A Natural History of the Moon Trivia Quiz

Answer: Collision or giant impact of planetoid on Earth

About 4.5 billion years ago (bya) a Mars-sized planet collided with the Earth. This impactor is sometimes called Theia, after the Greek Titan who was the mother of Selene (the personification of the moon). The impact threw debris into space, while Theia melted and merged with the Earth. The Moon was in a molten state, orbiting close to the Earth, resulting in tidal forces that in turn deformed the molten Moon into an ellipsoid, with its axis pointed toward the Earth. This hypothesis explains the similar orientation of the Earth's spin and the Moon's orbit. The Moon's relatively small core is because the iron in the core of the Earth and Theia remained on Earth, and most of the material involved was in the Earth's crust and mantle. It also explains why moon rocks appear "baked", have a composition similar to the Earth's mantle, and have no volatile compounds, which simply remained vapors.

Apollo and further study have not supported the other three hypotheses, which were popular in the 1960s before the Apollo missions. If the Earth and the Moon formed at the same time, they would have nearly identical composition, and they don't. The Earth lacks sufficient gravity (due to its size) to capture a wandering body such as the moon and keep it. Lastly, the Earth couldn't spin fast enough to eject a moon-sized blob.

Answer: maria (seas)

The darker areas are 'maria' (Latin for "seas") as, in ancient times, it was thought they really were seas, and the brighter areas are 'terrae' (Latin for "lands"). The maria are actually vast lowlands or plains of basaltic lava cooled into igneous rock. The basaltic lava flowed into depressions and flooded large parts of lunar surface, about 15% of the Moon.

Some of the maria include:
- Mare Tranquilitatis (Sea of Tranquility): where the first astronauts landed in the Apollo 11 mission.
- Mare Imbrium (Sea of Showers): the largest mare (700 mi or 1100 km in diameter)
- Mare Serenitatis (Sea of Serenity)
- Mare Nubium (Sea of Clouds)
- Mare Nectaris (Sea of Nectar)
- Oceanus Procellarum (Ocean of Storms)

Some maria lie within the walls of large circular basins such as Serenitatis and Imbrium, whereas others like Oceanus Procellarum occupy much larger, irregular depressions.

Answer: highlands or terrae (lands)

The highlands, or terrae, are older formations. After the giant impact of 4.5 bya, the magma of the molten Moon cooled and iron and magnesium silicates crystallized and sank. Feldspar-rich minerals crystallized and rose. As the ocean of magma cooled, it formed a lithosphere of material similar to the Earth's mantle. As the Moon continued to lose heat, the upper mantle (athenosphere) shrank toward the core and the lithosphere thickened. The rapid growth of the lithosphere hindered a plate tectonic system and made the interior of the moon quite different from the Earth's. But essentially, what you had were the terrae, without the maria just yet.

The maria would come during a later period of volcanic eruption which also produced domes or 'montes' (mountains), such as Mon Rücker, and fissures called 'rimae' (channels) or 'rilles' (grooves), such as Rima Hadley.

Answer: craters

The origin of the Moon's craters as impact features became widely accepted only in the 1960s! Between 4.5 and 3.9 bya, the moon underwent a period of intense bombardment from asteroids and other objects, which created the cratered surface in the lunar highlands. Impact craters tend to be perfectly round even though the objects creating them can be any shape.

When the body impacts the Moon and disintegrates, the force of the explosion excavates the ground and creates a round impression larger than the impactor itself.

Some of these bombardments produced large, multi-ringed basins and mountains. The most recent impact craters have a sharp-edged rim.

Answer: It has fewer maria (dark areas).

Once the meteoric bombardment ceased, from 3.9 to 3.2 bya the Moon underwent a period of vulcanism. Lava flowed through volcanoes and extruded from cracks in the crust. This lava filled the lowlands (maria) and cooled to basalt, an igneous rock. Much of the moon's heat was lost during this period. Because there were more heat-producing elements in the near side, and because the moon's crust is slightly thinner on the near side, lava could erupt more easily to fill the maria basins, and this may explain the far side is so lacking in maria. The heat from the molten Earth, which would have affected the near side more, may also be a factor.

The far side of the moon was never seen by us Tellurians until the Soviet satellite Luna 3 took its first pictures in 1959. Impact craters dominate the surface, with only a couple of small maria.

Answer: The Moon is tidally locked.

Tidal locking is also called gravitational locking or captured rotation. It means an object's orbital period matches its rotational period. In the case of the Moon, it takes about 28 days to revolve around the Earth, and it also takes about 28 days to rotate once around its axis. So when the moon is visible to us, it is always the one side that we can see. The moon used to spin faster, but the gravitational force of the Earth slowed it down, hence gravitational or tidal locking.

It is misleading to call it the "dark side", however, because the far side does get sunlight. It's just that at that point in its rotation, we cannot see the moon from Earth. During the New Moon phase, the Moon is between the Earth and Sun, so the far side is completely lit, but as the Moon is also on our daytime side, our nighttime side can't see it.

In one sense, however, it may be called the "dark side": when spacecraft are on the far side, they cannot be contacted; communications are dark.

Answer: regolith

The regolith is a fine powder of rock fragments and volcanic glass particles interspersed with larger rocks. The lunar regolith may extend 4-5 meters down in some places, perhaps 15 meters in the older highland areas. Some scientists object to calling lunar regolith "soil"; the regolith on the Earth is soil because it contains organic compounds. (There is also regolith on Mars.)

Astronaut Buzz Aldrin's footprint in the photograph will likely be there another billion years, as there is no wind or weather to blow it away. Consequently the lunar regolith is more jagged than earthly regolith; nothing wears down the sharp edges.

Answer: There is no measurable magnetic field around the Moon.

Although there is no significant source of internal energy, moonquakes have been recorded. The Moon's diameter is 2160 miles (3476 km) or 27% of the Earth's diameter, and the mass of the Moon is 7.35 10^22 kg, or 1.2 % of the Earth's mass. Consequentially, its gravitational force is weaker.

A falling object accelerates at 1.62 m/s^2, or 16.6% of Earth's gravitational pull. The atmosphere of the moon is so tenuous that it is rather more accurate to say it has none, for all practical purposes. The Moon has a north pole (pictured) and a south pole, but there is no geodynamo in its core to generate a magnetic field like Earth's. Yet it does have a very weak but measurable magnetic field that appears to come from the crust.

Answer: Copernicus

After the volcanic period ended, the Moon's heat was gone. In this Copernican period there has been little geologic activity other than impacts, and these have grown fewer and farther in between than before. Some more recent bombardments that produced Tycho and Copernicus, two craters on the maria, and the regolith. Impacts by meteorites and comets are the only abrupt geologic force acting on the Moon today.

The lunar Copernican period precedes and includes the entirety of the Phanerozoic eon on Earth (which includes everything from the Paleozoic era to the present).

At the time that animal life was beginning to flourish on the Earth, geologic activity on the Moon was dwindling.

Answer: Stabilized its orbit

As the Earth rotates on its axis, it wobbles, but the Moon's gravitational pull reduces this somewhat. Without the Moon, the Earth might wobble 10, 45, perhaps even 90 degrees off axis, in one direction and then the other. The climate in turn would be unstable as equatorial regions would become polar and then equatorial again. (Even a wobble as little 10 degrees would wreak havoc on seasons and climate.) Without the stability provided by the Moon, it is quite possible there would be no life on Earth.