March 7, 2012
Staring Back At The Man In The Moon, Researchers Look For Answers
Have you ever wondered where the man in the moon comes from, or perhaps more interestingly, why he always fixes his gaze on us rather than showing his dark backside?
Pondering those questions further, researchers from the California Institute of Technology (Caltech) have come up with some answers.
The moon´s near side facing us is covered with dark splotches of lunar maria (smooth plains formed by ancient volcanic eruptions) that resembles a man´s face when seen from Earth. The far side of the moon has many craters and elevated topography, and looks quite different, explain Caltech researchers.
As the moon revolves around Earth, the near side constantly faces Earth. It is locked in what is called a synchronous orbit -- meaning the moon rotates exactly once every time it orbits Earth. Some scientists had previously believed the fact that the man faces Earth was the result of a coincidence.
Oded Aharonson, professor of planetary science at Caltech, explained that coincidence as a lunar coin toss, giving it a 50/50 chance of landing left or right. But he and his colleagues have determined this is not the case. In the past, the moon spun around its axis at a much faster rate than it does today, and new analysis shows that the fact that the man is facing us may be the result of the rate at which the moon slowed down before becoming fixated into its current rotation.
The team, consisting of Aharonson; Peter Goldreich, the Lee DuBridge Professor of Astrophysics and Planetary Physics, Emeritus; and Re'em Sari of the Hebrew University of Jerusalem, described their findings in a paper published in the journal Icarus.
The team explains that, although the moon looks spherical, it is actually elongated, kind of like a football. Not long after the moon formed -- about 4 billion years ago -- while it was still hot and molten, Earth´s gravity began to stretch it out. And when the moon finally cooled, its oblong shape stuck. The man in the moon occupies one of the two elongated ends.
The reason we always see the near side has to do with the fact that the moon rotates on its axis once with every revolution around the Earth, so that the same side is always facing Earth. About 2 billion years ago, the moon rotated much more rapidly, and someone watching the moon back then would have seen all the different sides of the moon at various stages.
Eventually, Earth´s gravitational pull on the moon slowed it down through a dissipative process first explained in the 1960s by Goldreich.
Earth´s tidal forces pulled at the moon, creating another slight bulge in the already elongated moon. That bulge moved to stay on whichever side was closest to Earth at that moment. The bulge continued to point toward Earth as the moon rotated, causing it´s interior to squish and flex as the bulge changed position. The internal friction from this squishing and flexing acted as a brake that slowed the spinning of the moon until it matched its revolution rate, which is where it is today.
As a result of Earth´s gravity, the moon became locked into an orientation with its long axis pointing toward Earth. But, based on this theory, wouldn´t it seem more plausible that the moon´s more mountainous side with an elevated topography face us rather than the smooth plain side?
Based on a raw analysis of the physics, it might be expected that the far side to face Earth, because its surface and mass would be closer to Earth.
But the researchers explained that, through a more detailed analysis of the physics of the moon, the rate at which the lunar satellite slowed down its spinning and how fast it dissipated its rotational energy is the reason why it shows us the side we see now. If the moon had lost its energy at a significantly different rate than it did, there would have been a chance that the far side would have been facing us.
Aharonson said, in that scenario, having the man face us would indeed have been the result of a coin toss. But, because the rate of energy dissipation was much slower, the man in the moon had about two-to-one odds of facing us.
The team used computer simulations to see at which rates of energy dissipation would result in either the man in the moon or the mountainous side facing us.
“The real coincidence is not that the man faces Earth,” said Aharonson. Instead, the real coincidence is that the moon´s dissipation rate was just the right amount to create such fascinating physics.
However, there is a stipulation to their theory. The teams´ analysis was based on the present-day moon.
“In the past, when the moon first locked, it could´ve had different properties,” noted Aharonson. If that was the case, then the explanation for why we see the man might result in different odds.
But, if the moon locked into its synchronous orbit relatively recently -- within the last billion years or so -- then there´s a good chance the researchers´ analysis is fitting.
Support for the research was provided by NASA´s Lunar Reconnaissance Orbiter project.
Image 2: The moon's near side (left) is covered with dark splotches of lunar maria that look like a man's face when seen from Earth. The moon's far side (right), with its many craters and elevated topography, looks quite different.
On the Net:
- California Institute of Technology
- Hebrew University of Jerusalem
- Lunar Reconnaissance Orbiter