomer and philosopher Seleucus correctly theorized that ocean tides were due to the attraction of the Moon, and that their height depended on the Moon's position relative to the Sun.  In the same century, Greek astronomer and mathematician Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance.

Greek mathematician Ptolemy (AD 90-168) greatly improved on the numbers of Aristarchus, calculating the values of a mean Earth-Moon distance of 59 times Earth's radius and a Moon diameter of 0.292 Earth’s diameter - close to the correct values of about 60 and 0.273 respectively. 

During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognized as a sphere, though many believed that it was "perfectly smooth.”

In 1609, Italian astronomer Galileo Galilei used an early telescope to make drawings of the Moon, and deduced that it was not smooth, but had mountains and craters.

Telescopic mapping of the Moon followed later in the 17^th century.  The efforts of Italian priests and astronomers/mathematicians Giovanni Battista Riccioli and Francesco Maria Grimaldi led to the system of naming of lunar features in use today.

In 1647, Polish astronomer Johannes Hevelius drew the first lunar map to include the libration zones on the edges of Moon’s disk.

 

Johannes Hevelius drew the first lunar map to include the libration zones.

In 1834-1837, German astronomers Wilhelm Beer and Johann Heinrich Mädler produced the first trigonometrically accurate study of lunar features, including the heights of more than a thousand mountains. 

Lunar craters, first noted by Galileo, were thought to be of volcanic origin until the 1870s proposal of English astronomer Richard Proctor that they were formed by lunar impact collisions.

Exploration:  1959 - 1970s.  The Cold-War-inspired “Space Race” between the Soviet Union and the U.S. led to an acceleration of interest in direct exploration of the Moon.

Soviet Missions.  Spacecraft from the Soviet Union's Luna program were the first to accomplish a number of goals:  following three unnamed, failed missions in 1958, the first human-made object to escape Earth's gravity and pass near the Moon was Luna 1; the first human-made object to impact the lunar surface was Luna 2; and the first photographs of the normally hidden far side of the Moon were made by Luna 3 - all in 1959. 

The first spacecraft to perform a successful lunar soft landing was Luna 9, and the first vehicle to orbit the Moon was Luna 10 - both in 1966.  Luna 17 deployed the first lunar rover, in 1970.  Rock and soil samples were brought back to Earth by three sample return missions (Luna 16 in 1970, Luna 20 in 1972, and Luna 24 in 1976).

Soviet lunar programs ended in 1976, and the Soviet Union dissolved in 1991. 

United States Missions.  Following President John F. Kennedy's 1961 commitment to a manned Moon landing before the end of the decade, the United States, under NASA leadership, launched a series of unmanned probes to develop an understanding of the lunar surface in preparation for human missions:  the Jet Propulsion Laboratory's Ranger program produced the first close-up pictures; the Lunar Orbiter program produced maps of the entire Moon; the Surveyor program landed its first spacecraft four months after Luna 9.  The manned Apollo program was developed in parallel; after a series of unmanned and manned tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar human landing, in 1968, Apollo 8 made the first manned mission to lunar orbit. 

Neil Armstrong, commander of the American mission Apollo 11, became the first person to walk on the Moon on July 21, 1969.  Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing) returned to Earth with about 840 pounds of lunar rocks, core borings, pebbles, sand, and dust - in 2,196 separate samples.  The Apollo program ended in 1972.

Apollos 11's Buzz Aldrin looks back at the lunar lander from a scientific instrument site, 1969.

Scientific instrument packages were installed on the lunar surface during all the Apollo landing missions.  Long-lived instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo landing sites, and directly transmitted data to Earth until 1977.

U.S. Apollo program lunar landing sites.

 

Exploration:  1970s - Present.  After a lunar mission pause in the late 1970s and 1980s, several more countries joined the U.S. in direct exploration of the Moon.  The table below documents the lunar missions of Japan, Europe, China, India, and the United States in the order of each country’s first mission in this time period, except for the U.S., which is covered last in the table.  Some countries sent, or plan to send, payloads to the moon in a rideshare mode, on another country’s space vehicle.

After the Soviet Luna and U.S. Apollo programs, direct exploration of the Moon became an international effort.

Country	Year:  Mission	Planned (funded) Missions
Japan	1990:  Became 3rd country to place spacecraft in lunar orbit. 2007-2009:  Lunar orbiter obtained geophysics data and first high-def movies from beyond Earth orbit.	Pinpoint Moon landing and roving (2022), Lunar lander (2024).
Europe	2004-2006:  Lunar orbiter made first detailed survey of chemical elements on Moon’s surface.	UK:  Rideshare rover exploration (2022), Lunar sample return (2025); Germany:  Private tech demo of lander and rover (2022); Netherlands:  South Pole radiation measurement (2023); Europe:  Commercial telecom orbiter (2024).
China	2007-2009:  Lunar orbiter obtained full image map of Moon. 2010:  Orbiter obtained high-res map of Moon’s surface. 2013:  Soft landing on Moon; deployed lunar rover. 2018:  First spacecraft to land on Moon’s far side. 2020:  Moon sample (about 4 pounds) return mission.	Lunar sample return from South Pole (2024); South Pole lander, rover, and flying probe (2024).
India	2008-2009:  Lunar orbiter created high-res chemical, mineralogical, and photo-geological map of lunar surface.  Confirmed presence of water molecules in lunar soil. 2019:  Failed attempt at soft landing on Moon.	Soft land on Moon (2022).
U.S.	1994:  Lunar orbiter obtained first near-global topographic map of Moon and first global multispectral images of lunar surface. 1998:  Lunar orbiter detected excess hydrogen at lunar poles, likely causes by presence of water ice in upper regolith. 2009:  Lunar orbiter (still) obtaining precise lunar altimetry and high-res imagery from high-eccentricity polar orbit.  Follow-up impact mission in Moon crater. 2012:  Two lunar orbiters on mission to determine Moon’s internal structure by obtaining high-quality gravitational field map. 2013-2014:  Lunar orbiter studied Moon’s exosphere and dust.	First Intuitive Machines lunar lander (2022), Lunar landing with private payloads (2022), Lunar lander with NASA experiments (2023), Lunar lander to prospect for lunar resources at South Pole (2023), Deliver Intuitive Machines payloads to Moon’s surface (2024), Artemis program to deliver the “first woman and next man” to the lunar surface (2025).

 

In addition to the future missions noted above for Japan, Europe, China, and India, other countries also have funded planned missions, including Russia, with a resource explorer lander (2022), an orbiter (2024), and another lunar lander (2025); the UAE, with a lunar rover demo (2022); Mexico, with a rideshare microrover (2022); and Turkey, with a hard landing (2023), and an unmanned soft lander (2028).  Also, Canada and Australia are developing payloads for lunar exploration.

Beyond these plans, Brazil, Israel, South Korea, and South Africa have proposed lunar efforts that are currently unfunded.

But the “heavy lifting” for future lunar missions will be done by the United States.  With the Artemis program, the U.S. will conduct missions to explore more of the lunar surface than ever before, and will collaborate with commercial and international partners to establish the first long-term presence on the Moon.  Then, the U.S will use what was learned on and around the Moon to take the next giant leap: sending the first astronauts to Mars.

Things You Didn’t Know About the Moon

Here are a few facts that you probably didn’t know about the Moon:

There are rules for how the Moon's features are named

The custom of applying personal names to lunar formations began in 1645 with Michael van Langren, an engineer in Brussels, who named the Moon's principal craters after kings and great people on the Earth.  On his lunar map, he named the largest lunar sea (now known as Oceanus Proacellarum) after his patron, Phillip IV of Spain.

But just six years later, Giovanni Battista Riccioli of Bologna completed his own great lunar map, which removed the names bestowed by Van Langren and instead derived names chiefly from those of famous astronomers.  In 1939, the British Astronomical Association issued a catalog of officially named lunar formations, "Who's Who on the Moon," listing the names of all formations adopted by the International Astronomical Union.  Today the IAU continues to decide the names for features on our Moon.  The names for lunar craters now tend to fall into two groups:  deceased scientists, scholars, and explorers; and artists who've become known for their contributions to their respective fields.  Historically, most of the lunar seas were named for weather or states of mind (See Sea of Rains, Sea of Tranquility.)

The figure below identifies some of the prominent features on the near side of our Moon.

 

Prominent named lunar craters and seas (mares) on the near side of the Moon.

There's a huge lump of heavy metal under the Moon's South Pole

Scientists aren’t sure where it came from, but there is a giant deposit of heavy metal lodged underneath the Moon’s South Pole-Aitken basin.  It is estimated to weigh around 2.4 quadrillion tons.

The Moon has at least two lunar pits

Data gathered from NASA’s Lunar Reconnaissance Orbiter, has shown images of at least two large holes in the Moon’s surface, known as lunar pits.  The holes are thought to form when a subterranean lava tube collapses, and are considered extremely rare.

 

Lunar pit.

The Earth and Moon trade rocks

Scientists and researchers have found rocks on Earth that were ejected from the Moon.  Asteroid impacts on the Moon expel rocks into space, bringing with them lunar material from all areas of the Moon’s surface.  Studies also suggest that rocks from Earth travel to the Moon, and computer simulations have led scientists to believe that the Moon may have asteroid debris from Earth, Venus, and Mars.

The Moon still has moonquakes

As part of the Apollo space program, seismometers were placed on the surface of the Moon.  They were retired in the late 1970s, but during the time they were actively recording seismic activity on the Moon, there were 28 moonquakes of varying magnitudes, up to 5.5 on the Richter scale.  Later research via the Lunar Reconnaissance Orbiter showed fault lines in the Moon that are less than 50 million years old, as well as the displacement of boulders across the surface - both of which point to the occurrence of regular moonquakes.

The Moon is shrinking

The Moon is shrinking as its interior continues to cool, getting more than about 150 feet skinnier over the last several hundred million years.  Just as a grape wrinkles as it shrinks down to a raisin, the Moon gets wrinkles as it shrinks.  Unlike the flexible skin on a grape, the Moon’s surface crust is brittle, so it breaks as the Moon shrinks, forming “thrust faults” where one section of crust is pushed up over a neighboring part.  There is evidence that these faults are still active and likely producing moonquakes today as the Moon continues to gradually cool and shrink.

The Moon is moving away from Earth

The pace is minuscule, but there’s no doubt the Moon is slowly receding from the Earth.  Apollo 11 astronauts placed a Laser Ranging Retroreflector on the Moon’s surface, which helps measure the distance between Earth and the Moon.  The Retroreflector, a series of mirrors that reflect laser pulses, shows that the Moon is retreating at a rate of about 1.5 inches per year.

The Moon's gravity helps strengthen Earth's magnetic shield

Earth has a magnetic field, created by the movement of liquid iron and nickel in the outer core, that helps protect it from destructive natural forces like solar wind or cosmic particles.  Scientific research indicates that the Moon’s gravity “tugs” on the Earth’s mantle, which sits on top of that liquid outer core, and causes the liquid to move around, helping to generate energy that strengthens Earth’s magnetic shield.

The Earth, seen from the Moon, also goes through phases

However, they are opposite to the lunar phases that we see from the Earth.  It's a “full Earth” when it's new Moon for us; last-quarter Earth when we're seeing a first-quarter Moon, etc.  From any spot on the Moon (except on the far side, where you cannot see the Earth), the Earth would always be in the same place in the sky.  From the Moon, our Earth appears nearly four times larger than a full Moon appears to us, and - depending on the state of our atmosphere - shines anywhere from 45 to 100 times brighter than a full Moon. 

 

Earthrise as seen from Apollo 8 in lunar orbit, 1968.

Eclipses are reversed when viewing from the Moon

Phases aren't the only things that are seen in reverse from the Moon.  An eclipse of the Moon for us is an eclipse of the Sun from the Moon.  In this case, the disk of the Earth appears to block out the Sun.  If it completely blocks the Sun, a narrow ring of light surrounds the dark disk of the Earth - our atmosphere backlighted by the Sun. 

When a total eclipse of the Sun is taking place here on Earth, an observer on the Moon can watch over the course of two or three hours as a small, distinct patch of darkness works its way slowly across the surface of the Earth.  It's the Moon's dark shadow that falls on the Earth, but unlike in a total lunar eclipse, where the Moon can be completely engulfed by the Earth's shadow, the Moon's shadow is less than a couple of hundred miles wide when it touches the Earth, appearing only as a dark blotch.

Mysteries of our Moon

As NASA prepares to send astronauts to the moon once again in 2025, scientists reaffirm that the Moon still holds many secrets that we need to explore.  With new ways of collecting data and new methods for analyzing it, some of our long-standing questions might soon have answers.  Here are few of the biggest scientific mysteries about the Moon still waiting to be explained.

What is the origin of the Moon?

The Apollo missions brought back Moon rocks that initiated decades of investigations focused on learning how the Moon formed and evolved.  Scientists used them to come up with the strongest existing model: that the Moon began as a magma ocean - a giant ball of molten lava orbiting Earth. This in turn led, decades later, to the hypothesis that the Moon was borne out of Earth itself, when a giant impact with a Mars-sized object ejected debris into orbit.  That’s the most widely accepted hypothesis for the origin of the Moon, but it’s by no means perfect.  There is no way yet to confirm a giant impact created the Moon, and our sense of how the magma ocean cooled down into the dry, gray rock we see now, is still very hazy.

Why is the Moon’s crust so much thicker on the Moon’s far side, than it is on the near side?

There are two theories, but no agreed-to answer to this question: (1) One theory is that in the early history of our Earth-Moon system, a young dwarf planet collided with the Moon on its far side.  This impact would have thrown up huge amounts of material which eventually fell back onto the Moon’s surface, burying the far side in three to six miles of debris.  This debris would go on to form a large part of the crust, and could theoretically still be detected today.  (2) The second theory is that while the Earth and Moon were forming, heat from the still-molten Earth slowed the cooling of the near side of the Moon.  The far side solidified faster, forming a thicker crust.  This resulted in meteoroid impacts on the near side, sometimes punching through the thinner surface to the still molten mantle, releasing lava to surface to create the lunar seas.  Alternatively, when meteoroids impacted on the far side of the Moon, the thicker crust was not punctured (as often), so more valleys, craters, and highlands would be created, but fewer seas.

Where did lunar water come from?

There’s water on the Moon, and not just a little sprinkling - but troves and troves of water ice that could be sitting just beneath the surface, especially at the lunar poles.  This water could be harvested to help generate a new form of spacecraft fuel, or used to help sustain a future lunar colony.  It’s unclear what form the water ice is in - big blocks, or crystals mixed with lunar regolith.

How did all that water ice get there?  Nobody really knows yet.  There are three main theories for how water originated on the moon.  (1) The most obvious theory suggests that the water ice was deposited by asteroid and comet impacts, where it vaporized and eventually made its way to the poles.  (2) It’s also possible that ionized hydrogen from solar winds binds with oxygen trapped in regolith, and is eventually released as vaporized water, due to temperature fluctuations on the surface.  (3) Finally, there’s a possibility that water was present in the material that originally formed the Moon and was forced to the surface by volcanic eruptions.  It could be that all three processes were at work, which makes it a question of how much water each mechanism contributed.

What’s the complete story on moonquakes?

We’re already aware of a few phenomena that cause Moonquakes, like thermal expansion, tidal stress induced by Earth’s gravity, meteorite impacts, and shrinking and cracking of the crust as the Moon continues to cool.  But with limited data, we’re not entirely sure how these processes work and exactly how the quakes behave.  

A global seismic network installed around the Moon would be tremendously helpful in understanding the trigger mechanisms behind all this quake activity.  Such a network could clearly identify exactly when these events occur, where they originate from, and the effect they have on the rest of the Moon.

How does tidal locking occur?

Tidal locking (only one side of the Moon faces Earth) is not uncommon for moons in our solar system, but it’s still unclear exactly when this occurs, what conditions encourage it, and how it happens.  This mystery of tidal locking may harken back to the question of the origin of the Moon, and what was happening in early lunar history.  It’s another piece of the 4-billion-year puzzle.

This mystery isn’t just local to our Moon.  As we continue to find more planets and moons outside of our solar system, it’s important to figure out what factors help incite features like tidal locking, and decide whether potentially habitable worlds are hospitable to life.

What’s the huge mass under the Moon’s South Pole?

One enigma that’s sprung up only recently has to do with the discovery that something massive is lurking underneath the south pole of the Moon, below the largest impact crater ever made in the entire solar system.  Scientists have no idea what it could be, but it’s certainly big enough to affect the gravitational force exerted by the Moon’s mass.  Predominant theories suggest it’s some sort of heavy metal body from an asteroid that impacted the Moon’s surface a long time ago, but this is far from certain.  And it’s hard to understand what this mass is doing suspended underground.  We know the Moon’s interior is still active to some degree (e.g., moonquakes), and heat should have caused this mass to shift around instead of staying trapped on one place.

What’s the history of vulcanism on the Moon?

We don’t see volcanoes erupting on the moon these days, but research suggests lunar volcanoes were active within the last 100 million years, and on the scale of the cosmos, that may as well have been last week.  The problem is, we just don’t know enough about vulcanism on the Moon to determine what this activity was actually like and what it did to the Moon’s geology.  Some surfaces seem younger than others, and it’s not clear why.  It all comes down to a need for fresh lunar samples.

The implications stretch beyond just learning more about the Moon.  If we’re able to assess exactly how young a surface on the Moon really is, we can use that knowledge to make predictions of how old the surfaces on other celestial bodies are.  In turn, this could give us a better understanding of the solar system’s history.  The Moon is really just a small step toward the giant leap of unraveling the origin of everything we know.

Future Lunar Habitation

Our Moon will soon see the beginning of a new chapter in its life - permanent human habitation. Beginning in 2025, humans will return to the Moon, after an absence of 53 years, to solve the mysteries discussed above, and to accomplish the tasks suggested in the figure below.  Perhaps some of you will be involved in these exciting efforts.

There are many options for future Moon habitation.