Earth's powerful gravity tugged the moon into its oddball shape long ago, shortly after both bodies formed, a new study suggests.
Tidal forces exerted during the early days of the solar system can explain most of the moon's large-scale topography, including its slight lemon shape, reports the study, which was published online on July 30 in the journal Nature.
The new findings could help scientists tackle longstanding lunar mysteries, such as why the moon's near side is dominated by dark volcanic deposits, while the far side is not, researchers said.
"What is the origin of that asymmetry?" said study lead author Ian Garrick-Bethell, of the University of California, Santa Cruz. "Chipping away at this problem of the shape of the moon can give us insight into those types of fundamental geology problems,".
A young, molten moon
Scientists think the moon formed from debris blasted into space when a mysterious planet-size body slammed into the young Earth about 4.5 billion years ago. The moon was born hot, and it came into existence quite close to our home planet. (The moon has been slowly spiraling away ever since.)
The newborn moon was thus primed to be sculpted by Earth's gravity, and that's exactly what happened, researchers say.
Indeed, scientists have posited for more than a century that tidal forces helped shape the molten moon, causing bulges that froze into place when Earth's natural satellite cooled down and solidified. But the new study provides a much more detailed understanding of how this likely happened.
Garrick-Bethell and his team studied topographic data gathered by NASA's Lunar Reconnaissance Orbiter and information about the moon's gravity field collected by the agency's twin GRAIL (Gravity Recovery and Interior Laboratory) spacecraft. Though the researchers took a global view of the moon, they focused on areas outside of the body's biggest impact craters, which can complicate such analyses.
The data strongly implicate tidal effects as a key shaper of the moon, researchers said. For example, tidal forces pulled on the lunar crust, stretching it out and heating it up in places. This process thinned out the crust at the lunar poles and thickened it in the regions that lined up with Earth, helping sculpt the moon into a lemon with two small bulges (one on the side facing our planet, and one on the side directly opposite).
Such tidal heating could have occurred only when the moon's crust was floating on a sea of molten rock, largely decoupled from the rest of the body, Garrick-Bethell said.
"This happened a long time ago, when the moon was not completely solid," he said. "This was in the first 100 to 200 million years of lunar thermal evolution."
Also contributing to the moon's overall shape were more straightforward tidal deformations, which Garrick-Bethell likened to squeezing the lemon with your hands, and rotational forces, which cause spinning bodies such as the moon to flatten at the poles and bulge out near the equator.
When the moon cooled, the changes wrought by all of these processes were frozen in place.
Interestingly, the long axis of the moon doesn't point directly toward Earth as it likely did long ago; instead, it's offset by about 30 degrees. This probably happened when volcanic activity, impact cratering and other events made the moon's interior a much less homogeneous place, researchers said.
"Internal density anomalies have formed, and they've kind of canted the moon," Garrick-Bethell said. "At some point in lunar history, you have these events that took place that caused these density anomalies and shifted the density axis away from the shape axis."
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