The Moon's Surface, Origin & Phases: Sixth Grade Science
Lighting our nocturnal sky with its silvery glow, the Moon is Earth's only natural satellite, orbiting at an average distance of approximately 384,400 kilometers while influencing planetary stability and night-sky cycles.
Earth's Celestial Companion
For billions of years, the Moon has accompanied Earth on its journey around the Sun. Unlike Earth, which is enveloped in a dense protective atmosphere and churns with active plate tectonics, running water, and thriving biological life, the Moon is a desolate, airless world preserving an ancient geological record. By studying the Moon's surface features, origin, orbital dynamics, and the shifting illumination patterns called lunar phases, we gain deep insight into the history of our planetary neighborhood.
THE EARTH-MOON SYSTEM
(Scale and distance compressed)
.--------. .---.
/ \ / \
| EARTH | | MOON |
\ / \ /
'--------' '---'
Diameter: 12,742 km Diameter: 3,474 km
<---------------- 384,400 km ---------------->
The Moon has a diameter of roughly 3,474 kilometers, which is slightly more than one-fourth the diameter of Earth. Its mass is only about 1.2 percent of Earth's mass, resulting in a surface gravity that is roughly one-sixth (about 16.6 percent) as strong as Earth's gravity. A person who weighs 60 kilograms on Earth would weigh only 10 kilograms on the lunar surface.
Surface Features of the Moon
When viewing the Moon from Earth with the naked eye or a pair of binoculars, clear patterns of light and dark areas emerge across its surface.
LUNAR SURFACE TOPOGRAPHY
Highlands (Anorthosite) Maria (Basaltic Plains)
Light-colored terrain Dark, smooth lowlands
Heavy cratering Fewer impact craters
Ancient crust Ancient lava flows
/\ /\ /\ __________________
/ \/ \/ \ / \
Lunar Maria
The dark, smooth plains visible on the Moon are called maria (singular: mare, the Latin word for sea). Early astronomers mistook these vast, dark patches for bodies of water. In reality, maria are enormous basins formed by ancient asteroid impacts that later flooded with molten basaltic lava between 3.1 and 3.9 billion years ago. As the iron-rich lava cooled and solidified, it formed flat, dark volcanic plains. Maria cover approximately 16 percent of the lunar surface, concentrated primarily on the near side facing Earth.
Lunar Highlands (Terrae)
The lighter, heavily cratered regions of the Moon are known as the highlands or terrae. These elevated, mountainous terrains represent the Moon's original, primordial crust, composed primarily of a light-colored silicate rock called anorthosite. The lunar highlands are significantly older than the maria, dating back roughly 4.4 billion years, and they preserve an intense bombardment record from the early solar system.
Impact Craters and Regolith
The most prominent features on the Moon are impact craters, circular depressions created when meteoroids, asteroids, and comets slammed into the surface at hypervelocity speeds. Large craters like Tycho and Copernicus exhibit bright radial lines called rays, which consist of pulverized debris ejected outward during the explosion.
Because the Moon lacks an atmosphere, it has no liquid water, no wind, and no weather to erode surface structures. Furthermore, the Moon has no active plate tectonics or active volcanoes to recycle its crust. As a consequence, footprints, rover tracks, and impact scars remain pristine for millions of years. Over billions of years, continuous micrometeorite bombardment has ground surface rocks into a fine, powdery, gray dust layer called lunar regolith, which blankets the bedrock to depths ranging from several meters to tens of meters.
The Origin of the Moon: The Giant Impact Hypothesis
How did Earth acquire such a disproportionately large natural satellite? Scientists have proposed several models, but the Giant Impact Hypothesis (often called the Theia Hypothesis) is supported by the strongest geological, physical, and isotopic evidence.
THE GIANT IMPACT HYPOTHESIS
1. Proto-Earth & Theia 2. Oblique Collision
O o ( \\\ ///// )
Proto-Earth Theia \ EXPLOSION /
(Mars-sized impactor)
3. Debris Ring Forms 4. Accretion into Moon
.---. :::::: .---. .---.
( E ):::::: ( E ) ( M )
'---' :::::: '---' '---'
Earth + Ring Earth Moon
According to this hypothesis, approximately 4.5 billion years ago, during the violent formative epoch of the solar system, a Mars-sized protoplanet named Theia struck the young proto-Earth in a glancing, oblique collision. The colossal energy released by the impact vaporized Theia and stripped away substantial portions of Earth's outer mantle. This molten and vaporized debris was propelled into orbit around Earth, forming a glowing ring of superheated rock.
Over a remarkably short period—likely within a few years or decades—the gravitational attraction among the debris particles caused them to coalesce and accrete into a single spherical body: the Moon. This model explains key scientific puzzles:
- Why the Moon has an identical isotopic composition of oxygen to Earth's mantle.
- Why the Moon has an extremely small iron core (the iron core of Theia sank and merged with Earth's core).
- Why the Moon lacks volatile elements like water, which boiled away during the high-temperature impact event.
Synchronous Rotation: The Near Side and Far Side
One of the most remarkable aspects of the Moon is that observers on Earth always see the exact same hemisphere. We never see the opposite side directly from Earth's surface.
SYNCHRONOUS ROTATION
Orbital Period (27.3 days) = Rotational Period (27.3 days)
(A) North
.---.
( M ) Side A faces Earth
'---'
^
|
Side A faces .---.|.---. Side A faces
Earth -----> ( M | M ) <----- Earth
'---'|'---'
|
v
.---.
( M ) Side A faces Earth
'---'
This phenomenon occurs because of synchronous rotation (or tidal locking). Over billions of years, Earth's gravitational pull exerted tidal forces on the Moon, distorting it slightly into an oblong shape and acting like a gravitational brake on its spin. Eventually, the Moon's rotation slowed until its rotational period exactly matched its orbital period around Earth: precisely 27.3 days (a sidereal month).
Because the Moon completes one rotation on its axis in the exact same amount of time it takes to complete one orbit around Earth, the same lunar face (the near side) remains permanently pointed toward Earth. The opposite hemisphere, known as the far side (erroneously called the dark side, even though it receives full sunlight during the new moon phase), was never seen by human eyes until the Soviet Luna 3 spacecraft photographed it in 1959.
The Phases of the Moon
The Moon does not produce any light of its own; it shines entirely by reflecting sunlight off its rocky, dusty surface. Like Earth, half of the Moon is always bathed in brilliant sunshine (the day side), while the other half is shrouded in darkness (the night side).
THE EIGHT LUNAR PHASES
(Sunlight coming from right)
First Quarter
(D)
Waxing Gibbous Waxing Crescent
(O) ( )
^ ^
\ /
Full Moon (O) <----- (EARTH) -----> ( ) New Moon
/ v v
Waning Gibbous Waning Crescent
(O) ( )
Third Quarter
(C)
As the Moon revolves around Earth once every 29.5 days (a synodic month or lunar cycle), our perspective of the illuminated portion changes continuously. The varying shapes we see from Earth are called lunar phases. The cycle consists of eight distinct phases:
Phase 1: New Moon
The Moon is positioned between Earth and the Sun. The illuminated side faces directly toward the Sun, while the shadowed side faces Earth. The Moon is invisible in the sky, rising and setting with the Sun.
Phase 2: Waxing Crescent
As the Moon moves eastward in its orbit, a sliver of the sunlit hemisphere becomes visible from Earth on the right side (in the Northern Hemisphere). Waxing means growing or increasing in illumination.
Phase 3: First Quarter
The Moon has completed one-quarter of its orbit around Earth. Half of the visible disk is illuminated on the right side, while half remains dark. It appears as a distinct half-moon.
Phase 4: Waxing Gibbous
The illuminated portion continues to expand beyond half, but is not yet fully illuminated. Gibbous comes from a Latin word meaning humpbacked. More than half of the visible disk shines brightly.
Phase 5: Full Moon
Earth sits between the Sun and the Moon (though slightly above or below the orbital plane). The entire illuminated hemisphere faces directly toward Earth, presenting a brilliant, fully rounded circle of light in the night sky.
Phase 6: Waning Gibbous
Following the full moon, the illuminated area begins to shrink or decrease. Waning means decreasing. Sunlight illuminates more than half of the disk, but the darkness begins creeping in from the right, leaving the left side illuminated.
Phase 7: Third (Last) Quarter
The Moon has completed three-quarters of its orbital journey. Once again, half of the visible disk appears bright, but now it is the left half that shines, while the right half is shrouded in shadow.
Phase 8: Waning Crescent
A final slender crescent of light remains visible on the left side before the Moon returns to the new moon position, ready to restart the 29.5-day cycle.
Exercises and Solutions
Exercise 1: Describe the Giant Impact Hypothesis regarding the origin of the Moon, and list two pieces of scientific evidence supporting it.
Solution 1: The Giant Impact Hypothesis proposes that roughly 4.5 billion years ago, a Mars-sized protoplanet named Theia collided obliquely with the young proto-Earth, vaporizing the impactor and ejecting mantle material into orbit that coalesced to form the Moon. Supporting evidence includes the nearly identical oxygen isotope ratios found in Moon rocks and Earth's mantle, as well as the Moon's unusually tiny iron core, which occurred because the dense iron core of Theia sank into and merged with Earth's core.
Exercise 2: Explain why observers on Earth can only ever see one side of the Moon. What is this phenomenon called, and what are the rotational and orbital periods involved?
Solution 2: Observers on Earth only see one side of the Moon due to synchronous rotation, also known as tidal locking. Earth's gravitational forces historically slowed the Moon's spin until its rotational period (27.3 days) became exactly identical to its orbital period around Earth (27.3 days). Because it spins on its axis at the exact same rate that it circles Earth, the same hemisphere remains permanently turned toward our planet.
Exercise 3: Differentiate between lunar maria and lunar highlands in terms of color, rock composition, and crater density.
Solution 3: Lunar maria are dark, smooth lowlands composed of iron-rich basalt formed by ancient volcanic lava flows, exhibiting relatively few impact craters because the lava erased older craters. In contrast, lunar highlands are bright, rugged mountainous terrains composed of anorthosite rock, dating back to the Moon's primordial crust and displaying extremely dense impact cratering accumulated over 4.4 billion years.