Simulations suggest a dramatic history for the Earth-moon duo
The moon, Earth’s closest neighbor, is among the strangest planetary bodies in the solar system. Its orbit lies unusually far away from Earth, with a surprisingly large orbital tilt. Planetary scientists have struggled to piece together a scenario that accounts for these and other related characteristics of the Earth-moon system.
A new , based on numerical models of the moon’s explosive formation and the evolution of the Earth-moon system, comes closer to tying up all the loose ends than any other previous explanation. The work, published in the October 31, 2016 Advance Online edition of the journal Nature, suggests that the impact that formed the moon also caused calamitous changes to Earth’s rotation and the tilt of its spin axis.
The research suggests that the impact sent the Earth spinning much faster, and at a much steeper tilt, than it does today. In the several billion years since that impact, complex interactions between the Earth, moon and sun have smoothed out many of these changes, resulting in the Earth-moon system that we see today. In this scenario, the remaining anomalies in the moon’s orbit are relics of the Earth-moon system’s explosive past.
“Evidence suggests a giant impact blasted off a huge amount of material that formed the moon,” said , professor of at the
Collisional physics calls for this ring of debris—and thus the moon’s orbit immediately after formation—to lie in Earth’s equatorial plane. As tidal interactions between the Earth and the moon drove the moon further away from Earth, the moon should have shifted from Earth’s equatorial plane to the “ecliptic” plane, which corresponds to the Earth’s orbit around the sun.
But today, instead of being in line with the ecliptic plane, the moon’s orbit is tilted five degrees away from it.
“This large tilt is very unusual. Until now, there hasn’t been a good explanation,”
“We already suspected that the Earth must have spun especially fast after the impact” Ćuk said. “An early high tilt for Earth enables our planet to lose that excess spin more readily.”
The model also suggests that the newly-formed moon started off very close to Earth, but then drifted away—to nearly 15 times its initial distance. As it did so, the sun began to exert a more powerful influence over the moon’s orbit.
According to the researchers, both factors—a highly tilted, fast spinning Earth and an outwardly-migrating moon—contributed to establishing the moon’s current weird orbit. The newborn moon’s orbit most likely tracked the Earth’s equator, tilted at a steep 60-80 degree angle that matched Earth’s tilt.
A key finding of the new research is that, if the Earth was indeed tilted by more than 60 degrees after the moon formed, the moon could not transition smoothly from Earth’s equatorial plane to the ecliptic plane. Instead, the transition was abrupt and left the moon with a large tilt relative to the ecliptic— much larger than is observed today.
“As the moon moved outward, the Earth’s steep tilt made for a more chaotic transition as the sun became a bigger influence,” Ćuk said. “Subsequently, and over billions of years, the moon’s tilt slowly decayed down to the five degrees we see today. So today’s five degree tilt is a relic and a signature of a much steeper tilt in the past.”
“There are many potential paths from the moon’s formation to the Earth-moon system we see today. We’ve identified a few of them, but there are sure to be other possibilities,”