The Moon has always been mysterious. And even after 12 American astronauts took strolls on the lunar landscape 40-plus years ago, many of the mysteries remain. One enduring puzzle has to do with the five-degree tilt of the Moon’s orbit. The Moon is thought to have formed when a Mars-sized proto-planet collided with the early, molten Earth in the Solar System’s formative times. The impact ejected a huge mass of molten rock which cooled, coalesced, and became the Moon we know today.
The evidence all lines up, but the prevailing math suggests such a body should have an orbital tilt of one degree, not five. So, what else happened?
“When the Moon first formed, it did so while the inner Solar System was still somewhat crowded,” says planetary scientist Kaveh Pahlevan, of the Université Côte d’Azur in Nice, France.
Pahlevan is part of a research team which believes it may have found a solution to the so-called “lunar inclination problem.”
“The jetsam and flotsam of planet foundation had not yet been swept clear,” he explains.
“These interloping bodies [other spheres and planetoids] would have passed through the Earth-Moon system thousands of times. These so-called collisionless encounters can – through gravitational interactions – pull the Moon out of its orbit of birth. We propose that the Moon’s orbit was therefore nudged through this process while the Earth swept up the last one percent or so of its mass from interplanetary space.”
The research took the form of thousands of complex computer simulations of the early Solar System. Each went its own way, but when averaged and aggregated, a clear new picture emerged.
“For the past 15 years, researchers have favored a ‘late’ formation of the Moon, placing the event toward the very end of Earth formation,” Kaveh says.
“For the first time, we are able to specify precisely how late. The Moon-forming event left the Earth with 99 per cent of its final mass. That the lunar orbit can be so easily altered also implies … that its late formation is not a coincidence. Any moons that formed earlier were simply lost.”