The moon still holds many mysteries for scientists, not least of which is its unusual tilted orbit. Unlike other satellites which orbit around their planet’s equatorial plane, our moon’s orbit is distinctly elliptical. To be precise, our moon isn’t completely in line with the elliptical plane – it’s actually tilted five degrees away from it. And scientists have come up with a theory that attempts to explain this unusual phenomenon.
Researchers at the University of Maryland believe the peculiar tilt may have been a result of an angled, giant impact that vaporised most of the early Earth and created the moon. The popular theory (the Giant Impact hypothesis) is that the moon was formed when a huge rock the size of Mars (which scientists call Theia) grazed the Earth and the debris from that collision condensed to form the moon. But this theory couldn’t explain why the isotopic composition of the moon is strikingly similar to Earth, simply because astronomers find it unlikely that Theia would have a similar chemical composition to Earth.
So using new computer simulations, University of Maryland researchers modified the Giant Impact hypothesis to explain the moon’s chemical composition. They suggest that an extraordinarily high energy impact collision took place billions of years ago that vaporised Theia completely as well as most of early Earth, almost stripping our planet down to its core.
The impact caused a dense vapour that turned into a massive cloud filled with debris. Some of the debris would have fallen back on to Earth as it cooled – but a few of these rocks may have condensed to form the moon. “Evidence suggests a giant impact blasted off a huge amount of material that formed the moon,” said Douglas Hamilton, professor of astronomy at the University of Maryland and a co-author of the paper published in Nature. “This material would have formed a ring of debris first, then the ring would have aggregated to form the moon.” The impact also caused massive changes to Earth’s rotation and the tilt of its spin axis.
Matija Cuk, lead author of the study, said the Earth span much faster and at a much steeper angle when it initially formed more than four billion years ago. The moon, which started off very close to Earth, slowly drifted away to nearly 15 times its initial distance and the sun began to exert a more powerful influence over the moon’s orbit. The Earth, on the other hand, was tilted by more than 60 degrees after the moon formed, which is why the moon could not transition smoothly from Earth’s equatorial plane to the ecliptic plane. Instead, the transition was abrupt and chaotic and left the moon with a large tilt in its elliptical orbit.
“We already suspected that the Earth must have spun especially fast after the impact,” says Cuk. “As the moon moved outward, the Earth’s steep tilt made for a more chaotic transition as the sun became a bigger influence. “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.”