Why does Earth have a moon, and how does it affect our planet?

From its pockmarked surface to its Earth-like core, there’s a lot to learn about our lunar companion.

By Maya Wei-Haas
Published 5 Jul 2019, 16:08 BST

The moon, Earth’s sole natural satellite, is our constant companion. It has circled our planet for billions of years, since before the first sparks of life flickered in the oceans—before Earth was even cool enough to have oceans.

But its seemingly tranquil position in modern night skies stems from a remarkably tumultuous past. It formed some 4.5 billion years ago, when the solar system was in its infancy and comets, meteors, and asteroids ricocheted throughout. One such collision, between Earth and a Mars-sized orb, likely flung molten rock out into space, some of which coalesced and cooled to form what we now know as the moon.

This early formation and close ties with young Earth makes the moon one of the most promising places to explore the birth and development of our solar system and home planet. The moon also preserves many of its ancient features: Unlike Earth, it doesn’t have plate tectonics to continually reface the landscape, nor does it have wind and rain wearing down ancient rocks.

Generations of astronomers have studied this small airless world, from its pockmarked surface to its dense iron core. It’s the only other world humans have yet set foot upon—and a prime candidate for future visits. (Explore 50 years of visits to the moon.)

Luna’s shifting face

There are more than 190 moons orbiting the planets and asteroids in our solar system, and Earth’s moon is the fifth largest of the lot. It spans about 2,160 miles across, nearly a third the width of Earth, orbiting at an average of 30 Earth-widths away. (Learn more about the solar system’s many moons.)

Every 27.3 days, the moon makes its way around our planet while also completing one turn on its axis. Known as synchronous rotation, this celestial dance means that the same lunar face always peers down at us. Viewed from Earth, the amount of the moon illuminated by the sun appears to wax and wane, creating the familiar cycle from new moon to crescent to full. This sequence is a combined result of the moon’s changing position relative to both the Earth and sun, requiring 29.5 days to complete one full lunar cycle. 

Even though the same side of the moon always faces Earth, there’s no true “dark side,” as many mistakenly call our lunar orb’s far side. Even the far side of the moon receives sunlight—we just can’t see it—and the section of the moon illuminated on any given day shifts depending on the moon’s position.

The moon rocks

During the Apollo missions, astronauts brought back 842 pounds of lunar rock, sand, and dust to Earth, allowing scientists to scrutinise the moon’s surface. From this they’ve learned a wealth of information about the moon’s formation and evolution. Early in its history, vast oceans of magma blanketed the moon, and as that magma slowly cooled and crystallised, the less dense minerals floated to the surface. Much of this ancient lunar crust is made up of the light-colored rock anorthosite, which we see from Earth as the bright sections of the moon. 

After billions of years, however, that dazzling surface is now rife with dark tracks, speckles, and splotches. Many of these dark zones are vast swaths of lunar basalts, similar to the rocks that make up the Hawaiian islands. Known as maria, which is Latin for seas, these zones were formed when ancient volcanic eruptions of molten rock flooded to the surface. Scientists don’t think these eruptions are ongoing, and most of the lava likely burst free between three and four billion years ago.

Some of the small wending dark tracks are also faults, or deep cracks in the surface. But these don’t form due to shifting tectonic plates, like fault lines on the surface of the Earth. Instead, many likely formed as the moon cooled and contracted, and others come from Earth’s deforming gravitational tug on the tiny world. While much of this activity happened long ago, a recent look at Apollo-era earthquakes suggests that not all is relegated to the past, hinting that the moon may not be not geologically dead as some once thought. 

One of the most quintessential features of the moon is the array of overlapping craters punched into its surface. Studying these craters, combined with the geological dating of rocks brought back during the Apollo missions, helps scientists not only illuminate the moon and Earth’s history of bombardment but also calibrate a timeline for the ages of other solar system bodies.

Like on Earth, beneath the lunar crust lies the mantle, but scientists still aren’t sure of its exact composition. Models and some recent finds suggest the upper zones of the mantle are composed of the minerals pyroxene and olivine. At the moon’s centre lies a small iron-rich core, spanning roughly 300 miles across, as revealed by analyses of seismic waves passing through the moon’s interior using Apollo-era seismic records of lunar tremors.

Watery world

Once thought to be a parched landscape, scientists have found an increasing number of signs that the moon is wetter than we once thought. While liquid water can’t persist at its surface, researchers believe that water ice lingers in some of its permanently shadowed zones. Tiny glass beads from ancient volcanic eruptions also suggest that there’s a surprising amount of water locked up in minerals deep inside the orb. Remarkably, water also seems to be released as meteors collide with the moon’s surface—as much as 220 tons of water each year.

Such reservoirs would provide a valuable resource for hydration and fuel for future human visitors, or even for long-term residents of proposed lunar bases that could serve as a jumping-off point for exploration deeper into space.

Life with—and without—our lunar companion

The moon’s steady change between new and full provided a rhythm for generations of humans, who eventually crafted calendars marking the moon’s many phases and their effects on Earth’s surface. One of the most obvious lunar influences is seen in Earth’s tides. The moon’s gravitational tug causes one bulge of water to form on the nearest side of our planet and another on the side farthest away. As the Earth rotates, the part of Earth affected by the lunar pull shifts, creating a high tide about every 12 hours at any given spot.

The moon also dampens the amount that Earth teeters on its axis, helping to keep our climate more stable. The wobble in our planet’s tilt affects how the sun’s energy is distributed across the Earth and can influence the frosty advance or retreat of ice ages. Without the moon, scientists estimate that our planet’s tilt could have varied by up to 85 degrees, causing wild swings in climate.

But each year, the moon’s grip on our planet grows just a smidgen weaker as it drifts about one and a half inches farther into space. This slow expansion in the moon’s orbit is a result of its role in Earth’s tides. Our planet rotates a little bit faster than the moon’s orbit. So the tidal bulge that rises on the side of Earth nearest the moon spins just ahead of the orb. This drags the moon along, slightly speeding up its orbit and inching it away.

Never fear, it’s unlikely the moon will fly away altogether. So our little glowing buddy will continue to loop around Earth as we continue our annual venture around the sun for millennia to come. That is, of course, until it’s shredded by our dying sun.

NASA science: The moon
The Moon: stepping stone to the planets
Oregon State: Volcanism on the moon
Lunar and Planetary Institute: About our moon
Cornell's Ask an astronomer: Is the moon moving away from the Earth?
NASA News: The moon has an Earth-like core
Smithsonian National Air and Space Museum: Lunar rocks
University of Hawaii: The oldest moon rocks
NASA Blogs: Why study the moon?
NOAA SciJinks: What causes tides?
Read More

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