Moon-forming crash sent Earth into a spin

There’s plenty about our familiar grey moon we still don’t know for sure. How did it form? Why is its orbit tilted slightly? And at around 380,000 kilometres distant, why is it so far away?

Matija Ćuk from the SETI Institute in California and a team of US planetary scientists have an explanation. Using modelling and simulations, they conclude that the collision that formed the moon sent Earth almost rotating on its side.

Over time, they write in Nature, interactions between the Earth, moon and sun smoothed out the whirling spin, leaving the duo in their current gravitational dance today.

A leading theory of the moon’s genesis is the giant impact model. It states that some 4.5 billion years ago, the young Earth collided with a developing planet, Theia.

Dust and rubble formed a disc around what was left of the Earth, which clumped together to become the moon.

“But this scenario does not quite work if the Earth’s spin axis was tilted at the 23.5 ° angle we see today,” says Douglas Hamilton from the University of Maryland and co-author of the study.

Physics says the debris – and thus, the moon – should have gathered into a ring around Earth’s equator. Then as tidal forces pushed the moon away, the moon should have made its way into an ecliptic plane, which is in the same plane as the Earth’s orbit around the sun.

Instead, the moon’s orbit is tilted five degrees away from the ecliptic plane today.

So what happened?

Ćuk and his colleagues ran different moon-forming scenarios. The ones that ended up with an Earth-moon system most like we see today involved a collision that sent the Earth spinning extremely fast – as much as twice the rate predicted by other models.

The impact also knocked the Earth’s tilt way off between 60 and 80 ° – almost on its side.

The newborn moon also started off very close to Earth, tracking closely with the Earth’s equator, but then drifted away. As it approached15 times its initial distance, the sun exerted its own influence over the moon’s orbit.

A highly tilted, fast-spinning Earth and an outward-migrating moon probably contributed to the moon’s current strange orbit, the researchers conclude. 

Hamilton acknowledges that the model isn’t perfect and doesn’t answer all questions about the moon’s orbit.

But “what we have now is a model that is more probable and works more cleanly than previous attempts”, he says.

“We think this is a significant improvement that gets us closer to what actually happened.”