The Two Sides of the Moon

One of the things everyone notices when looking at the Moon are the dark spots or the Man in the Moon.

The Near Side of the Moon

These dark places are called “maria” from the Latin word for seas. I suppose early astronomers speculated that they really were oceans on the Moon. There is, of course no water on the Moon and the “seas” are, in fact, plains of basalt formed by volcanic eruptions when the Moon was first formed. The maria are richer in iron than the lunar highlands and so are darker.

Naturally, astronomers expected the far side of the Moon to be much like the near side. Until the space age, however, there was no way to know what the Moon’s far side was like. When the Soviet space probe Luna 3 sent back the first photographs of the far side of the Moon, in 1959, astronomers were surprised to discover that there are fewer maria on the far side.

The Far Side of the Moon
The Far Side of the Moon

Why there should be so few maria on the far side has been something of a mystery. It seems clear that the Earth must have something to do with it, but what exactly? Does the fact that the Moon has two faces have anything to do with the way it was formed? The study of exoplanets, planets orbiting other stars, may have helped to solve the mystery. Here is the article I read at Yahoo News.

Heat radiating from the young Earth could help solve the more than 50-year-old mystery of why the far side of the moon, which faces away from Earth, lacks the dark, vast expanses of volcanic rock that define the face of the Man in the Moon as seen from Earth, researchers say.

The Man in the Moon was born when cosmic impacts struck the near side of the moon, the side that faces Earth. These collisions punched holes in the moon’s crust, which later filled with vast lakes of lava that formed the dark areas known as maria or “seas.”


Now scientists may have solved the 55-year-old mystery; heat from the young Earth as the newborn moon was cooling caused the difference. The researchers came up with the solution during their work on exoplanets, which are worlds outside the solar system.

“There are many exoplanets that are really close to their host stars,”lead study author Arpita Roy, also of Penn State, told “That really affects the geology of those planets.”

Similarly, the moon and Earth are generally thought to have orbited very close together after they formed. The leading idea explaining the moon’s formation suggests that it arose shortly after the nascent Earth collided with a Mars-size planet about 4.5 billion years ago, with the resulting debris coalescing into the moon. Scientists say the newborn moon and Earth were 10 to 20 times closer to each other than they are now.

“The moon and Earth loomed large in each other’s skies when they formed, ” Roy said in a statement.

I would have liked to have seen such a sight.

Since the moon was so close to Earth, the mutual pull of gravity was strong. The gravitational tidal forces the moon and Earth exerted on each other braked their rotations, resulting in the moon always showing the same face to Earth, a situation known as tidal lock.

The moon and Earth were very hot shortly after the giant impact that formed the moon. The moon, being much smaller than Earth, cooled more quickly. Since the moon and Earth were tidally locked early on, the still-hot Earth — more than 4,530 degrees Fahrenheit (2,500 degrees Celsius) — would have cooked the near side of the moon, keeping it molten. On the other hand, the far side of the moon would have cooled, albeit slowly.

The difference in temperature between the moon’s halves influenced the formation of its crust. The lunar crust possesses high concentrations of aluminum and calcium, elements that are very hard to vaporize.

“When rock vapor starts to cool, the very first elements that snow out are aluminum and calcium,” study co-author Steinn Sigurdsson of Penn State said in a statement.

Aluminum and calcium would have more easily condensed in the atmosphere on the colder far side of the moon. Eventually, these elements combined with silicates in the mantle of the moon to form minerals known as plagioclase feldspars, making the crust of the far side about twice as thick as that of the near side.

“Earthshine, the heat of Earth soon after the giant impact, was a really important factor shaping the moon,” Roy said.

When collisions from asteroids or comets blasted the moon’s surface, they could punch through the near side’s crust to generate maria. In contrast, impacts on the far side’s thicker crust failed to penetrate deeply enough to cause lava to well up, instead leaving the far side of the moon with a surface of valleys, craters and highlands, but almost no maria.

“It’s really cool that our understanding of exoplanets is affecting our understanding of the solar system,” Roy said.

I did not know that astronomers had progressed so far in the study of exoplanets that they were able to learn some details about the geology of those planets. I am glad to be living in an age in which we are learning so much about the universe we live in.




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