Shortly after the Moon was birthed from the Earth as a giant, molten blob, it cooled and hardened into solid rock. Lacking a protective atmosphere, its surface was pocked and marred by billions of years of asteroid strikes. Scientists have long known that not all of the Moon's dark surfaces are due to impacts. But the origin of one of the Moon's most identifiable features, the Ocaenus Procellarum—the large dark splotch on the Moon's Earth-facing side—has been a matter of some debate.
According to the latest research, says Nature, this dark spot, the one that creates the image of the Man on the Moon, was not formed not by a massive asteroid strike.
Many of the dark spots—“lunar maria”—are due to volcanic eruptions: the dark area forms when lava cools on the Moon's surface. In some cases, both factors worked together—an asteroid hit the Moon, and lava filled the base of the crater. But, in this case, previous research has shown the rocks in the Oceanus Procellarum to be high in radioactive elements like uranium, says Nature, a sign interpreted by scientists as evidence of an impact.
In this new study, however, the authors suggest that the radioactive material was there all along, and it was the heat emitted by the radioative material that caused the valley to form:
They say that the region's high concentrations of radioactive elements would have made the area hotter than its surroundings, and quicker to cool... As the region cooled and contracted, the crust at its edges would have stretched, forming valleys in the distinctive rectangular pattern, they say. The process is similar to the way [that] fractures... form around a mud puddle as it dries, says Andrews-Hanna.
The thinning lunar crust would have developed cracks, says the Telegraph, through which lava could have flowed, darkening the surface.
After this dark lava layer cooled the region continued to be battered by asteroids, but by then the dark canvass on which the Man on the Moon was drawn had already been set.