How Earth’s natural satellite came to be


Over the years I’ve learned quite a lot about the moon, by which I mean Earth’s only natural satellite. I can relate many of those facts with a good deal of certainty.

However, I am uncertain about whether to capitalize the word “moon.” In that regard, astronerds and grammar geeks are at odds.

On the one hand, “moon” is not the satellite’s name, astronomically speaking. If it were, we’d be calling it “Moon” and not “the moon” the way we called Jupiter “Jupiter” and not “the Jupiter” or Ganymede (Jupiter’s largest satellite) “the Ganymede.”

The Associated Press Stylebook says not to capitalize. In doing so, it implicitly recognizes that “moon” is a synonym for “satellite.” Thus, I could write that Mars has two moons, Demos and Phobos.

True astronerds often refer to Earth’s natural satellite as Luna. The name derives, as is the astronomical custom, from one of Jupiter’s many consorts.

However, if I follow that convention, I should probably be referring to Earth as Terra or Gaia.

Grammar aficionados point out that “moon” derives from the medieval Old English word “mōna,” which refers to our “month.” Humans used to measure time in “moonths,” the time it takes for the moon to go through a set of phases.

“Moon” is thus a proper name, albeit one out of our “vulgar” and not our high-toned Latin past. Therefore, writers should capitalize it.

However, if I did that, I ought to write “Moon” instead of “the moon.” I’d sound like a troglodyte. “Look. (Grunt.) Moon.” Also, if I said “Luna,” some folks in my audience wouldn’t know what I was talking about.

Sigh. Lunar astronomer Paul D. Spudis has termed the controversy “a study in arrant pedantry.” Perhaps he’s right.

Nevertheless, I still have to decide what to do.

A drumroll, please. I’ll follow the AP Stylebook to save my editor trouble.

Here’s what we do know with a relatively high degree of certainty.

Luna (oops, the moon) is a 2,000-miles-wide, spheroid hunk of rock, Earth’s only natural satellite. It maintains an elliptical orbit at an average distance of 238,900 miles as it travels once a “moonth” around our planet.

In binoculars or a small telescope, we see craters formed billions of years ago. The moon and Earth, both still in the process of formation, continued to accrete new material. Pieces of space debris, large and small, crashed into the moon’s still-soft surface.

We see larger, dark areas, the lunar maria, or “seas,” the results of even more enormous impacts. We know that those collisions led to gigantic craters, basins that filled with liquid rock from the moon’s still molten interior. The giant lakes of lava slowly cooled, leaving those enormous flat plains we call lunar “seas.”

These things we know with a high degree of certainty. Astronomers are far less sure about the moon’s origin.

Here’s the current working hypothesis. Theia, a protoplanet the size of Mars, slammed into the still-molten Earth. Out of that collision splattered forth the material that would eventually coalesce to form the moon.

The long path to that conclusion illustrates science’s dogged determination to get to the facts.

Up to the beginning of the 1940s, many astronomers believed the theory proposed by George Darwin, son of famed naturalist Charles Darwin.

Darwin suggested that Earth was still molten just after its formation. It spun so fast that it threw off molten rock, eventually forming our only satellite.

Darwin also claimed that the moon had orbited much closer to Earth in its distant past, and he was right. Radar measurements of the moon’s orbit reveal that the moon is receding from Earth at about 1.5 inches yearly.

By the 1940s, the scientific consensus about the moon’s origins began to change. Astronomers proposed two alternate theories.

American chemist Harold Urey suggested the “capture model.”

In that scenario, our moon formed as a small planet somewhere else in the solar system. Eventually, it passed by Earth and was captured into orbit by our planet’s gravity.

An examination of other solar-system planets with moons makes Urey’s model unlikely.

Mars has two captured satellites called Demos and Phobos. But they are tiny compared to the mass of Mars.

Larger captured satellites, like Neptune’s moon Triton, are similar in size to our moon. However, Neptune has 17 times the mass of Earth.

In all cases but Earth’s moon, the ratio between the masses of the planet and a captured satellite is quite large. Urey’s capture model is not impossible, but it is highly improbable.

What turned out to be the correct conclusion (or at least movement toward it) was proposed by Canadian geologist Reginald Daly in 1946.

Daly proposed the “impact model.”

Daly agreed with Darwin on one point. He assumed that the Earth and the moon were composed of the same material. The best way to explain that similarity was a cataclysmic impact between Earth and a Mars-sized object.

The impact would have thrown hot Earth magma into space. That material eventually formed a sphere of liquid rock that finally cooled to form the moon.

Most astronomers ignored Day’s hypothesis until the early 1970s, and with good reason. Without evidence of the moon’s chemical composition, the assumption that the Earth and moon are similarly composed remained unproven.

But then came the Apollo missions to the moon. American astronauts brought back 842 pounds of lunar rock.

When geologists and chemists went to work, they discovered that the rocks’ mineral content closely matched the composition of Earth’s mantle.

Earth’s mantle is the layer between Earth’s thin outer crust and its relatively large metallic core. Both Earth’s mantle and the lunar rocks are rich in silicates but low in metals.

Most of Earth’s metals sank to its core because they are heavier than the rocky material in our planet’s mantle. In the moon’s case, the rocky silicates could have resulted from material gouged from Earth’s mantle by a glancing blow from another small planet.

The collision created a ring of scattered material that eventually coalesced into Earth’s moon.

However, the impact model needed refinement to explain Earth’s and the moon’s structure and composition. Why, for example, does Earth have a relatively sizeable metallic core?

Astronomers began developing the Big Splash hypothesis during the last decade to answer those questions.

In the Big Splash model, Earth did not suffer a glancing blow. Instead, it was more like a high-speed, head-on collision.

As Theia whacked into Earth, the cores of the two planets merged. Earth’s liquified magma squirted out the other side and formed a ring of material around Earth. In the fullness of time, the material cooled and recombined to form the moon.

However, problems remain.

The moon is tidally locked to Earth. One of its sides is always facing Earth. The other side is always facing away.

The two sides are strikingly different in their structure. The near side is relatively low and flat, but the far side has a thicker crust and is far more mountainous.

The impacts that formed the maria explain the flatness of the near side to some extent. They do not explain the differences in crust thickness.

In 2011, astronomers Erik Asphaug and Martin Jutzi created a computer model to explain the anomaly. They concluded that Earth once had a second, smaller moon in the same orbit as Luna.

According to the model, the ring of molten material created by the Big Splash produced at least two (and maybe more) Earth satellites. Because they formed out of the same ring of material, they were in the same orbit when they coalesced.

Moon #2 trailed the current moon by about 60 degrees of arc, about 1/6 of its orbit.

Soon after the two moons formed, their mutual gravitational pull drew them together. Moon #2 eventually collided with the moon’s far side at a relatively slow speed of 4,400 miles an hour.

The collision wasn’t fast enough to carve out a gigantic crater or even melt lunar rock. Instead, it splattered a large quantity of material on the moon’s far side.

Asphaug compared the collision to a low-speed automobile fender-bender. “It’s like a car crash, where you have crumpled bumpers, but you don’t melt the cars as they’re colliding.”

Thus, the Big Splash model has become the best explanation for the moon’s formation — so far. In the minds of scientists, the Big Splash is more like a hypothesis than an established set of facts.

As educated guesses go, it seems to fit the current data. However, as we return eventually to the moon and more evidence becomes available, the hypothesis may need to be refined or even replaced. Stay tuned.

By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

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